Degeneration of the vestibular nerve in unilateral Meniere’s disease evaluated by galvanic vestibular-evoked myogenic potentials
Introduction
Meniere’s disease is an inner ear disease characterized by relapsing-remitting aural symptoms and vertigo. The diagnosis is established on the most widely-used guidelines proposed by the American Academy of Otolaryngology-Head and Neck Surgery (AAO-HNS) and by clinical presentation (Committee on Hearing and Equilibrium, 1995). AAO-HNS defines definite Meniere’s disease as two or more spontaneous episodes of vertigo, tinnitus or aural fullness, documented hearing loss on at least one occasion, and other causes excluded. The pathogenesis for Meniere’s disease has been widely supported to be endolymphatic hydrops, which may result from over-production or inadequately resorption of endolymph in the inner ear (Paparella, 1985). Although the cochlea is the most involved area, vestibular endorgans, including the saccule, utricle and semicircular canals, may also be involved in the endolymphatic hydrops (Okuno and Sando, 1987). According to audiometric criteria with 4-frequency pure-tone averages at 0.5, 1, 2, and 3 kHz of the worst audiogram during the interval of 6 months before treatment, the AAO-HNS staging system defined Meniere’s disease with 4 stages of different disease severities. However, because pure tone audiogram only reflects the decline of cochlear function, additional information of vestibular function seems to be necessary for mapping the disease in detail.
Newly-developed cervical vestibular-evoked myogenic potential (cVEMP) has been progressively utilized for assessing the descending sacculo-collic reflex (SCR) pathway and saccule function (Colebatch et al., 1994), whereas the novel ocular vestibular-evoked myogenic potential (oVEMP) has been successfully demonstrated for evaluating the ascending vestibulo-ocular reflex (VOR) pathway and utricle function (Iwasaki et al., 2007). Air conducted sound (ACS) was first proposed to be best for generating cVEMPs and bone conducted vibration (BCV) for eliciting oVEMPs by Welgampola and Carey (2010), in order to achieve the best otolith afferent selectivity. However, even though abnormal ACS-cVEMPs or BCV-oVEMPs may indicate impaired SCR or VOR pathway, it is still insufficient to discriminate between labyrinthine and retrolabyrinthine deficits. VEMPs via galvanic vestibular stimulation (GVS) are recognized as fundamental in the assessment of retrolabyrinthine function because GVS directly elicits vestibular afferents and bypasses otolithic organs. In other words, GVS-VEMPs combined with the results of ACS-cVEMP or BCV-oVEMP testing may differentiate a labyrinthine lesion from a retrolabyrinthine one (Murofushi et al., 2002, Iwasaki et al., 2005, Cheng et al., 2009).
Most patients with Meniere’s disease who manage their illness by medication have an acceptable control of associated symptoms. Few patients will undergo vestibular destructive surgery, such as vestibular neurectomy and labyrinthectomy due to intractable vertigo. Clinically, even without neurectomy, vertigo will subside gradually after deterioration of the vestibular nerve and compensation of the central nervous system. Previously, various studies have investigated the correlation between Meniere’s disease and VEMPs via ACS or BCV modes for evaluating the otolithic function of the disease (Huang et al., 2011, Katayama et al., 2010, Manzari et al., 2010a, Young et al., 2003). This study further investigates the physiological function of vestibular nerve and brainstem by adding GVS-VEMP tests into the inner ear test battery. Considering the retrolabyrinthine involvement of Meniere’s disease pathologically (Ritter et al., 1981a, Ritter et al., 1981b, Tsuji et al., 2000), physicians may offer patients additional information regarding the likelihood of future vertigo attacks due to electrophysiological GVS-VEMP testing.
Section snippets
Patients
This prospective study enrolled seventy patients with unilateral Meniere’s disease. The diagnostic criteria of an ear with Meniere’s disease were based on the AAO-HNS guidelines proposed in 1995. If Meniere’s disease was established in one ear while the opposite ear had inner ear symptoms, such as documented hearing loss, tinnitus, or aural fullness, the case was considered as bilateral Meniere’s disease and excluded from this study. Within 2 weeks of the disease diagnosis, all subjects
Pure tone audiometry
There were 70 patients with unilateral Meniere’s disease enrolled in this study, including 35 men and 35 women. Their ages ranged from 22 to 77 years, with a mean of 58 years. The right ear was affected in 33 patients (47%), and the left in 37 patients (53%). There were 6 patients (9%) in stage I Meniere’s disease, 18 patients in stage II (26%), 27 patients in stage III (38%) and 19 patients in stage IV (27%). The abnormal rate of hearing with 4-tone average >26 dBHL was 91%. The mean duration of
Discussion
For further evaluation of Meniere’s disease localization and prevalence, the inner ear test battery, including audiometry, cVEMP, oVEMP and caloric tests, was utilized for exploration of inner ear function, namely, the cochlea, saccule, utricle and semicircular canals (Yang et al., 2010, Young, 2013). Involvement of labyrinth or retrolabyrinth was further defined by combined GVS-VEMP results with ACS-VEMP or BCV-VEMP results. In general, abnormal ACS-VEMPs plus normal GVS-VEMPs may imply a
Conclusions
According to the functional evaluation of unilateral Meniere’s disease via the inner ear test battery, the impairment of otolithic organs was found to be more than that of vestibular afferents. Furthermore, the deterioration of the saccule was more than that of the utricle, whereas the degeneration of the retrolabyrinthine part of SCR and VOR was similar. No obvious differences in latency, interval or amplitude between affected and unaffected ears may imply some contralateral ears of unilateral
Acknowledgements
This work was supported by Far Eastern Memorial Hospital, Taiwan: Grant no. FEMH-2013-C-002.
Special thanks to Hillary Chiao Lee for text editing.
Conflicts of interest statement: None of the authors have potential conflicts of interest to be disclosed.
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