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:
1. Nadol JB Jr, Young YS, Glynn RJ. Survival of spiral ganglion cells in profound sensorineural hearing loss: implications for cochlear implantation. Ann Otol Rhinol Laryngol 1989;98:411-16.
2. Fayad JN, Linthicum FH Jr. Multichannel cochlear implants: relation of histopathology to performance. Laryngoscope 2006;116:1310-20.
3. Seyyedi M, Eddington DK, Nadol JB Jr. Interaural comparison of spiral ganglion cell counts in profound deafness. Hear Res 2011;282:56-62.
4. Seyyedi M, Viana LM, Nadol JB Jr. Within-Subject Comparison of Word Recognition and Spiral Ganglion Cell Count in Bilateral Cochlear Implant Recipients. Otology & Neurotology 2014;35:1446-50.