BIPOLAR RECORDING OF THE COCHLEAR NERVE ACTION-POTENTIALS DURING CEREBELLOPONTINE ANGLE SURGERY

A technique of bipolar recording of the cochlear nerve compound action potentials (CAPs) to make identification and preservation of the coch lear nerve easier during cerebellopontine angle surgery is described. Four patients, aged 32 to 65 years, with acoustic tumors smaller than 2 mm and serviceable hearing, participated in the study. To investigat e the resolution power of bipolar CAP recordings, a pilot study had be en performed in a group of four patients submitted to vestibular neure ctomy. Two silver electrodes insulated by Teflon up to the exposed end s were utilized for bipolar recording of the CAPs. They were twisted w ith each other and the distance between the two electrode tips could b e varied from 0.3 to 0.7 mm. Rarefying polarity clicks (31/s) at 100 t o 125 dB SPL were utilized as stimuli. Bipolar recording from the eigh th nerve was extremely selective: a good response was obtained only wh en the electrode was positioned on the cochlear portion of the eighth nerve. No response was present when the electrode was placed on the ve stibular nerve and on the tumor. Frequent probing of tumor and eighth nerve with the bipolar recording electrode during acoustic neu roma re moval facilitated the task of identifying the cochlear nerve and allow ed its preservation in all patients.