Cochlear implant electrode arrays are engineered to optimize auditory outcomes while minimizing intracochlear trauma. This includes proximity to neural structures, atraumatic insertion and explantation, and minimal cochlear damage with preservation of residual hearing. Three widely accepted goals for the development of CI electrode arrays are deep insertion into the scala tympani (ST) to access lower frequency cochlear neurons; greater operating efficiency by reducing the stimulus charge required to produce a comfortable loudness level; and reduced intracochlear damage during insertion [1]-[4]. Adequate cochlear coverage is believed to be crucial in the final auditory outcomes. Straight lateral wall (LW) electrodes are believed to stimulate the nerve fiber endings at the organ of Corti, whereas pre-curved perimodiolar (PM) electrode arrays are believed to stimulate the spiral ganglion cells [2]. Straight electrodes are claimed to cause less damage, less scalar translocation, and allow for deeper cochlear penetration, and may provide more adequate coverage. They are also easier to explain [5][6]. PM electrodes are designed to be closer to their hypothetical target, the spiral ganglion cells, and thus provide adequate coverage. Being closer to their target means less energy consumption and increased efficiency. However, they are sometimes difficult to insert due to anatomical restraints [7][8].

Whether these technical differences significantly affect the functional outcome is still unclear.

The aim of this study is to study a series of pediatric patients implanted with different electrodes and report on their short-term outcomes.

This was a prospective observational cohort study, conducted at two tertiary referral centers between January 2022 and January 2023. Children with bilateral congenital profound sensorineural hearing loss who underwent unilateral cochlear implantation were included. Patients with inner ear malformations, post-meningitic deafness, or revision surgeries were excluded. A variety of electrodes were used [Medel Flex series; Cochlear CI 512, CI 522; AB Slim J, MS, and Oticon Evo]. The choice of electrode depended on the availability, the surgeon’s or patient’s choice, or on specific anatomical considerations (mastoid pneumatization, cochlear size, cochlear orientation). Patients were operated through a standard mastoidectomy-posterior tympanotomy approach and a round window or extended round window electrode insertion. Intraoperative impedance and measurements were routinely performed in all patients. All patients were evaluated by pure tone audiometry (PTA) and word discrimination threshold (WDT) 6 months after first fitting.

The study included 259 patients,120 females, and 139 males. The mean age was 33.4 months (11 - 48, ±8.719). There were 116 MED-EL^TM Flex series, 86 of the Cochlear^TM corporation (38 Slim straight CI 522^TM and 48 Contour advance CI512^TM), 44 of the Advanced Bionics^TM corporation (25 Slim J^TM and 19 Midscala MS^TM), and 13 for the Oticon company (Neuro 2, Evo^TM electrode). The age and sex distribution were proportionally equal between both LW and PM electrodes (LW: 98 males, 84 females; PM: 41 males, 26 females).

Surgical problems and completeness of insertion were comparable. Incomplete insertion was more frequent with Medel Flex 28 electrodes (2 cases with one extracochlear contact point), one Cochlear Contour Advance was misplaced and had to be replaced, and one MS of Advanced Bionics had to be repositioned.

Audiological Results:

The mean PT threshold for all electrodes was 32.521 (20 - 45 ± 6.894), and the mean WDT was 63.766 (40 - 85 ± 15.053). At that stage, APAL scores were not reported due to the wide disparity of results.

We then calculated the results for each brand and compared them to the general mean. The second step was to compare different types of electrodes within each brand. The third step consisted of comparing the results between all LW and PM electrodes. Finally, all electrodes were compared.

The ultimate goal of a cochlear implant is to achieve adequate auditory function with structural preservation of all intracochlear structures. Many variables are at play, electrode array insertion depth, cochlear coverage, tonotopicity matching, spread of excitation, electrode modiolus distance, scalar dislocation, atraumatic array insertion, and the proper choice of the array that matches the particular anatomy of each individual [2][10][11]. Various electrode designs were devised and are either flexible following the lateral wall of the cochlea [LW electrodes] or preformed hugging the modiolus [PM]. Each design has its advantages and disadvantages regarding frequency matching, hearing results, energy consumption, and intracochlear trauma [7][12]-[14]. Electrical stimulation is best provided by perimodiolar electrodes, while straight electrodes are less traumatic and may be better in residual hearing preservation [7].

