Effects of muscle contraction on cervical vestibular evoked myogenic potentials in normal subjects

•The effect of sternocleidomastoid (SCM) contraction on cVEMP amplitude is strong and linear.•Small muscle contraction differences have the same effect on cVEMP amplitude as 5-9 dB stimulus intensity changes.•Minimum contraction levels are required for accurate interpretation of cVEMPs.

ObjectiveCervical vestibular evoked myogenic potentials (cVEMPs) are vestibular-dependent muscle reflexes recorded from the sternocleidomastoid (SCM) muscles in humans. cVEMP amplitude is modulated by stimulus intensity and SCM muscle contraction strength, but the effect of muscle contraction is less well-documented. The effects of intensity and contraction were therefore compared in 25 normal subjects over a wide range of contractions.MethodscVEMPs were recorded at different contraction levels while holding stimulus intensity constant and at different intensities while holding SCM contraction constant.ResultsThe effect of muscle contraction on cVEMP amplitude was linear for most of the range of muscle contractions in the majority of subjects (mean R2 = 0.93), although there were some nonlinearities when the contraction was either very weak or very strong. Very weak contractions were associated with absent responses, incomplete morphology and prolonged p13 latencies. Normalization of amplitudes, by dividing the p13-n23 amplitude by the muscle contraction estimate, reduced the effect of muscle contraction, but tended to underestimate the amplitude with weak contractions.ConclusionsMinimum contraction levels are required for accurate interpretation of cVEMPs.SignificanceThese data highlight the importance of measuring SCM contraction strength when recording cVEMPs.