Investigating long-term effects of cochlear implantation in single-sided deafness: a best practice model for longitudinal assessment of spatial hearing abilities and tinnitus handicap

Abstract

Objectives: To evaluate methods for measuring long-term benefits of cochlear implantation in a patient with single-sided deafness (SSD) with respect to spatial hearing and to document improved quality of life because of reduced tinnitus.

Patient: A single adult male with profound right-sided sensorineural hearing loss and normal hearing in the left ear who underwent right-sided cochlear implantation.

Methods: The subject was evaluated at 6, 9, 12, and 18 months after implantation on speech intelligibility with specific target-masker configurations, sound localization accuracy, audiologic performance, and tinnitus handicap. Testing conditions involved the acoustic (NH) ear only, the cochlear implant (CI) ear (acoustic ear plugged), and the bilateral condition (CI+NH). Measures of spatial hearing included speech intelligibility improvement because of spatial release from masking (SRM) and sound localization. In addition, traditional measures known as "head shadow," "binaural squelch," and "binaural summation" were evaluated.

Results: The best indicator for improved speech intelligibility was SRM, in which both ears are activated, but the relative locations of target and masker(s) are manipulated. Measures that compare performance with a single ear to performance using bilateral auditory input indicated evidence of the ability to integrate inputs across the ears, possibly reflecting early binaural processing, with 12 months of bilateral input. Sound localization accuracy improved with addition of the implant, and a large improvement with respect to tinnitus handicap was observed.

Conclusion: Cochlear implantation resulted in improved sound localization accuracy when compared with performance using only the NH ear, and reduced tinnitus handicap was observed with use of the implant. The use of SRM addresses some of the current limitations of traditional measures of spatial and binaural hearing, as spatial cues related to target and maskers are manipulated, rather than the ear(s) tested. Sound testing methods and calculations described here are therefore recommended for assessing performance of a larger sample size of individuals with SSD who receive a CI.

Figures

Fig. 1

(A) A schematic view of the setup used in the localization and speech-in-noise tasks. For the localization task, the subject was positioned at the center of an arc of loudspeakers with a 1.2 m radius. The positions of each loudspeaker were concealed by a curtain. Nineteen loudspeakers were placed at 10° intervals. (B) The spatial configurations tested in the speech-in-noise task. Target speech (T) was always presented from 0° azimuth. Maskers (M) were presented from 0° (i), +/−90° (ii), −90° (iii), or + 90° (iv). Three listening conditions (cochlear implant alone (CI), acoustic (normal-hearing) ear alone (NH), and bilateral (CI+NH)) were tested in each of the four spatial configurations.

Fig. 2

Sound localization results. Root-mean squared (RMS) localization error is shown as a function of post-activation time for the CI-only (normal hearing ear plugged, black bars), acoustic-only (light bars), and bilateral (gray bars) conditions. For each of the three listening condition at all four testing time periods, 20 trials were performed for each of the 19 speaker locations in a random order (380 trials per condition). Error bars represent the standard deviation from the mean for each of these testing conditions at each time period. For reference, an RMS error of 90° represents localization accuracy no greater than chance alone. CI = cochlear implant with opposite ear plugged, NH = acoustic (normal-hearing) ear, CI + NH = bilateral listening condition, AVG = average RMS error over all four testing periods.

Fig. 3

Results of the binaural benefit calculations are shown for (A) head shadow, (B) binaural summation, and (C) binaural squelch. Calculations were made with respect to the NH (acoustic) ear (black circles) and the implanted (CI) ear (white circles). (D) SRM was calculated for the symmetric masker (crosses) and asymmetric masker (to right, triangle; to left square) conditions. Normal values for binaural hearing measures are as follows: Head shadow 3-10 dB; binaural squelch 3 dB; binaural summation 6-10 dB (20). Normal range of SRM in normal hearing adults is as follows: Masker left or right 6-7 dB; Maskers left and right 2-3 dB (19).

Fig. 4

Results for (A) the Tinnitus Reaction Questionnaire (TRQ) and (B) the Tinnitus Handicap Inventory (THI). The scores were obtained at one week pre-implantation (PRE), one week post-implantation (POST), one week post-activation (ACT) of the implant, and at 2 month intervals thereafter. After activation of the implant, TRQ scores decreased into the normal range (normal