Diffusion tensor imaging of the subcortical auditory tract in subjects with congenital cochlear nerve deficiency

Cochlear nerve deficiency (CND), including aplasia and hypoplasia, is a rare cause of congenital hearing loss. The purpose of this study was to evaluate the neuronal integrity of the subcortical auditory pathway in subjects with CND by diffusion tensor MR imaging (DTI).

Twelve subjects with unilateral congenital CND were included in this prospective study. Twelve normal-hearing subjects served as controls. DTIs were acquired from these subjects on a 3T MR imaging scanner. Several indices including axial diffusivity (lambda( ||)), radial diffusivity (lambda( perpendicular)), mean diffusivity (MD), and fractional anisotropy (FA) along the auditory pathway were extracted. Two regions of interest were selected bilaterally for evaluation: the lateral lemniscus (LL) and inferior colliculus (IC). The mean values of lambda( ||), lambda( perpendicular), MD, and FA at both regions of interest were compared between the ipsilateral and contralateral sides of subjects with CND with those in the control group.

Significant decrease of FA and increase of MD on both the ipsilateral and contralateral sides of patients with CND as compared with normal subjects in both LL and IC were found. The decrease of FA was due to the increase of lambda( perpendicular), whereas lambda( ||) remained unchanged.

The maintained axial diffusion implied that the orientation of neural fibers along the subcortical auditory pathway in subjects with unilateral CND is intact. However, the reduction in FA and increase in lambda( perpendicular) on both the ipsilateral and contralateral sides might be attributed to axonal loss and/or demyelination of the subcortical auditory tract in these subjects.