Outcomes of Cochlear Implantation in Mumps-Induced Single-Sided Deafness: A Retrospective Analysis

Introduction

Mumps is a disease caused by a virus from the Paramyxoviridae family, which includes viruses such as measles virus and respiratory syncytial virus (RSV) [1]. The disease is primarily transmitted through respiratory droplets, making it highly contagious. The virus’s incubation period is approximately 2 weeks, after which the first clinical symptoms appear, such as characteristic swelling of the parotid glands, fever, and muscle pain [2]. Mumps usually has a mild course but can lead to serious complications. The most common complications include: meningitis, pancreatitis, orchitis, and aseptic encephalitis [3,4]. While mumps was once a common childhood illness, the introduction of preventive immunization programs, particularly the mandatory administration of the measles–mumps–rubella (MMR) vaccine, has significantly reduced its incidence in developed countries [5–8].

One of the more serious, although relatively rare (approximately 1 in 1000 cases) [9,10], complications of mumps is sensorineural hearing loss (SNHL), which may be permanent or temporary. In the majority of cases, SNHL is unilateral, although bilateral involvement has been reported in up to 6% of cases [11–14]. This condition results from damage to inner ear structures, primarily the cochlea. Histopathological studies have demonstrated that the damage is largely confined to the cochlear endolymphatic compartment, with the basal turn being most severely affected. A hematogenous route of infection is postulated [15,16], leading to degeneration of the stria vascularis, and consequently also of the organ of Corti and the tectorial membrane. The extent of damage diminishes progressively toward the apex. Despite these structural changes, the spiral ganglion and a substantial portion of the cochlear nerve in the middle and apical turns seem to remain preserved, suggesting that central auditory pathways may be functionally intact [17,18]. Mumps virus together with cytomegalovirus (CMV) represent the most common viral etiologies of sensorineural hearing loss (SNHL). Both pathogens are well-documented causes of auditory impairment, with CMV recognized as the leading cause of congenital viral hearing loss and mumps as a predominant cause of acquired viral hearing loss in children [19,20].

The choice of treatment for patients with SNHL largely depends on the severity of the impairment. Individuals with mild to moderate hearing loss often experience satisfactory outcomes with conventional hearing aids, which amplify sound and improve auditory perception. However, when the degree of hearing loss is severe to profound, traditional amplification frequently fails to provide sufficient benefit, leaving patients with significant communication challenges. In such cases, cochlear implantation emerges as a viable and effective intervention, as it bypasses the damaged cochlear structures and directly stimulates the auditory nerve, thereby restoring access to sound and improving speech understanding. Consequently, early assessment and timely consideration of cochlear implantation are essential to optimize auditory rehabilitation and quality of life in these patients [21,22]. Therefore, this retrospective study, which included 14 patients with unilateral SNHL and a history of mumps, aimed to evaluate outcomes from audiometry testing before and 1 year after cochlear implantation surgery

Material and Methods

INCLUSION CRITERIA:

The present retrospective study was approved by the ethics committee (IFPS: KB/Statement No. 2/2023) and was conducted in accordance with the Helsinki Declaration. All patients gave their consent for their anonymized data to be used for research purposes. Patients with single-sided deafness caused by mumps were qualified for cochlear implantation surgery. The inclusion criteria included: (1) average hearing threshold for frequencies of 0.5, 1, 2, and 4 kHz of 90 dB hearing level (HL) or worse, and bone conduction thresholds at the limit of the audiometer’s capabilities; (2) word-recognition score with hearing aids below 50%; (3) lack of benefit from conventional hearing aids; (4) hearing loss due to congenital mumps virus.

STUDY GROUP:

The study included 14 patients: 10 female (71.4%) and 4 male (28.6%). All patients had unilateral deafness following mumps. Each patient received a cochlear implant to enable hearing on both sides. At the time of surgery, patients were aged between 20 and 62 years (M=39.4; SD=12.7). Some patients experienced additional difficulties besides hearing loss such as dizziness (n=4, 28.6%), tinnitus in the deaf ear (n=8, 57.2%), and tinnitus in the opposite ear (n=8, 57.2%).

The detailed distribution of outcomes is shown in Table 1.

AUDIOMETRIC EVALUATION:

Before the surgery, patients underwent a series of diagnostic tests. The first test was tonal audiometry for both air and bone conduction in the frequency range of 125–8000 Hz. The test was repeated 12 months after activation to assess the preservation of residual hearing. Preoperatively, each patient underwent speech audiometry without devices and free-field speech audiometry with a fitted hearing aid, to assess the possibility of compensating for the hearing loss using conventional methods. Speech material was presented at 70 dB HL. The result of the test was the percentage of correctly repeated words, designated as the word-recognition score.

