Vestibular-evoked myogenic potentials: a method to assess vestibulo-spinal conduction in multiple sclerosis patients

Vestibular-evoked myogenic potentials (VEMPs), elicited by acoustic stimulation, have been proposed in the assessment of the vestibulo-cervical reflex pathways. The procedure has been previously validated in several otovestibular disorders. The aim of this study was to investigate patients affected by multiple sclerosis (MS) in the attempt to clarify the underlying physiopathogenetic mechanisms and the clinical utility of VEMPs in detecting vestibulospinal involvement in this disease. VEMPs were obtained according to the technique described by Colebatch and Halmagyi [Neurology 42 (1992) 1635]. We averaged the surface tonic electromyogram from right and left sternocleidomastoid muscle, after bilateral click stimulation (click duration 0.1 ms, repetition rate 3 Hz, intensity 140 dBSPL, 256 stimuli, repeated at least twice). In all cases, we obtained the biphasic, initially positive, p13-n23 wave pattern. P13 peak latency was bilaterally or unilaterally delayed in 8 out of 15 patients (mean delay: 2.2 ms; p < 0.01 on right and <0.05 on left side) and peak-to-peak amplitude significantly reduced (mean amplitude loss: 130 microV; p < 0.01 on right and <0.05 on left side). Their overall diagnostic yield resulted in 60%. In conclusion, the present findings prove that VEMPs are delayed in p13 component and altered in amplitude in MS patients. We hypothesise that these changes might be the result of a conduction impairment in vestibulo-spinal fibres, producing a morphologic alteration of the myogenic responses.