Assessment of auditory pathway microstructure in vestibular schwannoma patients: a quantitative diffusion tensor tractography study

OBJECTIVE: Vestibular schwannomas (VSs) are cerebellopontine angle tumors that can result in cranial nerve dysfunction, most commonly sensorineural hearing loss. Conventional structural MRI is unable to provide correlative information on cranial nerve function. In this study, the authors used multitensor tractography to study the white matter microstructural properties of the auditory neural pathway as a correlate of cranial nerve function in a cohort of VS patients. They evaluated the relationship between the auditory neural pathway microstructural properties using pure-tone audiometry (PTA) and the speech discrimination score (SDS). METHODS: Retrospective chart review of 258 patients with VS treated at the Toronto Western Hospital Gamma Knife Radiosurgery Unit was conducted. Of these, 3T MR images were analyzed for 57 surgically naive patients with unilateral VS who had preoperative diffusion tensor imaging (DTI) and PTA and SDS results. Patients were excluded if they had bilateral tumors, previous surgical treatment (Gamma Knife radiosurgery or resection), or did not undergo DTI. DTI-derived metrics (fractional anisotropy [FA], radial diffusivity [RD], axial diffusivity [AD], and mean diffusivity [MD]) of five regions of interest positioned along the auditory neural pathway (ipsilateral superior olivary nucleus [SON] and trapezoid body [TB] and contralateral inferior colliculus, lateral lemniscus [LL], and medial geniculate body [MGB]) were measured bilaterally in all subjects. The diffusion metrics were correlated with quantitative average high-frequency (4000 and 8000 Hz) PTA and SDS results. RESULTS: Salient areas of neuroanatomical correlation included the LL and SON (affected side), where a statistically significant diffusion metric change was seen. This was characterized by higher FA and lower RD values (LL and SON) and a higher AD value (SON). SDS positively correlated with the TB AD. PTA showed a significant negative relationship with MD at the LL and a positive relationship with AD at the inferior colliculus (affected side). PTA also showed a significant negative relationship with RD and MD at the SON and TB, and a positive relationship with FA and AD at the MGB on the contralateral side (all p < 0.05). CONCLUSIONS: This work outlines that quantitative DTI is a useful tool to evaluate the white matter microstructural alterations in the auditory neural pathway. Importantly, as a noninvasive tool, diffusion metrics can help in understanding the pathophysiology of hearing impairment in this group of patients.