Post SRS Target Diffusion Metrics for Response Prognostication in Idiopathic Trigeminal Neuralgia

Abstract

Diffusion tensor imaging (DTI) is used to measure the movement of water molecules in biological tissue, which permits a thorough characterization of white matter microstructure through quantitative diffusivity metrics. The most commonly used diffusivity metric, fractional anisotropy (FA), provides insight into the overall white matter microstructural properties, whereas radial diffusivity (RD), axial diffusivity (AD), and mean diffusivity (MD) suggest changes in myelination, axonal integrity & underlying neuro-inflammation and edema respectively. Study purpose is to find whether trigeminal nerve diffusivity metrics at a single, early time point after frameless SRS is predictive of long-term pain relief in patients with TN.Study cohort comprised of 44 pts with inclusion criteria were: 1) diagnosis of classic, type 1 TN 2) SRS treatment with no prior surgical procedures for TN; 3) 3 tesla MRI scans at 6 months posttreatment (range 5-7 months); and 4) at least 12 months of clinical follow-up. Patients with TN secondary to multiple sclerosis, cranial tumors, or vertebrobasilar dolichoectasia compressing the brainstem were excluded from the study. For each patient, diffusivity metrics (FA, MD, RD, and AD) were extracted from both the affected and unaffected trigeminal nerves at 6 months posttreatment 6 months diffusivity metrics were compared with long-term clinical outcome. Patients were identified as long-term responders if they achieved at least 75% reduction in preoperative pain for 12 months or longer following SRS. Average microstructural diffusivity differences (within group - pre and post treatment; across group - responder & non-responders) were analyzed using paired / Independent T test.Trigeminal nerve diffusivity at 6 months post-SRS was predictive of long-term clinical effectiveness, where long-term responders (n = 36) showed significantly lower fractional anisotropy at the radiosurgical target of their affected nerve compared to their contralateral, unaffected nerve and to nonresponders. Radial diffusivity and mean diffusivity, correlates of myelin alterations and inflammation, were also significantly higher in the affected nerve of long-term responders compared to their unaffected nerve. Nonresponders (n = 8) did not exhibit any characteristic diffusivity changes after SRS.DTI, FA specifically, appears to be a strong, objective measure to assess the effects of radiation on a nerve. Incorporating early DTI assessment can provide prognostic information that supplements clinical measures.