Selecting the appropriate ear for cochlear implantation is one of the most controversial points when dealing with bilateral profound hearing loss because of the lack of evidence-based recommendations (1,2). Patients with long-term auditory deprivation usually have a history of a long-term dead ear and a non-useful hearing aid on the other ear. This particular situation raises the question of which ear should be implanted. Some authors state that a cochlear implant (CI) should be placed on the non-deprived ear to obtain better hearing results (3), as continuous neural stimulation may provide better CI outcomes. On the other hand, other groups support implantation of the deprived ear to benefit from bimodal stimulation and avoid risks over the “best” ear (4,5).
The general tendency in our center in the past decades has been to implant the “worst” ear, i.e., the long-term auditory deprived ear. Over years of clinical practice, we have observed patients with excellent results with a CI after long periods of auditory deprivation (as much as 30 yr). These observations contrasts with the theory that long-term auditory deprivation is a negative factor for CI performance (6,7). In fact, some authors suggest avoiding placing a CI in an ear that has been auditory deprived for more than 10 years (8). The rationale for supporting this recommendation is that the absence of auditory input over the years could entail the loss of spiral ganglion cells, inducing itself retrograde neural degeneration (9). However, the degree of neural degeneration is not consistent in patients with long time auditory deprivation and good results have been obtained with very low number of spiral ganglion cells (10,11).
Furthermore, studies comparing implantation of the best or worse ear (following different criteria based on the residual pure tone average, speech reception threshold, and the use of a hearing aid) have demonstrated similar results in both groups irrespective of the strategy of implantation (1,2,12).
To date, there are not any single-center studies with large samples focused on the impact of auditory deprivation time on CI performance. Actually, the role of long-term hearing deprivation in CI performance is not well established, and the limits for implanting an auditory deprived ear are still unknown.
The purpose of this study was to evaluate the effect of long-term auditory deprivation time on CI results.
MATERIAL AND METHODS
After institutional review approval, a retrospective study was conducted in the postlingual adult patients implanted in a tertiary referral center from 2001 to 2015. Patients under 18, those with bilateral CIs or patients with concomitant neurotologic or skull base disease were excluded (cerebellopontine angle tumors, petrous bone cholesteatomas with cochlear erosion or cochlear nerve involvement, Menière's disease, after temporal bone radiation therapy, etc.). Furthermore, recipients were excluded from the study if Spanish was not their first language, and if there was evidence of surgical complication or device failure. Data were extracted from an extensive CI database.
Data regarding age at implantation, sex, etiology of hearing loss, and duration of auditory deprivation in the implanted ear were collected.
Auditory deprivation was considered from the moment in which the patient him/herself had reported complete hearing loss of the affected side, not being able to use the telephone or use a hearing aid. In those patients who had used a hearing aid, the onset of auditory deprivation was considered as the moment in which the hearing aid was not useful for the patient, leading to cease its use.
CI results were expressed in terms of vowel identification (VI), disyllabic word recognition (DWR), and sentence recognition (S). All tests were conducted by the same examiner, with monitored live voice in free field inside an audiometric cabin. The examiner spoke through the audiometer microphone, outside of the visual field of the patient. The stimuli were adjusted to an intensity of 65 dB and were delivered through a loudspeaker located at 0 degree in the horizontal plane, at 1 m of distance, at the level of the patient's head. The interval time between two stimuli was 3 seconds. A standardized words and sentences list for Spanish language speakers (13) was used for the evaluation.
Data were collected for the evaluations at 6, 12, 24, and 36 months after CI activation.
Hearing results were analyzed 1) globally, with the entire sample by means of a correlation analysis between the hearing results and the years of auditory deprivation, 2) divided into two groups, comparing patients with less than 10 years of auditory deprivation (group A) with those with more than 10 years (group B).
