Electrical Vestibular Stimulation after Vestibular Deafferentation and in Vestibular Schwannoma

Abstract

BackgroundVestibular reflexes, evoked by human electrical (galvanic) vestibular stimulation (EVS), are utilized to assess vestibular function and investigate its pathways. Our study aimed to investigate the electrically-evoked vestibulo-ocular reflex (eVOR) output after bilateral and unilateral vestibular deafferentations to determine the characteristics for interpreting unilateral lesions such as vestibular schwannomas.MethodsEVOR was recorded with dual-search coils as binocular three-dimensional eye movements evoked by bipolar 100 ms-step at EVS intensities of [0.9, 2.5, 5.0, 7.5, 10.0]mA and unipolar 100 ms-step at 5 mA EVS intensity. Five bilateral vestibular deafferented (BVD), 12 unilateral vestibular deafferented (UVD), four unilateral vestibular schwannoma (UVS) patients and 17 healthy subjects were tested with bipolar EVS, and five UVDs with unipolar EVS.ResultsAfter BVD, bipolar EVS elicited no eVOR. After UVD, bipolar EVS of one functioning ear elicited bidirectional, excitatory eVOR to cathodal EVS with 9 ms latency and inhibitory eVOR to anodal EVS, opposite in direction, at half the amplitude with 12 ms latency, exhibiting an excitatory-inhibitory asymmetry. The eVOR patterns from UVS were consistent with responses from UVD confirming the vestibular loss on the lesion side. Unexpectedly, unipolar EVS of the UVD ear, instead of absent response, evoked one-third the bipolar eVOR while unipolar EVS of the functioning ear evoked half the bipolar response.ConclusionsThe bidirectional eVOR evoked by bipolar EVS from UVD with an excitatory-inhibitory asymmetry and the 3 ms latency difference between normal and lesion side may be useful for detecting vestibular lesions such as UVS. We suggest that current spread could account for the small eVOR to 5 mA unipolar EVS of the UVD ear.

Ocular torsion responses to sinusoidal electrical vestibular stimulation

“…The stimulation and recording techniques we describe here offer potential for clinical diagnostic use, since it is affordable, non-invasive, comfortable and relatively quick. To assess the function each ear separately would simply require a monaural stimulus, with a reference electrode distant from the ear ( Aw et al, 2013 , MacDougall et al, 2005 )…”

“…Clinical studies have shown that EVS has potential as a vestibular diagnostic ( Aw et al, 1996 , Aw et al, 1995 , Aw et al, 2013 , MacDougall et al, 2005 , Welgampola et al, 2013 ). When applied in a monaural configuration (with a reference electrode distant from the ears), diminished EVS-evoked ocular responses have been demonstrated in the affected ears of patients with a variety of vestibular disorders.…”

Ocular torsion responses to electrical vestibular stimulation in vestibular schwannoma

“…In contrast, assessment of the ocular response to EVS can be performed whilst seated. Aw et al (2013b) measured the ocular torsion response to brief pulses of square-wave EVS in four unilateral VS patients with large tumours. They reported longer response latencies as well as reduced velocity in the affected ear.…”

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