Diffusion Tensor Imaging of Axonal and Myelin Changes in Classical Trigeminal Neuralgia

Trigeminal neuralgia (TN) is commonly associated with pathologic factors of axonopathy and demyelination resulting from neurovascular compression at the trigeminal root entry zone (REZ). Decompression surgery can relieve TN pain, likely by resolving such structural abnormalities. To test this hypothesis, we used diffusion tensor imaging (DTI) to capture the full extent of trigeminal microarchitecture changes in vivo in patients with TN.

Twenty-four patients with TN were compared with 28 controls. DTI metrics of fractional anisotropy (FA) and mean, parallel, and perpendicular diffusivities (MD, λ_||, and λ_⊥, respectively) were calculated in isolation at each trigeminal REZ. In 6 patients with pain relief following decompression surgery, repeated studies were performed 2 times (1 week and 4-6 months) after surgery to detect dynamic changes in FA, MD, λ_||, and λ_⊥.

We observed significant FA reductions and increased diffusivity at the affected trigeminal REZ, corresponding to known underlying pathologic changes, including axonal edema and demyelination. Specifically, our results showed that these DTI-derived metrics are discriminating features for patients with TN according to the support vector machine approach. After effective treatment, diffusion recovery at 1 week was mainly due to the decrease in λ_|| (consistent with axonal membrane stabilization), whereas at 4-6 months it was due to the predominant reduction in λ_⊥ (consistent with remyelination).

Together, these results support that DTI permits the noninvasive detection of the trigeminal microstructural abnormalities underlying TN in vivo, and DTI-derived metrics could be considered surrogate markers of the axonal and myelin states for monitoring patients.