Miniature cochlea: a study of radiological measurements and its implications during the cochlear implant surgery

Abstract

En Abstract Hypothesis Approximately 25% of patients with congenital sensorineural or mixed hearing loss show bony inner ear malformations on computed tomography (CT) of the temporal bone, with significantly smaller cochlear height (CH) compared with normal-hearing patients. The miniature cochlea has an apparently normal radiological appearance and could be missed if proper measurements are not taken. Inner ear measurements not only aid in the duplication of radiologically diagnosed inner ear malformation but can also provide additional information about which specific part is abnormal. Aims The aims of the present study were to establish the normal measurements of the height and width of fully developed Egyptian cochleae using high-resolution CT scans of the temporal bone in normal individuals, and to predict the characteristic radiologic sings and measurements of miniature cochlea and its implication during cochlear implant surgery. Study design The authors conducted a prospective, comparative study. Patients and methods The study comprised three groups: the control group, which comprised 50 cochleae with normal hearing and negative history for head trauma or ear surgery; the patients group, which comprised candidates for cochlear implanting with a negative history for meningitis or head trauma, and was further subclassified into group A, which comprised 22 cochleae with CH small but not less than 2 SD from the controls, and group B, which comprised nine cochleae with CH less than 2 SD from the controls. High-resolution 64-slice CT scan of 1 mm slice thickness was obtained; in addition, CH, basal turn height and width, upper turn height and width, and oval window–round window distance were estimated according to well-established definitions in the literature. Results CH in the control group ranged between 4.8 and 6.9 mm, with a mean of 6.1 ± 0.29 mm, whereas, in group A, CH showed a mean of 5.9 ± 0.34 mm, which was statistically significant compared with the control group. In group B, CH showed a mean of 3.8 ± 0.31 mm, which was also statistically significant compared with the other two groups. The basal turn width, upper turn width, and oval window to the round window distance–all showed statistically significant difference when compared within the three groups. In contrast, both basal turn height and upper turn height showed no significant difference when compared within the three groups. Conclusion In cochlear implant surgery, the detailed and good radiological analysis aids in a safe, effective, and well-planned surgery. The development of standardized measurements to complement visual inspection improved the diagnostic accuracy and helped in the subclassification of hypoplastic cochleae. We found CH to be the most essential differentiating measurement in the subclassification and diagnosis of hypoplastic and dwarf cochleae. If dwarf cochlea is radiologically diagnosed, special surgical modifications regarding electrode length and site of cochleostomy during cochlear implanting should be applied.

Keywords

• cochlear height
• cochlear implant
• cochlear measurements
• dwarf cochlea
• hypoplastic cochlea
• miniature cochlea