Intraoperative monitoring during surgery for acoustic neuroma: benefits ofan extratympanic intrameatal electrode

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.