gms | German Medical Science

Objective: Vestibular schwannomas cause progressive hearing loss by direct damage to the vestibulo-cochlear nerve. After surgical resection of the tumor, improvement of hearing loss is unusual. The cerebral mechanisms of degeneration or plasticity of the associated white matter tracts and cortices are scarcely understood. The goal of our study was to quantify volume (Vol) and fractional anisotropy (FA) of partial sections of the hearing tract ipsi- and contralateral to the tumor in order to identify differences between the hemispheres.

Method: 15 patients with vestibular schwannomas (T2a to T4b) were investigated before surgery. Diffusion Tensor Imaging (25 directions) and T1-weighted MPRAGE were realized on a 3T MR system. Probabilistic tractography was performed using the software probtrackx, part of FSL. Three partial sections of the auditory pathway were tracked: lateral section from auditory cortex to sylvian fissure, long lateral section from auditory cortex to medial geniculate corpus, and diencephalic section from medial geniculate corpus to inferior colliculus. Vol and FA were determined and compared ipsilateral (il) and contralateral (cl) using the Wilcoxon test. Laterality quotient (il parameters divided by cl) was correlated with level of hearing loss.

Results: Anatomically reasonable tracts were depicted in all patients for the lateral section. Vol was significantly decreased on the hemisphere cl to the tumor site (mean Vol il: 1.957 ± 0.668, mean Vol cl: 1.480 ± 0.516, p = 0.017), while FA did not differ significantly. Tracking did not yield meaningful tracts in 3 patients for the long lateral section and in 5 patients for the diencephalic section. No significant differences of the parameters were found for the long lateral section (mean Vol il: 1.753, ± 0.756 mean Vol cl: 1.021 ± 0.677, p = 0.091). Vol of the cl diencephalic section was significantly decreased (mean Vol il: 0.572 ± 0.249, mean Vol cl: 0.411 ± 0.390, p = 0.009), while FA did not differ. No significant correlations between laterality quotient and classification of hearing loss were found.

Conclusions: This study showed for the first time that visualization of different sections of the hearing tract between the inferior colliculus and the auditory cortex is feasible using probabilistic tractography. The significant decrease in Vol of the lateral and diencephalic section on the cl hemisphere might correspond to transsynaptic degeneration of the crossing hearing tract secondary to the damage of the il cochlear nerve.