Differential Contribution of Vestibular Evoked Myogenic Potentials for Diagnosis of Superior Semicircular Canal Dehiscence Syndrome

Superior semicircular canal dehiscence syndrome (SSCDS) is described as a disorder of the inner ear denoted by the absence of a bony covering, causing vestibular and auditory symptomatology. The vestibular evoked myogenic potentials (VEMPs) provide information regarding the vestibular function, which is useful in the diagnosis of vestibular disorders. Our aim is to assess the differential role of ocular VEMP (oVEMP) and cervical VEMP (cVEMP). oVEMPs and cVEMPs were collected from four male and five female (64.5 ± 3.6 years old) with SSCDS validated by computerized axial tomography between 2017 and 2019. The neurophysiological assessment included the calculation of latency and amplitude of both the pathological ear (PE) and the healthy ear (HE) and the asymmetry ratio (AR) of both ears. A control group comprising 14 people without vestibular pathology and similar age was utilized. Results: The control group fitted well to the Gaussian distribution. The most persistent symptom was vertigo, followed by hearing loss. One patient had bilateral SSCDS. The latency was higher for cVEMPs at PE but was not disparate for oVEMP latency. In contrast, amplitude was higher at PE than at HE for both tests. Nonetheless, AR was >34% in all oVEMPs, while there were only in two of eight cases for cVEMPs. Conclusion: Though cVEMP abnormalities in amplitude may help to suspect the pathology, oVEMP is the most sensitive and strong test for the diagnosis of SSCDS. This technique is a rapid, reliable, inexpensive test without side effects and thus has a very vital use as a screening and follow-up test in SSCDS.

402 external auditory canal (Hennebert sign). This medical entity may also emerge in various other several clinical expressions, such as conductive hearing loss with a normal stapedial reflex [5], autophony, and hyperacusis with increased sensitivity for bone conduction, otic fullness [6], and pulsatile tinnitus [7]. The gold standard to validate the presence of the superior semicircular canal dehiscence (SSCDS) is high-resolution computed tomography (CT) of the temporal bone [8,9]. Nevertheless, neurophysiological studies to evaluate vestibular function are also noted [10].
Vestibular evoked myogenic potentials (VEMPs) have been proven useful in the diagnosis of vestibular disorders. However, there has not been too much attention to the SSCDS. VEMPs are time-locked muscular responses prompted by an auditory stimulus and give indispensable information regarding the otolithic organs (utricle and saccule), nerves, and vestibular nuclei ( Figure 1) [11,12].
Ocular vestibular evoked myogenic potentials (oVEMPs) are ascending and excitatory utriculo-sacular responses that allow us to assess the integrity of the vestibular-ocular pathways [13,14].
On the other hand, cervical vestibular evoked myogenic potentials (cVEMPs) are descending and inhibitory saculocolic responses that give information regarding the vestibulospinal pathways [15]. The principal objective of this work was to evaluate the various roles of both types of VEMPs in the SSCDS. To give a useful as well as objective information, findings have been compared with a control group of people without vestibular pathology.

Figure 1:
Neural pathways involved in VEMPs. The utricle-sacular pathway (red), which is valued with oVEMPs, is an excitatory and cross-pathway. The sacculal-colic pathway (blue) is an inhibitory and direct response, which is studied by cVEMPs.

Subjects
The study involved nine patients (four male and five female) with vestibular pathology, referred by an otolaryngologist that has a specialization in vestibular disorders (YLC), between 2017 and 2020 ( Table 1). All of them had a diagnosis of SSCDS validated by CT. Furthermore, we conducted VEMPs among them and in a control group of 14 volunteers who had no vestibular pathology. The ethics committee of the Hospital Universitario La Princesa approved this study.

