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ORIGINAL ARTICLE
Year : 2015  |  Volume : 2  |  Issue : 2  |  Page : 45-53

Vestibular evoked myogenic potentials and video head impulse tests in different stages of Meniere's disease


1 Audiology Unit, Faculty of Medicine, Alexandria University, Alexandria, Egypt
2 Department of Otorhinolaryngology, Faculty of Medicine, Alexandria University, Alexandria, Egypt

Date of Submission02-Nov-2015
Date of Acceptance09-Nov-2015
Date of Web Publication10-Dec-2015

Correspondence Address:
Doaa M Elmoazen
182 Omar Lotfy, Sporting, Alexandria, 21524
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/2314-8667.171518

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  Abstract 

Introduction
Meniere's disease (MD) is characterized by intermittent episodes of vertigo, fluctuating sensorineural hearing loss, tinnitus, and aural pressure. Histopathologically, endolymphatic hydrops is seen initially in the cochlear duct and the saccule; with the progression of the disease, subsequent affection of the utricle and semicircular canals (SCC) occur.
Objectives
The aim of this study was to assess the function of the saccule, the utricle, and the three SCCs using cervical vestibular evoked myogenic potential (cVEMP), ocular vestibular evoked myogenic potential (oVEMP), and video head impulse test (vHIT), respectively, in Meniere's patients and to correlate the findings with the different stages of MD.
Patients and methods
Forty patients diagnosed with unilateral definite MD according to the American Academy of Otolaryngology and Head and Neck Surgery's 1995 criteria [AAO-HNS (1995) criteria] were tested. They were divided into four subgroups based on the stage of MD using the average of 500, 1000, 2000, and 3000 Hz pure-tone thresholds of the worst documented audiogram during the 6-month interval before examination. A control group of 40 healthy age-matched adults were equally divided into four corresponding subgroups based on their pure-tone average. In all, 500 and 1000 Hz tone burst air conduction cVEMP and oVEMP tests, bithermal caloric test, and vHIT for all SCCs were recorded for both groups.
Results
The results of this study showed that the highest prevalence of abnormalities in Meniere's patients were seen in the pure tone average (PTA) (85%), followed by cVEMP (72.5%) and oVEMP (67.5%). Caloric test abnormalities follow with a lower prevalence of 32.5%. The vHIT presented the lowest prevalence of abnormalities, with 20% in the lateral canal, 7.5% in the anterior, and 5% in the posterior canal. This pattern of abnormality is in agreement with the reported pattern of hydrops formation in the vestibular system.
Conclusion
The findings of the current study suggest that with the advancement in the stage of MD consequent abnormal cVEMP, oVEMP, caloric, and vHIT responses occur following the histopathological evidence of progression of the disease from the saccule up to the SCCs.

Keywords: cervical vestibular evoked myogenic potential, Meniere′s disease, ocular vestibular evoked myogenic potential, video head impulse test


How to cite this article:
Elmoazen DM, Sobhy OA, Elbaky FA. Vestibular evoked myogenic potentials and video head impulse tests in different stages of Meniere's disease. Adv Arab Acad Audio-Vestibul J 2015;2:45-53

How to cite this URL:
Elmoazen DM, Sobhy OA, Elbaky FA. Vestibular evoked myogenic potentials and video head impulse tests in different stages of Meniere's disease. Adv Arab Acad Audio-Vestibul J [serial online] 2015 [cited 2017 Oct 21];2:45-53. Available from: http://www.aaj.eg.net/text.asp?2015/2/2/45/171518


  Introduction Top


Meniere's disease (MD) is a clinical syndrome that consists of episodes of spontaneous vertigo, lasting several minutes to several days, usually associated with unilateral fluctuating sensorineural hearing loss, tinnitus, and aural fullness. Histopathologically, the most notable feature is the endolymphatic hydrops. Hydropic changes initially involve the cochlear duct and the saccule, linking the vestibular manifestations with the hearing loss. However, with the progression of the pathology, the entire endolymphatic space is involved with subsequent affection of various endorgans, including the utricle and the semicircular canals (SCC) [1] . Schuknecht and Gulya [2] reported a prevalence for endolymphatic hydrops of 100% in the cochlea, 86.3% in the saccule, 50% in the utricle, and 36.4% in the SCCs based on pathological temporal bone studies.

