Cochlear function following implantation with a full electrode array

Clinicians traditionally advise patients that implantation with a conventional full cochlear implant array will lead to loss of any remaining hearing in the implanted ear. We sought to assess the extent to which 'standard' insertion of a full electrode array affects cochlear function.

Air conduction pure tone audiometry was performed as part of the routine 6-month postimplantation audiological assessment for 81 adult patients who preoperatively had identifiable pure tone thresholds. All patients were implanted with the nucleus CI24 device, with complete insertion of a straight array in 68 and a contour array in 13 patients. A separate cochleostomy was fashioned anterior to the round window using a 1 mm diamond burr, and the array inserted without lubrication. The operating surgeon was blinded to the inclusion of patients in this study at the time of surgery.

Of 81 patients, 58 preserved some residual hearing at 6 months. The rate of preservation of hearing was higher in the low frequencies. At 500 Hz 77 patients had an identifiable preoperative pure tone threshold, still identifiable postoperatively in 48. At 4 kHz only 25 patients had an identifiable preoperative threshold, preserved in 9, and at 8 kHz 6 of 13 patients still had identifiable pure tone thresholds postoperatively. There was no correlation between the depth of insertion and hearing preservation, both groups having a mean of 5 stiffening rings inserted in addition to the 22 active electrodes. The mean deterioration in threshold in those who had residual hearing was 15 dB at 500 Hz, and in 29 patients the deterioration in the threshold was less than 15 dB.

Insertion of a full conventional electrode array is not as damaging to cochlear function as often assumed. Residual low-frequency hearing may be preserved in approximately two-thirds of patients 6 months postoperatively. This study supports our counselling patients that despite the delicate nature of the inner ear there is a significant chance that their air conduction hearing will not be lost.