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Updated: Sep 23, 2025
Electrically Evoked Stapedius Reflex Measurements in Cochlear Implantation and Its Application in the Postoperative Fitting Process
Published on: June 21, 2024
Investigating the association of electrically-evoked compound action potential thresholds with inner-ear dimensions
Samuel Söderqvist1, Ville Sivonen1, Satu Lamminmäki1
1Department of Otorhinolaryngology - Head and Neck Surgery, Head and Neck Center, Helsinki University Hospital and University of Helsinki, Helsinki, Finland.
Insights
Larger cochlear size and bony cochlear nerve canal diameter are linked to higher electrically-evoked compound action potential (eCAP) thresholds in pediatric cochlear implant patients. This suggests neural excitability varies with inner ear anatomy.
Area of Science:
- Otolaryngology
- Neurosurgery
- Biomedical Engineering
Background:
- Narrow bony cochlear nerve canals (BNCN) and small cochlear nerves (CN) have been linked to increased electrically-evoked compound action potential (eCAP) thresholds.
- Larger cochleae may activate fewer spiral ganglion neurons with electrical stimulation.
- A detailed understanding of inner ear and internal auditory canal (IAC) dimensions' impact on eCAP thresholds is lacking.
Purpose of the Study:
- To investigate the relationship between inner ear and IAC anatomical dimensions and intraoperative eCAP thresholds in pediatric cochlear implant patients.
- To compare eCAP thresholds with cochlear diameter, bony cochlear nerve canal (BCNC) size, and internal auditory canal (IAC) and cochlear nerve (CN) dimensions.
Main Methods:
- 52 pediatric patients with congenital severe-to-profound hearing loss underwent bilateral cochlear implantation.
- Preoperative CT and MRI scans were used to measure cochlear, BCNC, IAC, and CN dimensions.
- Anatomical measurements were correlated with intraoperative eCAP thresholds.
Main Results:
- eCAP thresholds increased from the apical to basal direction.
- Higher eCAP thresholds were observed in larger cochleae and those with larger BCNC diameters.
- No significant effect of IAC or CN cross-sectional areas on eCAP thresholds was found.
- Significant correlations existed between cochlea and BCNC diameters, and between IAC and CN cross-sectional areas.
Conclusions:
- Higher stimulation levels are needed for neural responses in the basal cochlear turn.
- Increased eCAP thresholds correlate with larger cochlear diameter and BCNC diameter, but not with small BCNC or CN size.
- BCNC diameter is significantly correlated with cochlear diameter, highlighting its importance in neural response.
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.
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