Dyslexia is considered a specific neurobiological learning disorder that affects basic reading and language skills. It occurs due to differences in the functioning of brain systems responsible for phonological processing that result in difficulty in processing the sounds of words and associating them with the letters or sequences of letters that represent them. Other factors that may be associated are deficits in executive functions, difficulties in auditory and/or visual processing and psychomotor development.

It is considered a specific learning disorder because its symptoms usually affect the academic performance of students without any other alteration (neurological, sensory or motor) to justify them.1

Reading and writing processes involve complex and independents functions, in which cognitive and motor abilities are used. These functions result in a correct writing and a correct word decoding during reading.2,3 Another important ability for reading and writing is the visual-motor perception, that coordenates visual information and motor programming.4

The importance of eyes movement for the reading and writing learing processes has been studied.5–8 It is not only visual accurance that it matters, but also the visual-motor behavior characterized by sacadde alternated eyes movements, tracing and fixation.9

The eyes movements acuity directly interferes on the visual fixation of the word during reading and may be altered in people with dyslexia. Different bahavior patterns of visual movements has been found in dyslexic individuals as they read.10–12

For an accurate vision, it is necessary to stabilize the image on the retina, even during head movement. In this case, there is a compensatory movement of the eyes to the opposite direction of the cephalic movement. This compensatory movement is called the Vestibulo-Ocular Reflex (VOR) and involves the integration of the vestibular system and the extraocular muscles.13

This activity can be measured by objective measures such as Video-Nystagmography (VNG) and Vectoelectro-Nystagmography (VENG), which are used for the functional evaluation of the vestibular system.7,14 Vectoelectro-nystagmography is one of the most used methods to record eye movements with greater diagnostic sensitivity. It allows the measurement of vestibulo-oculomotor function parameters, by comparing stimuli and responses. It also allows the identification of the ocular direction by recording the vestibulocochlear reflex, saccades, tracing, optokinetic and ocular fixation.15–17 Videonystagmography is a computerized system that uses infrared sensors with special glasses to record, measures and analyzes eye movements.18

The aim of this study was to investigate the measurements of ocular tests from vectoelectro-nystagmography and videonystagmography exams in children with dyslexia and to compare them with typical children.

The aim of this study was to investigate ocular movements measures of vectoelectro-nystagmography and video-nystagmography in dyslexic children and compare with measures of typical children. In order to achieve it, a systematic review of the literature on observational studies was carried out in order to answer the following guiding question: “Do dyslexic children have different results in ocular tests of VNG and VENG when compared to typical children?”. The review was structured according to the items defined by the Preferred Reporting Items for Systematic Reviews and Meta-Analyzes Statement (Prisma)19 and a full protocol was published in the Prospero database (http://www.crd.york.ac.uk/PROSPERO) under the registration number CRD42018081954.

Approximately half of the analyzed studies shows immaturity of the cortical areas that control the visual fixation system in dyslexic children when compared to children with typical development.29,32,34 In dyslexic people, the percentage with poor binocular coordination during and after the saccades, is greater than in control cases. In this way, listening to the teacher in the classroom, copying and transcribing written lessons on the blackboard, reading the books lessons, writing and concentrating become complex activities for dyslexics since they lack the integrity of oculomotor functions and vestibular interconnections.18,35–37 A clear vision requires a quick alignment of the fovea with the object of interest (saccade) and keep it aligned to this object for a sufficient period of time so that the visual system can perform a detailed analysis of the image (fixation duration). In general, saccadic movements are interesting due to their close relation with attention. Its measurement may reveal disturbances in oculomotor activation and help diagnosis of neurological disorders.14

Data from the selected articles confirms the existence of a considerable difference in the ocular motor parameters between the groups. This disparate behaviour has been described in a study31 of 112 children with a mean age of 10.4 years, divided into two groups: 56 typical children and 56 with dyslexia. The authors compared latency values and found an increase in the dyslexic group. Similar results were found, in another French study24 of 30 children, with a mean age of 11.6 years, also divided into dyslexic (16) and non-dyslexic (14).

In addition, measurements of gain and mean velocities were reduced in the dyslexic groups with the occurrence of anticipatory saccades and large numbers of long-lasting, unstable fixations compared with the control group (Kraljević and Palmović 2011; Bucci et al., 2012; Bucci et al., 2004; Seassau et al., 2014). Several hypotheses were raised in order to justify such a difference: reduction or deficiency in the processing of visual attention and the precision of the search; immaturity of the saccadic ocular interaction and vergence systems; and the possibility of visual/ocular motor imperfections were frequently cited.

The impairment in convergence, ocular saccadic interaction, and fusion capacity of divergence may be due to a lack of cortical maturation, and the poor oculomotor performance in children with dyslexia is possibly related to a deficit in the allocation of visual attention, suggesting a processing deficiency.22–24,28,33

Learning disability, related to dyslexia, can still be justified by the interaction between the saccadic and vergence subsystems, on which knowledge acquisition may be based.27

VNG is a computerized system that uses the principle of capturing eye movements by infrared sensors placed on special glasses or mask. The movements are measured and analyzed on a video monitor and further recorded. In VENG, the most commonly used method for recording ocular movements, the variation of corneal-retinal potential, is used to record and analyze features of the vestibulo-ocular reflex and saccadic visual systems involved in the persecution, optokinesis and fixation.18,36

The use of VNG and VENG exams has already been consolidated in several pathologies that involve the cerebral cortex, the brainstem, the superior colliculus, the basal ganglia, the cerebellum and the sensory organs (semicircular canals and otolithic organs). This neural system controls the ocular fixation in various conditions such as the movement of the visual object, movement of the observer and changes in vision. Among the most common pathologies associated with this network are: petrositis (infection in the petrous part of the temporal bone that can reach the fourth cranial nerves, multiple sclerosis (demyelinating immune disease), optic neuritis (visual or subacute visual loss (vogt-koyanagi-harada syndrome, an autoimmune disease affecting eyes, skin, ears, and meninges) and congenital rubella (viral infection by vertical transmission, which mainly affects vision, hearing and the cardiac function of a newborn). Scientific and clinical evidence should be reasons to extend the use of these exams to the differential diagnosis on learning disorders, such as dyslexia, which does not yet have specific neurobiological cause described in the literature.13,38–41

When speaking of otoneurological evaluation we usually refer to practices performed by the speech-language pathologist related to the investigation of auditory and vestibular function, with the objective of assisting in the diagnosis of the disturbances of balance. However, the range of possibilities offered by the instruments such as VENG to the researcher is immense, especially from the perspective of making the diagnosis of language disorders more accurate, as in the case of dyslexia.42

It turns out, however, that the qualitative analysis of the studies of this review have not showed data or discussion about vestibular complaints in dyslexic population. This must be due to the lack of research using VNG and VENG for these purposes. These tests are fundamentally recommended to people with vestibular problems, and it is important to reinforce the need for more studies.

The study revealed that children with dyslexia have longer duration of fixation and fewer saccades during ocular movements on vectoelectro-nystagmography and video-nystagmography when compared to children without dyslexia. However, all these findings are not intended to reduce dyslexia to a purely ocular disorder, since, as noted above, it is a complex disorder that may be associated with deficits in executive functions, difficulties in auditory and/or visual processing and psychomotor development.1

The authors declare no conflicts of interest.