The assessment of patients with MD can be challenging, and the availability of investigations that examine each component of the vestibular apparatus would facilitate a more complete evaluation. Tests for utricular and saccular function include ocular (oVEMP) and cervical (cVEMP) vestibular evoked myogenic potentials, respectively [3-5] . Caloric test with video nystagmography or the head impulse test have long been in clinical use as measures of the lateral SCC function. Lately, recordable video head impulse tests (vHIT) that measure vestibulo-ocular reflex (VOR) to rapid impulsive head accelerations in the plane of each SCC have allowed quantitative monitoring of individual canal function [6] . When the results of oVEMP and cVEMP tests are combined with the results of vHIT, a full understanding of the state of the peripheral vestibular function for each end organ of the labyrinth ought to be achieved with the progression of MD.


  Objective Top


The aim of the current study was to assess the function of the saccule, the utricle, and the three SCCs using cVEMP, oVEMP, and vHIT, respectively, in Meniere's patients and to correlate the findings with the different stages of MD. The hypothesis is that, with the advancement in the stage of MD, consequent abnormal cVEMP, oVEMP, and vHIT responses are expected in order to follow the histopathological evidence of progression of the disease.


  Patients and methods Top


Forty patients diagnosed with unilateral definite MD according to the American Academy of Otolaryngology and Head and Neck Surgery's 1995 criteria (AAO-HNS) were divided into four subgroups [7] . According to these criteria, patients should experience two or more spontaneous vertigo episodes lasting more than 20 min, accompanied by tinnitus or fullness in one ear with audiometrically documented hearing loss on at least one occasion. The subgroups were divided on the basis of the stage of MD using the AAO-HNS (1995) criteria based on four-tone average (0.5, 1, 2, and 3 kHz) pure-tone thresholds of the worst-documented audiogram during the 6-month interval before examination. Stage I has a pure-tone average less than 26 dB (six patients), stage II has between 26 and 40 dB (10 patients), stage III between 40 and 70 dB (15 patients), and stage IV more than 70 dB (nine patients). Patients were not examined in the acute attack and were instructed to cease their MD medication for 48 h before the examination. The study was approved by the local ethics committee, and informed consent was obtained from each participant before inclusion. All patients had a normal brain MRI. Patients with eye motility disorders, conductive hearing loss, evidence of MD in the other ear, or an alternative cause of vertigo were excluded from the group. The control group consisted of 40 healthy age-matched adults divided equally into four subgroups according to their pure-tone average matching the four patient subgroups.

The following audiovestibular test battery was carried out for each participant on the same day. Both affected and unaffected ears were tested. All patients were subjected to detailed history taking of the character, frequency, and duration of the vertigo and the disease duration. Pure-tone audiograms were obtained for each ear of all participants using the Interacoustics AC33 (Denmark) using supraural TDH-49 headphones. The patients' four-tone average (500, 1000, 2000, and 3000 kHz) pure-tone thresholds were calculated. cVEMPs and oVEMPs were recorded individually in a random order using the GSI AUDERA evoked potential system (Minneapolis, USA). Individual random 500 and 1000 Hz tone burst air conduction stimuli of condensation polarity were monaurally delivered to the tested ear at a fixed intensity of 105 dBnHL via GSI TIP50 insert earphones. Blackman gated 2-1-2 cycle stimuli were used at a rate of 5 cycles/s. One channel recordings were obtained. An active electrode was placed on the upper third of the ipsilateral sternocleidomastoid muscle, the reference electrode on the sternoclavicular junction, and the ground electrode on the forehead. Electrode impedance was typically less than 5000 W. The patient was in the sitting position with the head rotated away from the side of stimulation. Myogenic variability was minimized by the use of target markers on the examination room walls. At least two runs were performed and 150 sweeps were averaged for each run with an analysis time of 50 ms. Electromyographic signals were amplified and band-pass filtered between 5 and 750 Hz and monitored to maintain background muscle activity.

cVEMP p13-n23 amplitude, p13 and p23 latency, frequency peak amplitude ratio (1000/500 Hz FPA) of the same ear, and interaural amplitude difference (IAD) were calculated.

