FACIAL AND ACOUSTIC NERVE PRESERVATION DURING EXCISION OF EXTRACANALICULAR ACOUSTIC NEUROMAS USING THE SUBOCCIPITAL APPROACH

The results are presented from a consecutive operative series of 62 ac oustic neuromas in 60 patients following the introduction of improved neurophysiological monitoring techniques. Twenty-two patients had usab le preoperative hearing. Thirty tumours were less than 2.5 cm diameter and 32 greater in size. Operation was via a 3-4-cm diameter retromast oid craniectomy. The internal auditory meatus was opened by an ENT sur geon (RM) using a drill and the facial nerve identified by stimulation . The tumour was then centrally evacuated by a neurosurgeon (MT/HC) us ing an ultrasonic aspirator, and the thin exterior part of the tumour carefully dissected off the nerves in or around the capsule with const ant stimulation and monitoring of facial EMG, BSAEP and electrocochleo graphy. A new type of stimulation probe has been designed and coupled to a stimulator/integrator/tone burst generator (SB) so that continuou s immediate direct feedback to the surgeon is possible. A variable amp litude discriminator rejects baseline EMG (> 50 muV) and a gating circ uit prevents stimulus artefact (during monopolar stimulation) from cau sing interference. By these means the VII nerve could be identified ev en when translucent and undefinable as a nerve bundle. Anatomical pres ervation was possible in 98% of VII nerves. Full facial function was p resent in 20 cases immediately postoperatively. Full delayed recovery occurred in 23 cases giving an eventual total in House Grade I of 69%. Seven other cases recovered to House Grade II. There was therefore 81 % satisfactory facial nerve function. This percentage is exactly the s ame for larger and for smaller tumours. Anatomical preservation of the VIII nerve was achieved in 24/62 (39%) of the whole series and 11/16 (69%) of those with a hearing loss of < 50 dB. Functional preservation of hearing described as usable by the patient (< 65 dB) was achieved in 7/22 cases (32%), 3/13 (23%) in tumours < 2.5 cm and 4/9 (44%) in t hose > 2.5 cm diameter. Hearing preservation of < 50 dB in patients wi th preoperative hearing threshold < 50 dB and tumours of < 2.5 cm was 3/11 (27%). Monitoring by BSAEP and ECochG was technically unsatisfact ory because the responses were affected by drilling and stimulation. A coustic nerve preservation should be attempted in all cases with measu rable hearing, regardless of tumour size.