Indications for CI surgery should be considered carefully and involve a number of factors before the procedure can be performed.8,9

• •

  Patient and family goals and expectations – It is crucial to understand what the family and patient expect from the procedure and align expectations with clinical reality.

• •

  Family involvement – Family support and involvement are critical to the success of CI, especially in children, ensuring consistent device use and active participation in speech therapy.

• •

  Consistent device use – The CI should be used for as long as possible each day to obtain the best results.

• •

  Realistic expectations – Although CI can significantly improve hearing, it is important that families and patients have realistic expectations. Progress may be slow, and patients will need ongoing support to develop their listening and speaking skills.

• •

  Prognosis assessment – The etiology of hearing loss, patient age, duration of auditory deprivation and the presence of other pathologies should be considered to estimate good or bad prognosis.

The indication for CI surgery requires coordination between otolaryngologists, speech therapists, audiologists, psychologists, and other health professionals, such as neurologists, pediatricians, radiologists, psychiatrists, social workers, and educators, varying according to the needs of each patient and the reality of the auditory rehabilitation center. The criteria to indicate CI are not uniform throughout the world.

The clinical visit should include a precise and comprehensive history-taking to support not only the diagnosis but also the prognosis regarding the progression of hearing loss, indications for CI, expectations for post-implant development, and counseling. The main questions that should be considered are as follows: (1) Onset and diagnosis of hearing impairment; (2) History of hospitalization and comorbidities; (3) Family history of hearing impairment; (4) Pregnancy history (mainly in children); (5) Perinatal history; and (6) Length of time using hearing aids (including current use or no use).

The worst postoperative outcomes are associated with increased duration of deafness (auditory deprivation), inconsistent use of hearing aids, presence of syndromic causes, perinatal problems, cochlear ossification, and atypical cochlear anatomy – especially changes in the anatomy of the Internal Auditory Canal (IAC) and Common Cavity (CC) deformity.1,10

Physicians evaluating patients who are candidates for CI should consider family and personal history. Family history allows a detailed assessment of the presence of premature, severe-to-profound, syndromic or nonsyndromic hearing loss. Medical history includes perinatal history (Cytomegalovirus [CMV] infection, prematurity, kernicterus, hypoxia), diabetes mellitus, hypertension, and uncontrolled arrhythmia, as well as otologic history (noise exposure, ototoxic drugs, chronic ear disease, ear surgery, trauma).

In addition to a detailed history and physical examination, audiologic evaluation is the first step in the investigation. The following tests are typically available:

• a)

  Pure-tone and speech audiometry;

• b)

  Transient-Evoked Otoacoustic Emissions (TEOAEs) and Distortion-Product Otoacoustic Emissions (DPOAEs);

• c)

  Auditory Brainstem Response (ABR);

• d)

  Tone-Burst ABR (TBABR) or Auditory Steady-State Response (ASSR);

• e)

  Free-field audiometry.

Laboratory tests are not necessary for etiologic investigation without any clinical suspicion due to the low diagnostic yield of these tests (0%–2%).42 There is a discussion in the literature about the indication of universal NHS for congenital CMV infection. This is a highly prevalent virus, and diagnosis is only possible if the infection is detected within the first 30 days of life. But it is still not performed routinely.43

Genetic testing should be considered for CI candidates, especially children. It has a high yield, detecting the condition in 44% of patients with bilateral sensorineural hearing loss.44 GJB2 gene mutations account for more than 50% of cases of sensorineural hearing loss due to autosomal recessive inheritance and 20% of cases of prelingual hearing loss in high-income countries.45 One of the most common GJB2 mutations is 35delG, accounting for 70% of cases.46,47 Therefore, it should be considered in children. Other mutations such as those in the SLC26A4 gene are less frequent.48

The investigation may be complemented by a renal ultrasound to check for congenital malformations, an electrocardiogram to rule out long QT syndrome (as seen in Jervell and Lange-Nielsen syndrome), and other tests based on clinical findings.

Imaging plays a role in determining the etiology, identifying abnormalities related to hearing loss progression, and surgical planning.49,50 It allows the evaluation of the anatomy of the structures of the cochlea, labyrinth, cranial nerves, and central auditory pathways. Some inner ear malformations may contraindicate CI surgery.51

The diagnostic yield of imaging studies in the evaluation of children with profound bilateral sensorineural hearing loss ranges from 7% to 74%, with high specificity and high positive predictive value.52 The choice of imaging modality should take into account radiation exposure, diagnostic accuracy, duration, need for sedation, and cost.53

Imaging modalities for the assessment of sensorineural hearing loss include Computed Tomography (CT) and Magnetic Resonance Imaging (MRI). CT provides better visualization of bone structures, lower costs, and faster imaging, which makes it easier to use. However, CT scans expose patients to ionizing radiation, and longitudinal studies have shown an increased lifetime risk of developing certain types of cancer in children.54 MRI provides better quality images of soft tissues and neural structures, but it is associated with higher cost and longer acquisition time, often requiring sedation to be accomplished in children.55,56

As an advantage, MRI can clearly identify the cochlear nerve on T2-weighted sequences, and abnormalities or tumors detected on MRI contraindicate CI. Concern has been raised about the role of gadolinium-based contrast agents in MRI due to controversy over gadolinium deposition in the brain.57 Thus, many recent pediatric hearing loss investigation protocols do not require gadolinium. In adults with postlingual deafness, CT has been advocated if there is concern for chronic otitis media or a history of middle ear disease, whereas MRI would be essential in patients with a history of meningitis or asymmetric sensorineural hearing loss.58,59

There is considerable variation between clinical practice and institutional protocols in diagnostic methods used.42,60 There is a correlation between diagnostic yield and hearing loss severity, with an increase of almost 20% in diagnostic yield in severe-to-profound hearing loss compared with mild-to-moderate hearing loss.61 The diagnostic yield is lower in patients who test positive for mutations in the GJB2 gene, which is the most common cause of nonsyndromic hearing loss.62

Two systematic reviews indicate that CT appears to have a higher diagnostic yield than MRI to diagnose Enlarged Vestibular Aqueduct (EVA) and cochlear abnormalities, whereas MRI can detect more cochlear nerve abnormalities.63,64 In a multicenter study evaluating the diagnostic yield of CT vs MRI in children with profound bilateral hearing loss, the concordance rate between CT and MRI was 92%, with the diagnostic yield of CT being 7% higher than that of MRI (p = 0.0001).65 In another study with a similar design comparing diagnostic yield between CT and MRI, the concordance rate between them was 86%, with MRI having a 14% higher yield for detecting diagnostic findings in children with profound bilateral sensorineural hearing loss.66

In cases of auditory neuropathy, MRI proved to be highly effective at detecting cochlear nerve hypoplasia, a common finding in these cases.67 Between 12% and 18% of children with congenital sensorineural hearing loss have hypoplasia or aplasia of the eighth cranial nerve. MRI has the highest diagnostic yield in these cases.68 The IAC may also be hypoplastic in these cases, which can be detected on CT.

The facial nerve has an abnormal course in 16% of children with inner ear malformations, representing increased difficulty in CI surgery, especially when it runs across the promontory or makes the facial recess narrower. In some children, the facial nerve may be split, and a second nerve may be located anteriorly, in addition to a branch located in the normal position of the facial recess.69 In these cases, a detailed study with MRI and CT is recommended.

The most common inner ear malformation is EVA, which can be detected by CT or MRI. The modern radiologic definition of EVA has been established as a mean diameter greater than 1.5 mm.69 Mutations in the SLC26A4 gene are a common cause of EVA. It often accompanies other inner ear anomalies, such as incomplete partition type II. CT is the best imaging modality to assess the vestibular aqueduct. MRI offers complementary visualization of the endolymphatic sac and duct that are usually also enlarged.48

Hearing loss after meningitis is the most common cause of acquired bilateral sensorineural hearing loss in the pediatric population. During the acute stage, the CT scan may be normal, but MRI reveals intense cochlear enhancement,70 and it is important to use contrast in these cases. In later stages, fibrosis is shown by loss of T2 signal in the cochlea. Cochlear ossification can be seen on both CT and MRI. MRI is essential to assess the extent of injury and feasibility of CI surgery in these cases.

Regardless of the choice of imaging modality, visualization of the cochlea is essential before CI. Although imaging often has little influence on the approach for CI surgery in adults with postlingual deafness, it provides necessary insights in the evaluation of hearing impairment in children that can have a major impact on CI decision-making.

Traumatic brain injury can lead to drastic conditions, such as facial paralysis and profound sensorineural hearing loss with involvement of the otic capsule. CT is essential in these cases to properly see the fracture lines. Longitudinal fractures, generally sparing the otic capsule, account for the majority of temporal bone fractures. In transverse fractures, due to the direction of the fracture line toward the otic capsule, patients often have some complications such as CSF leak, damage to the otic capsule, and injury to the facial nerve.71,72

The vestibular system consists of 3 semicircular canals (anterior, posterior, and lateral) and 2 otolith organs (saccule and utricle). Each of these 5 structures has sensory receptors consisting of highly specialized hair cells, with the ampullary cristae located in the ampullae of the semicircular canals, and the utricular and saccular maculae located in the respective otolith organs. The ampullary cristae are responsible for detecting angular accelerations, such as moving the head in different directions, whereas the maculae are responsible for detecting linear accelerations and head tilt, both in the vertical plane (saccular macula) and in the horizontal plane and in inclination (utricular macula).73,74

Body balance in humans depends on the integration of multiple systems and organs that send information to the central nervous system, which processes this information and sends signals and reflexes to regulate posture and gait. Among these multiple sensory systems, the following stand out: vision, vestibular system, and proprioception (a bundle of nerves, muscles, and joints).74 The vestibular system has a close anatomic relationship with the cochlea, sharing the same bone scaffold and membranous ducts, having similar sensory hair cells, and sharing the same embryonic origin. Thus, disturbances in one system can affect the other system.74,75

CIs can damage several vestibular organs, resulting in poor vestibular function, dizziness, and balance disorders.76 Multiple mechanisms contribute to vestibular dysfunction during or after surgery: (1) Direct trauma to the basilar membrane, modiolus, and spiral ligament from electrode insertion; (2) Acute serous labyrinthitis due to cochleostomy; (3) Foreign body reaction causing an inflammatory process, fibrosis, and osteogenesis; (4) Endolymphatic hydrops; and (5) Electrical stimulation of the labyrinth by the implant.77,78

Most studies investigating vestibular function in patients before and after CI surgery have been conducted in adults because of the technical difficulties associated with performing vestibular tests in children. Some studies have reported an incidence of vertigo after CI surgery in adults ranging from 2% to 35%, whereas vestibular dysfunction ranges from 20% to 80% after surgery, depending on the method used to assess labyrinthine function.79

The literature has shown a close relationship and high prevalence of vestibular disorders in children with congenital sensorineural hearing loss, ranging from 20% to 70%.80,81 Some studies even suggest a relationship between the etiology and severity of hearing loss and vestibular disorders. Children with severe-to-profound bilateral hearing loss would be more likely to have vestibular dysfunction, and some etiologies, such as Usher syndrome, congenital CMV infection, and GJB2 gene mutations, would also be more closely related to vestibular deficits.81–83

Achieving age-appropriate spoken language in pediatric CI users has been associated with higher levels of nonverbal intelligence and maternal education, higher levels of residual hearing after surgery, early intervention, a focus on auditory and oral instruction, and the use of newer speech processor technologies.94 Improved educational attainment and quality of life have been described in children receiving a CI.95 However, among CI users, long-term educational and occupational levels continue to be significantly lower than the population average.96

