CI Outcomes: The Effect of Spiral Ganglion Survival
Although CI outcomes are generally good and consistently improving, there continues to be a wide range of performance in speech recognition. Disparate outcomes have been attributed to several clinical variables including:
Age at implantation
Use of signed language
Duration of deafness
Pre-op hearing aid use
Percent active electrodes
Scalar position of electrode
Device, electrode, program
However, in examining these factors a retrospective study of 2,251 CI recipients showed that even a combination of the most significant variables accounted for only about 10 to 20% of outcome variability.(1,2) Some other determinant(s) must have a substantial impact on performance.
McClellan et al (2014)(3) of the University of North Carolina employed a novel analysis of ECOG potentials (which include contributions of both hair cell and neuron function, but are dominated by the latter.) The ECOG-TR sums response amplitudes at multiple frequencies to derive a “total response”. This test alone accounted for 40% of CI outcome variance in 32 subjects. Aside from its predictive value, this study suggested that residual spiral ganglion cell function may play an important role in CI outcomes. Support for this notion was to come from genetic studies using massively parallel sequencing techniques.
In 2015, Wu et al(4) of the National Taiwan University reported that 12 children with poor CI performance (in speech perception, receptive and expressive language) tended to have more genetic mutations affecting the spiral ganglion than a matched group of 30 children with good outcomes. The authors found that organ of Corti mutations were associated with “good” results and ganglion cell mutations were associated with “poor” results. This makes sense since CIs bypass hair cells and synapses.
A larger study of 155 adult CI subjects was recently (PAP 2017) reported by Shearer and colleagues from the University of Iowa(5) The authors categorized subjects as a sensory-genetic group (deleterious variants among 89 genes affecting hair cells or synapses) or neural-genetic group (deleterious variants among seven genes affecting the spiral ganglion.) Mutation of spiral ganglion-associated genes was associated with poorer results than mutation of organ of Corti-associated genes and accounted for 18% of the variance in speech perception outcomes, underscoring the importance of spiral ganglion cell function in CI outcomes.
The importance of residual spiral ganglion function has been largely overlooked in assessing possible causes of CI outcome predictability and variability. This will change as physiologic and genetic methods of evaluating spiral ganglion survival and function in CI candidates become more prevalent.
ECOG-TR can account for 40% of outcome variability and genetic testing for mutations in spiral ganglion cells account for 18%. Although widespread application of these techniques awaits further study, one thing has become clear: spiral ganglion cell survival and function play an important role in CI performance.
1. Lazard DS, Vincent C, Venail F, Van de Heyning, P, Truy, E, et al. Pre-, Per- and Postoperative Factors Affecting Performance of Postlinguistically Deaf Adults Using Cochlear Implants: A New Conceptual Model over Time. 2012 PLoS ONE 7, e48739–11.
2. Blamey P, Artieres F, Baskent D, Bergeron F, Beynon AS, et al.. Factors affecting auditory performance of postlinguistically deaf adults using cochlear implants: an update with 2251 patients. 2013 Audiol. Neurootol. 18, 36–47.
3. McClellan J.H, Formeister EJ, Merwin WH, Dillon M, Calloway N, Iseli C, et al. 2014. Otology & Neurotology 35, e245-e252.
4. Wu CC, Lin YH, Liu TC, Lin KN, Yang WS, et al. Identifying Children With Poor Cochlear Implantation Outcomes Using Massively Parallel Sequencing. 2015. Medicine 94, e1073.
5. Shearer AE, , Eppsteiner RW, Frees K, Tejani V, Sloan-Hagen C Black-Ziegelbein EA, et al. 2017 Hear Res (PAP, DOI: 10.1016/j.heares.2017.02.008).