Is Word Recognition Correlated With the Number of Surviving Spiral Ganglion Cells and Electrode Insertion Depth in Human Subjects With Cochlear Implants?

Abstract

Objectives/Hypothesis: Speech perception scores using cochlear implants have ranged widely in all published series. The underlying determinants of success in word recognition are incompletely defined. Although it has been assumed that residual spiral ganglion cell population in the deaf ear may play a critical role, published data from temporal bone specimens from patients have not supported this hypothesis. The depth of insertion of a multichannel cochlear implant has also been suggested as a clinical variable that may be correlated with word recognition. In the current study these correlations were evaluated in 15 human subjects. Study Design: Retrospective review of temporal bone histopathology-.Methods: Temporal bones were fixed and prepared for histological study by standard techniques. Specimens were then serially sectioned and reconstructed by two-dimensional methods. The spiral ganglion cells were counted, and the depth of insertion of the cochlear implant as measured from the round window was determined. Correlation analyses were then performed between the NU6 word scores and spiral ganglion cell counts and the depth of insertion. Results: The segmental and total spiral ganglion cell counts were not significantly correlated (P > .50) with NU6 word scores for the 15 subjects. Statistically significant correlations were not achieved by separate analysis of implant types. Similarly, no significant correlation between the depth of insertion of the electrode array and postoperative NU6 word score was identified for the group. Conclusion: Although it is unlikely that the number of residual spiral ganglion cell counts is irrelevant to the determination of word recognition following cochlear implantation, there are, clearly, other clinical variables not yet identified that play an important role in determining success with cochlear implantation.

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“…2 E, F, and H), which is clinically relevant because serviceable function can be achieved with a relatively small number of functional neurons. With only 5-10% of the total axonal population, patients with a cochlear implant are able to hear (49). Coincidentally, a similar value of functional motor neurons can enable patients to become mobile (50).…”

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“…This may be the result of sensory deprivation for long periods, which adversely affects connections between and among neurons in the central auditory system (Shepherd and Hardie, 2001) and may allow encroachment by other sensory inputs of cortical areas normally devoted to auditory processing (i.e., cross-modal plasticity; see Lee et al, 2001;Bavelier and Neville, 2002). Although one might think that differences in nerve survival at the periphery could explain the variability, either a negative correlation or no relationship has been found between the number of surviving ganglion cells and prior word recognition scores, for deceased implant patients who in life had agreed to donate their temporal bones for post-mortem histological studies (Blamey, 1997;Nadol et al, 2001;Khan et al, 2005;Fayad and Linthicum, 2006). In some cases, survival of the ganglion cells was far shy of the normal complement, and yet these same patients achieved high scores in speech reception tests.…”

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“…However, to date it has not been possible to show a positive correlation between cochlear implant performance and SGN density [45,46,57,58]. The conductive nature of the cochlear fluids leads to relatively low spatial precision of SGN activation by cochlear ES meaning that higher SGN densities may not offer improvements in resolution with contemporary devices.…”