The main goal of NHS programs is to diagnose hearing loss as early as possible and refer the infant to a specialized center to confirm the diagnosis, investigate the etiology, and initiate appropriate treatment. Every effort should be made to limit the period of auditory deprivation, encouraging treatment as early as possible. Hearing rehabilitation using PSAPs before 6 months of age significantly improves vocabulary, speech intelligibility, general language skills, and socioemotional development.8,97

Children who receive a CI in infancy will benefit from auditory stimulation and demonstrate development in the brainstem and the thalamocortex.98,99 However, if deprived of access to sound, the auditory cortex will experience developmental arrest and demonstrate cortical reorganization over time.100 Children with bilateral hearing loss receiving only unilateral CIs will also demonstrate reorganization in the form of aural preference toward the implanted ear. There is evidence supporting that a longer inter-implantation delay (sequentially) increases asymmetry in access to sound.96,101

In children with congenital hearing loss, CI before 12-months of age offers the opportunity to promote auditory development during infancy and early childhood. The development of neural connections, a process known as functional synaptogenesis, peaks within the auditory cortex at around 2–4 years of age.102 Synaptogenesis decreases over time, a phenomenon that supports the idea of a critical period for the cochleotopic organization of the developing auditory cortex. The immature brain, with its increased plasticity, is especially suited to benefit from early CI.103 Therefore, access to sound and appropriate therapy during early childhood offers lifelong benefits to children with hearing loss.104

Lack of auditory stimulation can cause deviations in neural development, with long-lasting effects on auditory development, language acquisition, and even higher-level cognitive skills.105,106 Likewise, unilateral auditory stimulation may cause asymmetric development and compromise how the auditory system responds to stimulation from a subsequent CI in the contralateral ear.107 Early CI improves auditory processing skills.108

CI in the first year of life has been associated with age-appropriate language gains, while comparatively poorer developmental outcomes have been observed in children undergoing surgery later in life.109 After 1-year of age, hearing gains decrease as the child’s age at the time of surgery increases. Colletti et al.110 found evidence of an additional benefit when CI is performed even earlier, before 6-months of age.

Although age at the time of CI may have only minor long-term effects on general language understanding, it has a lasting effect on more specific and complex skills within the domains of phonetics, expressive vocabulary, grammar, and semantics. These skills depend on the functional specialization of certain networks in the brain that are triggered by sensory input during early sensitive periods of neuronal development.110

While studies of central auditory development reflect the importance of a sensitive period for cortical plasticity that peaks around 2–4 years of age, the functional importance of CI in children under 12 months of age is highlighted by studies specifically investigating expressive and receptive language outcomes. Comprehensive literature reviews on language development in children with congenital hearing loss suggest that early CI may prevent long-term spoken language deficits in children implanted before 12 months of age.111,112

Colletti et al.,113 investigating a cohort of 19 children implanted before 12 months of age, compared their speech and language development over 10 years of follow-up with that of children implanted after 12 months of age. The authors found that, after 5 years of CI use, all 19 children implanted before 12 months of age had achieved speech that was intelligible to the average listener, compared with only 67% of children implanted between 12 and 23 months of age.

Dettman et al.,114 evaluating a cohort of more than 400 pediatric CI users, found that children implanted before 12 months of age demonstrated significantly better speech perception, language acquisition, and speech production accuracy and were also more likely to score within the normative range of language performance upon school entry.

Bruijnzeel et al.115 conducted a systematic review of studies investigating potential speech and language benefits for children undergoing CI surgery before 12 months of age. The authors identified 14 studies and found that children implanted before 12 months of age performed better on speech production, auditory performance, and some receptive language scores. Ching et al.,116 in a prospective study of 350 children with hearing loss, found a strong positive benefit to early intervention; for example, children implanted at 6-months of age had significantly higher global language scores (a summary score of 20 language measures) at 5-years of age than those implanted at 24-months of age.

In infants, all tests should support a diagnosis of bilateral profound sensorineural hearing loss. Any inconsistencies in the testing should be addressed before making any recommendation. Evidence-based guidelines suggest that infants with a pure tone average better than 65 dB HL should continue with hearing aids, while infants with a pure tone average poorer than 80 dB HL should proceed to CI.117 Multidisciplinary CI teams should often closely monitor infants with hearing thresholds between 65 and 80 dB HL.117

Protocols for hearing aid fitting in infants with permanent hearing loss include monitoring of auditory and language development with parental reports/questionnaires, in addition to therapy to promote language.118 Teams should be cautious about proceeding to CI when infants are making progress in language development with consistent hearing aid use. Using a family-centered approach, multidisciplinary CI teams can help the child’s caregivers decide upon the best approach for language development, considering the unique characteristics of each family. Identifying an etiology of deafness, such as congenital CMV infection or genetic mutation, can help inform decision-making and reinforce confidence in the results of both objective and behavioral measures of hearing in infants.119

Binaural hearing is the ability of the auditory system to use sound information from both ears. In normal patients, it is considered to play an important role in speech recognition in noise, sound localization, and 3-dimensional perception of the sound environment. Three auditory mechanisms are responsible for the efficacy of binaural hearing: the head-shadow effect, the squelch effect, and the binaural summation effect.120

Bilateral implants are the most viable option for spoken language development.121 Simultaneous bilateral CI, compared with unilateral or sequential surgery, improves auditory performance, auditory pathway development in the brain, and, consequently, spoken language acquisition.109 If sequential CI is selected, the need to rapidly implant the second CI should be discussed with the family, which should occur within a few months of the first implant (up to 6-months),122 in addition to the use a PSAP on the non-implanted ear until surgery.34

Better language outcomes can be expected for children who receive early bilateral CIs.123 Therefore, it seems likely that the best intervention to promote oral language development for children with profound hearing loss is early simultaneous bilateral CI.124 Children with bilateral CIs achieve better language outcomes than those with unilateral CIs.125,126 Young children spend most of their time learning and socializing in noisy environments.127 In such situations, bilateral stimulation helps reduce listening effort and facilitates learning.125,126,128

Patients using a unilateral CI without contralateral residual hearing have, by definition, a monaural hearing system. They have difficulty locating sound sources and understanding speech in noisy environments, a common situation in everyday life. These data support the increasing appeal in recent years for bilateral CI. Several authors have evaluated the performance of adult users of bilateral CIs on objective auditory tasks, most often through speech discrimination and spatial localization tests, in addition to self-assessment of auditory skills and quality of life.120,129–131

Recognition between speech and noise when sound sources are spatially separated is more accurate in bilateral than in unilateral CI conditions because of the head-shadow effect. Better results in bilateral conditions are also observed for speech discrimination in quiet, which may be partly explained by the binaural summation effect and the increased likelihood of correctly detecting the signal. For spatial localization tasks, performance measured in bilateral conditions is significantly better than that measured in unilateral conditions.132 The patients who perform best in localization tests are also those who achieve the best scores in speech discrimination in noisy environments.120

Studies comparing the benefits of bimodal devices and bilateral CIs in oral language skills (receptive or expressive language) suggest that, in children with profound bilateral hearing loss, bilateral CI should be indicated. Children with thresholds close to 70 dB in one ear and profound hearing loss in the contralateral ear may benefit from bimodal stimulation. However, they should be closely monitored due to the risk of worsening hearing in the ear using the PSAP.34,133

A difficult issue for unilateral CI users may be the decision to pursue a second CI. Two implants provide the benefits of binaural hearing, including sound localization, binaural summation and squelch, and speech recognition in noise.134 CI users with residual hearing in the contralateral ear can use PSAPs (bimodal stimulation). However, for patients who are not clearly receiving bimodal benefit, the decision to pursue a second CI is not straightforward. Gifford and Dorman,134 comparing speech recognition between adults using bimodal stimulation and adults with bilateral CIs, observed no significant differences in speech perception or binaural summation using traditional clinical measures. The patient’s perception of the need for a second CI appeared to be of greater clinical utility than routine objective testing.

Indications for CI have increased substantially in recent years. Over the past few decades, electrode design and surgical techniques have evolved so much that hearing preservation is now possible. Current indications for adult CI are primarily based on open-set speech recognition. Typically, less than 50% of speech recognition scores are required for patients to meet CI candidacy. These score requirements are complex and vary slightly among centers in each country.

As the criteria for CI continue to broaden, the materials and methods used to test potential CI candidates remain highly variable and open to interpretation, even for traditional candidates with bilateral deafness. There are several parameters to help determine the suitability of a referral to CI and predict the likelihood that the applicant will qualify. The use of objective audiological criteria helps hearing care professionals make high-throughput and appropriate CI assessment referrals. However, it is essential to recognize that there is a poor correlation between unaided speech recognition scores and eventual CI application. Therefore, if a patient demonstrates limited benefit from appropriately fitted hearing aids, it is essential to consider referral for a formal CI assessment, regardless of performance on any single test or group of test measures.

The sentence recognition test has been the standard for measures of assisted speech in CI candidacy tests. Sentence recognition scores reflect less on how well a person can detect and process spectral and temporal components of speech and more on how well someone can use “top-down processing” to “fill in the missing pieces”. Cognitive features can also alter top-down processing. Post-deaf lingual voters score much higher on the sentence test than on the word test during the CI assessment.15,135

When monosyllabic word recognition scores are used for CI candidacy qualification, performance results show significant improvement from pre- to post-CI. There is a trend towards better performance in the CNC word test when less restrictive criteria (<40% CNC) are used for application qualification rather than more restrictive criteria (<30% CNC).15,136 Patients with bilateral deafness who undergo implantation in their worst ear, using a CNC score of ≤50% in the ear to be implanted, had a sensitivity of 99.7% to identify candidates who ultimately qualified based on previous CMS criteria (≤40% sentence recognition test). Pre- and postoperative CNC word score comparisons in the implanted ear demonstrated a significant improvement for those who scored up to 50% preoperatively.137

One of the best-assisted speech recognition tests is defined as the speech perception score in the individual ear(s) using optimized hearing aid(s) on a monosyllabic word test (Consonant-nucleus-consonant, CNC).138 The target presentation level for stimuli is 60 dB A.139 When unaided hearing thresholds are 60 dB HL or better in the untested ear, tamponade and muffling or masking using noise in the form of speech is performed. Patients who score ≤50% on the CNC in the ear with worse hearing should be considered for CI unless contraindicated for other reasons during CI evaluation. The CNC word score in the contralateral ear should not be considered when determining candidacy using this test.137

Communicating with background noise is a universal challenge because there is so much noise around us. Speech-in-noise tests have become integral to CI candidacy assessments and post-surgery monitoring. Cochlear implant candidacy guidelines are based on an individual's degree of hearing loss and assisted speech perception scores. These results are necessary to document that an individual is not receiving sufficient benefits from acoustic amplification alone and, therefore, may benefit more from a CI.

