Electrically- and mechanically-evoked vestibulo-ocular reflex in central vestibular disorders

Abstract

In the current study, vestibulo-ocular reflex (VOR) in acute and chronic brainstem and cerebellar lesions was examined topographically using search coils for electrical vestibular stimulation (EVS) and head impulse test (HIT), and surface electrodes for vestibular evoked myogenic potentials (VEMP). In medullary lesions angular VOR (aVOR) is bilaterally reduced only horizontally, but electrically- evoked VOR (eVOR) either increased or was inappropriately directed. In pontine lesions VOR may be reduced or normal, with concordance between mechanical and electrical vestibular function. Lesion of medial longitudinal fasciculus lead to asymmetric horizontal and vertical semicircular canal involvement, impaired eVOR and more frequent ocular VEMP abnormalities. Midbrain lesions result in increased vertical-torsional aVOR, prolonged eVOR onset latency, and delayed ocular VEMP. In cerebellar lesions horizontal aVOR is variable and vertical-torsional aVOR may be increased. The eVOR and VEMP findings are signified in part by prolonged latency and increased response. These findings suggest direct involvement of root entry zone and/or vestibular nuclei in lower brainstem lesions, the medial longitudinal fasciculus as the common conduit for semicircular and otolith signals, mediation of otolith-ocular reflexes and modulation of vertical-torsional aVOR in midbrain lesions, and defective nodulo-uvular and floccular inhibitory input on the VOR in medullary and cerebellar lesions.