Preliminary results and technique for electrophysiological intra-operative monitoring of residual hearing during cochlear implantation.

AIMS

To assess the feasibility of intra-operative monitoring of residual hearing using electrocochleography (ECOG) during cochlear implantation. We present our methods and preliminary results.

METHODS

We attempted to monitor 21 consecutive paediatric and adult subjects during cochlear implantation. All subjects were implanted via a promontory cochleostomy with full electrode insertions of Cochlear(TM) Nucleus® (Cochlear Nucleus, Cochlear Ltd, Australia) devices: 8 with straight electrodes and 13 with contoured electrodes. Baseline recordings were made after the completion of the posterior tympanotomy, and subsequently at stages of the cochleostomy and electrode insertion.

RESULTS

We were unable to obtain recordings on five patients because the ear-insert tubing kinked, which disrupted transmission of the stimulus. Five had insufficient cochlear function to be able to record a baseline ECOG. No subjects in whom we were able to obtain a satisfactory baseline ECOG (n=11) had a reduction in ECOG action potential (AP) amplitude, morphology or threshold up to completion of the cochleostomy. Seven maintained an unchanged ECOG throughout the procedure to full insertion. An implant had to be replaced because of a faulty straight electrode in one patient. The amplitude significantly reduced during insertion of the replacement array. Two had a reduction in amplitude, threshold, and shape of the wave related to inadvertent suction of the perilymph. Subsequently, one maintained this changed ECOG to the end of the procedure, but the other progressed to complete loss of the ECOG during insertion of the array. One other subject had a significant reduction in the amplitude during insertion of the electrode from a depth of approximately 18mm to full insertion.

CONCLUSIONS

Intra-operative monitoring of residual hearing may be possible in most patients undergoing cochlear implantation. This pilot study suggests that cochleostomy is not associated with intra-operative loss of residual hearing; ECOG can be preserved during the procedure in most patients; intra-operative loss of hearing is most likely to occur when the tip of the array reaches the basal turn of the cochlea. This risk may increase if the array has to be removed and re-inserted. Suction of perilymph causes immediate changes, which may not recover.