Twelve months after the surgery, a similar test was conducted when the patient had an active implant during the test. The speech material was presented at 65 dB HL [23,24].

SURGICAL PROCEDURE:

In each patient’s ear for which cochlear implantation was planned, the procedure was conducted using the 6-steps Skarżyński method. The intention was to insert electrodes through the round window approach, using different electrodes which were selected individually for each patient, depending on anatomical conditions.. Steroids were administered routinely in every case: 0.1 mg/kg/day dexamethasone intravenously in 2 doses per day for 3 days beginning when surgery started [25,26].

The steps of the 6-steps Skarżyński method [27,28] were:

Regarding specific surgical findings, there is an important factor related to round window niche. In comparison with the typical middle ear, in patients after mumps there is a tendency to a more ossified round window niche, and due to that, the approach often requires drilling out bone overhang.

STATISTICAL ANALYSIS:

Data were analyzed using descriptive statistics, with results expressed as means and standard deviations for continuous variables and percentages for categorical variables. Comparisons between preoperative and postoperative outcomes, including hearing thresholds and speech discrimination scores, were performed using the Wilcoxon signed-rank test for paired samples. A P-value of <0.05 was considered statistically significant. We used IBM SPSS Statistics 25 for the analysis.

Results

COCHLEAR IMPLANTATION:

Implantation was performed according to established procedures. The procedure was carried out on the right ear in 28.6% of cases (n=4) and on the left ear in 71.4% of cases (n=10). The mean time from hearing loss to implantation was 28.1 years (SD=9.6). A round window tympanotomy approach was used in all patients. The implant and processor model as well as the type of electrode were selected by the surgeon on an individual basis according to the patient’s anatomical conditions after analyzing the computed tomography (CT) scan and reviewing the patient’s medical history and test results. Cochlear implantation data are presented in Table 2.

Most of the cochlear implant surgeries were successful with no adverse events. Surgeon-reported adverse events during surgery included 1 severe bleeding event and 1 difficult electrode insertion. Both adverse events were resolved. The bleeding was stopped, and the difficulties with inserting the electrode were overcome, with the surgeon successfully inserting it into the cochlea.

AUDIOMETRIC OUTCOMES:

All patients had profound hearing loss on the operated ear. Their average air conduction threshold was at least 82.5 dB. For the purpose of the study, the response deficits were replaced by the maximum audiometer output for each frequency. Each subject had normal hearing in the contralateral ear. After surgery, hearing thresholds were slightly higher in the operated ear, while there was no significant change in mean air conduction hearing thresholds. A comparison of preoperative and postoperative hearing thresholds is shown in Figure 1.

The average preoperative hearing threshold, averaged across the 7 frequencies from 0.125 to 8 kHz was 100.98 dB HL (SD=11.57), while the average postoperative hearing threshold was 106.02 dB HL (SD=5.82). The average change was 5.04 dB and was statistically significant (Z=−2,6 P=0.009).

SPEECH PERCEPTION OUTCOMES:

Pre-operatively, the speech discrimination score measured with headphones was 0% for the majority of patients (n=12), 5% for 1 patient, and 10% for another. The free-field speech discrimination percentage 12 months after surgery was 48.9% without an active processor and 66.8% with an active processor (Figure 2).

The hearing preservation (HP) levels were then divided into 4 categories: (1) no measurable hearing (full loss of hearing); (2) minimal HP (0–25%); (3) partial HP (>25–75%); and (4) complete HP (>75%) [29].The minimum preservation of residual hearing was 4.3, and the maximum 96.6, with a mean of 39.2. Two patients who had total deafness preoperatively (ie, no residual hearing) were not included in this analysis. Data on HP, categorized into minimal, partial, and total, are presented in Table 3. Most of the patients had partially preserved hearing (58%).

Discussion

LIMITATIONS:

A limitation of the study was the small group of patients. The number of patients studied was related to strict inclusion criteria. Due to the MMR vaccine in Poland, mumps is a relatively rare disease, so the study was retrospective (the authors could only use the patients’ medical histories).

Conclusions

Looking ahead, further research is needed to better understand the long-term outcomes of cochlear implantation in mumps-induced hearing loss. Large-scale, multicenter studies could help identify predictors of success, refine patient selection criteria, and optimize surgical and rehabilitative strategies.