The benefits of low-frequency hearing preservation in adults has been clearly demonstrated, as residual low-frequency hearing allows for enhanced pitch perception, improved speech recognition in quiet and in noise, and musical discernment (1–3).
The benefits of preserved low-frequency hearing make hearing preservation surgery attractive in the pediatric population. There are potentially greater concerns in this group, that make implantation of children with a goal of hearing preservation more complicated. Children may have a greater propensity for progression of their hearing loss, and in the setting of a shorter electrode, the risk for suboptimal coverage of the cochlea, with potential poor outcomes is a concern (4).
Comparisons within the literature of functional hearing preservation in pediatric cochlear implant recipients is limited by the current indications for pediatric cochlear implantation, and the variable definitions of functional hearing (5–7). Two commonly used criteria for hearing preservation include preservation of low-frequency pure tone average <90 dB (LFPTA; 125, 250, and 500 Hz) (8). This was previously demonstrated to be the threshold at which patients benefited from acoustic amplification. Another recent publication also used criteria of <80 dB at 250 Hz (9). Both of these are useful parameters as they avoid the “floor” effects that evaluating only change in low-frequency are prone to, instead these define preservation of functional low-frequency hearing. The “floor” effect is where patients who already have severe hearing loss appear to paradoxically preserve this hearing given the limits of audiometric testing.
In this study, our objective was to determine factors that influence hearing preservation in the pediatric population by reviewing data from 105 subjects implanted over the last 10 years. We then use information from our regression analysis on this larger group of subjects to define and report on hearing preservation rates for a subgroup of subjects implanted over the last 3 years with lateral wall electrodes inserted by round window.
The University of North Carolina Institutional Review Board approved this research. All subjects received an implant before 18 years of age. Potential subjects with vestibular or cochlear abnormalities were excluded, except for those with enlarged vestibular aqueduct (EVA). Progressive hearing loss was defined as a change of >15 dB HL preoperatively comparing an audiogram from one year preoperatively to the audiogram taken directly prior to cochlear implantation. This dataset included recipients of different electrode arrays from each cochlear implant manufacturer. The initial portion of this study was a retrospective chart review of 105 subjects implanted over the last 10 years, who presented preoperatively with a LFPTA ≤ 70 dB HL. A subset of subjects implanted within the last 3 years with lateral wall electrodes was evaluated separately. Patient demographics can be found in Table 1.
Cochlear implantations for the second population of subjects (n = 45) were performed from December 2014 to December 2017 by the senior author. All of these subjects had at least 12 months of device use.
Cochlear implantation for the initial population of subjects (n = 105) was performed from 2007 to 2017 by multiple past and current surgeons of our cochlear implant team. A standard posterior tympanotomy approach was used in all cases. Cochleostomy or round window insertions were performed at the discretion of the operating surgeon.
In the second population of subjects (n = 45), cochlear implantation was performed using currently recommended hearing preservation techniques. Patients were given a one-time dose of IV decadron, 0.25 mg/kg, prior to the start of the case. Intratympanic dexamethasone was likewise administered (10 mg/ml). Routine postoperative steroids were not given. All patients had a posterior tympanotomy approach with a round window insertion. Insertion was performed slowly, typically over a 30 second period of time. Circumferential exposure of the round window annulus was performed to provide the greatest degree of freedom for electrode insertion angle. The round window insertion site was not packed, but had a “washer” of fascia placed at the round window opening following insertion. The 22 electrode Cochlear Nucleus® CI522 and CI422 arrays were inserted to a 20 mm depth, per the manufacturer recommendations for hearing preservation cases. For the MED-EL Flex-24, devices were inserted to the full extent up to 24 mm. Systemic antibiotics were administered before the start of every case and the cavity carefully irrigated with antibiotic saline after dissection and before electrode insertion.
A postoperative skull x-ray was taken in each case to verify implant position and determine depth of insertion. Depth of insertion was determined by assessing degrees of insertion on plain film x-ray. As this study was retrospective, computed tomography analysis of each pediatric subject was impractical.
Pure tone audiograms were performed preoperatively and postoperatively at an average of 2.4 years for the 105-subject group. Around 19 of these subjects had preoperative audiograms performed at <5 years of age. Children under the age of 7 were tested using conditioned play audiometry or visual reinforcement audiometry depending on the developmental abilities of the child. A two-tester setup is used routinely, with one audiologist performing testing and an assistant in the booth with the child. For the 45-subject lateral wall electrode only group, audiograms were performed preoperatively, at 1 month, at 3 months, 6 months, and 12 months following surgery. About 35 of the 45 subjects had data at both 6 and 12 months following surgery. LFPTA for each subject was computed as the average of results from 125, 250 and 500 Hz.
