Clinical characteristics of patients with narrow bony cochlear nerve canal: is the bilateral case just a duplicate of the unilateral case?

OBJECTIVES/HYPOTHESIS To evaluate whether there is a genetic contribution to the development of narrow bony cochlear nerve canal and analyze the differences between unilateral and bilateral narrow bony cochlear nerve canal. STUDY DESIGN Retrospective review. METHODS Patients diagnosed with unilateral (n=44) or bilateral (n=13) sensorineural hearing loss with narrow bony cochlear nerve canals from October 2004 to July 2011 were evaluated. The segregation ratio of hearing loss among siblings of probands was calculated. The differences in segregation ratio and other clinical characteristics were also compared. RESULTS The segregation ratio of hearing loss in siblings of patients with unilateral narrow bony cochlear nerve canal was close to zero, whereas the ratio in siblings of patients with bilateral canals was 20%. Internal auditory canal narrowing was significantly more frequently combined in unilateral than bilateral narrow bony cochlear nerve canal cases. Cochlear nerve deficiency based on magnetic resonance imaging was found in 63.6% of bilateral narrow bony cochlear nerve canal cases. Milder hearing loss was exclusively detected in bilateral cases. CONCLUSIONS Narrow bony cochlear nerve canal is not likely to have a simple Mendelian inheritance pattern, especially in unilateral cases. The contribution of a Mendelian genetic etiology to bilateral narrow bony cochlear nerve canals remains elusive. More frequent coexistence of internal auditory canal narrowing among unilateral narrow bony cochlear nerve canal cases suggests that bilateral narrow bony cochlear nerve canals are not just a duplicate of the unilateral narrow bony cochlear nerve canal.