M. Torrens et al., FACIAL AND ACOUSTIC NERVE PRESERVATION DURING EXCISION OF EXTRACANALICULAR ACOUSTIC NEUROMAS USING THE SUBOCCIPITAL APPROACH, British journal of neurosurgery, 8(6), 1994, pp. 655-665
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.