Multiple logistic regression analysis was performed for the first group using Statistical Package for the Social Sciences (SPSS, IBM 2015). The effect of preoperative LFPTA, side, stable or progressive hearing loss, array type (lateral wall or perimodiolar), round window or cochleostomy insertion, and the presence or absence of enlarged vestibular acqueduct was determined on change in LFPTA following cochlear implantation.
Preservation rates for the lateral wall electrode only group were determined utilizing two established criteria that enable the determination of functional hearing. The first of these was from the cochlear hybrid trial and was preservation of hearing <90 dB LFPTA (8). The second criteria were from a recently published paper that used preservation of hearing at 250 Hz <80 dB (9). Preservation rates were calculated for this second group as a whole and for each array. Depth of insertion for both of these devices was also computed and compared by unpaired T-test using Prism software.
Patient demographics for both the 105 subject all electrode group and the 45 subject lateral wall only groups are shown in Table 1.
Multiple logistic regression analysis of the 105-subject cohort implanted between 1997 and 2017 was performed. The effect of preoperative LFPTA, time after surgery, side of implant, stable versus progressive hearing loss, lateral wall versus perimodiolar electrode, and the presence or absence of enlarged vestibular aqueduct on change in LFPTA was determined. A significant effect in the model was demonstrated for preoperative LFPTA and method of insertion. Specifically, better preoperative hearing was associated with less change after surgery (Table 2). Round window insertion of electrodes was highly correlated with less change in low-frequency pure tone average (Table 2). None of the other variables reached statistical significance.
An evaluation of hearing preservation by device demonstrated differences in hearing preservation between devices, as well as in the case of the 20 mm depth, 22 electrode array, differences in preservation rates between progressive and stable hearing loss subjects (Table 3). As was suggested by the regression analysis, there is a clear difference in preservation rates between lateral wall electrodes inserted by round window and perimodiolar electrodes inserted by cochleostomy. With at least one manufacturer, increased depth of insertion appeared to be associated with reduced rates of preservation.
To further explore the effect of electrode array, hearing preservation within a second group of subjects was reviewed. This group underwent cochlear implantation with a lateral wall electrode with consistent hearing preservation surgical techniques, and with a single surgeon. Both systemic and intratympanic steroids were routinely used. Soft surgical technique including optimal round window exposure, soft insertion, and slow introduction was performed. This 45-subject lateral wall electrode only group was then evaluated for hearing preservation. Frequency specific changes over time were determined for each of the individual subjects. In addition, changes in LFPTA over time were also evaluated (Fig. 1). Mean LFPTAs were 46 dB HL (±19) preoperatively, 56 dB HL (±21) at 1 month after surgery, 56 dB HL (±22) at 3 months, 62 dB HL (±25) at 6 months, and 64 dB HL (±26) at 12 months after surgery.
The comparison between lateral wall electrodes demonstrated a significant difference in depth of insertion. Specifically, the 24 mm depth, 12 electrode array achieved an average insertion depth of 454° while the 20 mm depth, 22 electrode array achieved an average depth of insertion of 314°. This difference was highly significant (Fig. 2). Low-frequency hearing preservation rates using a 90 dB LFPTA threshold demonstrated a higher preservation rate with the 24 mm depth electrode array (100%) compared to the 20 mm depth electrode array (78%). The 24 mm depth electrode also maintained at least 80 dB at 250 Hz more consistently compared to the 20 mm depth, 22 electrode array (86% vs 63%).
Preserved acoustic hearing during cochlear implantation has led to substantial benefits in the adult population (1–3). Recent studies have suggested similar benefits in the pediatric population (5,6,10,11). We wished to determine the factors associated with hearing preservation in children and then identify the optimal outcomes achievable for pediatric subjects.
Our multiple logistic regression model determined that both preoperative LFPTA and method of implant insertion were correlated with less change in preoperative hearing (Table 2). The preoperative residual hearing has been demonstrated to be predictive of higher rates of hearing preservation in other studies in adults (12). This appears to be similar in children. A recent multivariate analysis of adult hearing preservation also suggested that device type, specifically lateral wall electrodes were associated with higher rates of preservation in adults (12). We did not see this as an isolated predictor of better hearing preservation, likely due to all lateral wall electrodes (with the exception of a few longer 28–31 mm lateral wall electrode) being inserted by a round window approach.
