ELECTROPHYSIOLOGIC IDENTIFICATION OF THE COCHLEAR NERVE-FIBERS DURINGCEREBELLOPONTINE ANGLE SURGERY

To facilitate identification and preservation of the auditory nerve du ring cerebello-pontine angle surgery, bipolar recording of cochlear ne rve compound action potentials (CNAPs) was performed. Two silver wires insulated with teflon up to the exposed ends were utilized as electro des. They were twisted together, the distance between the two tips bei ng 1 mm or less. Rarefaction polarity clicks (31/s) ranging from the p sychoacoustical threshold to 120 dB pe SPL were used as stimuli. The i nvestigation was performed in three groups of patients. The first grou p consisted of 9 patients submitted to vestibular neurectomy and 4 pat ients operated on by microvascular decompression of the eighth nerve. The second group comprised 8 patients with acoustic tumors smaller tha n 2 mm and serviceable hearing. Postoperative audiometric results in t he subjects in the second group were compared with those obtained in w ell-matched homogeneous controls consisting of patients with acoustic neuroma operated on without the aid of CNAP recording. Bipolar recordi ng from the eighth nerve was extremely selective, a good response bein g obtained only when positioning the electrode on the cochlear portion of the eighth nerve. During removal of the acoustic neuroma, repeated bipolar probing of the tumor and eighth nerve facilitated the task of distinguishing the cochlear nerve from other nervous structures and f rom the tumor, and contributed to preserving hearing in most patients.