Simpler and effective radiological evaluations for modiolar proximity of a slim modiolar cochlear implant electrode

Two Discrete Types of Tip Fold-Over in Cochlear Implantation Using Slim Modiolar Electrodes: Influence of Cochlear Duct Length on Tip Fold-Over

OBJECTIVE

Precise electrode positioning is crucial for achieving optimal audiological outcomes in cochlear implantation. The slim modiolar electrode (SME), a thin, flexible, and precurved electrode, exhibits favorable modiolar proximity. However, tip fold-over can affect optimal electrode placement. Herein, we share our experiences with tip fold-over in SMEs and present an analysis of conditions that may predispose to tip fold-over.

STUDY DESIGN

Retrospective medical record review.

PATIENTS

In total, 475 patients (671 ears) underwent cochlear implantation using SMEs (Nucleus CI532 or CI632 from Cochlear) performed by a single surgeon at a tertiary center between June 14, 2018, and December 1, 2022.

INTERVENTIONS

Intraoperative x-ray scans (cochlear view), operative records, and cochlear duct length (CDL) were reviewed.

MAIN OUTCOME MEASURES

Tip fold-over patterns on plain x-ray images (proximal versus distal).

RESULTS

Electrode tip fold-over was observed in 18 (2.7%) of the 671 ears with SMEs. This fold-over occurred more frequently in cases with long CDL (>36 mm). Among the 14 cases with available initial x-rays before correction of the tip fold-over, half were classified as proximal and the other half as distal. A predilection for proximal tip fold-over was found in those with a CDL of 36 mm or longer, and longer CDLs were observed for proximal cases than for distal cases. Our pilot data suggest that identifying the type of tip fold-over can aid in correcting it more efficiently.

CONCLUSIONS

Tip fold-over of SME does not occur uniformly and is more common in ears with long CDL. This tendency is particularly pronounced for the proximal type of tip fold-over. Therefore, preoperative measurement of the CDL and meticulous examination of intraoperative imaging are essential for customized correction.

Effects of in vivo repositioning of slim modiolar electrodes on electrical thresholds and speech perception

The slim modiolar electrode has been reported to ensure better modiolar proximity than previous conventional perimodiolar electrodes and consistently high scala tympani localization. Nonetheless, variability in modiolar proximity exists even among slim modiolar electrodes, still leaving room for further improvement of modiolar proximity, which may positively affect functional outcomes. Given this, the pull-back maneuver was reported to increase the modiolar proximity of slim modiolar electrodes in a cadaveric study, but in vivo repositioning effects remain to be established. Here we identified that the pull-back maneuver led to better modiolar proximity than conventional insertion while maintaining a similar angular insertion depth. Notably, the reduced electrode-modiolus distance from the pull-back maneuver was associated with significantly lower impedances across electrodes postoperatively as well as reduced intraoperative electrophysiological thresholds than conventional insertion. Among adult cochlear implant recipients, this maneuver resulted in significantly better sentence recognition scores at three months postoperatively when compared to those with a conventional insertion; however, this benefit was not observed at later intervals. Collectively, slim modiolar electrodes with the pull-back maneuver further enhance the modiolar proximity, possibly leading to better open-set sentence recognition, at least in the early postoperative stage.

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

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.