IAD = [(A r − A 1 )]/(A r − A 1 )] × 100

where Ar and Al represent the peak amplitude of the right and the left side [8],[9] . The IAD in Meniere's patients was as follows:

IAD = [(A u − A a )]/(A u − A a )] × 100

where Au and Aa represent the peak amplitude of the unaffected side and the affected side [8] .

A positive value for the IAD in Meniere's patients indicated reduced VEMP amplitude in the affected ear, whereas a negative value signified an augmented amplitude in the diseased ear. During statistical analysis, the sign of the IAD value in controls and Meniere's patients was omitted and the mean and SD of the absolute value was used.

Ocular vestibular evoked myogenic potential

Similar parameters as cVEMP were used. The contralateral eye to the tested ear was selected. Participants were in a seated position and were asked to maintain an upward gaze at a fixed mark in the ceiling mounted at 30° from the neutral gaze point according to Govender et al. [10] The active electrode was placed just inferior to the center of the lower lid margin and the reference electrode was 1 cm below and the ground electrode on the forehead .The n1-p1 amplitude, latency, 1000/500 Hz FPA of the same ear, and IAD were calculated using the aforementioned equations.

Bithermal caloric test

Bithermal caloric testing was performed using the GN Otometrics ICS Chartr video-nystagmography (Denmark). The patient was made to lie in the supine position with the head flexed 30° upward [11] . Four irrigations were performed separately (two for each ear) with water at a temperature of 30 and 44°C with at least 5 min of rest between irrigations. A unilateral weakness in the caloric response was quantified according to the conventional Jongkees' formula. A value of 25% or greater was considered pathological. vHIT was performed using GN Otometrics ICS Impulse system (Denmark) in the lateral, left anterior right posterior, and right anterior left posterior SCC planes. A pair of lightweight, tightly fitting goggles was worn. A small nine-axis motion sensor on the goggles measures the head movement in free space [12] . The participant was made to sit at 1 m from the targeted mark. For the horizontal plane, the target was directly in front of the patient and the head was tilted anteriorly by 30° and unpredictably thrust to the left or right into a 15-20° angle [Figure 1]. For the vertical planes, Migliaccio and Cremer's [13] modified procedure was used, with the gaze directed in the plane of head rotation to mainly elicit vertical eye movement for two-dimensional video pupil tracking. The patient's head was rotated 30° to the right (left anterior right posterior test) or left (right anterior left posterior test) while fixating on the target out of the corner of the eye for the gaze to be in the plane of the tested canals [Figure 1]. Brief, abrupt, forward and backward head impulses were given in the plane of the vertical canals that allow stimulation of the anterior and the posterior SCC, respectively [13] . A total of five accepted head impulses in each direction for each plan were performed. The average gain of the VOR responses for each canal and the gain asymmetry (GA) were calculated.
Figure 1: Modified video head impulse test positions for lateral (a), left anterior right posterior (b), and right anterior left posterior (c) canal stimulation with the participant fixating on a target placed 1 m ahead. For the vertical planes the gaze is directed in the plane of head rotation

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GA = [(Gc - Gi)/(Gc + Gi)] × 100

where Gc and Gi represent the gain for head thrust exciting the contralateral (unaffected) canal and the ipsilateral (affected) canal [14] . Abnormal overt and/or covert saccades were measured. The vHIT results were defined as abnormal, if canal gain or canal gain asymmetry were outside the normative range and/or when refixation saccades (overt and/or covert) were present.

Statistical procedures

The statistical evaluation of data was carried out by means of SPSS software. Mean ± SD was recorded for each parameter. The c2 -test was used for quantitative data. A paired t-test and Student's t-test were used for comparing the control, affected, and unaffected ears for normally distributed data. Comparison between two dependent means for nonparametric data was made using the Wilcoxon signed-ranks test and for two independent means the Mann-Whitney test was used. Comparisons across the four different subgroups of MD in VEMP, vHIT, and caloric tests was made using the F-test (analysis of variance) for normally distributed data and using the Kruskal-Wallis test for the nonparametric data. Correlation of disease duration with the stage of MD was tested using Spearman's correlation. VEMP, vHIT, and caloric test results were defined as abnormal if they fall outside the 95% limits (mean ± 2 SD) of normal hearing control data (subgroup I). The prevalence of abnormality in all tests was compared using McNemar's test. Receiver operating characteristic curves were used to obtain cutoff values for the diagnosis of MD using the 1000/500 Hz FPA of the cVEMP and oVEMP. Statistical significance was set at P value of 0.05 or greater.


