The bony cochlear nerve canal in children with absent or hypoplastic cochlear nerves

Abstract

Objective To correlate presence and size of the bony cochlear nerve canal [BCNC] with size of the internal auditory meatus [IAM] on CT in children with absent or hypoplastic cochlear nerves [CNs] as compared to age matched controls. Methods This retrospective case-notes review was based in the departments of Cochlear Implantation and Neuroradiology at a tertiary paediatric hospital. Twenty-five ears of fifteen children (subjects) with profound sensorineural deafness (SND) and absent or hypoplastic CN on MRI scan were compared to age matched controls. Two groups of controls were included; a control group of nineteen ears of twelve children with normal hearing or conductive hearing loss [control group 1] and a second control group of twenty one ears of eleven children with severe to profound hearing loss related to GJB2 mutations [control group 2]. Both control groups had evidence of the presence of the CN. Two neuroradiologists independently assessed presence and size of BCNC and IAM on CT and presence of CN on MRI in subjects and controls. The BCNC and IAM size was compared between subjects and both control groups. The presence of BCNC was correlated with the IAM size on CT, presence/absence of CN on MRI and audiological evaluation in subjects. Results The mean IAM width was significantly smaller in subjects as compared to controls. The BCNC was absent in 17/25 subject ears and present in all control ears. Absent BCNC correlated with a narrow IAM in 13/17 subject ears. Presence of the BCNC supported presence of a CN although this was not seen on MRI. However, BCNC absence may be associated with presence of a CN as was seen in two subject ears. Five subject ears out of 22 [22%] with absent CN on MRI had other evidence of a present cochlear nerve. Conclusions BCNC is an additional parameter to assess presence of the cochlear branch of the CN. Presence of the BCNC may indicate cochlear nerve presence. Caution should be used in assessing candidacy of cochlear implants based on MRI alone and a combination of imaging and audiological tests should be used to assess presence of the CN.