Cervical Vestibular Evoked Myogenic Potentials in Vestibular Neuritis

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INTRODUCTION
Vestibular neuritis [VN] is described as a degenerative neuropathy of the vestibular nerve trunks [1] . It is one of the most common reasons for vertigo [2] . VN is distinguished by sudden onset of rotatory vertigo, nausea, vomiting, & nystagmus [3] . It is usually severe for few days & progressively subsides within 2-3 weeks [4] .
Reason for VN is not totally understood, but several hypotheses are elucidating its pathophysiology [5] . The most possible hypothesis is reactivation of dormant neurotropic virus [6] . Other mechanisms may be involved, are autoimmune or microvascular ischemia affecting the vestibule [7] .
Superior division of vestibular nerve has been identified as being affected by VN, however after vestibular evoked myogenic potentials [VEMPs] testing has been introduced, it became evident that both vestibular nerve divisions could be impacted, either together or independently [8] . When both divisions are impacted, spinal ganglion is regarded to be impacted [9] . Depending on location of lesion, VEMPs response may be normal, abnormal or even absent totally [10] .
Diagnosis of inferior vestibular neuritis is difficult since typical symptoms of vestibular neuritis are absent in this condition [11] . As a result, isolated inferior vestibular neuritis may be mistakenly attributed to central pathology unless inferior vestibular function is not thoroughly evaluated [12] .  [13] .
Using AC sound, Chihara et al. [14] and Chou et al. [15] , noted absence of cVEMPs from affected ears, but these researches were small & presented inconsistent proof of inferior nerve insight to AC evoked reflexes [16] . By Using BC stimulation, Brantberg et al. [17] , proved that forehead & mastoid taps generated more cVEMPs abnormalities than AC stimulation. This is consistent with BC evoked cVEMPs being mediated at least in part by superior vestibular nerve afferents. On the other hand, Curthoys [18] declared that in VN, BC stimulation generated normal cVEMPs reaction. This was explained by regardless of stimulus modality, otolith-collic projections originate primarily from saccule [19] . This discrepancy of findings of preceding researches, leaded us to study the effect of type of stimulus on cVEMPs test results.

THE AIM OF THE WORK
This study aimed to study and compare AC and BC cVEMPs in patients with vestibular neuritis.

PATIENTS AND METHODS
This observational case control research included 40 subjects. Their age ranged from twenty to sixty years old, recruited from Audio vestibular clinic of Al Zahraa university hospital. Written consents were obtained from all participants. They were categorized to 2 groups: [1] Control group: comprised of twenty normal healthy subjects with no complaint of dizziness or history of vestibular disorders.
[2] Study group: comprised of 20 patients identified with vestibular neuritis [they did not receive any vestibulo-suppressant medications for at least 48 hours before examination] according to clinical criteria of Taylor et al. [7] . They gave history of vertigo [at least one attack of rotational vertigo that increases significantly with head movement], nausea/vomiting and imbalance. On vestibular assessment, patients showed spontaneous horizontal rotational nystagmus toward lesion side, deviation in the opposite direction to nystagmus and unilateral caloric weakness.
All participants in this study had no hearing complaints, normal hearing sensitivity in the frequency ranges of two hundred fiftyeight thousand Hz as shown in pure tone audiometry. Middle ear functions were normal as evidenced by tympanometry and acoustic reflexes threshold. Also, they had no history of chronic diseases. cVEMPs testing for all participants was carried out using Interacoustics Eclipse [EP25, Inc., Middlefart, Denmark].
• Electrode montage: skin was cleaned to verify that impedance was less than five kΩ. Positive electrode was located on upper 3rd of stimulated side's SCM, negative and ground electrodes were placed on sternum and forehead respectively.
• Instructions to patients: they were instructed to turn their head to opposite side of stimulation with slight head flexion to enhance muscle contraction.
• AC and BC 500 Hz tone burst stimuli with intensity of 95 dBnHL & 70 dBnHL respectively, presented at a rate of 5/ second, with a total sweep of 200 and analysis time of 50 milliseconds were used [11] .
• Wave analysis: during the study, equipment system was observing EMG levels.

RESULTS
In this work, there was no statistically significant difference between Rt and Lt ears as regard AC and BC cVEMPs in both groups [ Tables 1 & 2].
Table [3] shows no significant difference between AC & BC cVEMPs measures in control group.
Table [4] shows that the differences between AC & BC cVEMPs in study group are not statistically significant.
As the differences between Rt & Lt ears in both groups were not statistically significant, we used 40 ears for statistical analysis in [tables 5,6].
Table [5] shows no significant differences between both groups as regard AC cVEMPs measures.
Table [6] shows no significant difference between both groups as regard BC cVEMPs.
Table [7] shows that the differences between affected ears & non-affected ears of VN group as regard AC cVEMPs P13& N23 latencies are significant.
Table [8] shows that there is statistically significant deference between intact ears & affected ears as regard BC cVEMPs P13& N23 latencies.
Table [9] shows that there are more abnormalities in BC cVEMPs than AC cVEMPs.

DISCUSSION
Statistical analysis was done on the results and the study showed that there is no significant difference of AC and BC cVEMPs measures between right and left ears of both groups [tables 1&2]. Also, we reported no significant difference between AC & BC cVEMPs in both groups [tables 3&4]. As a result, we used 40 ears for statistical analysis in tables 5&6 and declared that the differences between study and control groups as regard latency of P13& N23, P13N23 amplitude and asymmetric ratios were not of significant difference [table 5&6]. These outcomes were in agreement with outcomes of Curthoys et al. [20] , Govender et al. [16] , and Oh et al. [21] , who reported that most VN patients shows normal cVEMPs. This can be explained by VN mostly affects fibers in superior division of vestibular nerve more severely than fibers in inferior division [22] . This could be due to the anatomical variations between the two divisions. When compared to inferior division, superior vestibular nerve has more length, reduced diameter, & increased bony trabeculae of its housing bony canal [23] . In agree with this postulation is the result of Sirige et al. [24] who get normal cVEMPs in 3 studied cases previously diagnosed with vestibular neuritis. Furthermore, these findings are consistent with findings of Welgampola et al. [25] , who declared that both AC & BC cVEMPs are more likely to exert their impacts via similar pathways, resulting in similar patterns of response in VN.
On comparing affected [diseased] and nonaffected[intact] ears in study group as regard AC cVEMPs and BC cVEMPs, important variation was showed with delayed P13 & N23 latencies and there was no significant difference as regard P13N23 complex amplitude [table  7&8]. This outcome is in agreement with Govender et al. [20] , who detected delayed latencies of affected ears when compared to the latencies of the nonaffected ears. Sirige et al. [24] provided objective proof that inferior vestibular neuritis exists a subtype of vestibular neuritis. His work showed normal caloric testing outcomes and abnormal cVEMPs outcomes and thus constructing firm diagnosis of inferior vestibular neuritis.
In this work about 25% of study group had abnormal AC cVEMPs while 35% had abnormal BC cVEMPs [table 9]. These outcomes were in agreement with Brantberg et al. [17] , who demonstrated that BC evoked cVEMPs showed more abnormalities than AC stimulation. This may be explained by the BC cVEMPs may be mediated by afferents within the superior vestibular nerve [utricular effect] [26] . While AC evoked cVEMPs is sensitive to many pathologies [25] , BC can be used when there is no response to AC stimuli and in patients suffering from conductive hearing loss [27] .