Hearing preservation in cochlear implant recipients: A cross‐sectional cohort study

OBJECTIVES
A surge of new developments and research regarding cochlear implants and hearing preservation resulted in several treatment options in the last 5 years. By reviewing our CI population of this period, we aimed to investigate hearing preservation rates and the effect of different treatment options on hearing preservation.


DESIGN
Retrospectively, all adult cochlear implant recipients with preoperative residual hearing at lower frequencies (threshold <80 dB hearing level) in a single tertiary referral centre between 2015 and 2020 were analysed. Patients were classified into four groups based on their hearing preservation outcome. Subsequently, differences between the four groups regarding several patient dependent and independent factors were investigated.


RESULTS
In this study, 140 patients were included, which is 46% of all adult CI recipients. Complete hearing preservation was achieved in 14 patients (10%), and complete loss of residual hearing in 48 patients (34%). The lateral wall array and local application of corticosteroids were associated with better hearing preservation. Intravenous corticosteroids, local hyaluronic acid and surgical experience had no effect on hearing preservation rates. Speech perception was not better in patients with residual hearing.


CONCLUSION
Approximately half of all adult cochlear implant recipients had residual hearing at lower frequencies before surgery. In current medical practice, only electrode choice seems to have a clear effect on hearing preservation rates. The majority of CI recipients lose their residual hearing after cochlear implantation. Much improvement in treating CI recipients is needed to preserve their residual hearing in the future.


| Data extraction
The following data were collected from the electronic medical records: age at implantation, cause of deafness, side of implantation, date of implantation, name of surgeon, electrode-array type, the use of perioperative corticosteroids (local or systemic), the use of hyaluronic acid, pre-and postoperative PTA low outcomes of the implanted and contralateral ear, and consonant/vowel/consonant (CVC)-word test outcomes.

| Data analysis
The pure-tone audiogram outcomes were subtracted from medi-  partial HP (>25%-75%), minimal HP (0%-25%) and complete loss of hearing (no measurable hearing). These HP scores were also checked manually. In cases with a difference between the pre-and postoperative hearing level at the same frequency of 5 dB, which is equal to the margin of error of the audiometry, HP on this frequency was considered as complete HP.
The CVC-word test outcomes were extracted preoperatively (approximately 6 months prior to surgery), and postoperatively at 3 and 12 months. The preoperative CVC scores were obtained with hearing aids in both ears. Postoperative CVC scores were obtained with activated CI and hearing aid contralaterally to adequately determine the speech perception shifts. These CVC scores were obtained in a situation without background noise. Patients with one-sided hearing impairment were included in the analyses for HP, but excluded for CVC-score analyses.
The pure-tone audiometry outcomes were extracted of the contralateral non-implanted ear in 45 patients to evaluate deterioration of hearing levels irrespective of surgery. The electrode-array type was categorised as perimodiolar or lateral wall. The mid-scala electrode array of Advanced Bionics was classified as perimodiolar electrode array, because it is precurved.  Table 1 for the demographics.

| Hearing preservation
Descriptive data are shown in Table 2.

| Patient independent factors
Looking at electrode array, PM arrays were used in 66 patients

Key points
• Half of all adult cochlear implant recipients had residual hearing at lower frequencies prior to surgery.
• Only a minority of CI recipients retain their residual hearing after cochlear implantation.
• Lateral wall electrode arrays are preferred for hearing preservation.
• Speech perception with CI not improved in CI recipients with preserved residual hearing.
• Surgical experience had no influence on hearing preservation outcomes.

| Surgical experience
The

| Speech perception
A total of 110 CI recipients had CVC scores available at 3 months after surgery, see Figure 2. Before surgery, average CVC score was 33 points (range: 0-77). Three months after surgery, 11 cases had no improvement of CVC score (i.e. CVC-score shift between −25 and 0), while remaining 99 cases had increased CVC scores compared with preoperative scores (range: 2-86). Cases with no residual hearing had largely same distribution of CVC-score shift as the whole cohort. The preoperative CVC-word test scores were comparable between groups. CVC-score shifts were not different between HP groups at 3 and 12 months after implantation (p > 0.05).

| DISCUSS ION
This retrospective cohort study provides a complete overview of a general adult CI population of the last 5 years. Almost half of the adult patients (46%) who underwent primary cochlear implantation had residual hearing at lower frequencies. Complete HP was achieved in 10% of these patients, partial HP in 26%, minimal HP in 30% and complete loss of residual hearing was seen in 34%. LW arrays in general, and intraoperative local corticosteroids usage in small sample set of 8 patients, were associated with better HP.
Speech perception of patients with preserved residual hearing was not better than patients without residual hearing after surgery.
Lastly, surgical experience had no effect on HP outcomes.

| Hearing preservation
Several different classifications are used to indicate HP at lower frequencies after CI surgery. 6 Studies, similar to our study, described complete HP rates ranging between 0% and 68%. 7-9 Based on these studies, and others, residual hearing at lower frequencies deteriorates over time. Direct comparison between our study and other studies is therefore somewhat limited, as most of the previously mentioned studies 8,9 measured at an earlier timepoint than our study (around 40 days vs. 88 days in this study). It is likely that HP depends on direct acute trauma during cochlear implantation resulting in inflammatory ototoxic processes, which impacts inner ear homeostasis and manifests as hearing deterioration at longer term. The deterioration over time could also be independent from cochlear implantation and might be related to progress of the disease itself. All in all, it is very difficult to establish final HP outcomes, if at all possible, considering that residual hearing is probably continuously deteriorating to some degree.

| Patient independent factors
In our study, patients with LW arrays had more often complete HP than patients with PM arrays (14% vs. 6%). Scalar translocation is regarded as severe insertion trauma, occurring more often with PM arrays, and negatively influences residual hearing of CI recipients. 10 Therefore, this difference is probably linked to scalar translocation. It is unknown whether these differences between LW and PM arrays remain the same on the longer term. Another factor, hyaluronic acid, had no effect on HP in our study. Another study showed a correlation between HP and the use of hyaluronic acid. 11 However, this was a weak correlation, and is the only study, to our knowledge, showing a direct effect of hyaluronic acid on HP rates.

| Speech perception
Preserved residual hearing can improve speech perception in patients with EAS. 2 In our cohort, only one individual made use of EAS.

| CON CLUS ION
Approximately half of all adult CI recipients had residual hearing at lower frequencies before surgery. The majority of these patients lost