Direct Monitoring of ABIs Evoked Potentials on the Cochlear Nuclei and Neighboring Cranial Nerves

Abstract

Objectives: The potentials evoked with Auditory Brainstem Implant (ABI) on the cochlear nuclei (CN) and on the neighboring cranial nerves are traditionally evaluated with Electrical Auditory Brainstem Response (E-ABR) and electromyography (EMG). To obtain a more precise evaluation of ABI insertion, a near field monitoring procedure that records electrical compound action potentials (E-CNAPs) directly from the CN and from each individual cranial nerve is presented. Methods: Between January 1997 and September 2012, 184 patients were fitted with ABI by the Verona team, 155 in Verona and 29 (13 children and 16 adults) elsewhere. The E-CNAPs utilizes a cotton-wick recording electrode locate on different anatomical structures: CN, root entry zone of the cranial nerve VII, IX, X, XI, and V. All subjects were monitored with E-ABR and 18 of them underwent also E-CNAPs recording. Results: Systematic significant lower E-CNAPs thresholds were observed compared with E-ABRs. The evaluation of the threshold for inducing extra-auditory effects with E-CNAPs on the neighboring cranial nerves allowed a more precise selection of the active electrodes to be used in the fitting. For the same electrical intensity value utilized for obtaining ABI responses, E-ABRs and E-CNAPs showed potentials of neighboring cranial nerves respectively on 4 and 10 electrodes. Conclusions: The adoption of near-field recordings offered the chance to improve the correct placement of the array and post-operative outcomes. E-CNAPs recorded on neighboring cranial nerves can replace EMG giving an on line demonstration of the effects on nerves on which electrodes induce myogenic responses.