  Results Top


The patient group consisted of 80 ears (40 unaffected ears and 40 affected ears). The left ear was more commonly affected (52.5%). A total of 25 men and 15 women were tested, with an age range between 21 and 69 years (mean = 43.30 ± 13.77 years). The control group consisted of 20 men and 20 women (80 ears), with a mean age of 40.82 ± 10.68 years (21-58 years).

Vertigo was the most common complain (82%) accompanied with tinnitus (77.5%), fluctuating hearing loss (77.5%), and ear fullness. In the earlier stages of MD, vertigo and vomiting were more pronounced, whereas dizziness was more evident in the late stages. Hearing loss was fluctuating in nature in the earlier stages of the disease with low frequency configuration and accompanied by tinnitus and ear fullness. In the last stage of the disease, hearing loss became permanent with a flat configuration due to repeated rupture of membranes with progressive destruction of the membranous labyrinth [15],[16] . The duration of illness ranged from 3 month to 15 years (4.34 ± 3.36) and the vertigo attacks lasted from 20 min to 2 h (33.88 ± 19.98 min). No significant correlation was found between the disease duration and the stage of the disease.

[Table 1] summarizes the descriptive data (mean ± SD) of the measured indexes of 500 Hz cVEMP and oVEMP; and vHIT in subgroup I of controls (normative data) and in the total population of controls, unaffected, and affected ears in MD.
Table 1 Descriptive data (mean ± SD) of vestibular tests indexes in the total population and normative data

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Cervical vestibular evoked myogenic potential results

  1. Prevalence of cVEMP response:

    The prevalence of cVEMP response to the 500 Hz and 1000 Hz ACS stimuli was equal (92.5%) in the control group. The affected ears of Meniere's patients had insignificantly lower prevalence of 82.5% in both stimuli.


  2. Latency of cVEMP: In response to the most commonly used 500 Hz stimulus, the mean latencies of cVEMP p13 and n23 waveforms were not significantly different in the affected ears compared with controls or the unaffected ears.


  3. Amplitude of cVEMP: In the affected ears, the 500 cVEMP amplitude was normal in 10 ears (25%), augmented in eight (20%), diminished in 15 (37.5%), and absent in seven (17.5%). After exclusion of absent cVEMP ears, a significant difference was found between the mean amplitude of affected ears and controls and also between the unaffected and affected ears. No significance was found between control ears and unaffected ears of Meniere's patients. In subgroup I of Meniere's patients, the mean cVEMP amplitude was significantly increased (augmented) in affected ears (508.58 ± 234.40 mV) compared with unaffected ears (179.14 ± 154.66 mV) and control ears (196.58 ± 53.81 mV). In the other three subgroups (II-IV), the mean cVEMP amplitude was decreased significantly in affected ears compared with unaffected ears and control ears. Augmented VEMPs can be explained by dilatation of the saccular hydrops, pressing against the footplate and enhancing the sensitivity of the saccular macula to loud sounds [17],[18] . The mean 500 Hz cVEMP amplitude was compared across the four studied subgroups in controls and affected ears to determine the effect of hearing loss and the progression of MD on the amplitude. No significant difference was found in the control group excluding the effect of hearing levels. In contrast, the affected ears of Meniere's patients showed a significant difference between subgroup I and all other subgroups (II-IV). This could be attributed to the augmented amplitudes of subgroup I. Nonsignificant amplitude difference between subgroups II, III, and IV of the affected ears of Meniere's patients was seen, although a trend toward decrease in amplitude with the advancement of disease stage was seen.


