The effects of amplitude normalization and EMG targets on cVEMP interaural amplitude asymmetry

The purpose of this investigation was to determine whether the use of visual feedback of tonic electromyographic (EMG) activity, or the use of amplitude normalization techniques would reduce significantly the variability in cervical vestibular evoked myogenic potential (cVEMP) P13-N23 interaural amplitude asymmetry data in otologically and neurologically intact children and adults.

There were 97 subjects, both pediatric and adult, from whom the authors recorded cVEMPs with and without the use of an EMG target and amplitude normalization techniques. The four conditions were: (1) conventional recording (condition 1), (2) conventional recording with an EMG target (condition 2), (3) same as condition 1, with the addition of postacquisition amplitude normalization techniques, and (4) same as condition 2, with the addition of postacquisition amplitude normalization techniques. The absolute peak to peak amplitude of positive-negative response (P13-N23), absolute latency of P13, and the left or right amplitude asymmetry of P13-N23 were measured.

Neither P13-N23 amplitudes nor latencies, neither mean root mean square (RMS) of the full wave rectified EMG activity, nor the standard deviation of the RMS EMG activity differed significantly when subjects were, and were not, asked to ensure their tonic EMG activity exceeded a visual target (i.e., representing >50 µV RMS EMG). Amplitude normalization of the cVEMP waveforms failed to reduce significantly the variability in the amplitude asymmetry data.

Activating the sternocleidomastoid muscle with the patient in a semirecumbent position, with head turned away from the stimulated ear and head elevated (i.e., an optimal activation technique) was sufficient to produce the highest amplitude cVEMPs with an acceptable amount of variability in subjects of all ages. Group data suggested that the use of visual targets and amplitude normalization routines did not reduce significantly the variability in cVEMP interaural amplitude asymmetry measures. However, in isolated cases amplitude normalization converted an "abnormal" cVEMP into a "normal" cVEMP although the reverse occurred as well, suggesting that these techniques may be beneficial for a subset of patients receiving a less than perfectly administered test procedure.