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 cochlear 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 investigate the resolution power of bipolar CAP recordings, a pilot study had been performed in a group of four patients submitted to vestibular neurectomy. Two silver electrodes insulated by Teflon up to the exposed ends were utilized for bipolar recording of the CAPs. They were twisted with each other and the distance between the two electrode tips could be varied from 0.3 to 0.7 mm. Rarefying polarity clicks (31/s) at 100 to 125 dB SPL were utilized as stimuli. Bipolar recording from the eight nerve was extremely selective: a good response was obtained only when the electrode was positioned on the cochlear portion of the eighth nerve. No response was present when the electrode was placed on the vestibular nerve and on the tumor. Frequent probing of tumor and eighth nerve with the bipolar recording electrode during acoustic neuroma removal facilitated the task of identifying the cochlear nerve and allowed its preservation in all patients.