Early indications specified the use of HINT; however, clinicians often completed the evaluations in silence rather than performing the test in the adaptive SNR format as originally designed. Therefore, the test was not representative of hearing difficulties in real-world settings. Subsequently, the research identified that the HINT was prone to ceiling effects when the trial was completed only in silence to assess the pre-implant versus post-implant benefit. Updated recommendations based on the revised Minimum Speech Test Battery (MSTB 2011) suggest using the AzBio sentence test instead of the HINT.140 It has been recommended that tests be completed in both silence and noise to better document an individual's performance in a more realistic range of listening scenarios.15,16,137

The aided speech perception test usually includes CNC words, silent AzBio sentences, AzBio sentences in noise, and the BKB-SIN (AzBio sentences provide a correct percentage score at a fixed level, while BKB-SIN provides an SNR50 score). A presentation level of 60 dB SPL is recommended for all speaking material; however, when administering AzBio in noise, the clinic may choose its own SNR based on that clinic's candidacy evaluation protocol.137

CI candidacy assessments should be performed using appropriately fitted and verified HAs, i.e., using real ear or simulated real ear measurements conducted in a test box or at the ear to confirm that the devices meet the prescriptive goals for sufficient audibility. The Best Aided Connected Speech Test (best aid defined as speech recognition test in the individual ear using optimized hearing aid) should be performed using AzBio phrases to determine if a patient qualifies for coverage for their CI. There is no consensus on the recommended SNR for testing, and it varies between clinics. Zitler et al.137 recommend testing the ear to be implanted in the best aid condition using AzBio phrases in noise starting with an SNR of +10 dB using a babbling of 10 speakers (AzBio phrases presented at 65 dB A and noise presented at 55 dB A). To better assess auditory status, the clinician should consider decreasing the adversity (phrases presented silently at 60 dB A) or increasing the adversity (phrases at +5 dB SNR with phrases presented at 65 dB A and noise presented at 60 dB A) of the hearing condition, as needed.

Although clinically meaningful improvements in speech perception in noise can be achieved after surgery for CI-eligible patients in noise, improvements tend to be smaller as the SNR becomes more adverse.141 People qualified for CI only in the SNR condition of +5 dB can obtain significant benefits from their device, but the objective results are more variable.142 Consideration of hearing needs and goals should be discussed with each candidate individually. The variation in SNR represents real-life hearing situations and may be beneficial in determining SNR in which substantial difficulty is noted. Testing AzBio Sentences on the person's daily hearing condition is recommended for postoperative comparison with the person's daily hearing condition. The daily hearing condition is defined as a test with the optimized auditory configuration typical of a patient's daily hearing (e.g., unilateral or bilateral non-occluded hearing aid(s), bimodal, unilateral or bilateral CI(s), EAS with contralateral HA).137

The decision on which ear to implant is differentiated, and it is not possible to create a single approach that leads to specific recommendations. Although the ear with the worst hearing has been routinely selected in the past, dogmatic methodology should not take precedence in all cases over these other factors.143 This less rigid approach may lead to a recommendation for implantation of the ear with better hearing in some cases when it also meets the candidacy. When the examination indicates significant pathology in one ear, implantation of the uninvolved ear (which may be the ear with the best hearing) is often indicated. However, in cases with a non-revealing history, normal examination and imaging, and symmetrical auditory thresholds, the ear with the worst performance on the CI candidacy test is routinely selected for CI.

Although the list of indications is expanding, contraindications to CI are becoming clearer as time passes and studies emerge. First, patients need a cochlear nerve to undergo CI. MRI is used to determine the presence of a cochlear nerve in children with congenital deafness; in adults, it can diagnose tumors or demyelination. Although definitive absence of a nerve is not always easy to determine, imaging that indicates absence or hypoplasia of a cochlear nerve is a relative contraindication to CI. The duration of deafness is another important factor. Adult patients with prelingual hearing loss who have not developed language are considered a relative contraindication.

Some challenging decision-making is necessary over time as more adult patients with natural low-frequency hearing find CI a possibility for rehabilitation. Despite evidence of the potential to preserve low-frequency hearing, CI surgery also poses the risk of losing residual hearing, and the durability of hearing preservation remains unclear.144 In children, this decision is often not as relevant, although the decision to perform bilateral CI in single or sequential procedures has become relevant in this population. As the criteria for CI evolve, the decision-making process becomes more complex, requiring comprehensive discussions among patients, families, physicians, and speech therapists.

CI has been increasingly performed in patients with unilateral deafness or Single-Sided Deafness (SSD). Traditional treatments included the use of PSAPs with Contralateral Routing of Signal (CROS) systems and bone-anchored hearing aids. CI was initially discouraged due to the misconception that electric stimulation would not integrate with contralateral acoustic input. However, van de Heyning et al.145 demonstrated feasibility after implanting 21 patients with SSD and debilitating tinnitus. Indications for CI in patients with SSD have been expanded to include even children with congenital pathologies who have a normal cochlear nerve.146,147 Adults have experienced tinnitus improvement and have become daily CI users.148 Overall, implanted patients with SSD experience a reduction in tinnitus in the implanted ear, in addition to improvements in speech perception in noise, sound localization, and quality of life.146,149,150

Previously, patients needed to have profound or total bilateral sensorineural hearing loss to be considered CI candidates. Technological advances have allowed the expansion of indication criteria to include individuals with significant levels of residual hearing. Because the goal of expanding CI indications is to maintain or preserve hearing, some patients with normal hearing or moderate low-frequency hearing loss and severe-to-profound high-frequency sensorineural hearing loss have used Electric-Acoustic Stimulation (EAS) or hybrid stimulation.151

Patients with residual low-frequency hearing often have poor outcomes with PSAPs. For these patients, CIs with EAS or hybrid CIs allow for low-frequency acoustic amplification and middle-to-high-frequency electrical stimulation.152 EAS improves speech recognition in noise and provides finer spectral resolution of complex auditory stimuli (e.g., music).17,151

Low-frequency acoustic cues are important for speech perception and other auditory skills that are transmitted more effectively than electric information provided there is useful residual hearing. Hybrid CI users show significantly increased perception of sound frequency compared with electric-only stimulation.153,154 Acoustic perception of low-frequency sounds allows patients to use the fundamental frequency and first formant of speech, timing cues, and spectral information more effectively than auditory electrical stimulation.155 The additional acoustic information can compensate for the limitations associated with the auditory stimulus generated by the CI, such as the reduction in the number of functional channels and poor pitch perception, leading to difficulty when listening in competing voice situations, even for CI users who achieve high levels of speech recognition in quiet.156

It is essential to use an atraumatic surgical technique, including a very delicate opening of the cochlea, with slow insertion of thin electrodes that are shorter than usual.157 This indication is particularly important for older patients, as aging also reduces hearing clarity with a well-fitted hearing aid.158 In these patients, postoperative EAS (low-frequency acoustic stimulation and high-frequency electrical stimulation) allows for a better understanding of speech in noise and the perception of music. CIs can improve speech understanding for this type of patient. Electrode length has been identified as a critical factor: greater insertion depth improves speech perception in “traditional” candidates but may destroy residual hearing. Shorter electrodes, however, may compromise outcomes for patients who ultimately lose residual hearing and require electric-only stimulation.159,160

Optimizing insertion depth (∼360 °), thinner electrodes, and Round Window (RW) insertion (vs cochleostomy) are strategies for “soft” surgery.156,161,162 Intraoperative monitoring using Electrocochleography (ECochG) has also become available as an adjunctive tool to minimize intracochlear trauma.163–165 Intraoperative ECochG may be obtained directly through the CI during insertion. Acoustic stimulation is provided by inserting an earphone, whereas cochlear hair and neural cell responses are detected through the inserted electrodes.165 The recorded signal provides intraoperative feedback on cochlear hair cell function and may influence the insertion technique or insertion depth when hearing preservation is intended.

Pillsbury et al.,166 in a prospective multicenter study of 73 patients with residual low-frequency hearing and severe-to-profound hearing loss in the middle-to-high frequencies treated with EAS, reported that 85% of patients performed better on the Speech Perception Score (SPS%) at 12-months with EAS use compared with the preoperative aided condition. Achena et al.158 reported that the mean percentage of SPS in quiet changed from 21.8% before surgery to 71.8% after surgery, while in noise it changed from 17% to 65% (p < 0.001). The indication criteria for hybrid CI have included the following parameters:144,167

• a)

  Audiometric thresholds less than 65 dB at frequencies from 125 to 500 Hz and greater than 75 dB at frequencies above 1000 Hz;

• b)

  Sensorineural hearing loss with air-bone gap less than 15 dB, if present;

• c)

  Result of the speech perception test, in quiet, less than 50% in the better ear, with the best possible amplification conditions;

• d)

  Stable hearing loss for more than 2-years;

• e)

  Having received counseling about CI expectations.

CI surgery is safe if performed by experienced surgeons. Although a variety of techniques have been described to access the cochlea, the most commonly used approach is posterior tympanotomy.168 It allows access to the RW membrane and cochlear promontory with a direct route for electrode insertion with few serious complications.169 Device fixation techniques have become less frequent as the subperiosteal pocket has become more popular.170 The use of the subperiosteal pocket decreased the likelihood of rare but potentially devastating intracranial complications reported with cortical drilling for fixation.171,172

Facial nerve monitoring is recommended in almost all otologic and neurotologic procedures that involve the intrapetrous facial nerve,161–173 and, although damage to the patient or the implant itself has not been demonstrated, there is concern and risk of inducing electric current through the CI electrode with monopolar cautery. Because of this risk, monopolar cautery should not be used routinely in patients with an existing CI.

Other techniques have been described, but they are largely reserved for patients with anatomic variations that require deviation from the facial recess.174 The most frequently described alternative approach is the suprameatal approach,175–177 which requires a tympanomeatal flap to be raised to access the RW. This approach avoids the need to drill the facial recess and thus reduces the risk of injury to the facial nerve.178 The combined approach technique described by Lavinsky et al. can also be used,160 in which a minimal, reduced posterior tympanotomy is performed, only large enough to pass the electrode cable, with the RW approach or cochleostomy being performed via the transcanal route. Therefore, in special cases such as malformations, it allows a safe approach via posterior tympanotomy to pass the cable providing a wide transcanal surgical field for insertion of the electrode array. Rarely used techniques such as middle fossa and closure of the External Auditory Canal (EAC) have been described for CI, but they are not routinely used.161–167

Regardless of the surgical approach, the goal is to place the CI electrode within the Scala Tympani (ST). Placement within the ST optimizes the interaction between the electrode and the spiral ganglion cells, which are being directly stimulated. It has been well demonstrated that user outcomes are improved with electrodes in the ST, 179,180 and any translocation to the scala vestibuli decreases CI performance. In addition, as soft surgery techniques become standard, RW membrane insertions have been favored because they avoid the potential trauma of drilling a cochleostomy.162

Additional intraoperative care is required for patients younger than 12-months. Pediatric patients have significantly lower blood volume than adult patients. Children younger than 12 months have a total systemic blood volume of approximately 80 mL/kg. Minor blood losses in the pediatric population can have catastrophic effects. Blood losses greater than 10% in this population can lead to hypovolemia.108,181 Bleeding from emissary veins and bone marrow oozing from the temporal bone are the main sources of blood loss during pediatric CI.182 Care should be taken to maintain meticulous hemostasis throughout CI surgery in children younger than 12-months to avoid potential complications.

The skin of children younger than 12-months is significantly thinner than the skin of adults. Flaps should be manipulated with caution.108,183 Skull thickness of younger pediatric patients may be less than a few millimeters. Dura mater exposure during pocket creation in the temporal bone cortex can be performed to reduce tension under the flap and improve aesthetic outcomes.170 Receptors have become increasingly thinner, making it possible to perform it from the pocket. Because the skull develops over time and migration of the internal component may occur, the device should be well positioned and the pocket should be created properly.31,170,182

Lehnhardt first described the concept of “soft surgery” cochleostomy (minimal cochleostomy inferior and anterior to the RW) in 1993.184 The principles of the hearing preservation technique include no perilymph suctioning, careful opening of the cochlea, slow and gentle insertion of electrodes, and use of intraoperative corticosteroids to reduce foreign body reaction. Hearing preservation has been reported in up to 90% of patients.156,185,186 Antibiotic prophylaxis may prevent biofilm formation on the electrode surface.185

Postoperative loss of residual hearing is linked to the direct physical trauma caused by electrode insertion and the associated acute inflammatory response. Postoperative hearing loss may be linked to chronic inflammation and the development of fibrotic tissue in the cochlea (in the long term).

