Early postsurgical diffusivity metrics for prognostication of long-term pain relief after Gamma Knife radiosurgery for trigeminal neuralgia

Abstract

OBJECTIVE Gamma Knife radiosurgery (GKRS) is an important treatment modality for trigeminal neuralgia (TN). Current longitudinal assessment after GKRS relies primarily on clinical diagnostic measures, which are highly limited in the prediction of long-term clinical benefit. An objective, noninvasive, predictive tool would be of great utility to advance the clinical management of patients. Using diffusion tensor imaging (DTI), the authors' aim was to determine whether early (6 months post-GKRS) target diffusivity metrics can be used to prognosticate long-term pain relief in patients with TN. METHODS Thirty-seven patients with TN treated with GKRS underwent 3T MRI scans at 6 months posttreatment. Diffusivity metrics of fractional anisotropy, axial diffusivity, radial diffusivity, and mean diffusivity were extracted bilaterally from the radiosurgical target of the affected trigeminal nerve and its contralateral, unaffected nerve. Early (6 months post-GKRS) 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 GKRS. RESULTS Trigeminal nerve diffusivity at 6 months post-GKRS was predictive of long-term clinical effectiveness, where long-term responders (n = 19) 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 = 18) did not exhibit any characteristic diffusivity changes after GKRS. CONCLUSIONS The authors demonstrate that early postsurgical target diffusivity metrics have a translational, clinical value and permit prediction of long-term pain relief in patients with TN treated with GKRS. Importantly, an association was found between the footprint of radiation and clinical effectiveness, where a sufficient level of microstructural change at the radiosurgical target is necessary for long-lasting pain relief. DTI can provide prognostic information that supplements clinical measures, and thus may better guide the postoperative assessment and clinical decision-making for patients with TN.

Trigeminal nerve and white matter brain abnormalities in chronic orofacial pain disorders

“…117 Furthermore, in patients who underwent gamma-knife surgery to treat TN, postsurgical microstructural metrics, such as FA, were shown to predict the level of pain relief 6 months after surgery. 101 More specifically, responders had lower FA and achieved at least 75% reduction in pain. Nonresponders did not show the expected decrease in diffusion metrics.…”

Hippocampal and trigeminal nerve volume predict outcome of surgical treatment for trigeminal neuralgia

“…While CNV volume and cross-sectional area (CSA) -typically measured on T2-weighted MRI scans -appear to be consistently reduced on the affected side in patients with TN (6,7), the exact relationship between these simple nerve-based measures and surgical outcome has been conflicting (5,6,(8)(9)(10). Some groups have also examined microstructural alterations in CNV and its REZ with diffusion tensor imaging (DTI), and identified possible links between preoperative diffusivity metrics and pain outcomes (11)(12)(13). However, quantitative DTI approaches are disadvantaged because their implementation requires a non-trivial degree of image processing expertise, and further because DTI acquisitions are not typically part of the routine workup in TN patients.…”

High spatial resolution nerve‐specific DTI protocol outperforms whole‐brain DTI protocol for imaging the trigeminal nerve in healthy individuals

“…The resulting partial volume effect with adjacent CSF will reduce FA and elevate MD erroneously. For example, MD values in the unaffected CNV of TN patients have ranged from 1.2‐4.3 x 10 −3 mm 2 /s, 4,5,7,11,14–17 which is 1.5‐5–fold greater than that expected of adult white matter (~0.8 x 10 −3 mm 2 /s). However, the effects of partial volume and CSF contamination may be reduced by improving spatial resolution.…”

“…Many previous nerve DTI studies of CNV in TN have employed low resolution “whole‐brain” acquisition protocols with thick slices of 2 mm or greater, yielding large voxel volumes of 2 x 2 x 2 = 8 mm 3 5,7,14 or 1.9 x 1.9 x 3 = 10.8 mm 3 4,6,15,16 relative to a comparatively small structure. The resulting partial volume effect with adjacent CSF will reduce FA and elevate MD erroneously.…”