The auditory brainstem response in surgical monitoring: the pathological significance of a reduced P1-P5 interwave interval.

The present case has provided material from which interesting observations can be made. The initial effects of CN VIII compression were a sudden increase in the latency of P1 that occurred 40 minutes prior to large changes in the latencies of the P3 and P5 components. It is possible that the blood supply to the cochlea was initially compromised. This vascular deficit may have affected the basal cochlea, which is also sensitive to the effects of noise exposure and ototoxicity. Later, when P3 and P5 showed large increases in latency, it is possible that the vascular compromise was affecting the entire cochlea, or that compression of CN VIII was beginning to impose a substantial deficit in neural transmission. The present observations underscore the need to be vigilant to changes in both the P1 and P5 components of the ABR. A paradoxical shortening of the P1-P5 interpeak interval beyond normal laboratory limits should be interpreted as reflecting the presence of significant high frequency hearing loss. The amount of time wherein it is safe not to take corrective measures is unknown, and therefore, action should be taken as soon as this phenomenon is observed.