The primary diffusion metrics (FA and MD) reflect overall white matter health, organization and maturation [
36]. In addition, AD reflects axon integrity and RD reflects myelin sheath integrity [
31]. Both AD and RD are of great significance in understanding the underlying physiological mechanism [
26]. Decreased FA values maybe based on predominantly increase of RD or both RD and AD [
30]. The study of DeSouza et al. showed the right-sided TN patients had significantly decreased FA and increased RD, MD, and AD in the brain white matter including the corpus callosum, posterior corona radiata, cingulum, and superior longitudinal fasciculus. Moreover, MD and RD changes of brain white matter in TN patients maybe have relation to central nervous system plasticity, neuroinflammation and edema [
18]. In our study, we found the similar results that reduced FA and elevated RD in the corona radiata (mainly concentrating on bilateral superior corona radiate and anterior corona radiata), corpus callosum and left cingulum. Additionally, we found reduced FA and elevated RD in the fronto-occipital fasciculus, internal capsule, external capsule, fornix cerebri and cerebral peduncle in the TN patients. Moreover, altered FA and RD was mainly located in white matter of left hemisphere. However, the differences of MD and AD were not statistically significant. As we all know, TN is involved in trigeminal nerve functional disorders, but other theories of central nervous system pathology is not clear [
18,
37]. Due to peripheral trigeminal nerve injury, central nervous system plasticity will most probably occur [
18,
38,
39], including fiber organization changes, astrocyte morphology and angiogenesis [
18,
40‐
42]. In our study, compared with healthy controls, TN patients showed decreased FA. The decreased FA may correspond to less fiber organization, such as more axonal sprouting/branching, larger axons, or more crossing fibers [
18,
42]. Besides, neuropathic pain is usually related to chronic painful influence on central nervous system. This leads to central sensitization, a process involving demyelination and neuroinflammation processes [
18]. The decreased FA is presumably caused by a significant increase of RD, inferring that demyelination and neuroinflammation processes may lead to the impairment of white matter integrity in the TN patients. The RD abnormalities maybe result from these mechanisms in the white matter of TN patients [
8].
Many studies have reported cortical and subcortical gray matter impairment in the cognitive-affective, sensory, modulation of pain, attention and motor regions of TN patients [
43‐
45]. In anatomy and/or function respect, these brain areas are connected [
13,
46‐
48]. In our study, we reported decreased FA and increased RD in the corona radiata, corpus callosum, cingulum, fronto-occipital fasciculus, internal capsule, external capsule, fornix cerebri and cerebral peduncle in the TN patients. These fiber connection of brain regions is related to rapid transmission of pain, attention and motor function [
49], and maybe lead to the unique sensory symptoms of TN. Our finding also revealed that altered FA and RD was mainly located in white matter of left hemisphere, suggesting contralateral white matter lesions of TN patients.
Correlation analyses found negative correlations between the disease duration and the FA values of left anterior corona radiata, genu of corpus callosum, left external capsule, left cerebral peduncle, indicating the white matter impairment is more and more severe as the disease progressed. With impairing progression of left anterior corona radiata, left external capsule and left cerebral peduncle, painful sensation is more serious. What is more, we infer these regions are probably related to transmission of pain [
50]. The RD and the disease duration also reveal positive correlations in the regions of left anterior corona radiata, right external capsule, left fornix cerebri, left cerebral peduncle, demonstrating the white matter demyelination and neuroinflammation of these regions is aggravated in the disease progression. And as demyelination and neuroinflammation of left anterior corona radiate and left external capsule progresses, painful sensation is also more serious. Regrettably, the results of these correlation analyses had not been able to withstand multiple comparison correction.