Objective: To assess and compare subjective and objective results after bilateral cochlear implantation with a special emphasis on time interval between the first and second implant.
Study design: Clinical trial.
Setting: Tertiary referral center.
Patients: All consecutively bilaterally implanted adult patients who had used the second implant for more than 6 months were selected for the study. They had to have the mental capacity to answer questions regarding their hearing abilities and fill out a questionnaire (n = 34). Twenty-nine patients (85%) finally could be included in the study (age at first implantation, 31.0 +/- 16 yr [mean +/- standard deviation {SD}], time to second implantation, 5.6 +/- 5.7 yr [mean +/- SD]). In all patients, a full insertion (21 electrodes) of the implant was achieved. For the subjective part of the study, the patients were matched with unilaterally implanted subjects selected according to specified criteria.
Interventions: Sequential or simultaneous cochlear implantation with a Nucleus device.
Main outcome measures: Speech comprehension measures were performed using the Oldenburger sentences in quiet and in noise with unilateral and bilateral implant use. Summation effect, head shadow effect, squelch effect, and interaural difference in quiet and noise were calculated. Advantage for binaural stimulation with respect to the unilateral condition was assessed for each individual. Additionally, a localization test was performed using 12 speakers arranged in a circle. Subjective benefit was assessed by a questionnaire (The Speech, Spatial and Qualities of Hearing Scale). Results were analyzed with special emphasis on effects of timing and intervals.
Results: There was a statistically significant advantage for the head shadow effect test (p < 0.05) when the sound source was located on the activated side. There was also a statistically significant correlation of the subjective and objective results and a strong correlation of the interaural difference of speech intelligibility in quiet and the time interval between the first and the second implant (p < 0.001; r = 55%). In the bi-implanted state, an interaural difference of 18 +/- 27% and 3 +/- 2.2 dB signal-to-noise ratio (mean +/- SD) was measured in quiet and noise, respectively. The mean results for the bilateral condition for the summation effect, the squelch effect, and speech discrimination in quiet were better than in the unilateral condition but were not statistically significant. The subjective results of the bilateral group were better in all categories than the results of the unilateral group but were just below statistical significance.
Conclusion: Speech understanding in noise is improved with bilateral cochlear implantation with unambiguous evidence that the second implant expands the sound field for effective speech recognition. Communication in daily life is facilitated, as determined by the subjective Speech, Spatial and Qualities of Hearing Scale test. The correlation of the subjective and objective results confirms the practical benefits in daily activities. Although there was improvement with a second Cochlear implant even after a long implantation interval, short intervals lead to better results.