  4. IAD of cVEMP:

    The absolute value of the IAD of the 500 Hz cVEMP had a highly significant increased mean compared with controls using the whole sample and in each of the four subgroups, indicating increased interaural peak asymmetry in MD. On comparison across the subgroups, no significant difference was found in the control group, whereas the affected ears of Meniere's patients showed a significant difference between most subgroups. In the current study, the suggested normal asymmetry limit (mean+2 SD) for the 500 Hz cVEMP IAD obtained from subgroup I of controls was 39.25%.


  5. Frequency tuning of cVEMP:

    In normal individuals, the cVEMP has been shown to be most robustly elicited by using a 500 Hz tone-burst, with the response reducing on either side of this frequency [19] . Reports published by several groups suggest that this frequency tuning is shifted to 1000 Hz in patients with MD [20-22] . Thus, tuning curves may be potentially used as a method of diagnosing of MD. Significantly larger 1000/500 Hz FPA ratio is seen on comparing the whole sample of the affected ears of Meniere's patients with the control ears and the unaffected ears. The increased FPA ratios denote the presence of altered tuning of ears affected by MD shifting toward the higher 1000 Hz frequency [Figure 2]. Unaffected ears of MD showed no significant difference in 1000/500 Hz FPA compared with controls. A highly significantly increased ratio in the affected ears was also seen in the four subgroups. Comparison across the subgroups in control ears and affected ears showed no significance demonstrating no effect of hearing levels on tuning. Receiver operating characteristic curves using the 1000/500 Hz FPA ratio of the cVEMP were used to obtain a cutoff value greater than 0.9 for the diagnosis of MD, with 78.79% sensitivity and 80% specificity.
    Figure 2: Cervical vestibular evoked myogenic potential waveforms in a Meniere's patient stage I with affected left ear .The upper and lower waveforms are responses to 500 and 1000 Hz stimuli, respectively. The right ear shows normal frequency tuning with the 500 Hz responses being slightly enhanced (a). The left ear shows altered frequency tuning with the 1000 Hz responses being greater in amplitude compared with the 500 Hz waveforms (b)

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Ocular vestibular evoked myogenic potential results

  1. Prevalence of oVEMP response:

    The prevalence of oVEMP response to the 500 Hz and 1000 Hz ACS stimuli was similar (92.5%) in the control group. The affected ears of Meniere's patients had an insignificantly lower prevalence of 80% using the 500 Hz stimulus compared with the 87.5% prevalence of the 1000 Hz stimulus. The prevalence of both cVEMP and oVEMP responses together in affected ears showed an insignificantly higher prevalence for the 1000 Hz stimuli (90%) compared with the 500 Hz stimuli (85%).


  2. Latency of oVEMP:

    No significant difference was found in 500 Hz oVEMP n1 and p2 mean latency of affected, control, or unaffected ears.


  3. Amplitude of oVEMP:

    The 500 oVEMP amplitude of affected ears of Meniere's patients was normal in 23 ears (57.5%) ears, diminished in nine (22.5%), and absent in eight (20%). Unlike the cVEMP, no augmented oVEMP amplitude was obtained in any of the studied cases.


  4. The oVEMP mean amplitude was significantly reduced in the affected ears of Meniere's patients compared with control ears and with the unaffected ears in the total sample and in comparing subgroups II, III, and IV separately. No significance in amplitude was found in the early stage of MD (subgroup I). No significance was found in oVEMP amplitude between control ears and unaffected ears of Meniere's patients. Comparison across the subgroups showed no significant difference in the control group, whereas the affected ears showed a significant difference between subgroup I and all other subgroups (II, III, and IV).


  5. IAD of oVEMP:

    Meniere's patient group showed a highly significant increased IAD compared with controls in subgroup III and IV only, indicating increased oVEMP asymmetry in Meniere's patients in the later stages of the disease. Comparison across the subgroups showed no significant difference in the control group, whereas the affected ears showed a significant difference in amplitude between earlier subgroups (I and II) and later subgroups (III and IV) and between subgroup III and IV. A pattern of increasing IAD with the increase in stage of the disease was seen. The calculated normal asymmetry limit for the 500 Hz IAD in oVEMP was 24.55 ± 9.24%.


