Residual hearing in cochlear implant patients

Abstract

Preservation of residual hearing should be a desirable outcome of implant surgery. Prevention of neural degeneration due to loss of residual hair cells, together with the continuous progress in cochlear implant technology should be able to preserve cochlear integrity as well as possible. The degree of hearing preservation may vary depending on surgical approach, maximum insertion depth and other factors not uniformly considered to date. The aim of this retrospective case controlled study is to evaluate residual hearing after cochlear implant surgery. In particular, we analyzed data obtained with use of two different kinds of electrode arrays, with and without rigid introductor (stylet). We report the results on 37 patients with measurable preoperative hearing thresholds, mean age of 28 years (5-70 years), having the following implants: seven Advanced Bionics, four Med-El, 24 Cochlear, two MXM; 19 of them were performed using the stylet and the other 18 without it. A minimally invasive surgical approach was performed with a short retroauricular incision and a 1.2 mm cochleostomy. A complete electrode array insertion was obtained in all patients. Responses to pure-tone stimuli were measured for each ear in pre-implantation conditions and 3-12 months after surgery. After implantation 14 patients (38%) showed no hearing threshold variation, 29 (78%) maintained an appreciable hearing threshold level in the implanted ear, 8 (22%) had a total loss of residual hearing. Median increases of threshold levels were, in all 37 studied patients, 5, 10, 10 and 5 dB HL, respectively, for 125, 250, 500 and 1 kHz. For the 18 patients having implants without the stylet median increases of threshold levels were 0, 10, 5 and 7 dB HL; in the stylet group, they were 10, 5, 5 and 10 dB HL. On a comparison between the stylet and the non-stylet group, no significant differences in mean hearing threshold worsening were found. Data seem to suggest that cochlear function is less sensitive to mechanical trauma during implant surgery than was thought. Besides, electrode array stiffness seems not to influence preservation of cochlear residual functional integrity. Finally, the authors hypothesize a direct spiral ganglion activation under strong mechanical stimulation.