Can radiological measurements of cochlear nerve parameters predict cochlear implant outcome? Our experience in 87 ears

Investigating the association of electrically-evoked compound action potential thresholds with inner-ear dimensions in pediatric cochlear implantation

OBJECTIVES

A narrow bony cochlear nerve canal (BCNC), as well as a hypoplastic and aplastic cochlear nerve (CN) have been associated with increased electrically-evoked compound action potential (eCAP) thresholds in some studies, suggesting poorer neural excitability in cochlear implantation. Also, in large cochleae the extent of activated spiral ganglion neurons with electrical stimulation is less than in smaller ones. However, a detailed description of the relationship between eCAP thresholds for a lateral-wall electrode array and dimensions of the inner-ear structures and internal auditory canal (IAC) is missing.

DESIGN

The study subjects were 52 pediatric patients with congenital severe-to-profound hearing loss (27 females and 25 males; ages 0.7-2.0 years; 1.0 ± 0.3 years, mean ± SD) implanted bilaterally with Cochlear Nucleus CI422, CI522, or CI622 implants with full insertion of the Slim Straight electrode array. Diameters of the cochlea and the BCNC as well as the widths and heights of the IAC and the CN were evaluated from preoperative computed tomography and magnetic resonance images. These anatomical dimensions were compared with each other and with the patients' intraoperative eCAP thresholds.

RESULTS

The eCAP thresholds increased from the apical to basal direction (r = 0.89, p < 0.001). After sorting the cochleae into four size categories, higher eCAP thresholds were found in larger than in smaller cochleae (p < 0.001). With similar categorization, the eCAP thresholds were higher in cochleae with a larger BCNC than in cochleae with a smaller BCNC (p < 0.001). Neither IAC nor CN cross-sectional areas affected the eCAP thresholds. Correlations were found between cochlea and BCNC diameters and between IAC and CN cross-sectional areas (r = 0.39 and r = 0.48, respectively, p < 0.001 for both).

CONCLUSIONS

In the basal part of the electrode array, higher stimulation levels to elicit measurable neural responses (eCAP thresholds) were required than in the apical part. Increased eCAP thresholds associated with a larger cochlear diameter, but contrary to the earlier studies, not with a small size of the BCNC or the CN. Instead, the BCNC diameter correlated significantly with the cochlea diameter.

Imaging and audiological features of children with cochlear nerve deficiency

Objective: Cochlear nerve deficiency (CND) accounts for 10-19% of hearing loss in children; this study investigated the imaging and audiological features of 25 CND children. Methods: A total of 563 children with an unpassed automatic auditory brainstem response were diagnosed with hearing loss in our department between December 2018 and December 2021, of which, the imaging and audiological features of 25 children (25/563, 4.4%) diagnosed with a CND were reviewed. Results: Twenty-one (21/25, 84.0%) CND children had unilateral deafness, and 4 cases of bilateral deafness. All deaf ears were diagnosed as severe hearing loss due to an auditory brainstem response. CM waves were recorded in 8 cases and DPOAE in 3 cases, suggesting the audiological characteristics of auditory neuropathy spectrum disorders (ANSD). MRI results indicated 23 cases had small cochlear nerves and 2 cases had absent cochlear nerves. No genetic mutations were identified in the 25 CND children. Conclusions: Most CND children had unilateral hearing loss; therefore, high-resolution MRI imaging of the internal auditory canal should be performed to detect the auditory nerve in children with severe hearing loss. Some CND children had characteristics of ANSD.