Vestibular evoked potentials in human neck muscles before and after unilateral vestibular deafferentation

Current clinical assessment of vestibular function relies heavily upon caloric testing: recording nystagmus caused by hot and cold water irrigation of the ears. The response reflects the integrity of the horizontal canal,' only one of the five vestibular end organs. Furthermore, the nature of caloric testing is such that differences in latency due to central neural conduction delay cannot be detected. Many patients who complain of vestibular symptoms have normal caloric tests. Brainstem potentials following natural stimulation of the horizontal canal are small and require special apparatus capable of rapidly and repeatedly applying high accelerations to the head. ' Attempts a t recording cortical vestibular evoked potentials have given conflicting results.3 We have recently investigated the effects of brief, intense clicks on averaged EMG activity from neck muscles and have identified a short-latency component that is specifically dependent upon the integrity of the ipsilateral vestibular nerve. The apparatus required is no more complicated than that used in conventional evoked response testing. Case report. A 38-year-old man developed symptoms of Meniere's disease 3 years before presentation. He was troubled by uncontrolled episodes of vertigo, nausea, and tinnitus, and was scheduled for selective vest ibular nerve section. Preoperative testing showed normal caloric responses from both ears. Audiometry showed a 20 dB loss up to 1 kHz, with greater loss a t higher frequencies, and a 20% loss of speech discrimination on the right. An average was made of the effects of 512 presentations (3 per second) of clicks of 0.1-msec duration on surface EMG recorded from over the right and left sternomastoid muscles during a gentle voluntary contraction. The click intensities were referred to the perceptual threshold for normal subjects (= 0 dB). The subject's perceptual thresholds for the clicks were 15 dB (R) and 10 dB (L), and clicks of 100 dB intensity were used for averaging EMG responses. Short-latency, largeamplitude myogenic responses were evoked in both the right and left sternomastoid muscles (figure). The responses began at an average latency of 8 msec, beginning with an initial positivity (p13: latency 12.8 msec [R], 12.5 msec IL]), followed by a negativity (1123: latency in this patient 20.6, 20.3 msec) and then a second positivity and negativity. The subject was restudied under identical conditions 3 months after selective section of the right vestibular nerve, following which his symptoms had been almost totally relieved. His subjective perceptual thresholds for the clicks were 20 dB (R) and 10 dB (L) above the reference. Retesting showed abolition of the p13-n23 wave previously recorded from the right sternomastoid, with preservation of the later potentials and unaltered responses from the left sternomastoid (figure). Discussion. We have since studied two additional subjects following selective vestibular nerve section with similar results: the p13-n23 wave is present only on the side ipsilateral to the intact vestibular nerve. Conversely, two subjects with sensorineural deafness and elevated perceptual thresholds (thresholds 25 to 80 dB above the reference) but intact caloric responses had p13-n23 evoked responses on both sides. Experiments on animals have shown that clicks similar to the ones we used can activate saccular afferents at short laten-