Video head impulse test results suggest that different pathomechanisms underlie sudden sensorineural hearing loss with vertigo and vestibular neuritis: Our experience in fifty‐two patients

Sudden sensorineural hearing loss with vertigo and vestibular neuritis are two disorders that could be caused by similar pathomechanisms. A quick and convenient tool, the video head impulse test (vHIT), was developed to measure and quantitatively analyze the function of each semicircular canal. Auditory and vestibular function in the two disorders, using PTA and vHIT, could help determine whether these disorders have similar degree of severity and scope of affected semicircular canals. The affected semicircular canals and severity of functional loss in the canals differed between the SHLV and VN groups by assessing the vestibulo-ocular reflex(VOR) gain (vHIT-G) and asymmetry values. vHIT results were different in VN and SHLV groups, which may underlie different etiologies of VN and SHLV. This article is protected by copyright. All rights reserved.


| INTRODUCTION
Vestibular neuritis (VN) is a most common disorder that has a comorbid presentation with acute vestibular syndrome (AVS) in the absence of cochlear symptom. Sudden sensorineural hearing loss (SSHL) with continuous vertigo (SHLV) that lasts more than 1 day is another disorder of AVS. VN and SHLV can be easily differentiated by reference to their clinical features, namely without or with cochlear involvement. Although VN and SHLV are considered as two different disorders, their pathophysiologies can be explained by similar mechanisms.
Although the mechanisms underlying SSHL remain unclear, the most widely acknowledged theories implicate vascular ischaemia and viral infections. 1,2 The inner ear and cranial nerves VII and VIII are supplied by different branches of the internal auditory artery (IAA; Figure 1), and vascular lesions at different segments of this artery lead to cochlear and/or vestibular symptoms. In contrast to SSHL, the most important aetiology underlying VN is viral infection. 3 Recently, a quick and convenient tool, the video head impulse test (vHIT), was developed to measure and quantitatively analyse the function of each semicircular canal. vHIT can easily and accurately analyse the degree of function of each canal in patients with SHLV and VN, and may also further our understanding of the pathomechanisms underlying these two disorders. Thus, this study enrolled patients with either SHLV or VN, as diagnosed based on clinical symptoms and signs. vHIT was performed in all patients to investigate whether SHLV and VN share a similar aetiology.

| Ethical considerations
Anonymised case note data were used and had no impact on patients. Formal ethical approval was not required.

| vHIT protocols
All patients underwent vHIT on the first or second visit, usually within 2 days to 2 weeks after onset of acute vertigo. a "wide window" from the beginning of the impulse until the head velocity returns to (or crosses) 0°/seconds. 7 Overt and covert saccades are also important for evaluation of impairments in the functions of the semicircular canals and to help determine the accuracy of vHIT-G.

| Variables
For each patient, a detailed history was obtained and standard bedside examinations were performed.

| Statistical analysis
All statistical analyses were performed with SPSS software (ver. 22.0; IBM Corp., Armonk, NY, USA). The rate of abnormalities was compared between SHLV and VN patients using independent-samples t tests. Normally distributed vHIT-G data were compared using independent-samples t tests and post hoc tests. The gender between two groups was compared using chi-square test. P values <0.05 were considered to indicate statistical significance.

| Clinical features
The clinical and demographic characteristics of the SHLV and VN patients are summarised in Table 1

| vHIT
In the two groups of patients, the mean vHIT-G values for the anterior semicircular canal (AC), horizontal semicircular canal (HC) and posterior semicircular canal (PC) are shown in Table 2 Asymmetry values among the bilateral HC, AC and PC were calculated in the VN and SHLV groups (Table 3). There were significant differences in the bilateral HC and AC between the SHLV and VN groups (P < 0 .05), but not in the bilateral PC.

Keypoints
• Sudden sensorineural hearing loss with vertigo and vestibular neuritis are two disorders that could be caused by similar pathomechanisms.
• A quick and convenient tool, the video head impulse test (vHIT), was developed to measure and quantitatively analyse the function of each semicircular canal.
• Auditory and vestibular function in the two disorders, using PTA and vHIT, could help determine whether these disorders have similar degree of severity and scope of affected semicircular canals.
• The affected semicircular canals and severity of functional loss in the canals differed between the SHLV and VN groups by assessing the vestibulo-ocular reflex (VOR) gain (vHIT-G) and asymmetry values.

| DISCUSSION
Sudden sensorineural hearing loss with vertigo and VN are two different auditory and/or vestibular disorders that can be easily differentiated based on their clinical symptoms. Although these two disorders are thought to have different aetiologies, they could be explained by similar pathomechanisms, namely a vascular pathology that theoretically underlies both disorders. If this is true, the severity and scope of semicircular canal impairment would be similar between the two disorders. Thus, the present study assessed auditory and vestibular function in the two disorders, using PTA and vHIT, and sought to determine whether these disorders have similar aetiologies.

| STREN GTHS OF THE STUDY
The present study found that the affected semicircular canals and severity of functional loss in the canals differed between the SHLV and VN groups. In the SHLV group, dysfunction typically included profound deafness or high-frequency hearing loss in conjunction with decreased function in the PC, suggesting that the entire basilar membrane (or basal turn) and PC were affected. In the present study, all three semicircular canals were typically affected equally in the VN group. Unlike the SHLV group in the present study, a vascular pathology could not explain the isolated vestibular function loss without cochlear damage seen in the VN patients with functional loss in either the AC, HC, or all three canals.
The present study also showed that asymmetry of the vHIT VOR can be used to evaluate the function of semicircular canals and may be more accurate than vHIT-G. Our results indicate that the PC in SHLV patients was vulnerable to injury, whereas the functions of all three semicircular canals in VN patients were susceptible to injury, which suggests that SHLV and VN do not have a common aetiology.
For making a diagnosis of VN, decreased function of semicircular canal is a necessary criterion. In the present study, spontaneous nystagmus only presented in 12 and h-HIT were abnormal in 18 of 22 patients with VN. When the static compensation is completed, spontaneous nystagmus will disappear, but abnormal h-HIT will still exist.  9 vHIT, as a high-frequency measure tool, is a useful complement to caloric and rotational tests.

| CONCLUSIONS
The present study showed that vHIT results were different in VN and SHLV groups, which may underlie different aetiologies of VN and SHLV; thus, further study will be necessary to confirm these results and determine the different aetiologies.

CONF LICT OF I NTEREST
None to declare.