The atraumatic electrode insertion is an important factor for hearing preservation. The following factors should be considered187:

• 1)

  Electrode insertion depth.

• 2)

  Electrode array insertion forces against intracochlear structures.

• 3)

  Selection of electrodes best suited to the individual patient’s cochlear anatomy, especially in the presence of inner ear malformations.

Electrode insertion via the RW requires a shorter drilling time in the cochlea and may result in less acoustic trauma. A systematic review of 16 studies comparing the RW approach vs. cochleostomy concluded that both insertion approaches resulted in comparable hearing preservation.156 However, some studies using window insertion showed better hearing preservation outcomes (about 20% higher)188 than those in which cochleostomy was used.156

ECochG can be used to monitor electrode array insertion in real time. Reduced ECochG responses after implantation are expected as a result of intracochlear trauma and mechanical changes induced by electrode insertion.189 However, ECochG amplitude responses after insertion may increase.163 There is a high degree of correlation between ECochG responses and speech perception performance in adults.190

The speed of electrode insertion is important. A slow and steady insertion speed of approximately 30 seconds reduced forces within the cochlea compared with faster insertion speeds.191 Insertion speed appears to have a significant impact on inner ear fluid dynamics and hearing and vestibular preservation. The optimal insertion speed remains unclear.192 Hyaluronic acid has been used as a lubricant to reduce trauma during electrode array insertion, but it has no benefit for hearing preservation.193

Although shorter electrodes may have better rates of early hearing preservation,156 complete electrical stimulation of the cochlea may not be possible in cases of residual hearing loss. Friedmann et al.194 reported significantly better speech understanding after loss of residual hearing in patients implanted with the CI422 (Cochlear) than in patients implanted with the shorter electrode. Hearing preservation with full insertion of a standard-length electrode is also possible.193 The advantage of using standard electrodes is the possibility of stimulating the distal cochlea if hearing loss progresses after surgery.

The use of steroids is common in protocols to reduce hearing loss caused by inflammation.156,188,193,195 The optimal timing of administration of corticosteroids also remains to be established. The use of preoperative steroids combined with intraoperative steroids has a positive impact on hearing preservation compared with other regimens or no steroid use at all.188 Topical steroid application has been associated with better hearing preservation outcomes, presumably by reducing the electrode-induced acute phase response.192

Parameters to define hearing preservation156:

• • • I

      Complete hearing preservation: postoperative loss in hearing thresholds of a maximum of 10 dB at each frequency to be preserved;

    • II

      Partial hearing preservation: postoperative loss in hearing thresholds above 10 dB at each frequency, but with residual hearing less than or equal to 80 dB for at least one frequency between 250 and 1000 Hz;

    • III

      No hearing preservation: the patient is unable to use acoustic stimulation because of thresholds worse than 80 dB.

• XLII

  Hearing preservation in CI surgery should be encouraged for better device performance, reducing the risk of vestibular trauma. (Strong Recommendation – Moderate-Quality Evidence).

• XLIII

  Slow and gentle insertion of electrodes through the RW and no perilymph suctioning are the principles of hearing preservation surgery. (Strong Recommendation – Low-Quality Evidence).

• XLIV

  The use of topical corticosteroids in the RW is associated with better hearing preservation. (Moderate Recommendation – Low-Quality Evidence).

All major CI manufacturers have their own electrode design that addresses patient and surgeon needs. The 3 main factors for the quality of insertion of the CI electrode array are insertion depth, proximity to the modiolus, and placement within the ST.196 The 2 types of commercially available CI electrode arrays are lateral wall and precurved (perimodiolar). The electrode known as “Mid-Scala” is considered perimodiolar.

There are several methods to attempt to obtain objective measurements of auditory nerve function after electrical stimulation in CI users. The most commonly used procedure during surgery is the electrically Evoked Compound Action Potential (ECAP). ECAP thresholds can be useful to predict the minimum and maximum levels that should be used in mapping the electrodes for programming the speech processor, facilitating this process in children, and determining the stimulation parameters that will result in programming with more appropriate levels, which can improve user performance.

The ECAP is recorded on the CI using specific software, called telemetry system. Telemetry is a mechanism for capturing distant events that consists of 2 systems. The first is used to measure the impedances of each electrode, monitoring the suitability of electric current generators. The second system, Neural Response Telemetry (NRT), allows the recording of the ECAP of the distal portion of the auditory nerve in CI users, using the implant itself to elicit the stimulus and record the responses.

Impedance is related to the resistive characteristics of the fluid and tissue surrounding the electrodes and is one of the factors that determine the energy consumption of the CI system. Impedance telemetry should always be performed before NRT to confirm the proper functioning of the receptor and stimulator and to check for the occurrence of an open-circuit or short-circuit fault in the intracochlear electrodes by measuring their electrical resistance.

NRT is performed after surgery. The CI stimulates the cochlea, and the ECAP is recorded by amplifying the signals from the intracochlear electrodes. After checking the patient’s ECAP, a plain transorbital X-Ray is undertaken to confirm the position of the electrodes in the cochlea or CT, which is recommended in some services around the world. The patient returns for postoperative care common to other ear surgeries. After 4–6 weeks, the patient returns for adjustment of the external component of the implant and its programming or “mapping”.

When CI started, it was recommended to delay initial activation until at least four weeks after CI surgery due to possible complications and to avoid wound disruption.212 However, advances in surgical techniques and fitting experiences have allowed initial activation within 24 h. Emerging evidence suggests that early activation is feasible and beneficial, giving patients faster access to sound and rehabilitation.213,214

Activation is considered early variably between studies (up to 14 days postoperatively). Some studies report activation within one day postoperatively.215,216 Some studies have indicated low impedance levels in the early activation group.216,217 Magnet force adjustment or using the processor outside the ear was often recommended. Complication rates are low, with no difference from late activation. Early activation has improved patient satisfaction and anxiety levels without impairing speech recognition or rehabilitation.213,215,216 Regular inspection of magnet strength is recommended as part of CI follow-up, as postoperative wound edema should be expected.218

Patro et al.219 evaluated 604 adult CI recipients, stratified by time of CI activation (Group 1: ≤10d, n = 47; Group 2: >10d, n = 557). CI activation within ten days of surgery is associated with increased early device use and superior speech recognition at early (AzBio in quiet performance at three months) and late (AzBio in quiet performance at 12-months) follow-up visits. The timing of activation and device use are modifiable factors that can help optimize postoperative outcomes in the CI population.

Alahmadi et al.217 compared the values of electrode impedance at different times within two years in patients who activated the CI early (one day after surgery ‒ n = 481) with patients who activated it four weeks later (classic activation ‒ n = 434). Early activation led to sustained reductions in electrode impedance compared to classical activation, suggesting that early activation may positively affect long-term CI outcomes.

The estimated risk of (immediate and long-term) postoperative meningitis after CI is low, approximately 0.07%.220 However, it is higher than the risk of meningitis in the general population – 20 cases per 100,000 population (0.02%),221 but in countries such as the United States and the Netherlands, the rate is very low (<0.002%).222,223 Bacterial meningitis is a disease with high morbidity and mortality rates. The most commonly detected organisms are Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitis. Even with adequate treatment, 5%–10% of patients die, and complications, such as severe brain injury, occur in 10%–20% of survivors.224 The pneumococcal meningitis mortality rate is even higher, ranging from 15% to 60%.225 Infants under 1-year of age would be at greater risk of having meningitis because of the immaturity of their immune system. However, systematic reviews and meta-analyses have not found such an association in this age group, with a negligible rate of meningitis.226

Some authors have suggested that Acute Otitis Media (AOM) may be a risk factor for post-implant meningitis.221,227 Bacteria from the infected middle ear could cause meningitis by passing into the inner ear through the electrode array. Other studies suggest that the risk of meningitis in those who develop postoperative AOM remains negligible.220 However, in cases of AOM where CI surgery occurred within 2-months, there is a concern about an increased risk of meningitis due to the spread of the middle ear bacteria into the CSF via the cochlear perilymph.228,229

The 2-main methods of electrode insertion are the RW approach and cochleostomy. A variant of the RW approach involves extending the edges of the RW prior to electrode insertion (extended RW technique).228 Recent evidence suggests no difference in cochlear trauma risk between these techniques, with improved speech perception using the RW approach.212 An international survey suggests that most surgeons now use the RW insertion approach.230 In a systematic review, the pooled rate of meningitis was negligible for both RW (1-case in 1557 patients) and cochleostomy (0 cases in 584 patients).220

The main agent involved in cases of CI-related meningitis is pneumococcus.231 There are more than 90 serotypes of S. pneumoniae. The maximum number of serotypes covered by vaccination is 23. Currently, there are 2 commonly available vaccines for S. pneumoniae: 1) Pneumococcal 13‐Valent Conjugate Vaccine (PCV13) (Prevnar 13, Pfizer Inc., New York, NY) and 2) Pneumococcal Polysaccharide Vaccine (PPSV23) (Pneumovax 23, Merck & Co., Kenilworth, NJ). A second dose of PPSV23 should be administered 5-years after the first.

The risk of meningitis is influenced by the combination of risk factors a patient has. Several risk factors can contribute to meningitis in CI users – otitis media, age, head trauma, surgical technique, cochlear malformations, and CSF leak.220 The CI electrode array is a route for spreading infection from the middle ear to the cochlea.

Given the underlying evidence of efficacy, pneumococcal vaccines are part of routine childhood immunization programs in many countries. It is recommended that CI candidates and users be fully vaccinated against the agents causing bacterial meningitis, especially S. pneumoniae due to its high prevalence. The U.S. Centers for Disease Control and Prevention (CDC) recommends that children with CIs should receive one dose of PCV15, followed by one dose of PPSV23.232,233 In 2022, the CDC updated its guidelines to include that unvaccinated adults with CIs should receive one dose of a conjugate vaccine (PCV20). If PCV15 is used in this group, it should be followed by one dose of PPSV23.234 The immunization schedule varies depending on the country.

Hearing in mammals relies on the ability of sensory hair cells to convert sound-evoked mechanical stimuli into electrochemical signals. Hair-bundle deflection induces rapid opening of sensory transduction channels, leading to the generation of an influx of cations into the IHC. This results in a depolarizing potential, allowing an influx of calcium through voltage-gated calcium channels. The coupling of Ca2+ channels at the presynaptic site triggers synaptic vesicle fusion and neurotransmitter glutamate release into the synaptic cleft.239 The release of glutamate in the synapse activates Ca2+-sensitive AMPA receptors. This initiates the generation of stimuli in the spiral ganglion neuron fibers, which encodes information about sound stimuli that is sent to the central nervous system.83,240

The synapse of the peripheral auditory system includes the presynaptic site (the base of the hair cell), the synapse itself, and the postsynaptic site (terminal dendrite of the spiral ganglion). The base of IHCs interfaces with terminal processes of spiral ganglion neurons. Presynaptic release of glutamate leads to the transmission of a sound signal to the spiral ganglion neurons and propagation of the signal in the central auditory pathway. The synapse between the IHC and the spiral ganglion terminal process consists of several unique features. First, each spiral ganglion neuron receives input from a single IHC, but each IHC synapses with multiple spiral ganglion neurons.