Continuous data were summarized as mean and interval of confidence (95%) and median. Categorical data were presented as absolute and relative frequencies. Visual observation of histograms and the Shapiro-Wilk test were used to assess normality. The correlation between speech recognition scores and deprivation time was studied by Spearman's rho correlation test. The differences in speech score recognition between group A and B (more or less than 10 years of AD) were performed by Mann–Whitney U test. Relations of two categorical variables were performed by chi-square test. All tests were two-sided and p values below 0.05 were considered significant. Data were analyzed with a statistical software program (SPSS Statistics for Windows version 20, Chicago, IL).
From a total of 275 patients implanted in the study period, 103 fitted the criteria and agreed to participate in the study. Demographic data, side of implantation and CI brand, and etiology of hearing loss are shown in Table 1.
The mean auditory deprivation time in the implanted ear was 9.5 years (range, 1–43 yr). Mean age at implantation was 53.1 years (95% IC 50.5–55.7).
As expected, hearing results improved progressively, stabilizing between the first and second year. Mean results for the whole sample are summarized in Table 2.
Deprivation time was not statistically correlated with age at implantation (r = 0.165, p = 0.097) (Fig. 1). Age at implantation was not statistically correlated with CI results (Fig. 2). There was only a weak negative correlation between sentence recognition at 6 months and age (r = −0.20; p = 0.04). This correlation was not found for any other of the speech recognition scores evaluated in the successive follow-ups.
Correlation Analysis Between Auditory Deprivation Time and CI Performance
Spearman's correlation test was applied to determine whether there was a relationship between auditory deprivation time and speech recognition scores. A weak negative correlation was found between VI, DWR and S, and deprivation time, which was not statistically significant at 6 months, 1, 2, and 3 years follow up (Spearman correlation coefficient ranged from −0.23 to −0.37) (Fig. 3, Table 3).
Comparison of Patients with Less and More than 10 Years of Auditory Deprivation
The sample was divided depending on the auditory deprivation of the implanted ear. Group A (less than 10 yr) was comprised of 61 patients with a mean age of 50.9 years (IC 95% 47.2–54.6) and a mean auditory deprivation time of 2.5 years (IC 95% 2.1–3.0). Group B was comprised of 42 patients with a mean age of 56.3 years (IC 95% 52.8–59.7) and a mean auditory deprivation of 19.6 years (IC 95% 16.7–22.6).
Both groups did not significantly differ regarding sex, age at implantation, and etiology of hearing loss (p > 0.05).
Mann–Whitney U test was applied to study speech recognition scores in these two groups. No statistically significant differences were found in the auditory results (VI, DWR, and S) between both groups in any of the successive follow-ups (Fig. 4).
Long-term auditory deprivation in the ear-to-be-implanted has been traditionally considered as a negative predictive factor for CI performance (6,7). The rationale to support this premise is the logical thought that the lack of auditory input could lead to a reduction in the number of spiral ganglion cells, compromising the results of the CI. However, while the absence of acoustic stimulation has been associated with a progressive degeneration of spiral ganglion cells, no clear correlation has been established between the spiral ganglion cells density and CI outcomes (11). To date no correlation has been established between the number of surviving ganglion cells and the performance of a CI (14), and the minimum number of ganglion cells required for successful cochlear implantation is still unknown. In fact, postmortem studies in patients that had useful auditory sensation after cochlear implantation showed as few as 3,000 surviving ganglion cells (15).
In consonance with these observations, our findings suggest that long-term auditory deprivation in the implanted side is not correlated with poorer CI outcomes. The results of this study are robust and there is concordance between the two statistical analyses performed. Neither the correlation analysis nor the comparison between deprived and non-deprived ears showed any significant relationship between the auditory deprivation and CI performance. Both groups were controlled for possible confusing factors such as sex, age at implantation, and follow up (same number of patients in both groups at 3 yr follow-up). However, the scatter plots for the speech recognition scores showed a ceiling effect (Fig. 2), losing information regarding variance among the best performers. This reduces the likehood of finding a relationship between speech recognition scores and auditory deprivation time, if one exists.
