EP 140. Vestibular evoked myogenic potentials: A retrospective analysis

Abstract

Diagnostic testing of the vestibular system is an integral part of assessment of patients with balance dysfunction. Rediscovery of Vestibular Evoked Myogenic Potentials (VEMP) has introduced a new dimension to vestibular assessment, especially otolith function. We present a retrospective analysis of both C-VEMP and O-VEMP in neurologic & otologic conditions. Data collated retrospectively from records of 98 patients referred for diagnostic VEMP testing to the Electrophysiology laboratory. Presenting symptoms were imbalance and giddiness. 45 male and 53 female patients in the age range of 17–79 years. 38 had Benign Paroxysmal Positional Vertigo (BPPV), 12 Acute Vestibular Neuronitis, 11 Meniere’s Disease, 7 Chronic Otitis Media ( 3 right, 2 left and 2 bilateral ). 1 was referred to confirm left Superior Canalicular Dehiscence, 9 Parkinson Disease, 1 ProgressiveSupranuclearPalsy and 4 Multiple Sclerosis. 15 remained unclassifiable and were categorised as Vertigo Not Otherwise Specified (NOS). Cervical and Ocular VEMP were performed on both ears on all the patients with Medicaid EP machine. Testing done in one session lasting about 30 minutes for each patient. Rest periods given between sets to prevent muscle fatigue. Cervical VEMP: active recording electrodes on Sternocleidomastoid belly ipsilateral to auditory stimuli. Reference on the manubrium sterni. Patients instructed to raise the head about 30 degrees above the bed and hold it actively at that level . Ocular VEMP: recording electrodes were placed contralaterally, active 1 cm inferior to lower eyelid and reference, 2 cm below that. Maximum upgaze was encouraged during testing. Ground electrode: forehead for both tests. Stimulation Parameters: Rarefaction clicks: 100 microsec duration, 5 Hz frequency , intensity 120–125 dB nHL, headphones. Recording Parameters: Filters: Low frequency filter- 20 Hz, High frequency filter- 3 KHz. Sweep: 10 msec/divn. Total sweep of 100 msec. Gain: 100 microvolts /divn. Notch: None. Averages: 100/set, 2 sets for each test. Parameters analysed : Latencies of P13, N23 and amplitude of cervical VEMP on both sides. Latencies of N10, P16 and amplitude of ocular VEMP on both sides. Comparison against normative values and two tailed t test with unequal variance were performed to look for statistically significant deviation in each subgroup of diseases. There was no statistically significant difference in latencies and amplitudes of cervical (p = 0.39 and 0.28 for left and right respectively ) and ocular VEMP (p = 0.4 and 0.64 for left and right respectively) in BPPV. In some patients with Acute Vestibular Neuronitis cervical and ocular responses were absent. Very significant difference in latencies and amplitudes (p < 0.01 on left, p < 0.001 bilaterally) were noted. In Meniere’s disease, p values < 0.01 were obtained uniformly for both cervical and ocular VEMP. p < 0.001 for cervical and ocular VEMP in Otitis Media except the left ocular VEMP which trended the disease. PD and PSP group showed statistically significant differences in cervical and ocular VEMP latencies and amplitudes (p < 0.01). Number of patients with Multiple Sclerosis is small and the results are inconclusive. In the sole patient with left Superior canal dehiscence, the cervical and ocular VEMP were larger than normal ( more than twice in amplitude as against average for the age) on the left thus underscoring its use in this condition. 15 patients had imbalance and giddiness, but not diagnosed with a CNS, PNS or vestibular disorder. Interestingly the Cervical VEMP was significantly abnormal (p < 0.001) but the Ocular VEMP did not show statistically significant deviation in latency or amplitude. This suggests a dichotomy between saccular & utricular pathology, the former being responsible for the patients symptoms. This is, to the best of our knowledge the first report of abnormalities of both Cervical and Ocular VEMP in patients with neurologic and neurootologic disorders from India. The role of VEMPs in studying degenerative neurological disorders (PD and PSP) with imbalance and falls has been explored and since abnormalities in both cervical and ocular VEMPs are noted,suggesting a role for the vestibular system and central connections in imbalance & loss of posture in these conditions. Role of VEMP in MS is routinely mentioned, but it is at best an adjunct diagnostic procedure. The lack of usefulness of VEMP in BPPV has been underscored by this study. But it may be useful in acute attacks of Meniere’s disease and Acute Vestibular Neuronitis to guage the extent and laterality of damage, especially since these disorders are bilateral.Our findings suggest that Ocular VEMP is more sensitive than Cervical VEMP in Vestibular Neuronitis. In chronic otitis media VEMPs are abnormal and can be useful to evaluate integrity of labyrinthine function. The main drawback is the paucity of numbers in different categories. Based on these findings we propose to enrol more patients, especially in the neurological subsets prospectively to elucidate the role of VEMP.