This synapse is able to encode a high degree of temporal precision due to graded release of glutamatergic vesicles via the synapse. The synapse consists of a long tether that holds synaptic vesicles close to the presynaptic release point in IHCs as it is anchored to the cell membrane at the presynaptic active zone in close proximity for release into the synaptic cleft. This presynaptic organization allows for transmission of a finely temporal encoded (up to 1 kHz) and indefatigable signal. The synaptic component also includes the postsynaptic glutamate receptors at the terminal dendrite of the spiral ganglion.

The neural partition of the human auditory system consists of the distal neurite of the spiral ganglion, the somata of the spiral ganglion that reside within Rosenthal’s canal, and the central processes of the spiral ganglion that form the cochlear partition of the vestibulocochlear nerve (eight cranial nerve) and branch to form synapses within the dorsal and ventral cochlear nucleus of the midbrain. Lossless conduction of this signal occurs through the myelinated distal and central dendrites of the spiral ganglion. Central components of the human auditory system include the brainstem and auditory cortex of the temporal lobe, which are together responsible for maintaining the tonotopic organization of sound and processing sound and speech, respectively.

A dysfunction at any level of this complex transduction machinery can make sound coding difficult. Potential sites of damage are diverse, including IHCs and their synapses with spiral ganglion neurons or synaptopathy (e.g., presynaptic glutamate release or postsynaptic terminal dendrites of spiral ganglion neurons), or can be due to demyelination and axonal loss of auditory nerve fibers and their targets in the cochlear nucleus (neuropathy). These auditory pathologies are called Auditory Neuropathy Spectrum Disorder (ANSD), in which the activity of OHCs is maintained.240,241

It has long been hypothesized that individuals with auditory neuropathy would be poor CI candidates because transmission of the signal from electrical stimulation of the spiral ganglion through the CI could be affected. A study of 260 individuals with ANSD showed positive, albeit variable, CI outcomes in these patients.83,242 Some individuals will have qualitative or quantitative deficits to the spiral ganglion or cochlear nerve and are hypothesized to have poor outcomes with CIs. However, some individuals with ANSD have a genetic lesion affecting the synapse itself, which would be bypassed by the CI, and these individuals would therefore be expected to have the same outcomes as those with a genetic lesion affecting IHCs.243,244 Hearing loss is characterized by the presence of OAEs and/or Cochlear Microphonics (CM), indicating normal cochlear function, and by altered auditory responses from the brainstem measured by ABR indicating abnormal transmission of the synaptic signal to the central nervous system.244–246 Furthermore, absence of the stapedial reflex and contralateral suppression of OAEs are commonly observed.222,245,247

The CI bypasses the sensory and synaptic partitions by directly stimulating the spiral ganglion somata, leading to the transmission of electrical signals to the midbrain. Therefore, it is concluded that the health of the organ of Corti or synapse will not affect the outcomes of CI. Conversely, the health of the spiral ganglion (or more generally the auditory nerve), the synapse between the spiral ganglion and the cochlear nucleus, midbrain, or auditory cortex may negatively affect the electrical transmission of sound by a CI and, theoretically, could lead to suboptimal CI outcomes.248

Environmental causes of ANSD primarily affect newborns and include hyperbilirubinemia, thiamine deficiency, and hypoxia. ANSD can also be noise-induced or age-related. Spiral ganglion neurons are bipolar, with cell bodies located in the modiolus of the cochlea, peripheral axons running through the spiral limbus toward the row of cochlear IHCs, and proximal axons that synapse at the midbrain. These neurons are tonotopically organized with the cochlea.242

Multiple spiral ganglion neuron fibers synapse with each IHC. Their axons are myelinated and project to the brainstem. In response to glutamatergic stimulation at the synapse, excitatory postsynaptic potentials at the peripheral axon lead to gradual Na + influx via a large number of channels, which allows for temporal encoding of acoustic stimuli.242,248

Genetic neuropathies often affect other neurons, leading to syndromic phenotypes such as Charcot-Marie-Tooth (CMT) disease, Autosomal Dominant Optic Atrophy (ADOA), Leber Hereditary Optic Neuropathy (LHON), Friedreich Ataxia (FRDA), Mohr-Tranebjaerg syndrome, Refsum disease, and Wolfram syndrome.

Congenital hearing loss is mostly caused (80%) by membranous malformations involving the hair cells of the inner ear, without macroscopic bony abnormality. Therefore, these cases show normal findings on high-resolution CT and MRI of the temporal bone. The remaining 20% show a wide variety of malformations involving the bony labyrinth and, therefore, can be demonstrated radiologically.294 The majority of these patients have severe-to-profound bilateral hearing loss and are candidates for CI surgery. Some severe malformations may require a different surgical approach in CI surgery, such as performing subtotal petrosectomy. There is a wide variety of classifications for these malformations, but the most used is the one proposed by Sennaroglu et al.295

Three factors should be considered before making a preoperative decision to indicate CI, selecting the type of electrode array, and choosing the most appropriate technique: classification of the malformation, presence of a cochlear nerve, and preoperative audiologic test results. The main challenges for CI surgery in patients with inner ear malformations are gusher, risk of meningitis, and facial nerve anomalies.

There are few settings where the cochlea can be obstructed with ossified tissues that make electrode insertion challenging. Cochlear ossification often occurs secondary to meningitis, although advanced otosclerosis and other diseases can induce a similar process.348,349 In this setting, efficient workup and surgical scheduling is necessary because ossification can progress rapidly and limit electrode insertion.

Ossification after bacterial meningitis presumably occurs by the spread of infection to the cochlea via the cochlear aqueduct.349 The inflammatory reaction within the cochlea progresses through several phases that begin as fibrosis and culminate in ossification that has its most dramatic effect on the basal turn of the cochlea, although the entire cochlea can be affected.350

In advanced otosclerosis, ossification occurs in the RW and extends to the basal turn of the cochlea.351 In most cases, ossification does not extend beyond the basal turn of the cochlea. To overcome the ossification encountered, several drilling techniques have been described that can be performed with the standard posterior tympanotomy approach.352 If access for drilling the ossified portion of the cochlea is limited, obliteration of the cavity and the canal wall has been described.353

Approximately 10% of patients with otosclerosis and conductive hearing loss also develop sensorineural hearing loss.354 Advanced otosclerosis is characterized by sensorineural hearing loss and decreased speech discrimination (<30% at 70 dB),355 associated with radiologic abnormalities.356 Sensorineural hearing loss in patients with otosclerosis occurs when ionic homeostasis of the cochlea is disrupted due to atrophy and hyalinization in the stria vascularis and spiral ligament. Consequently, dysfunction or loss of hair cells and loss of spiral ganglion can occur.357

The severity of sensorineural hearing loss in otosclerosis is correlated with radiologic abnormalities on high-resolution CT, which can detect oval window abnormalities in 80%–90% of cases.358,359 On CT, the finding of pericochlear lucencies is highly specific for otosclerosis. It presents as a double halo.360,362 T1-weighted MRI images may show a ring of intermediate signal in the pericochlear area with mild-to-moderate gadolinium enhancement.362 T2-weighted sequences are the best method to assess the patency of the cochlear duct.358,363,364

Intraoperative difficulties include ossification, partial obliteration of the basal turn and RW, and false tract insertion of electrode array into the cochlea.351 In a case series of advanced otosclerosis treated with CI surgery, the RW membrane was ossified in 60% of cases and the ST in 30% of cases.365 Some software programs have been developed by CI manufacturers with the purpose of conducting a detailed planning of the surgery as well as choosing the best electrode insertion method and the best electrode array system based on high-resolution CT data.366,367

Facial nerve stimulation can occur both at the time of electrode activation and during subsequent device monitoring visits.368 To eliminate or at least minimize its effects, mapping adjustments should be done, such as decreasing the electric charges by changing the stimulation mode – by reducing the amplitude of the maximum current levels, keeping them below the stimulation threshold for the facial nerve, or even by adjusting the biphasic pulse width.369 More recently, triphasic pulse patterns have also been successfully used to alleviate the symptoms of facial nerve stimulation.370,371

Switching off the problematic electrodes has been another procedure frequently described in studies as an alternative method to manage facial nerve stimulation.372 There is no consensus on which electrodes (basal, medial, or apical turn electrodes) are the most affected in facial nerve stimulation.361,373 The fact is that, depending on the number of deactivated electrodes, speech perception can be significantly affected. In these cases, reoperation using another CI model with characteristics suitable for targeted electrical stimulation (related to the positions of intracochlear electrode contacts, electrode geometry, and stimulation parameters) as well as sequential CI may be viable alternatives to be considered.369,373,374

Most studies point to promising auditory perception outcomes in patients with advanced otosclerosis. Numerous studies have not found significant differences in word and speech recognition performance scores between implanted patients with advanced otosclerosis and those with other etiologies.368,369,374 In fact, it is assumed that this disease has little effect on the spiral ganglions of the auditory nerve.366 However, although not significant, some authors have reported a trend toward poorer performance in the group with advanced otosclerosis. The poorer results obtained in patients with advanced otosclerosis were associated with factors such as long deafness periods, older age, extensive ossification, presence of facial nerve stimulation, and a greater number of deactivated electrodes.368,375,376

The progression of otosclerotic foci often occurs in the basal and medial regions of the cochlea and, due to decreased impedance of the otic capsule and flow of the electric current through the bone, the electric current required to stimulate the fibers of the auditory nerve is increased. Mapping adjustments are essential to manage this situation. With the increase in stimulation levels, if the perceived intensity is not adequate, there is a need to increase pulse duration, which can potentially result in a decrease in stimulation frequency and compromise the proper functioning of the selected processing strategy. In certain situations, there may be a need to switch off the electrodes to avoid the negative effects generated by the significant increase in stimulation levels.359,369,371

Facial nerve stimulation resulting from a shunt of current from the otic capsule that reaches the labyrinthine segment of the facial nerve362,371,376 has been described as one of the most common postoperative complications in patients with advanced otosclerosis, with an average incidence of 20% in this population, reaching up to 75%.361,373,374 Authors have suggested that the high incidence of facial nerve stimulation is associated with the type of electrode array used (straight or perimodiolar), with the straight or more distal array showing a higher incidence.359,361,373,375

Other techniques describing scala vestibuli insertion377,378 and retrograde electrode insertion have been reported.379 Surgically, in addition to being challenging, ossification in the setting of meningitis and otosclerosis has higher reported rates of facial nerve stimulation than in traditional CI.348 There is also evidence demonstrating that patients with cochlear ossification who undergo CI often experience limited hearing benefit,380 given the reduction of spiral ganglion neurons due to ossification.349

Varying degrees of cochlear ossification may occur in more than 10% of all CI candidates.381 Diseases implicated in cochlear ossification include meningitis (bacterial),348,382,383 otitis media,382,384 otosclerosis,351,384 and autoimmune disease (Cogan syndrome).384–386 Several disorders such as temporal bone trauma, labyrinthine artery occlusion, temporal bone tumors, and Wegener’s granulomatosis are also associated.348,387

The basal turn is the site most affected by most diseases. Two types of ossification have been described: metaplastic and osteoplastic.388 The metaplastic form (meningitis and otitis media) has high cellularity, low osteoblasts, and ill-defined margins. It is confined to the cochlear lumen with preservation of the endosteum. The osteoplastic form causes disruption of the endosteum (trauma and otosclerosis), leading to new bone formation that is lamellar, less cellular, with poorly defined margins.388

Bacterial meningitis is the most common cause of cochlear ossification reported in the literature.383 Infection reaches the cochlea through the cochlear aqueduct that communicates with the subarachnoid space.389 Spread to the cochlea is rapid and may occur within a few hours of the diagnosis of infection. The most affected region is the ST, reaching the organ of Corti posteriorly.349S. pneumoniae is the most harmful agent.389 Meningitis has been associated with negative post-implantation speech outcomes, regardless of ossification or extent of electrode insertion.390,391

Cochlear ossification remains a challenge in CI due to considerable changes in surgical techniques ranging from different surgical approaches (posterior tympanotomy, middle fossa, and subtotal petrosectomy),348,352,387 choice of electrode arrays (standard, compressed, and double),352,383,392,393 and extent of drilling (RW, basal turn, middle turn/circum-modiolar)348,352,387,392 to location and extent of electrode insertion (ST and scala vestibuli) (partial and complete).382,387,393,394

Postoperative hearing outcomes are variable due to extent of electrode insertion, reduction in spiral ganglion cells,382–384,388 increased impedance,395 and increased risk of electrode migration.396 Preoperative radiographic evaluation is essential for preoperative planning. CT and MRI are complementary modalities. The technique to be used varies depending on the degree of cochlear ossification.

