Intraoperative monitoring of early auditory evoked potentials in hearing acoustic neuroma patients complemented by cochlear nerve action potentials]

Hearing preservation was attempted in eight cases of acoustic neurinomas with good preoperative hearing. This was successful in 3 cases. Brain-stem auditory evoked potentials (BAEP0 were monitored in all cases, and compound nerve action potentials (CNAP) were recorded from the cochlear nerve in the last 3 cases. The BAEP was extremely sensitive to intraoperative manipulation. Intraoperative loss of wave V in BAEP was observed not only in all 5 cases with postoperative hearing loss, but also in 2 out of 3 cases with successful hearing preservation. CNAP correlated better with the hearing outcome than BAEP. In the cases where hearing was preserved, intact CNAP responses were demonstrated at the end of the operation. Conversely, deterioration of CNAP was seen in a case of postoperative hearing loss. CNAP was 10-15 times larger in amplitude than BAEP, so that the new responses were obtainable in shorter intervals. This capability of frequent examination seemed to increase the possibility of avoiding irreversible damage to the hearing by changing surgical tactics. The addition of CNAP monitoring is a good supplement to conventional BAEP monitoring in acoustic neurinoma surgery.