COCHLEOVESTIBULAR AFFERENT PATHWAYS OF TRAPEZIUS MUSCLE RESPONSES TO CLICKS IN HUMAN

Brief intense clicks cause short-latency cervical muscles microcontrac tions which are supposed to be of vestibular origin. Averaging these m icrocontractions allows myogenic vestibular evoked potentials (MVEP) t o be obtained. MVEP from the trapezius muscles were investigated in no rmal subjects, cochleovestibular nerve-damaged patients and patients w ith a vestibular or a cochlear lesion. Muscular responses were recorde d on right and left trapezius by averaging from surface electrodes fol lowing right and left monaural 100 dB hearing level click stimulation. In normal subjects, responses to monaural stimuli were bilateral, of equal amplitude and latency in left and right trapezia. Normal respons e consisted of four consecutive waves, labelled p13, n23, p32 and n40 according to their polarity (p, positive; n, negative) and mean peak l atency in msec. In total unilateral cochleovestibular damaged patients , auditory stimulation of the affected side gave no MVEP either ipsila teral or contralateral to the stimulation. In the case of selective co chlear lesion, stimulation of the affected side gave MVEP which was pr esent on ipsilateral and controlateral trapezius muscles. The four suc cessive waves were present with a normal latency; however, amplitude w as lower than that obtained after stimulation of the healthy ear. In t he case of selective vestibular lesion, the four waves of MVEP were ag ain present with normal latency but with reduced amplitude. Responses were present on both the ipsilateral and controlateral trapezius muscl e. it is concluded that normal MVEP recorded on the trapezius muscles are bilateral and consist of four waves, the amplitude of which could depend on the simultaneous stimulation of both cochlear and vestibular afferents. In the case of unilateral cochlear and/or vestibular impai rments responses were present on both ipsilateral and contralateral tr apezius muscles. Latencies had normal values but amplitudes were reduc ed. MEVP recorded on trapezius muscles were absent in the case of tota l cochleovestibular damage.