Patients with profound deafness require effective auditory rehabilitation and, when deafness is concomitant with chronic middle ear diseases, rehabilitation should be combined with disease eradication. While bone-anchored hearing aids can be positioned far from an open cavity or middle ear with secretion,397 CI is not placed in this same situation.

Many factors can hinder the use of CI in patients with open mastoid cavities, such as the presence of secretion, inflammatory tissue, residual cholesteatoma, patent eustachian tube, stenosis of the EAC, or extensive meatoplasty, which can lead to exposure, infection, and extrusion of the CI. Numerous surgical techniques have been described to minimize complications. These procedures include maintaining the mastoid cavity open by covering the electrode with fascia or bone pate, overclosing the EAC, reconstructing the posterior bony canal wall, and performing a middle fossa craniotomy to access the cochlea.177,398–400

In favor of the maintenance of an open canal wall down mastoid cavity after CI are the speed and simplicity of the procedure, improved postoperative visualization, the ability to clean the cavity, and reduced risk of postoperative infections, breakdown of the EAC closure, development of residual cholesteatomas (due to cavity control), mucocele, and complications related to the fat donor site.401,402 However, maintaining an open cavity is not without risk. Electrode extrusion may occur with cavity cleaning, in addition to being an open area susceptible to contamination from the external environment.403

To mitigate the risk of extrusion, some authors emphasize the elevation of the cavity’s fibroepithelial lining, minimizing the risk of leaving residual squamous tissue while providing adequate soft tissue coverage of the electrode.404,405 Others describe additional electrode coverage using combinations of bone pate, fibrin glue, bone cement, perichondrium, bone matrix, cartilage, abdominal fat, and local musculoperiosteal flaps.400,406,407 Furthermore, to prevent the electrode array from looping within the open cavity, the implant can be placed more posteriorly than normal, creating gutter or zigzag grooves in the cortex of the temporal bone.406,407 Olgun et al.400 described creating a tunnel underneath the facial nerve to the sinus tympani for passage, protection, and tethering of the electrode.

Subtotal petrosectomy was first described by Rambo in 1958408 and later modified and popularized by Fisch in 1965.409 The main steps of the procedure are: (1) Blind sac closure of the EAC; (2) Exenteration of the middle ear and mastoid, including perisigmoid, perilabyrinthine, perifacial, and hypotympanic cells; (3) Removal of the middle ear epithelium and mucosa; (4) Closure of the tympanic orifice of the eustachian tube; and (5) Obliteration of the cavity with abdominal fat.410

In 1998, Issing et al.403 and Bendet et al.411 proposed subtotal petrosectomy as the technique of choice for CI in ears affected by chronic otitis media. Subtotal petrosectomy allows complete isolation of the surgical cavity from the external environment, reducing the risk of postoperative infection, CSF leakage, and meningitis.412,413 Furthermore, improved view and illumination of the surgical cavity provide excellent control and visualization of all available landmarks.414,415 Properly performed subtotal petrosectomy results in a “safe and dry” cavity that is optimal for CI positioning.416 The wide angle of approach to the RW area, obtained by removing the posterosuperior wall of the EAC, facilitates electrode insertion.

Mastoid obliteration options include abdominal fat grafts, hydroxyapatite cement, and vascularized pedicled muscle flaps, including the temporalis muscle and even free flaps.400,404,417,418 Abdominal fat is widely used due to its easy accessibility and reduced risk of necrosis and infection compared with muscle.419 However, creating a new surgical site introduces new risks, including hematoma and infection.402 Although large preoperative meatoplasties may discourage EAC overclosure due to the risk of postoperative breakdown, a meta-analysis has shown that EAC overclosure leads to fewer complications than maintenance of a canal wall down mastoid cavity with CI.418

Some algorithms have been developed for the management of CI in chronically infected ears.401,420,421 In order to reduce the risk of meningitis resulting from the insertion of an electrode into a potentially contaminated field415,422 and residual cholesteatoma,423 CI has already been performed at different stages after surgery, ranging from concomitantly with surgery for removal of the disease up to 1-year later.417,421,424,425 Most authors prefer to perform a second surgery before placing the CI, according to a meta-analysis that showed that 42.6% of cases with cholesteatoma were operated on more than once, while only 20.1% of cases were operated on in a single stage.426 However, another meta-analysis compared complication rates between patients who had the CI performed simultaneously with EAC overclosure and those who had the CI placed later, and there was no difference.418 Furthermore, depending on the case, especially in children, performing CI in a second stage can cause a significant delay in the hearing rehabilitation process.

In the presence of radical cavity or chronic otitis media with cholesteatoma, despite all efforts to meticulously remove all squamous epithelium, residual cholesteatoma may occur even many years after surgery.411,413,415 Therefore, prolonged follow-up of these patients is required. One of the protocols used includes monitoring patients with high-resolution CT at 1, 3, 5, and 10-years for the presence of cholesteatoma.414 The radiologic interface created by the fat in the cavity increases the diagnostic efficacy of high-resolution CT for residual cholesteatoma.415 Diffusion-weighted MRI, considered the gold standard for residual cholesteatoma, cannot be performed in the presence of a CI due to the enormous artifacts produced by this specific sequence. However, new generation implants allow the magnet to be removed and repositioned under local anesthesia immediately before and after MRI. This procedure can be performed once after 3-years. MRI-induced artifacts produced by a CI may not allow checking for cholesteatoma recurrence: even if tailored magnet positioning makes it feasible to monitor specific intracranial structures, its role in cholesteatoma detection is still debated.401,427

Given the concerns about cholesteatoma development after EAC overclosure, some authors overclose the EAC without mastoid obliteration to allow for postoperative imaging to assess the modified middle ear for possible cholesteatoma, providing easier access in a revision.398,428 However, postoperative imaging in unobliterated overclosed cavities shows very little air and mostly soft tissue in the mastoid cavities, suggesting the limited utility of postoperative imaging to assess for possible cholesteatoma.428 In addition, there are case reports of complications secondary to retrograde contamination of the middle ear through a patent eustachian tube.429,430 Thus, closure of the eustachian tube is indicated. There are several techniques involving scarring the orifice mucosa with bipolar coagulation, stripping the mucosa, and/or obliterating with bone wax, muscle, cartilage, bone pate, myofascial grafts, perichondrium, periosteum, and/or fibrin.415,419,425

Farinetti et al.431 analyzed CI complications in 403 adults and children and found no significant difference in the rates of major complications between adults and children, in agreement with 2 other studies.418,432 In a multicenter case series, with a mean follow-up of 44-months, only 12.7% of patients undergoing subtotal petrosectomy experienced complications.433 There are no significant differences in the rates of complications, reoperations, or device failures between the pediatric and adult populations.426 An additional, non-negligible benefit provided by subtotal petrosectomy is quality of life improvement in terms of clinical monitoring and daily living (swimming, vertigo induced by changes in pressure or temperature).433

While most mastoid cavities are either maintained or overclosed in conjunction with CI, 2 other surgical techniques have been described: reconstruction of the posterior wall of the EAC and the middle fossa approach.176,177,407,434,435 Regarding the reconstruction of the EAC, Ito et al.436 reported a case of malformation of the inner ear and facial recess in which the posterior canal wall was recreated with a temporal bone plate and bone pate and reinforced with fibrin glue and a vascularized pedicle of temporal muscle. Tamura et al.435 reported on 3 patients who had posterior canal wall reconstruction with bone plates obtained from the mastoid cortex, followed by CI. However, there is always a risk that patients will develop complications such as postauricular incision dehiscence and development of residual cholesteatoma, but few reports are available in the literature. The middle fossa approach may be another option, but it requires craniotomy with retraction of the temporal lobe and also risks damage to the facial nerve.

Tinnitus is a common symptom in CI candidates, with a prevalence ranging from 66% to 86%,437 and it can be disabling in some cases.438 A meta-analysis and systematic review demonstrated that CIs lead to improvements in tinnitus. Tinnitus Handicap Inventory (THI) scores significantly reduced by more than 50% after surgery in patients with different types of hearing loss and at different follow-up times.439

CI has been indicated as a rehabilitation method for hearing loss associated with sporadic Vestibular Schwannoma (VS) or Neurofibromatosis type 2 (NF2) resulting from the disease itself or treatment.440–444 It has been indicated for unilateral and bilateral hearing loss, especially when associated with tinnitus.440,445–447 One of the main arguments against CI in patients with VS is the impossibility of follow-up with MRI to assess the IAC postoperatively. Artifact reduction techniques have progressed and allowed good visualization of even the membranous labyrinth of CI users.448

Hearing rehabilitation in NF2, or sporadic VS in the only hearing ear, is a greater challenge due to the threat of bilateral hearing loss, bilateral hearing loss, from either the natural course of the disease or treatment, often at a young age. As a result, the traditional approach has been to wait for hearing loss to occur before surgical resection to preserve hearing function for as long as possible. However, the resulting larger tumor confers a greater risk at surgery, including facial nerve palsy and intracranial complications, in addition to loss of candidacy for hearing preservation surgery.449 A move toward earlier intervention in smaller tumors, with the goal of cochlear nerve preservation for CI, may reduce these additional surgical risks. In the case of NF2, a successful unilateral CI may also impact decision-making for a contralateral VS, with surgical treatment in an ear with residual hearing being considered in the knowledge that the patient will continue to function from an audiologic perspective.450

Most patients with VS experience a decline in hearing in the affected ear, regardless of whether observation, radiation, or surgery is selected as the primary treatment modality. Unilateral deafness has commonly been accepted as a consequence of disease progression or intervention in sporadic tumors. Rehabilitation options have previously been limited to contralateral rerouting of hearing aids or bone conduction devices (active or passive), with no attempt to provide auditory input to the affected ear.451 However, studies of CI in non-tumor-related unilateral deafness have shown significant benefits in spatial perception, sound localization, speech intelligibility, and quality of life in patients with a contralateral hearing ear.452 Some patients have difficulty integrating the signal from their CI with their natural hearing. However, this is not universal, and trends show improvement over time. CI is a feasible method of hearing rehabilitation in selected cases of sporadic VS regardless of the hearing status of the contralateral ear and is being increasingly used.453

