Improving NVC diagnosis in type 2 TN by MRI has received attention of many researchers [
5‐
7]. High-resolution 3D MR imaging reconstruction was adopted in patients with constant facial pain (Type 2 TN) to help determine the presence/absence of neurovascular compression [
5]. This MRI technique demonstrated the relation between vascular and TGN, but the alteration of TGN was not clear. It was reported demyelination without significant axonal injury was the essential pathological basis of the affected TGN [
7]. DTI can quantitatively assess the microstructural abnormalities of the affected TGN in patients with TN by multiple diffusion metrics. The MRI DTI metrics FA representing the most valuable diffusion tensor imaging index that has been widely used to investigate white matter changes, decreased at the root entry zone of affected nerves [
21,
22]. Furthermore, the decreasing of FA value was observed in many studies of the affected TGN and was more consistent than other DTI metrics, for example, the mean diffusivity (MD) [
22,
23]. FA value quantitatively reveals the myelin structure change of trigeminal nerve and may be a potential objective MRI biomarker related with clinical severity [
24]. All the knowledge mentioned above indicates the more decreasing of FA value, the more definite and serious of the vessel compression. This also means the prognosis of MVD is potentially more satisfactory. As we know, the features of vein include thinner vessel wall, relatively lower pressure and no pulsatile pressure compared with artery. These structural characteristics result in the compression of vein less severe than artery. This is in consistent with our findings that the declining proportion of FA value of artery compression was more significant than that of vein. These findings are especially valuable for type 2 TN as the clinical symptom is not typical and diagnosis is relatively difficult compared with type 1 TN. With the help of DTI sequences, a patient with constant facial pain excluded for other etiology (dental problem, intracranial tumor and so on) accompanied with the NVC and obvious decreasing of FA value may more reliably refer to MVD. Furthermore, the outcome of MVD may be more optimistic.
There are some limitations about this study. First of all, the sample size is relatively small and large scale study is required for further research. Secondly, the diagnosis of type 2 TN is mainly based on the symptom described by the patients. Selection bias may be unavoidable. The last, information of follow-up including MRI will contribute to the comprehensive understanding of the significance of FA value.