The effects of high-dose barbiturates on the acoustic reflex and auditory evoked responses. Two case reports.

The effects of high-dose barbiturates (pentobarbital) on the acoustic reflex, and the auditory brainstem (ABR) and middle-latency (AMR) responses, are illustrated with two case reports. Auditory electrophysiologic data were recorded serially during recovery from therapeutic barbiturate coma. ABR latency remained within normal limits in barbiturate coma, but amplitude of the wave I component was abnormally augmented. Contralateral and ipsilateral acoustic reflex activity, and the Pa component of the AMR, were not observed in barbiturate coma, and reappeared with the emergence of brainstem neurologic signs. These findings suggest a fundamental difference in the neurophysiologic substrate of the ABR vs. acoustic reflex and AMR. Possible mechanisms for the differential influence of barbiturates on these three auditory electrophysiologic measures are offered.