N. Mullatti et al., Intraoperative monitoring during surgery for acoustic neuroma: benefits ofan extratympanic intrameatal electrode, J NE NE PSY, 66(5), 1999, pp. 591-599
Objectives-To assess the utility of an extratympanic intrameatal electrode
for intraoperative monitoring during acoustic neuroma and other cerebellopo
ntine angle tumour surgery and to define the neurophysiological and surgica
l factors which influence hearing preservation.
Methods-Twenty two patients, 18 with acoustic neuromas and four with other
cerebellopontine angle tumours, underwent intraoperative monitoring during
tumour excision. The extratympanic intrameatal electrode (IME) was used to
record the electrocochleogram (ECoG) and surface electrodes to record the b
rainstem auditory evoked response (ABR).
Results-The compound action potential (CAP) of the ECoG was two and a half
times greater in amplitude than wave I of the ABR and was easily monitored.
Virtually instant information was available as minimal averaging was requi
red. Continuous monitoring was possible from the commencement of anaesthesi
a to skin closure. The IME was easy to place, non-invasive, and did not int
erfere with the operative field. Operative procedures which affected CAP or
wave V latency or amplitude were drilling around the internal auditory mea
tus, tumour dissection, nerve section, and brainstem and cerebellar retract
ion. Hearing was achieved in 59% of patients.
Conclusions-The IME had significant benefits in comparison with other metho
ds of monitoring. The technique provided information beneficial to preserva
tion of hearing.