We presented the longitudinal auditory performance of CI in children with bilateral CN aplasia. Our study has some merits for the following reasons. This study included the largest number of patients with bilateral CN aplasia to explore outcomes of CI based on cross-sectional and longitudinal audiological analyses. The overall CAP score showed significant and gradual improvement from 0.29±0.55 to 2.67±1.73 throughout the 3-year follow-up period, suggesting that CI may elicit favorable auditory performance even in children with CN aplasia. However, improvement in auditory performance showed variability between individuals, and the benefit appeared to be predominant in patients with a CAP score >1 at 1 year postoperatively. The results of this study may serve as a possible prognostic factor to aid in the decision-making process of surgical management of children with CN aplasia. Specifically, in pediatric cochlear implantees with a CAP score ≤ 1 at 1 year postoperatively, transiting auditory rehabilitation from CI to ABI may be considered for better auditory performance.
Since the first report proposing that CI could be applied to children with CN aplasia,12 very few studies on CI outcomes in patients with CN aplasia have been reported. Wu et al.13 demonstrated significantly lower CAP scores in children with CN aplasia than in those with normal CNs matched for demographics. They reported that none of the seven patients with bilateral CN aplasia achieved favorable CI outcomes after 3 years of surgery, and their CAP scores ranged from 0 to 5, which is in line with our results. Furthermore, Birman et al. demonstrated that approximately 50% of children with CN aplasia achieved some verbal understanding, as evidenced by a CAP score of 5 to 7. Meanwhile, Yousef et al. reported a mean postoperative 2-year CAP score of 1.29 in seven cases with CN deficiency.2 Of these five patients with CN aplasia scored 0, indicating no awareness of environmental sounds. The auditory performance of pediatric cochlear implantees with apparent CN aplasia presented in this study fell within the middle range based on reported studies in the literature. Although differences in methodologies, such as inclusion criteria and follow-up period, may lead to significant discrepancy, cross-sectional assessment and lack of evaluation time points would limit CI outcomes in patients with CN aplasia.2
There is insufficient evidence on the criteria for determining the treatment modalities (CI versus ABI) in children with CN aplasia. A recent study by Yousef et al.2 compared and analyzed auditory performance after implantation in 14 patients with CN deficiency. Of these, seven patients underwent CI, and the other seven patients underwent ABI. Five patients in the CI group and all patients in the ABI group had bilateral CN aplasia. The mean CAP score at 2 years postoperatively was 1.29 in the CI group and 2.87 in the ABI group, demonstrating better outcomes in the ABI group. However, not all studies in the literature fully support this, probably due to small sample sizes for statistical significance, confounding factors, or heterogeneous assessment of auditory and speech performance in patients with CN aplasia. Specifically, a meta-analysis demonstrated that among pediatric patients with CN deficiency, 25% (27/108) attained open-set speech perception and 34% (37/108 attained close-set speech perception after CI,6 suggesting that CI may serve as an initial treatment before ABI in children with CN deficiency. The rationale behind this could be the presence of residual CN fibers that were too hypoplastic to appear on MRI, even when CN aplasia is documented. For instance, a subset of children with CHARGE syndrome benefited from CI because the residual CN fibers exist, although they are very small and follow an alternative course. Additionally, children with apparent CN aplasia on MRI could benefit from electrical stimulation to develop auditory performance, which is likely due to connections or anastomoses between the CN and adjacent vestibular nerve based on anatomical studies.14,15 Corroborating this, our results also support that CI may be useful as an initial treatment modality before ABI in children with CN aplasia, as evidenced by the result that 57.1% (12/21) obtained a CAP score ≥3 at 3 years post-CI.
However, ABI could be an alternative treatment modality for CI in a subset of children with CN aplasia. Our data presented herein suggest that transiting auditory rehabilitation from CI to ABI may be considered for better auditory performance, especially in cases with limited benefit at the early postoperative stage (i.e., CAP score ≤1 at 1 year postoperatively). To the best of our knowledge, reproducible and reliable prognostic markers, including imaging and audiological data, that can predict CI outcomes in cases with CN aplasia are scarce. We, for the first time, suggest that changes in auditory performance at the early postoperative stage might serve as a possible prognostic marker to predict the trajectory of auditory performance postoperatively. Indeed, possible cofactors, such as demographic and surgical approaches, were well controlled between the groups according to the CAP score at the early postoperative stage. Supporting this, Vesseur et al.6 suggested that progress towards alternative treatment modalities such as ABI should be considered to obtain better outcomes when there is no response within several months following CI. Indeed, moderate hearing benefit after switching to ABI was observed in cases with unsuccessful CI outcomes. Furthermore, a meta-analysis of non-tumor pediatric ABI,16 predominantly with CN aplasia (103/162, 64.6%), reported that 47.9% of ABI recipients achieved CAP scores >4 at 5 years postoperatively.16 Interestingly, Aslan et al.17 showed that pediatric ABI recipients with late implantation (age at implantation ≥ 3 years) had poorer auditory performance than those with early implantation even after 5 years of ABI insertion. In other words, delayed transition from CI to ABI, up to 3 years after CI surgery, would be inappropriate, precluding sensitive time for auditory and language development. However, further studies are required to elucidate additional evidence on the optimal time point for transiting auditory rehabilitation.
In clinical practice, intraoperative ECAP is measured to confirm electrode placement, which is correlated with the SGN population. Typically, ECAP is useful for determining the initial programming level and estimation of audiologic outcomes,18,19 and significant correlations between speech perception after CI and ECAP parameters have been documented in the literature.20–22 Similarly, some authors have suggested that the absence of ECAP is associated with poor audiologic outcomes in children with CN deficiency.21,23 However, poor responsiveness and the possibility of electrical artifacts between electrodes and CN fibers make it difficult to use ECAP in patients with CN aplasia.24 Furthermore, previous studies have indicated that the electrically evoked auditory brainstem response elicited by CI was more sensitive than the ECAP. Yamazaki et al. showed that electrically evoked auditory brainstem response testing, coupled with CN integrity on MRI, is clinically meaningful for predicting postoperative CI outcomes. In this study, we observed that the ECAP response rate was not significantly correlated with language development after CI in children with CN aplasia, albeit with a weak relationship. Perhaps, diverse electrodes and a small cohort may have hindered the drawing of firm conclusions regarding the relationship between intraoperative ECAP and postoperative CI outcomes.
This study had some limitations that should be addressed in future studies. In particular, the retrospective nature of this study may limit the generalizability of our results. Potential confounding factors that may affect language outcomes after CI, such as developmental delay and comorbidities such as medical syndromes,5,25 were not completely evaluated in our cohort, which might have biased the results of this study. Furthermore, the lack of a control group, comprising patients with CN aplasia without CI, may weaken our findings. Therefore, future case-control studies with a prospective study design, as well as matched cofactors, would have a stronger significance. Although we observed that the CAP score at 1-year postoperatively may serve as a prognostic value for longitudinal improvement of auditory performance in children with CN aplasia, further investigation is needed to support our results.
In conclusion, we elucidated the outcome of CI in the largest number of patients with bilateral CN aplasia in the literature, based on cross-sectional and longitudinal audiological analyses. Our results further refine those of previous studies on the clinical feasibility of CI as the first treatment modality to elicit favorable auditory performance in children with CN aplasia. However, this benefit may be predominant in a subset of pediatric CI recipients manifesting a CAP score >1 at the early postoperative stage. Thus, special attention should be paid to switching auditory rehabilitation from CI to ABI, especially in pediatric patients with unsuccessful cochlear implants at the early postoperative stage