Neurophysiological tests
VEMPs were obtained by means of a four-channel Neurosoft® with no pre-stimulus normalization. For oVEMPs, the latency was defined as the first negative deflection (n10), and, for cVEMPs, it was the first positive deflection (p13), where n is negative (upward deflection) and p is positive (downward deflection). The numbers refer to the time in ms. The amplitude (in µV) was calculated to be between p13-n13 for oVEMPs and cVEMPs, respectively. We also have measured the asymmetry between the amplitudes, wherein usually a significant symmetry is greater than 34% [20]. The asymmetry ratio (AR) was measured with the use of the following formula: Where VEMP1 refers to the higher potential and VEMP2 to the lower one.
Pathological findings have been defined in accordance to a double criteria: a) Own standard control tables acquired from people with no pathology and, also, as an additional control system, as seen in published tables [21,22], and b) Furthermore the AR. Fiducial limits have been defined as mean ± 2.5 standard error of the mean (SEM) for our data and mean ± 2.5 standard error for published data.
Imaging tests were conducted before VEMPs in five patients, and SSCDS has been validated using both tests. In the remaining cases, the pathological side was determined with VEMPs, and the diagnosis of SSCDS was then validated with the imaging test. We conducted an evaluation of the data with the use of responses from the pathological ear (PE) and the healthy ear (HE).

Statistical analysis
For the patient's group, we categorized the sample into two groups:

a)
The pathological ear (PE) and b) The healthy ear (HE).
For the control group, both ears were considered to be non-

Control group
We have obtained the control values from people with no vestibular pathology which are picked up from our environment.
The mean age was 54.3 ± 3.9 years. Results are outlined in

Patients group
The mean age was 64.5 ± 3.9 years, which is no different from  Table 3. Latencies overall considered either from PE or HE were between normal limits. In the case of oVEMPs, no differences were noted between both groups (9.8 ± 0.2/10.1 ± 0.4 ms for PE and HE, respectively; n.s paired Student t-test). Nonetheless, in the case of cVEMPs, the mean latency for PE was higher than for HE (13.7 ± 0.7/12.6 ± 0.8 ms, p = 0.023 paired Student t-test). However, amplitudes were always higher for PE, such as for oVEMPs (41.5 ± 8.3/6.4 ± 1.2 µV for PE/HE, p < 0.005 paired Student-t test) and cVEMPs (148.8 ± 39.4/87.2 ± 28.0 µV, p = 0.029, paired Student-t test). PE amplitudes were higher for oVEMP (p < 0.001, Mann-Whitney rank test) and cVEMP (p = 0.045, Mann-Whitney rank test) as well, compared to the control group, but values for HE were between the control limits. See Figure 2 for a typical example of unilateral SSCDS. AR was greater than 34% in all cases (8/8) for oVEMPs (pooled 62.8 ± 6.2%) but only 2/8 for cVEMPs (pooled 26.9 ± 10.7%). AR was < 34% in the case of the patient with bilateral SSCDS (12.2% for oVEMP and 32.3% for cVEMP). Thus, the sensitivity for oVEMPs was 100%, whereas the sensitivity for cVEMPs was only 25%. Figure 2 shows an example of a patient with left SSCDS. We evaluated the association between latencies and amplitudes using the least-squares fit, for either PE or HE, but no correlation was found.

Discussion
The principal aim of our study was to evaluate the specific role for oVEMP and cVEMP in the diagnosis of SSCDS. We have shown that even though cVEMP anomalies in amplitude may help to suspect the pathology, oVEMP is the most sensitive and strong test for the diagnosis of this syndrome. SSCDS could be due to the modification of fluid dynamics, where dehiscence would act as a "third window" in the inner ear, thus reducing the resistivity of the vestibular system and the resistance to the transmission of pressure and sound [1,3]. Because of this, the high intensity of the movement of the perilymph would generate a greater stimulation of the receptors of the vestibular system, leading to an increase in the amplitude of the response [24]. Presently, there are no amplitude or latency values capable of defining SSCDS. Usually a decrease in amplitude is a commonly accepted criterion in the neurophysiological test for one to say that it is pathologic. However, in the case of a disturbance of the vestibular system, an abnormally large potential would show the presence of a mobile "third window" in the labyrinth, as in SSCDS. Increased latency from VEMPs is not common, and it is usually correlated with conduction abnormalities in the central nervous system, as in the case of patients impacted with multiple sclerosis (ME) [11,23]. An own control group has been utilized from the same population as patients. This is a recommendation in most of the neurophysiological tests but seldom conducted.
Values for latencies and amplitudes are similar to those found in literature [13,14], which is not unexpected. What is slightly more surprising was the observation that most of the variables fitted well to a Gaussian distribution. We also determined that the upper limit for AR is lower than the commonly accepted 34%.
Nonetheless, this criterion to define pathology has not been used since we need a greater number of people to be more confident.
There are various studies that describe the decrease of the threshold and the increase in the amplitude of VEMPs [25][26][27] or the use of