  6. Frequency tuning of the oVEMP:

    Similar to cVEMP results, a significantly larger 1000/500 Hz FPA ratio was seen in the affected ears of Meniere's patients compared with the control and unaffected ears in the total sample and in subgroups II-IV. An example of the oVEMP frequency tuning is shown in [Figure 3]. Unaffected ears of Meniere's patients were insignificantly different from controls. Comparison across the subgroups showed a non significant difference in the control group and the affected ears. The cutoff value for the diagnosis of MD for oVEMP 1000/500 Hz FPA ratio was more than 0.83, with 90.32% sensitivity and 50% specificity. Other cutoff points were evaluated, but yielded unacceptably high false-negative values.
    Figure 3: Ocular vestibular evoked myogenic potential waveforms in a Meniere's patient stage IV with affected left ear. The upper and lower waveforms are responses to 500 and 1000 Hz stimuli, respectively. The right ear shows normal frequency tuning with the 500 Hz responses being slightly enhanced (a). The left ear shows altered frequency tuning with the 1000 Hz responses being greater in amplitude compared with the 500 Hz waveforms (b)

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Bithermal caloric test results

Caloric asymmetry was found in 32.5% of Meniere's patients, with weakness ranging from 28.0 to 66.0% (mean = 43.23 ± 12.58%). [Table 2] summarizes the prevalence of abnormal caloric response in stages I to IV.
Table 2 Caloric response in the four studied subgroups of Meniere's patients

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Video head impulse test results

  1. Canal gain:

    The lateral canal gain was normal in 32 ears (80%), increased in three (7.5%), and diminished in five (12.5%). The anterior canal gain was normal in 38 ears (95%) and diminished in three (7.5%), whereas the posterior canal gain was normal in 37 ears (92.5%) and diminished in two (5%), with no increased responses in both canals. No refixation saccades, occult or overt, were recorded in any of the studied cases in any canal. No significant difference in gain was found between controls, unaffected, and affected ears of Meniere's patients for all SCCs.


  2. Canal gain asymmetry:

    The gain asymmetry of lateral canal was normal in 36 ears (90%) and increased in four ears (10%). No decreased gain asymmetry was seen in the lateral canal. The anterior and posterior canal gain asymmetry were normal in all 40 ears (100%). No significant difference was found between control, affected, and unaffected ears for the lateral, anterior and posterior SCCs. No effect of hearing loss on canal gain or asymmetry in control ears or affected ears was seen.


Prevalence of abnormalities in Meniere's disease

Comparison of the prevalence of abnormalities in all parameters of audiovestibular tests performed in Meniere's patients were made using McNemar's test. The highest prevalence in abnormalities in Meniere's patients was seen in PTA (85%), followed by cVEMP (72.5%) and oVEMP (67.5%). Caloric test abnormalities follow with a lower prevalence of 32.5%, followed by vHIT lateral (20%), anterior (7.5%), and finally posterior (5%) SCC abnormalities [Figure 4].
Figure 4: The prevalence of abnormal test results across all tests

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  Discussion Top


MD is associated with the accumulation of endolymph in the cochlear duct and the vestibular organs in histopathological studies. However, endolymphatic hydrops does not explain all clinical features, including the progression of hearing loss and the frequency of vertigo attacks. Many researchers now believe that the abnormality related to Meniere's symptoms may be more physiological than obstructive anatomical - that is, MD is the outcome of a complex cascade of pathophysiologic processes that ultimately dysregulate the ionic composition of the endolymph [23],[24] . As a consequence of impaired homeostasis, hearing and balance functions become vulnerable to internal and external factors, such as stress, sleep deprivation, etc. [24] . cVEMP and oVEMP abnormalities in the form of absent waveforms, reduced amplitude, elevated threshold, increased peak latencies, increased IAD, or altered frequency tuning in ears affected by MD have been reported in the literature [9],[21],[22],[25] .

In the current study, cVEMP amplitude showed a significantly enhanced response in stage I of the disease, followed by a significant decrease in amplitude in the later stages. The IAD and 1000/500 Hz FPA ratio showed significantly increased values in all stages of the disease. Thus, the cVEMP test demonstrated abnormalities starting from stage I of MD, indicating saccular affection early on in MD. The oVEMP parameters were insignificant from controls in stage I of MD denoting sparing of the utricle in the early stage of MD. In the later stages, significantly decreased amplitude and increased IAD and 100/500 Hz FPA ratio of oVEMP were seen.

