Combined intra-operative monitoring of hearing by means of auditory brainstem responses (ABR) and transtympanic electrocochleography (ECochG) during surgery of intra- and extrameatal acoustic neurinomas

Background. Although being established as a standard procedure in intra-ope rative monitoring in acoustic neurinoma surgery, auditory brainstem respons es (ABR) represent a far-field technique bearing some technical limitations . This prospective study was designed to evaluate electrocochleography (ECo chG) as a supplementary tool for hearing preservation. Method. 84 patients with unilateral intra-/extrameatal acoustic neurinomas (extrameatal diameter: 5-55 mm) preserving serviceable hearing, were operat ed on using a combined (neuro-/otosurgical) suboccipital approach. ECochG w as recorded simultaneously to ABR following transtympanic insertion of a st eel needle electrode into the promontory under otoscopic view. Findings. Serviceable hearing (Class 1-3 according to Gardner/Robertson) wa s preserved in 43 out of 84 patients (51.2%), of whom 40 showed both ECochG and ABR being preserved. All 24 patients with loss of both modalities beca me deaf. Hearing preservation was observed in 4 out of 12 patients with pre served ECochG but loss of ABR (waves IH-V). The reverse was observed in 2 c ases with postoperative deafness. While both ECochG and ABR amplitudes were significantly correlated with pre- and postoperative hearing, latencies of ECochG summating (SP) and action potential (AP) proved to be more reliable indicators for preserved hearing than ABR (peak I/III/V) latencies. The pr edictive value of baseline ABR amplitudes for postoperative hearing, howeve r, was superior to ECochG parameters. Only in large neurinomas (extrameatal diameter: >2 cm) tumour size was found to be a significant predictor for t he preservation of hearing. Apart from three cases with postoperative otoli quorrhea and one further case presenting with local bleeding within the ext ernal acoustic meatus, no side effects were observed. Conclusions. In combination with ABR monitoring, ECochG-proved to be a usef ul supplementary tool for hearing preservation in acoustic neurinoma. surge ry. It is particularly helpful during electrocautery and drilling, since no averaging is required. Special applications are: (1) small tumours with go od serviceable hearing; (2) and/or a large intrameatal portion; (3) cases w ith lost or endangered contralateral hearing (e.g. bilateral acoustic neuri nomas), when the preservation of poor or even non-functional hearing is des irable.