Background & Aims
Trigeminal Neuralgia (TN) is a chronic neuropathic facial pain disorder characterized by unilateral attacks of severe stabbing pain in the distribution of the trigeminal nerve (CN V)(1). Medication inadequately controls pain in many TN patients, who go on to have surgery directed at CN V(2). A significant minority of surgically-treated TN patients with initial pain relief develop delayed pain recurrence, for reasons that are incompletely understood(3,4). Few neuroimaging studies have systematically examined the structure of the trigeminal nerve after surgery through to pain recurrence when it occurs(5-11). This study aimed to use diffusion MRI (dMRI) to characterize microstructural changes occurring in the trigeminal nerve as a result of surgery, and to track these changes over time. We hypothesized that specific patterns of microstructural nerve alterations may be associated with durable post-operative pain relief and that these alterations are likely to be specific to surgery type.
Methods
Forty (40) TN patients treated with either microvascular decompression (MVD—21) or percutaneous balloon compression rhizotomy (PR—19) were prospectively scanned using a custom, high-resolution, 3T FLAIR-diffusion tensor imaging (FLAIR-DTI) sequence(12), at pre-operative and multiple post-operative time points, including at pain recurrence (when it occurred). CN V tractography was carried out with a deterministic algorithm(13). Diffusivity metrics reflecting whole-nerve microstructure were calculated for each treated nerve at each time point: fractional anisotropy (FA); axial diffusivity (AD); radial diffusivity (RD); mean diffusivity (MD)(14). Post-operative diffusivity metric changes were compared between patients with sustained pain relief (responders) and those with recurrent pain (non-responders). Within-subject diffusion metrics between time points were compared with a non-parametric independent measures Kruskal-Wallis test, followed by Wilcoxon signed-rank post hoc testing(15).
Results
We found novel, consistent, temporal patterns of trigeminal nerve diffusivity changes associated with durable TN pain relief following MVD or PR. Responders to either MVD or PR demonstrated significant reduction in operated-nerve FA by 1 month after surgery (p=0.02), which persisted at 6 and 12 months post-surgery. In contrast, operated nerve FA, MD, AD, and RD were stable at all postoperative time points in non-responders to MVD, while non-responders to PR demonstrated transient operated nerve FA reduction at 1 month (p=0.03) and 6 months (p=0.009) after surgery which resolved by the 12 month time-point (p=0.74). In a subset of patients undergoing multiple successive procedures for recurrent pain, we found no alterations of operated nerve diffusivity metrics either at 1 month or delayed time points.
Conclusions
Patients undergoing surgery for TN show unique microstructural changes in the operated trigeminal nerve. The post-operative evolution of these changes is associated with response to surgical outcome. Specifically, long-term responders to surgery show a significant reduction of FA in the operated nerve 1-month after surgery, which persists at 6 and 12 months. These findings suggest that surgery needs to induce specific structural changes in the operated trigeminal nerve in order to produce lasting pain relief. Further work is necessary to understand how sustained nerve changes may interact with central pain processing mechanisms in order to maintain the pain-free state. Nevertheless, incorporating dMRI of the trigeminal nerve into routine post-operative imaging assessments after TN surgery may be a useful clinical tool for prognostication.
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Presenting Author
Tejas Sankar
Poster Authors
Topics
- Specific Pain Conditions/Pain in Specific Populations: Orofacial Pain