Delayed changes in auditory status in cochlear implant users with preserved acoustic hearing

Abstract

This retrospective review explores delayed-onset hearing loss in 85 individuals receiving cochlear implants designed to preserve acoustic hearing at the University of Iowa Hospitals and Clinics between 2001 and 2015. Repeated measures of unaided behavioral audiometric thresholds, electrode impedance, and electrically evoked compound action potential (ECAP) amplitude growth functions were used to characterize longitudinal changes in auditory status. Participants were grouped into two primary categories according to changes in unaided behavioral thresholds: (1) stable hearing or symmetrical hearing loss and (2) delayed loss of hearing in the implanted ear. Thirty-eight percent of this sample presented with delayed-onset hearing loss of various degrees and rates of change. Neither array type nor insertion approach (round window or cochleostomy) had a significant effect on prevalence. Electrode impedance increased abruptly for many individuals exhibiting precipitous hearing loss; the increase was often transient. The impedance increases were significantly larger than the impedance changes observed for individuals with stable or symmetrical hearing loss. Moreover, the impedance changes were associated with changes in behavioral thresholds for individuals with a precipitous drop in behavioral thresholds. These findings suggest a change in the electrode environment coincident with the change in auditory status. Changes in ECAP thresholds, growth function slopes, and suprathreshold amplitudes were not correlated with changes in behavioral thresholds, suggesting that neural responsiveness in the region excited by the implant is relatively stable. Further exploration into etiology of delayed-onset hearing loss post implantation is needed, with particular interest in mechanisms associated with changes in the intracochlear environment.

Keywords: Cochlear implant; Electrically evoked compound action potential; Electrode impedance; Hearing preservation.

Conflict of interest statement

Conflict of Interest

B.J.G. is a consultant for Cochlear Corporation and Advanced Bionics companies. He holds a patent on the hybrid CI, but receives no royalties. The authors declare that they have no other conflicts of interest.

Copyright © 2017 Elsevier B.V. All rights reserved.

Figures

Figure 1

Individual examples of implanted and unimplanted ear audiometric thresholds (PTA) as a function of time for each hearing loss category. Note that the x-axis is not the same for each panel.

Figure 2

ECAP amplitude growth function for one participant and one electrode. Visual detection threshold (CL), the steeper slope (μV/CL), and the amplitude (μV) at a stimulus level of 198 CL are indicated.

Figure 3

Grand mean unaided behavioral audiometric thresholds (± 1 standard deviation) at low frequencies measured prior to surgery (black) and at the first post-operative visit (gray). Pre- and post-operative data points are shifted horizontally for easier visualization of error bars. Panels separate data by device type.

Figure 4

Duration of data available for each participant, ranked from least to greatest. Color coding indicates category of change observed in audiometric thresholds (see legend).

Figure 5

Example of one participant showing a precipitous drop in acoustic hearing (a) and coincident increase in electrode impedance (b). Symbols are the means across frequencies (a) or electrodes (b), vertical lines extend ± 1 standard deviation.

Figure 6

Impedance trajectories separated by hearing loss category (a. stable/symmetrical; b. gradual; c. precipitous). Each symbol is the change in average impedance across electrodes at a given time point for an individual. Individual data are connected by thin lines.

Figure 7

Average (a) and maximum (b) impedance changes across time separated by hearing loss category. Note the different y scales. Individual data are represented with gray circles, and group means with horizontal black bars.

Figure 8

Scatterplots relating changes in ECAP measures (a. threshold, b. slope, c. amplitude) to changes in PTA. Symbols denote hearing loss category (white circles: stable; light gray squares: symmetrical; dark gray triangles: gradual; black triangles: precipitous).