CI is considered an option in cases where there is anatomic and functional preservation of the cochlear nerve, since CI outcomes may be superior to those of ABI with lower morbidity rates.445,454 EABR has been used for intraoperative assessment of cochlear nerve function after VS resection, assisting in the selection of patients who may benefit from a CI.447,454 The electrode array is placed in the RW niche or within the cochlea, with the recording electrodes being placed on the scalp. Although it can be used during hearing preservation surgery, sound stimulation with recording electrodes optimally either on the cochlear nerve (measuring the cochlear nerve action potential) or on the brainstem in the foramen of Luschka (measuring the dorsal cochlear nucleus action potential) can cause substantial artifact, making these recording sites less useful. However, a robust EABR has been correlated with a good post-CI auditory outcome.455,456

Preservation of the cochlear nerve and improved CI response occur more frequently in tumors up to 2 cm.457–459 CI has been used in patients with VS treated with observation, with imaging follow-up and growth stability over the last 2–3 years,458,460,461 and in patients treated with microsurgery, even with a hearing preservation technique that evolves with hearing loss.446,458 In patients undergoing translabyrinthine resection, cochlear ossification has been reported postoperatively, with the percentage of completely patent cochleae decreasing from 75% to 38% in 6–12 months after surgery. When performing CI in a second stage, placement of a dummy electrode in the cochlea is recommended to maintain cochlear patency.462–464 Furthermore, there is evidence that spiral ganglion cells can deteriorate after surgery, leading to poorer hearing outcomes.465

West et al.,446 in a review of 86 patients undergoing tumor surgery (via middle fossa, retrolabyrinthine, retrosigmoid, or translabyrinthine approach), found no difference in CI outcomes between patients with sporadic VS or NF2, with mean discrimination scores of moderate-to-high performances and only a few cases where there was no response. The authors emphasized the difficulty of comparing different studies since the tests used for audiologic evaluation were different.

Iannacone et al.,458 in a review study, recommended that CI be performed simultaneously to translabyrinthine resection of VS, based on reports of up to 40% of cochlear ossification at 1 year after tumor resection. Conversely, Wick et al.466 found no significant difference between simultaneous and sequential CI in a literature review of 93 patients.

Arnoldner et al.443 established hearing loss as ≤50% monosyllable recognition at 80 dB, Deep et al.457 as mean pure-tone thresholds >70 dB, and Mukherjee et al.462 as thresholds >90 dB. Sorrentino et al.441 considered CI indication for patients with moderate-to-severe hearing loss in the contralateral ear and an indication for translabyrinthine surgery in the ipsilateral ear. This prospective study considered the presence of positive intraoperative EABRs after translabyrinthine resection of sporadic VS an indication of cochlear nerve function preservation to allow simultaneous CI. Of 17 patients, 10 received a CI.443 They also observed improved postoperative responses in patients who had residual speech recognition preoperatively, ranging from 20% to 65% in 9 patients 6-months after surgery. One patient had no response.

Bartindale et al.,467 in a systematic review of 45 studies including 15 patients with sporadic VS, found that the mean speech discrimination score improved from 30% to 56.4%, with better results in patients who showed poorer discrimination preoperatively. Tumor location and size, duration of deafness before CI, and type of CI (sequential or concomitant) were not associated with postoperative hearing outcomes.

Deep et al.457 retrospectively evaluated 24 patients with NF2 treated with microsurgery, irradiation, or observation who received a unilateral CI. Mean patient follow-up was 4-years. The rate of open-set speech perception was 42%, with better results in tumors 1.5 cm or less.

Sanna et al.447 retrospectively evaluated 41 patients (33 with sporadic VS and 8 with NF2) undergoing unilateral resection of tumors 2 cm or less via the translabyrinthine approach with simultaneous CI. One year after surgery, 48.8% of patients still used their CI, with a sentence recognition rate of 60.7%. Those who stopped using the CI had a sentence recognition rate of 15.3%. There was no statistically significant difference between patients with sporadic VS and NF2. Unlike the study by Bartindale et al.,467 better preoperative audiologic test results had a positive correlation with postoperative results, which may indicate better preservation of neural elements.

Longino et al.461 retrospectively evaluated 7 patients with sporadic VS treated with observation and showed tumor growth stability. Consonant-nucleus-consonant word scores improved from 6% to 55% and AzBio scores in quiet improved from 9% to 56% 6-months after CI, and these results were maintained at 12-months.

Sorrentino et al.441 evaluated 8 patients with sporadic VS and 9 with NF2 24-months after CI; one of the patients received a bilateral CI. CI was simultaneous in 11 ears and sequential in 7 and occurred between 1- and 3-months after VS resection. The results were better in patients with sporadic VS and worse in those with better contralateral hearing, while preoperative hearing had no effect on the implanted side. The mean word recognition score was 40%.

Regarding tinnitus, West et al.446 retrospectively evaluated 22 patients, 21 with sporadic VS and 1 with NF2. Nineteen patients underwent tumor resection, 1 received radiotherapy, and 2 were treated with observation. Sixteen patients had unilateral deafness. The THI was administered to 17 patients both preoperatively and postoperatively, with a significant score reduction in 76%, no changes in 18%, and increase in 6%. Conway et al.445 prospectively evaluated 10 patients with sporadic VS who underwent translabyrinthine resection with simultaneous CI and found a significant reduction in THI scores (from 41.3 to 23.3) 3-months after surgery. Tian et al.468 reviewed 33 patients who received radiotherapy and reported improved speech discrimination and reduced tinnitus in 70% of cases after CI surgery.

There is concern about whether imaging can confirm the effectiveness of tumor resection, detect recurrences, or control for residual lesions. However, satisfactory evaluation has been obtained. Novel bipolar magnets align with the magnetic field and reduce the likelihood of CI displacement and pain during imaging. Changes in CI positioning have allowed MRI to be performed with 1.5-T and 3.0-T scanners without causing many artifacts in the IAC region.469,470 As for the surgical technique, positioning the receiver 8–9 cm from the EAC at an angle of 90 ° or 160 ° in relation to a line that passes through the nasion (anterior portion of the frontonasal suture) and the EAC allows adequate visualization of the IAC and the cerebellopontine angle.469,470

Long-term post-CI hearing outcomes in VS are scarce in the literature. Neff et al.471 reported that hearing performance did not significantly deteriorate over an extended postoperative follow-up, with a maximum follow-up of 93-months. Given the less predictable growth pattern of NF2-associated VS, the proportion of patients with NF2 who will potentially lose benefit from CI caused by tumor regrowth and will require ABI for hearing rehabilitation remains to be determined.457 Overall outcomes for CI in patients with VS show favorable results with observed or irradiated tumors, sporadic and NF2-related, but outcomes after tumor resection are more variable with a greater chance of no benefit.450,457 Despite poorer outcomes in patients with VS than seen in the general CI population, a CI should always be considered in a stable or resected tumor with cochlear nerve preservation because it will almost always outperform an ABI.471,472

As highlighted by Bartindale et al.;467 the priorities of treatment in VS are in order of importance to preserve life, facial function, and hearing, as well as to minimize other complications. Hearing preservation should remain a key factor in decision-making for patients with VS. Schlacter et al.,472 in their meta-analysis, reported that 82% of patients had improved speech perception scores after CI. The same proportion were regular daily users (defined as more than 8 h per day). Likewise, Smith et al.473 reported that 84.9% of recipients remained regular daily CI users after a median follow-up of 3.6 years. Although uncertainty remains regarding long-term hearing outcomes, CI offers significantly better results than ABI.450,467,471 Neither the primary treatment modality nor the cause of the tumor (NF2-related or sporadic) appears to have a significant effect on outcome. CI in VS serves to offer patients, with a functioning cochlear nerve, the probability of open-set speech discrimination with a consequent positive impact on quality of life.

There are complications inherent in mastoidectomy and posterior tympanotomy, presence of an internal device in the patient (foreign body), and failure of the implanted device. The rate of these complications has decreased over time due to improvements in surgical techniques and technological developments, currently standing at approximately 9%.431 It is important that patients, family members, and health professionals be aware of potential complications. In addition to the consequences on patients’ health, there is also the economic impact on the public health system and private health insurers.

In 2009, Hansen et al.432 defined complications as minor or major (Table 5). The authors also divided them into medical and surgical, adult and pediatric, perioperative (occurring up to 24 h after the end of surgery), early postoperative (within 1 week of surgery), and late postoperative (beyond 1 week of surgery). Complication rates reported in the literature are 11.8% for minor complications and 3.2% for major complications.

Major complications are defined as those requiring revision surgery or hospitalization for medical treatment: 1) Serious medical problem (e.g., meningitis); 2) Complication requiring additional, major surgical intervention (e.g., cholesteatoma or explantation); 3) Any degree of permanent disability (e.g., permanent facial paralysis).

Minor complications are defined as those requiring conservative treatment or minimally invasive surgery, such as ventilation tube placement, that do not meet the criteria for major complications.

Patients rarely develop cholesteatoma after CI. It may result from damage to the tympanic membrane and/or EAC during surgery. Treatment consists of CI explantation at the site of infection and biofilm formation, in addition to conversion to a canal wall down mastoid cavity and excision of the entire lesion. It is important to maintain only the electrode array inside the cochlea and remove the rest of the device to preserve cochlear patency and avoid fibrosis. CI reimplantation should be performed as soon as possible, after infection control, combined with subtotal petrosectomy.487

Peripheral Facial Palsy (PFP) is one of the worst complications that can occur in ear surgery. When opening the mastoid to resect tumors or treat infections, although undesirable, PFP can be an iatrogenic complication of the procedure. However, in CI surgery, in patients with normal anatomy, PFP may be disastrous. The incidence of PFP is low (0.4% to 0.7%).488,489

Continuous IFNM has been used for CI surgery.490 There is scarce literature on the effectiveness of IFNM in reducing the risk of postoperative facial paralysis in CI surgery. Only 2 articles have briefly discussed the topic, but the results are conflicting.489,491 Fayad et al.489 reported that there is no relationship between the use of IFNM and delayed facial nerve palsy. However, Thom et al.491 observed a 4.5-fold increased risk of delayed postoperative facial nerve palsy in cases where IFNM was not used.

Although some studies imply a weak association between IFNM use and postoperative facial nerve function and, therefore, fail to validate the standardized use of IFNM in CI surgery, the merit of using a monitoring system as an additional precautionary device to prevent further nerve injury should be recognized. IFNM is also useful in training surgeons and enhancing their skills under the guidance of experienced surgeons.492 Furthermore, the use of IFNM is strongly encouraged in cases of otitis media, mastoiditis, or abnormal inner ear morphology due to poor visibility and low predictability of facial nerve location in these patients.298,476

Intraoperative factors, including mechanical trauma and thermal injury, can lead to progressive inflammation, neural edema, and ischemia, which, in turn, can result in immediate-onset or early-onset delayed palsy. Thermal injury may occur due to inadequate irrigation or excessively vigorous drilling of the facial recess. In several previous studies, thermal injury has also been reported when the “neck” of the drill is accidentally placed against the facial nerve during drilling to identify the RW membrane or cochleostomy.491,493 The use of copious irrigation may reduce the possibility of burr heating and thus reduce the risk of thermal injury and neural edema. Although necessary, the use of IFNM does not replace the surgeon's knowledge of anatomy and proper technique.

