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.
Highlights
Human electrical vestibular stimulation has been used for over a century to probe the human vestibular system [1,2]
We showed that the evoked vestibulo-ocular reflex (eVOR) was completely abolished from all bilateral vestibular deafferented (BVD) with the mean torsional (x), vertical (y) and horizontal (z) eVOR positions at #0.01u (Figure 1B)
We confirmed that human electrical (galvanic) vestibular stimulation (EVS) did not stimulate any residual vestibular nerve or the central vestibular pathways
Summary
Human electrical (galvanic) vestibular stimulation has been used for over a century to probe the human vestibular system [1,2]. One of the most intriguing findings in numerous studies is the presence of vestibular responses to bipolar (binaural) cathodal excitation of the lesion side [4,5,10] These responses have been attributed either to stimulation of the vestibular afferent after an intra-labyrinthine lesion or residual functions from incomplete lesions [5,9,10]. The evidence suggests that it is difficult to stimulate the vestibular afferents after intra-labyrinthine lesions We wondered if this often measured ‘‘residual’’ vestibular response evoked by bipolar electrical stimulation of a unilateral lesion could instead be due to anodal inhibition of the intact labyrinth by EVS suppression of the vestibular afferent discharges as proposed by the above hypothesis on the mechanism of EVS. 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
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