We at The Hearing Blog have been banging the drum for residual hearing preservation after CI surgery since June, as the post-implant performance is much better, and there is a much lower risk of triggering tinnitus. Because of this, we are Seriously Pissed Off at a Prominent CI Surgeon at a Major University Hospital in Atlanta: In August, one of this author’s reverse slope SNHL hearing aid patients used this “surgeon” instead of much better ones at Vanderbilt… And sure enough, despite using the AB mid-scala electrode designed for atraumatic insertion, he wiped out her residual hearing, which was still at 10dB at 4kHz when this author last measured her hearing. What’s more, we have since found out from a local audiologist who MAPs CI’s that this surgeon does not (more likely, cannot) preserve residual hearing.
With that as a background, we present even more evidence on the necessity of residual hearing preservation in Spiral Ganglion Cell Survival and CI Performance by UMiami’s Thomas J Balkany, MD:
The key neural elements that are stimulated by cochlear implants (CI) are spiral ganglion cells (SGC). So it would seem logical that the more SGCs that survive, the better CI performance would be. Nonetheless, histopathologic studies have suggested that SGC survival rates do not correlate with CI performance. 1,2
However, prior temporal bone studies could not control for variables that might affect performance (age, cause of deafness, degree of hearing loss, duration of deafness, cognitive ability, etc.) Failure to control these variables cast some doubt on the validity of the findings above.
In short, the best data available over the past 25 years indicated that SGC survival did not correlate with CI performance, but those studies created an intuitive dissonance. In order to settle the issue, the critical variables would need to be controlled.
Joe Nadol’s group at Harvard re-investigated this issue by studying temporal bones from bilateral implantees. Seyyedi, Eddington and Nadol 3 first demonstrated that individual temporal bone donors who were deafened bilaterally by the same etiology and had similar hearing loss in each ear, had similar numbers of surviving SGCs.
Based on this information, Seyyedi, Viana and Nadol 4 studied 12 temporal bones from six ‘bilateral’ subjects with both ears deafened by the same etiology. Age and cognitive function were also controlled by their study design that compared right vs left ears of individual subjects.
The authors found that word recognition scores were directly correlated with SGC counts ((R = 0.934, p = 0.006). There was no significant correlation between CI performance and depth of electrode insertion, duration of CI usage, or age at implantation in this sample.
These findings suggest that greater SGC survival may improve CI performance. If that is the case, several corollaries may exist:
- Hearing-preservation surgical techniques and electrodes may be more important than previously thought, even in profoundly deaf patients.
- The neurotrophic effects of electrical stimulation that supports SGC survival may be another reason for early implantation.
- Pharmaceutical support of SGCs (e.g.: neurotrophins) may play an important role in improving CI performance.A
- Efforts to improve low-trauma electrode arrays should continue.
- Large cooperative clinical studies comparing performance among categorical etiologies may provide further insight into determinants of outcome.
- Joseph B Nadol, Yi-Shyang Young, and Robert J. Glynn. “Survival of spiral ganglion cells in profound sensorineural hearing loss: implications for cochlear implantation.” Annals of Otology, Rhinology & Laryngology 98, no. 6 (1989): 411-416;
- Jose N Fayad, and Fred H. Linthicum. “Multichannel cochlear implants: relation of histopathology to performance.” The Laryngoscope 116, no. 8 (2006): 1310-1320;
- Mohammad Seyyedi, Donald K Eddington, and Joseph B Nadol. “Interaural comparison of spiral ganglion cell counts in profound deafness.” Hearing research 282, no. 1 (2011): 56-62;
- Seyyedi, Mohammad, Lucas M. Viana, and Joseph B. Nadol Jr. “Within-subject comparison of word recognition and spiral ganglion cell count in bilateral cochlear implant recipients.” Otology & Neurotology 35, no. 8 (2014): 1446-1450.
- Dr Balkany’s article was lightly edited to conform to the Chicago Manual of Style’s notes and bibliography system; and also we added links to abstracts of the original articles;
- Separate but related, we understand there’s a showstopper bug in AB’s Naida Q90 speech processor when it’s used for hybrid (EAS): When you snap on a Roger 17 FM receiver the audio is only routed to the electrode array, and not the receiver. We’re sure Phonak will fix this.