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 acoustic neuromas in 60 patients following the introduction of improved neurophysiological monitoring techniques. Twenty-two patients had usable preoperative hearing. Thirty tumours were less than 2.5 cm diameter and 32 greater in size. Operation was via a 3-4-cm diameter retromastoid craniectomy. The internal auditory meatus was opened by an ENT surgeon (RM) using a drill and the facial nerve identified by stimulation. The tumour was then centrally evacuated by a neurosurgeon (MT/HC) using an ultrasonic aspirator, and the thin exterior part of the tumour carefully dissected off the nerves in or around the capsule with constant stimulation and monitoring of facial EMG, BSAEP and electrocochleography. A new type of stimulation probe has been designed and coupled to a stimulator/integrator/tone burst generator (SB) so that continuous immediate direct feedback to the surgeon is possible. A variable amplitude discriminator rejects baseline EMG (> 50 muV) and a gating circuit prevents stimulus artefact (during monopolar stimulation) from causing interference. By these means the VII nerve could be identified even when translucent and undefinable as a nerve bundle. Anatomical preservation was possible in 98% of VII nerves. Full facial function was present 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 same 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 those > 2.5 cm diameter. Hearing preservation of < 50 dB in patients with preoperative hearing threshold < 50 dB and tumours of < 2.5 cm was 3/11 (27%). Monitoring by BSAEP and ECochG was technically unsatisfactory because the responses were affected by drilling and stimulation. Acoustic nerve preservation should be attempted in all cases with measurable hearing, regardless of tumour size.