Identification of the facial nerve is essential to avoid severe injuries and complications. Preoperative imaging, especially in patients with malformations, can prevent complications. If immediate PFP is detected, it is essential to re-approach the mastoid and decompress the facial nerve as quickly as possible. The prognosis is more favorable when there is only edema. However, the occurrence of damage to the facial nerve fibers increases the risk of permanent paralysis.

The rate of facial nerve stimulation ranges from 0.9% to 14.9% in CI users. It is usually associated with otosclerosis (most common), cochlear malformation, and temporal bone fracture. It may also result from extrusion or partial insertion of the electrodes and direct stimulation of the facial nerve, mainly in the mastoid portion. The most common symptoms are paresthesia, visible facial twitching, and pain.357

Facial nerve stimulation during CI use is more common in patients with advanced otosclerosis, reaching up to 17% of cases in some studies.358,495 It may be a minor or major complication, often resolved by reprogramming or deactivating part of the electrodes. However, when many electrodes need to be deactivated, CI performance can be negatively impacted. In some cases, explantation may be necessary.

The risk of facial nerve stimulation in advanced otosclerosis is lower with perimodiolar electrodes. Migration of the electrode array into the IAC may also occur. In this case, imaging can be useful in making the diagnosis and assessing the need for electrode repositioning. Patients with cochlear malformations, such as CC deformity, IP-I, and IP-III, are at increased risk of electrode migration into the IAC. Other unpleasant symptoms that may develop after CI in far advanced otosclerosis are tinnitus, dizziness, and headache.

It is one of the most common errors made by inexperienced surgeons. Incomplete electrode insertion occurs in up to 2% of patients with normal cochlear anatomy, being more prevalent in straight banded arrays than in perimodiolar arrays.496

For optimal hearing outcome, the electrode array must be inserted into the ST. The best way to ensure that the electrodes will be positioned at this location is to insert them via the RW. Sometimes, the RW can be confused with hypotympanic cells that are inadvertently opened, and when attempting to insert the CI, the internal component may break. To avoid this type of error, in addition to properly identifying the RW, the pyramidal eminence and stapes should be located. Drilling of the superior lip of the RW niche exposes the window more adequately. Another way to confirm the proper location of the RW is to move the handle of the malleus and check whether the RW membrane also moves.

Some authors consider it a major complication because the patient requires rehospitalization and revision surgery to replace the device. It occurs in approximately 2%–4% of cases. It is more common in children than in adults due to trauma. Over the years, failure rates have decreased due to improved device quality and increased strength of the internal component. When a device failure is confirmed, a mastoid CT should be performed to check whether the electrodes are properly positioned inside the cochlea and reimplantation should be performed as soon as possible.195

The use of MRI has been expanded particularly to include non-neurologic applications such as the heart, abdomen, pelvis, and musculoskeletal system. The use of MRI is expanding at a rate of approximately 20% per year.497

Pain is the most common complication in CI users undergoing MRI and can occur in up to 70% of patients, but only in about 6%–18% it prevents scanning from being completed.498 Other significant complications are magnet dislocation (0.6%–15%),488 depolarization, and polarity reversal.497 Artifacts caused by CI remain a problem but can be reduced by using specific sequences. Noise perception during MRI has rarely been described in CI users, but there are reports of interruption of the scanning procedure, in a 1.5-T scanner, due to pain and noise perception.499

The induction of electric currents in response to rapidly changing electromagnetic fields and radiofrequency pulse emission could lead to heating of muscle tissue, skin damage, and internal device malfunction. However, studies of CIs have not yet demonstrated relevant temperature increases that would pose a risk to the surrounding tissue.500,501 There is no correlation between pain and MRI duration or head positioning.502,503 In the studies evaluated, from 2.25% to 17% of MRI scans had to be stopped because of patient pain.498,502,504 Transient erythema or pressure ulcers may also occur, most likely due to patient wearing a tight bandage around the head.503 Other studies and case reports have described severe pain often associated with magnet displacement, but which could also result from wearing a tight headband.505,506

Displacement of the internal magnet caused by the MRI magnetic field has been considered a serious risk and even a contraindication to CI in the past.507 But CI has evolved over the years. Currently, there are devices compatible with MRI up to 3.0 T. However, some studies have shown CI-related impairment and complications, especially in earlier implant models. Tam et al.,508 in a prospective study, found a magnet displacement rate of 3.5% for MRI scanning in CI or ABI users using a 1.5-T scanner. Loth et al.509 reported a magnet dislocation rate of 7% in their retrospective study of 711 patients. Demagnetization and magnet polarity reversal are rare and poorly reported events.510,511 Magnet polarity reversal or demagnetization during MRI is a relatively rare complication that may result in reduced retention of the external speech processor.

A study conducted in a cadaveric model showed that a pressure head bandage over the CI reduces the risk of magnet dislocation during MRI.512 However, a head bandage does not appear to completely prevent magnet dislocation.513 Hassepass et al.,497 in a retrospective study with an observation period of 13-years, found 23 cases of magnet dislocation in CI users. Ten of these cases occurred despite the use of a bandage without a counter-pressure element. This suggests that either the bandage does not reliably reduce dislocation, or the dressing technique needs to be improved. Finally, it does not appear to make a significant difference whether the preventative head bandage is applied by a trained radiologist or an otolaryngologist.514

In the available prospective and retrospective studies, the use of head bandages has not been reliably documented in all patients and the method of implementation is variable.497,502,504,508 It is important to always follow the manufacturer’s recommendations for limiting the field of MRI scanning. In the study by Kim et al.,498 2 of the 18 study patients did not receive a head bandage and were also scanned at 3.0 T, which contradicts the manufacturer’s recommendations for the hearing implants used.

Most CIs are compatible with 1.5 T field strength. Devices that support field strengths of up to 3.0 T are now available on the market.515 The manufacturer’s recommendations should be followed to reduce the risk of complications, although they can occur even when the guidelines are followed.

To ensure that implants at risk can also undergo MRI scanning, it is necessary to provide specialized care for CI users during the imaging procedure. This could be regulated by standard operating procedures, of which the procedure described here could be a variant. This would help ensure compliance with the manufacturer’s precautions and the MRI parameters required for each implant type.

Additional studies would be useful to consolidate MRI safety conclusions for currently implanted CIs and to increase the safety of imaging older models. This may also help address the current refusal of CI users by imaging centers.

Electrode positioning complications represent a significant proportion of perioperative CI complications and compromise patients’ hearing gain. Careful surgical planning and appropriate preoperative and intraoperative imaging can reduce the risk and impact of electrode positioning complications.

In 2003, Eshraghi et al.516 developed a grading system for intracochlear trauma caused by insertion of 3 different types of perimodiolar electrodes in cadavers (Table 6).

In a non-ossified cochleae with normal anatomy, the electrodes should remain fully in the ST after insertion. Scalar Translocation (STL) of the electrode array occurs when the electrode migrates from the ST to the scala vestibuli or media, piercing the cochlear partition. It reduces the patient’s residual hearing, in addition to leading to a more unfavorable position of the electrode array for stimulation of the cochlear nerve. Straight electrodes have a significantly lower risk of STL than perimodiolar electrodes (7% vs. 43%), even when only RW insertion is considered (2% vs. 22%).517

Electrode tip fold-over or rollover is another complication of CI insertion. It is uncommon, less than 2% of surgeries, being more frequent in perimodiolar electrodes. It occurs because the electrode tip folds during insertion, which may not be noticed by the surgeon. Typically, intraoperative neurotelemetry is normal. It can be corrected before waking the patient if identified by intraoperative imaging or in revision surgery or if identified postoperatively if patients experience vertigo, unfavorable hearing performance, or tinnitus.518

CI can have a significant impact on vestibular function. Multiple mechanisms that may lead to vestibular dysfunction during or after CI surgery have been proposed: (1) Direct trauma from electrode insertion; (2) Acute serous labyrinthitis due to cochleostomy; (3) Oreign body reaction with labyrinthitis; (4) Endolymphatic hydrops; and (5) Electrical stimulation from the implant itself.77 The occurrence of vestibular dysfunction after CI surgery has a very wide range as assessed by caloric testing and VEMP.77,519 However, not all CI users experience postoperative dizziness.519,520 Kluenter et al.519 reported different forms of dizziness after surgery. Given the increasing use of bilateral implants, it is important to quantify the effects of CI surgery on the vestibular system. This information is of great benefit to both the CI team and patients.

The reported incidence of vestibular dysfunction after CI surgery ranges from 39% to 74%, lasting from 2 days to 2 years.521 Only a few patients report significant postoperative symptoms. However, vestibular assessment is important in cases that will undergo unilateral surgery. If the chosen side has good vestibular function and the contralateral side has vestibular hypofunction, there is a high risk of causing bilateral vestibular hypofunction, which may lead to loss of balance with great limitation in daily activities. In these cases, it is preferable to choose the side with worse vestibular function.522

The onset of vertigo can occur immediately after surgery or as a delayed phenomenon. The electrode array impairs the physiologic function of the organ of Corti and adjacent labyrinthine structures.523 Other possible mechanisms include changes in vestibular receptors and effects on the central nervous system.524 Patients with profound hearing loss may have vestibular dysfunction and report vertigo before surgery. Children rarely experience long-term vertigo.525 However, vestibular dysfunction and balance impairment have been identified as important risk factors for CI failure in children.526 Older patients more frequently complain of vertigo after CI.527

The saccule is the most affected organ. CI surgery has a significant negative effect on caloric testing and VEMP in adults. The factors that most influence symptoms are advanced age and etiology of hearing loss.92 Younger patients may compensate better after vestibular dysfunction.528 Hearing preservation techniques and electrode insertion via the RW also appear to better preserve vestibular function.529 Therefore, although rare, the possible effects of CI surgery on the vestibular system should be communicated to patients before surgery.

Most patients with Meniere’s Disease (MD) will have different degrees of permanent hearing loss, and for 15%–38% of patients with unilateral MD, hearing loss will progress to severe sensorineural hearing loss.530 Bilateral MD rarely presents with both ears affected simultaneously. Most cases occur sequentially with initial unilateral symptoms progressing to bilateral disease.531 Management of this condition aims to minimize vestibular symptoms while preserving hearing as much as possible. Most patients are controlled by lifestyle modifications and medication use. If unsuccessful, intratympanic injections (gentamicin or corticosteroids), endolymphatic sac decompression, or vestibular neurectomy can be proposed, with these procedures entailing the risk of hearing loss. Surgical labyrinthectomy is an effective treatment for vertigo attacks, but this procedure forfeits residual hearing and may trigger cochlear ossification.532

CI has also been described for patients with MD, yielding favorable hearing outcomes.533 A systematic review evaluated 17 studies that showed CI safety and efficacy in 182 patients with MD.534 Despite the heterogeneity of studies, patients with DM had significant hearing improvements after CI and a complication rate comparable to that of CI users without MD. The results were similar to those in the general population of CI users. Only 3 patients (1.6%) had a decline in speech perception scores. Of the patients reporting postoperative vertigo, none had undergone simultaneous or delayed labyrinthectomy with CI.

However, patients with MD may pose a challenge postoperatively due to fluctuating performance that requires device reprogramming.533,535 In patients with bilateral MD, the status of the contralateral vestibular system should be considered in CI, as surgery may result in additional vestibular damage. Patients with MD who have previously undergone labyrinthectomy may require updated imaging (MRI) to ensure the presence of a patent cochlear duct. Approximately one-third of patients undergoing labyrinthectomy develop cochlear ossification, which could preclude electrode array insertion.535,536