Stage I of the disease was spared of caloric asymmetry. No significant difference was found between the stage of the disease and the prevalence or the degree of unilateral caloric weakness. However, a trend of an increase in the mean caloric asymmetry was seen with disease progression [Table 2]. The bithermal caloric test was reported to be asymmetrical in 28-73% of affected ears in MD in previous studies [8],[14],[26] . Caloric unilateral weakness values are variable, and weakness values greater than 75% have been reported in some Meniere's patients, with no correlation with the stage of the disease [8] .

The recorded percentage of vHIT lateral canal gain asymmetry (10%) was significantly lower than the percentage of unilateral caloric weakness (32.5%), empathizing the finding of the differential response of Meniere's ears to high and low frequency vestibular stimulation [27],[28] . Selective loss of type II hair cells was thought to occur in Meniere's patients and was expected to result in selective loss of low frequency labyrinthine sensitivity, yielding poor caloric test results. The presumed preservation of type I hair cells was hence thought to preserve the high frequency sensitivity of the labyrinth necessary for the response to HIT [14] . Nonetheless, the actual contribution of the different fibers to vestibular function and signal processing remains unclear [27] .

McGarvie et al. [27] recently proposed the hydropic expansion of the lateral canal membranous duct as an alternate explanation. They referred to the model by Gentine et al. [29] , who proposed that the underlying drive to the caloric test is the hydrostatic drive. In this model, the temperature variation across the temporal bone creates a density difference in the endolymph between the medial and lateral arms of the lateral canal, which then creates a pressure that, in turn, deflects the cupula and produces nystagmus [27] . As the duct expands, the caloric thermal gradient dissipates owing to local convective flow, reducing the hydrostatic driving force across the cupula and causing caloric weakness. The hydrops does not reduce the endolymphatic flow in angular acceleration and so VOR gain during the vHIT is not reduced. This is because the membranous canal lies along the outermost wall of the bony canal; hence, even when the canal is hydropic, its radius of curvature remains unchanged and therefore so would its dynamic response [27] .

The vHIT gain and gain asymmetry of the three SCCs were insignificantly different from that in controls in all stages of the disease. Stage I of MD was free of vHIT abnormalities in all SCCs. Augmented gain responses were recorded only in the lateral canal in the last three stages. All three SCCs showed decreased gain and increased gain asymmetry in stages II-IV and were more frequently encountered in the lateral canal, whereas the posterior canal had the least percentage of abnormality.

The recorded prevalence in abnormalities in Meniere's patients was as follows: PTA>cVEMP>oVEMP>caloric test>lateral SCC>anterior SCC>posterior SCC [Figure 4]. This pattern of abnormality is in agreement with the reported pattern of hydrops formation in the vestibular system by and Schuknecht and Gulya [2] and Horner [18] in pathological temporal bone studies.

The percentage of abnormalities in PTA (indicating cochlear abnormality) was not significantly higher compared with cVEMP abnormalities, pointing to saccular dysfunction accompanying the cochlear dysfunction. PTA abnormalities were significantly higher compared with the abnormalities in the oVEMP (utricular dyfunction), caloric test (lateral canal function at low frequencies), and vHIT (high frequency function of SCC). cVEMP abnormalities were significantly higher compared with the caloric test, and the vHIT and the caloric test was significantly higher compared with the anterior and posterior canals only. There was no significant difference in the prevalence of abnormality between the three SCCs .These findings suggest consequent affection of the cochlea and the saccule, followed by the utricle and the low frequency function of the lateral SCC with the progression of MD. The affection of higher frequency function of the lateral canal follows. The anterior and posterior canals are the least affected in MD.


  Conclusion Top


The findings of the current study confirm the proposed hypothesis that, with the advancement in the stage of MD, consequent abnormal cVEMP, oVEMP, caloric, and vHIT responses occur following the histopathologic evidence of progression of the disease from the saccule up to the SCCs. On the basis of the findings, it can be suggested that, the cVEMP followed by the oVEMPs are better tools to separate Meniere's from non-Meniere's ears rather than the caloric or vHIT tests, especially in the earlier stages of the disease.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
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    Figures

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    Tables

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