Clin Exp Otorhinolaryngol.  2021 Aug;14(3):287-294. 10.21053/ceo.2021.00752.

Potential Implications of Slim Modiolar Electrodes for Severely Malformed Cochleae: A Comparison With the Straight Array With Circumferential Electrodes

• Lee SY
• Choi BY

• KMID: 2519183
• DOI: http://doi.org/10.21053/ceo.2021.00752

Abstract

Objectives
. Malformations of the inner ear account for approximately 20% of cases of congenital deafness. In current practice, straight arrays with circumferential electrodes (i.e., full-banded electrodes) are widely used in severely malformed cochleae. However, the unpredictability of the location of residual spiral ganglion neurons in such malformations argues against obligatorily using full-banded electrodes in all cases. Here, we present our experience of electrically evoked compound action potential (ECAP) and radiography-based selection of an appropriate electrode for severely malformed cochleae.
Methods
. Three patients with severely malformed cochleae, showing cochlear hypoplasia type II (CH-II), incomplete partition type I (IP-I), and cochlear aplasia with a dilated vestibule (CADV), respectively, were included, and the cochlear nerve deficiency (CND) was evaluated. A full-banded electrode (CI24RE(ST)) and slim modiolar electrode (CI632) were alternately inserted to compare ECAP responses and electrode position.
Results
. In patient 1 (CH-II with CND), who had initially undergone cochlear implantation (CI) using a lateral wall electrode (CI422), revision CI was performed due to incomplete insertion of CI422 and resultant unsatisfactory performance by explanting the CI422 and re-inserting the CI24RE(ST) and CI632 sequentially. Although both electrodes elicited reliable ECAP responses with correct positioning, CI24RE(ST) showed overall lower ECAP thresholds compared to CI632; thus, CI24RE(ST) was selected. In patient 2 (IP-I with CND), CI632 elicited superior ECAP responses relative to CI24RE(ST), with correct positioning of the electrode; CI632 was chosen. In patient 3 (CADV), CI632 did not elicit an ECAP response, while meaningful ECAP responses were obtained with the CI24RE(ST) array once correct positioning was achieved. All patients’ auditory performance markedly improved postoperatively.
Conclusion
. The ECAP and radiography-based strategy to identify an appropriate electrode may be useful for severely malformed cochleae, leading to enhanced functional outcomes. The practice of sticking to full-banded straight electrodes may not always be optimal for IP-I and CH-II.

Keyword

Cochlear Implantation; Cochlea; Congenital Abnormalities; Electrodes

Figure

• 

  Fig. 1. Radiological demonstration and electrically evoked compound action potential (ECAP) threshold-based selection of the electrode in the case (case 1) with cochlear hypoplasia type II (CH-II). (A) Schematic illustration showing CH-II (C, cochlea; V, vestibule). Temporal bone computed tomography (CT) displays that the cochlea affected by CH-II has smaller dimensions with defective modiolus and interscalar septa, but a normal external outline. On parasagittal oblique magnetic resonance (MR) images, cochlear nerve (CN) deficiency was observed. Based on reformatted high-resolution temporal bone CT images using multiplanar reconstruction algorithm (i.e., cochlear view), the previously implanted conventional lateral straight electrode (CI422) was incompletely inserted. (B) Initially, ECAP threshold responses were achieved only from four electrodes among the entire CI422 electrode array that had been incompletely inserted. ECAP thresholds and electrode array position on post-insertion radiographs were evaluated for both CI24RE(ST) and CI632. (C) Final selection of the electrode to be inserted for this case. Lt, left; NRT, neural response telemetry.

• 

  Fig. 2. Radiological demonstration and electrically evoked compound action potential (ECAP) threshold-based selection of the electrode in the case (case 2) with incomplete partition type I (IP-I). (A) Schematic illustration showing the IP-I anomaly (C, cochlea; V, vestibule). Temporal bone computed tomography (CT) shows a clear differentiation between the cochlea and vestibule, demonstrating the cochlea without a modiolus and interscalar septa, accompanied by a dilated vestibule. On parasagittal oblique magnetic resonance (MR) images, the cochlear nerve (CN) may be deficient on both ears. (B) Sequential trials for selecting ideal electrode: comparison of ECAP thresholds and electrode array position on post-insertion radiographs between the CI24RE(ST) and CI632. (C) Final selection of the electrode to be inserted for this case. Rt, right; Lt, left; NRT, neural response telemetry.

• 

  Fig. 3. Radiological demonstration and electrically evoked compound action potential (ECAP) threshold-based selection of the electrode in the case (case 3) with cochlear aplasia with a dilated vestibule (CADV; right [Rt] ear) and incomplete partition type I (IP-I; left [Lt] ear). (A) Schematic illustration showing (CADV, Rt ear) and IP-I (Lt ear) (C indicates cochlea; V indicates vestibule). Temporal bone computed tomography (CT) exhibits a vestibular dilatation (Rt ear) located at the posterolateral part of the fundus, representing CADV. The cochlear nerve (CN) is absent on parasagittal oblique magnetic resonance (MR) images, but notably, the cochleovestibular nerve entering a dilated vestibule was observed on heavily T2-weighted axial images. (B) Sequential trials for selecting the ideal electrode: Comparison of ECAP thresholds and electrode array position on post-insertion radiographs between the CI24RE(ST) and CI632. (C, D) Final selection of the electrode to be inserted for this case. NRT, neural response telemetry.

• 

  Fig. 4. A schematic diagram of repositioning the full-banded straight electrode array in the cochlear aplasia with a dilated vestibule. (A) Insertion direction parallel to the lateral wall via a labyrinthotomy approach. (B) Insertion direction perpendicular to the lateral wall via a labyrinthotomy approach.

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