In spite of theoretical and technical differences, there doesn’t seem to be any functional difference between the two designs [15][16]. A number of studies were conducted to compare the functional outcomes between the two. Speech perception outcomes for patients who received a lateral wall or perimodiolar electrode array (Slim Straight and Contour) were found to be similar at 3 or 9 months after implant activation. Similarly, in a study comparing the Hifocus 1 and the 1 J electrodes (AB corporation), the authors did not find any demonstrable influence on performance [17]-[19].

A negative correlation was observed between electrode-modiolus distance (EMD) at the cochlear base and monosyllabic word discrimination 6 months after implantation. However, this difference disappeared after 12 months [11]. PM arrays appear to offer superior and faster speech perception and superior melody perception during the first 6 months after implantation, but the differences do not persist in the long term, with performance equalizing between groups by 24 months [20]-[24].

A mismatch was observed between the predicted frequency and the default frequency provided by every electrode for all electrode arrays. The pitch mismatch was smaller for the perimodiolar electrode array than for the lateral wall electrode array [25][26].

Superior audiologic outcomes are observed for electrode arrays inserted entirely within the scala tympani [27][28]. Scalar translocation and tip fold-overs occurred more frequently with perimodiolar arrays than with lateral wall arrays [29]. Perforation of the SL, in contrast, predominantly occurred with straight electrodes and was not associated with localized ossification [30]. However, a meta-analysis suggested that there is no significant correlation between hearing preservation and either insertion depth or scala position [31]-[33]. Similar results were found in patients with inner ear malformations and in hearing preservation surgeries [34]-[39].

Most studies report adult outcomes. Our study was carried out exclusively in pediatric patients. A wide variety of electrodes from different manufacturers were used. This allows greater versatility in the comparison between suggested performances and programming strategies. The choice of electrode depended on availability, surgeon choice, patient choice, or anatomical restraints. This may be a minor limitation in the true intergroup comparisons. We included 259 congenitally deaf patients with a mean age of 33.4 months. There were 192 LW electrodes and 67 PM electrodes. For LW electrodes, the mean PTA was 33.01 dB and 32.02 dB for PM electrodes. Regarding the WDT, the mean value for LW electrodes was 64.02 dB whereas it was 62.72 dB for the preformed ones. These values were not statistically significant. Electrode-wide comparison did not reveal any statistically significant difference in PTA or WDT. A study on pediatric patients with malformations did not reveal any significant difference between both types of electrodes [40], whereas another study suggested that PM electrodes may have offered a more predictable and consistent programming [41].

These findings in congenitally profoundly deaf children are comparable to results reported in adults [11][20]-[23]. We chose the 6-month benchmark as some studies have shown that the most significant gains occur from 3 to 6 months with more gradual progress over the next 2 years [42][43]. Long-term follow-up in the same cohort is ongoing to determine if there will be a difference in long-term outcomes or not. Additionally, an age-stratified outcome analysis will be performed to complement the results at the end of the intended extended evaluation period.

In our cohort of pediatric congenitally deaf patients, different electrode designs did not show any short-term difference in PTA thresholds or speech discrimination scores. Each of the electrode designs has its pros and cons, but for the time being, none seems to offer a significant functional advantage. The future designs should aim at taking advantage of each to achieve more optimal results, both on the anatomical and functional levels [2][15][44].

This study protocol was reviewed and approved by the Ain-Shams University IRB [ASUMed-4932-24]. Written informed consent for the study was not required as it did not involve any change in the preplanned surgical intervention. Written informed consent was obtained from participants (or their parent/legal guardian/next of kin) for the surgery, including the type of device, which was predetermined during counselling.

Badr Eldin Mostafa: Inception, surgeries, data collection, writing.

Lobna El Fiky: Inception, surgery, data collection.

Naglaa Nasser: Audiological evaluation.

Ahmed Mostafa: Surgery, data collection.

Abir Omara: Audiological evaluation.