The results of this study suggest that auditory deprivation time in the implanted side does not play a determining role in the CI performance in postlingually deafened adults. In our study, there were groups with auditory deprivation time as different as 2.52 years and 19.5 years with equivalent results. In fact, previous studies have demonstrated that the cerebral cortex is able to reorganize even when hearing is restored after 40 years of auditory deprivation (16).
Unexpectedly, there was no correlation between deprivation time and the age of the patients at the moment of implantation. However, the fact that this study was comprised entirely of postlingually deafened adults must be taken into consideration. While it is known that auditory deprivation plays a determinant role during the sensitive period of maturation of the auditory system (up to the age of 3.5 yr) (17,18), deprivation time may be a secondary factor in postlingual deafness. Our results are in keeping with the findings made by Boisvert et al. (19), suggesting that auditory deprivation was not correlated with CI performance. In consonance with these findings, Gantz et al. (20), observed that there was not a better performance of the ear with less auditory deprivation in patients with binaural CI. Canale et al. (1) obtained similar results to the present study, obtaining no significant correlation between auditory deprivation time and speech recognition scores in a group of 30 adults.
Furthermore, the duration of auditory stimulus of the implanted ear and the overall auditory experience of the individual are factors that should be considered in addition to the auditory deprivation time.
This was expressed previously by other authors (19,21) and has been named by Boisvert et al. (19) as “bilateral significant hearing loss”. It refers to the time in which the patient presents bilateral severe hearing loss, is unable to use the telephone and has less than 30% of speech recognition score on both ears (with hearing aids optimally fitted).
Interestingly, this author found that the CI outcomes were inversely correlated with the duration of bilateral significant hearing loss. These findings suggest, as mentioned by the author, that the overall auditory experience of the individual was a better predictor of the CI outcomes than the auditory deprivation in the ear-to-be-implanted. Our future research will focus on this point.
Furthermore, as expressed by Lazard et al. (17,18), the results of a CI are conditioned by peripheral factors, such as the position and characteristics of the electrode array or the condition of the cochlea. But CI results are also determined by central factors related to higher cognitive influences, phonological memory, and reorganization of auditory areas. These factors may explain the individual variability observed in these patients. In this sense, auditory deprivation plays a double role. It influences the peripheral organ by reducing the number of spiral ganglion cells, and more importantly, it induces the reorganization of the auditory cortex at the central nervous system. All this suggests that the ability to process the information may be more important that the quality of the signal itself.
Blamey et al. (22) in a multicenter study including 15 international centers and 2,251 patients observed that the duration of CI experience had the largest effect on CI performance, followed by age at implantation, duration of severe-to-profound hearing loss and etiology of hearing loss. However, it is worth to mention that, according to these authors, these four factors together were able to explain only around 10% of the existing variability. According to this model there would be a 90% of variability that remains unexplained. Thus, there may be some effect of duration of deafness in the implanted ear, but the total auditory experience of the patient may be more important.
Our results seem to contradict the general rule that implantation in patients with long duration of deafness and absence of auditory stimulus should not be performed (as long as language as been acquired previously) (6–8). In contrast, they support previous observations that implanting the poorest side does not yield worse performance (1,2,12,21), leaving a chance for bimodal stimulation and reserving the best ear for future therapies. Future studies should focus on the individual ability to process information, cognitive status, and overall auditory experience of the subject, as monaural auditory deprivation has become a secondary factor on CI performance.
Long-term auditory deprivation in the ear to be implanted does not negatively influence CI results. CI performance in these cases is similar to patients with very short time of auditory deprivation. Age at implantation does not constitute a negative prognosis factor for CI performance in cases with long-term auditory deprivation. Long-term auditory deprivation should not be considered criterion to reject cochlear implantation.
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Keywords:Copyright © 2017 by Otology & Neurotology, Inc. Image copyright © 2010 Wolters Kluwer Health/Anatomical Chart Company
Auditory deprivation; Cochlear implant; Cochlear implantation; Duration of deafness; Postlingual deafness