Diffusion tensor imaging of microstructural alterations in the trigeminal nerve due to neurovascular contact/compression

Background Several recent studies have focused on microstructural changes in the trigeminal nerve in trigeminal neuralgia using diffusion tensor imaging (DTI). However, alterations after microvascular decompression (MVD) have rarely been investigated. Furthermore, the trigeminal nerve of asymptomatic individuals also presenting with neurovascular contact/compression (NVC) has not yet been studied. Methods Thirty-four patients suffering from trigeminal neuralgia and 34 healthy age-matched controls, who were identified as having unilateral NVC signs, underwent both DTI and high-resolution magnetic resonance imaging (MRI) for comparison. All trigeminal neuralgia patients underwent a post-surgical MRI scan after 7 days and a follow-up MRI scan within 6–8 months after surgery. The apparent diffusion coefficients (ADCs) and fractional anisotropy (FA) values were measured from coronal images in which the nerves from the root exit point to the distal segment were clearly shown. Results In 34 trigeminal neuralgia patients, the absolute FA value was significantly lower on the affected side (mean FA, 0.34 ± 0.03) than on the unaffected side (mean FA, 0.37 ± 0.05, p < 0.001). The FA ratio was also significantly different between the trigeminal neuralgia group (Rs FA , 0.92 ± 0.06) and the control group (Rs FA , 0.99 ± 0.09) ( p = 0.001). The absolute ADC value between the two sides in patients and the ratios of ADC between the trigeminal neuralgia and control groups did not show any significant differences ( p = 0.21 and 0.29, respectively). However, in 34 healthy subjects presenting with signs of NVC, neither the FA value nor the ADC showed a difference between sides ( p > 0.05). The FA ratio of patients showed a significant increase on two follow-up MRI scans compared to the preoperative FA ( p = 0.02 and 0.002, respectively), while the ADC ratio showed a significant decrease at 6 months after MVD ( p = 0.004). Conclusion This study of trigeminal neuralgia due to NVC found that DTI indexes could reflect alterations in the affected trigeminal nerve. Furthermore, a reversible change after MVD surgery could be potentially valuable for monitoring the change in white matter of the trigeminal nerve.