ECOCHG, CNAP AND ABR MONITORING DURING VESTIBULAR SCHWANNOMA SURGERY

Identification of the specific pathophysiological processes and correl ation with post-operative hearing are the prerequisites for utilizing electrophysiological audiomonitoring techniques in preventing damage t o auditory structures during vestibular Schwannoma (VS) surgery. The p resent paper compares the value of auditory brainstem responses (ABRs) , electrocochleography (ECochG) and directly recorded cochlear nerve a ction potentials (CNAPs) in detecting damage to auditory structures du ring VS surgery and predicting postoperative hearing. Eighteen consecu tive patients operated on for VS, in an attempt at hearing preservatio n, participated in the investigation. The ipsilateral hearing level (p ure tone average [PTA] 0.5-3 kHz) ranged from 10 to 50 dB HL (mean: 30 .7 dB HL), with a speech discrimination score equal to or better than 50 per cent. CNAPs furnished the highest predictive score for post-ope rative hearing. In particular, when a permanent loss of CNAPs occurred the sensitivity and specificity were 100 per cent. The discrepancies between the ECochG and CNAP findings were attributable to high prevale nce of cochlear nerve damage, capable of 'disconnecting' the ear from the central auditory pathways, causing persistence of peripheral audit ory function and no propagation of the neural input. ABR monitoring wa s highly sensitive in detecting auditory damage but its prognostic uti lity was marred by its poor specificity.