Surprisingly, progressive hearing loss was not predictive of a postoperative loss of residual hearing. There was, however a suggestion of a difference in pediatric subjects with progressive compared to stable hearing loss with the 22-electrode lateral wall array inserted to a 20 mm depth. We also did not see a correlation of presence of an enlarged vestibular aqueduct with changes in LFPTA. This is similar to outcomes reported for eight pediatric subjects with confirmed bi-allelic SLC26A4 mutations and an EVA on temporal bone computed tomogram. Their results demonstrated an average hearing deterioration from preoperative to post-operative of only 8.1 dB, which was maintained at follow-up at 18 months (13).
The second analysis of subjects who received a lateral wall array demonstrated high rates of hearing preservation. Overall, at 12 months after surgery, we saw hearing preservation of 82% at < 90 dB LFPTA and preservation of 68% at < 80 dB at 250 Hz. When separated between lateral wall electrode designs, preservation with the 22-electrode array inserted to a 20 mm depth had a preservation rate of 78% for LFPTA < 90 dB and 63% had maintenance of at least 80 dB at 250 Hz (Fig. 2). In contrast the 12-electrode array inserted to a 24 mm depth had a preservation rate of 100% for the LFPTA < 90 dB HL criteria at 12 months and 86% had maintenance of at least 80 dB at 250 Hz. These higher rates of preservation were achieved in spite of the fact that the 12-electrode array inserted to a 24 mm depth achieved a depth of insertion (454 degrees on average) significantly greater than the 22-electrode array inserted to only 20 mm (314 degrees on average) (Fig. 2). These preservation rates compare favorably with a recent study on pediatric hearing preservation, which found preservation of low-frequency hearing (LFPTA ≤ 85 dB) in 65% of pediatric patients that underwent implantation, albeit with a conventional full-length electrode (10).
The finding that higher hearing preservation rates are possible with a more deeply inserted electrode is compelling for two reasons. First, several studies have found adult patients that underwent hearing preservation surgery with a short electrode and subsequently lost low-frequency hearing, had significantly improved speech perception outcomes after being re-implanted with a full electrode array (14,15). With greater cochlear coverage, it is possible that subjects who ultimately lose low-frequency hearing would not require re-implantation with a deeper electrode. Second it suggests that length is not the only factor important in electrode design for hearing preservation. Thickness of the electrode as well as the density of metal in the electrode and the inherent stiffness it gives to the array may also be important factors.
The other component to hearing preservation, aside from electrode design, is the potential need for pharmacological treatment of the inner ear. Studies have demonstrated that the potential for hearing loss after cochlear implantation is not limited to the trauma of electrode insertion alone. Hearing can be lost in areas apical to the electrode and occur days to months after implantation suggesting that inflammatory changes as a result of implantation also play a significant role (16,17). The majority of the research has focused on the impact of glucocorticoids, often dexamethasone, on this inflammation. Glucocorticoid receptors present in hair cells, the spiral ligament and spiral ganglion neurons allow for steroids to inhibit apopotosis and down regulate pro-inflammatory cytokines (18). Another drug found to be effective in animal models to decrease implantation trauma is the apoptosis inhibitor AM111 (D-JNK-1 inhibitor). The other consideration is drug application, systemic versus local, and the mode of delivery. More recent research has focused on creating a steroid eluting electrode array (19).
Additional studies are needed to monitor pediatric hearing preservation long term. One recent study examined hearing preservation in adolescents with an average follow-up of two years and ten months, and demonstrated progression of hearing loss in three of their fifteen patients with initial postoperative successful hearing preservation. The increase in hearing thresholds was seen at 2 years 11 months, 2 years and 2 years 3 months postoperatively (11). Given patients’ young age and etiology of deafness, it is possible that if patients from this study are followed for a longer period that loss of residual hearing may be seen.
Our study does have the limitation of a retrospective data collection. Our results and conclusions are limited secondary to the retrospective nature of our data as there is less control over the variability of the surgery. Particularly in our larger group, with several surgeons participating, there may be variation between the surgical experiences of each patient. Our smaller group has the benefit of a single surgeon following a consistent surgical method. A prospective study would be ideal to continue to evaluate hearing preservation in the pediatric population.
This study contributes to the small, but growing data on pediatric hearing preservation supports the idea that hearing preservation can be consistently achieved in the pediatric population. We have demonstrated that mode of insertion coupled with electrode style are important factors in hearing preservation.
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