PHYSIOLOGICAL IDENTIFICATION OF THE AUDITORY-NERVE DURING SURGERY FORACOUSTIC NEUROMA

We report the design and clinical use of an electrode which can locate the acoustic nerve fibres in the normal eighth nerve and also in eigh th nerves deformed by acoustic neuromas. The improvement in facial ner ve preservation during acoustic neuroma surgery is partly due to the u se of a facial nerve stimulator to anatomically locate the fibres. Our new acoustic nerve detector has the capability of anatomical location of cochlear fibres which may help to improve hearing preservation in selected cases of acoustic neuroma. The device functions by detecting the compound action potential evoked by no frequency auditory simulati on at 500 Hz. The 500 Hz compound action potential is detected with a bipolar probe and then amplified and filtered. This results in a 500 H z tone when the probe contacts the auditory nerve. Detection is virtua lly instantaneous. The acoustic nerve detector (AND) is demonstrated i n a normal eighth nerve complex and its use is then described in the t otal removal of an acoustic neuroma with a 1 cm extracanalicular exten sion in which useful hearing was saved post-operatively. The present p rototype may not be sensitive enough to detect the very low signals th at may result when cochlear fibres are widely distorted around a large tumour or in cases where slight contusion of the nerve occurs during dissection. In all other cases the real time anatomical information is extremely helpful in guiding acoustic nerve dissection and also in mo nitoring the effects of petrous bone drilling.