Spiral Ganglion Cell Survival and CI Performance

By Thomas J. Balkany, MD

1.1.16

Background

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.

New Information

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.

Take Home

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.
  • 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.

References

    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.

Leave a Comment

Mario Svirsky 1 year ago

When reading the take home messages in this post or in the original article, please remember that they are based on one ear from one single subject. A quick read and reanalysis of the data from Seyyedi, Viana and Nadol (2014) shows that the correlation they found (based on only six patients) is driven by one single outlier (patient 1). Once you remove that outlier, the correlation ceases to be significant. The differences in word recognition scores between left and right ears for the other five subjects were between zero and six percentage points, well within test-retest reliability error for most tests of word recognition (Seyyedi et al, do not say which test they used). This means that for all but one patient in the study, the differences in word recognition scores between left and right ears were neither statistically significant nor very meaningful. Patient 1 had much higher scores in the left than the right ear (25% vs 5%), but there were other important left-right differences: the left ear had been implanted for 11 years versus only 1 year for the right, and the depth of insertion was almost twice for the left ear than for the right (11 mm vs. 6 mm). These are suspiciously short and atypical insertion depths, particularly the one in the right ear. I don't remember any documented cases of excellent speech perception in CI users with an insertion depth of 6 mm. In summary, it is fair to say that the correlation in Seyyedi et al. is based on one single, highly atypical ear. This is not enough to draw any conclusions. The hypothesis that higher SGC counts may result in better CI outcomes remains as reasonable as ever, and just as unsupported.

Ravi Samy 1 year ago

Dear Dr. Balkany, Nice topic for this month's blog. I am curious if there is any thought about use of transtympanic steroids to reduce chance of intracochlear fibrosis, ossification, and subsequent loss of spiral ganglion cells. Have a blessed and Happy New Year! RNS

Basharat Nadeem 1 year ago

With warm greetings,with respect to the SGC preservation ,round window insertion of implant with pre insertion placement of dexamethsone soaked gelfoam may be a better method .