To evaluate the changes in muscles of mastication in patients with idiopathic TN using Magnetic Resonance Imaging (MRI), before and after upfront GKRS (Gamma knife radiosurgery).

Patients with severe pain due to TN often complain of pain on chewing and tend preferentially use the other side for chewing. We hypothesize that there could be changes in the muscles of mastication in these patients. 

In this single institution retrospective cohort study, a total of 68 patients with idiopathic, unilateral TN treated with primary GKRS at University of Virginia from 2007-2017 were included. The thicknesses of temporalis, pterygoids and masseter muscles were measured on MRI, pre and post-GKRS in a blinded fashion in axial, sagittal and coronal planes. The muscles on the normal side were used as control.

Of the 68 patients, 136 temporalis muscles, 136 medial & 136 lateral pterygoid muscles and 136 masseter muscles were studied. There is a differential atrophy in muscles of mastication on the involved side at baseline before GKRS. Pre-GKRS atrophy of masseter, medial pterygoid, lateral pterygoid and temporalis muscles was seen in 18(26%), 16 (23%), 9 (13%) and 16 (23%) of the cases respectively. Logistic regression analysis showed that distribution of pain in the V3 territory (p=0.01, OR:5.43, 95% CI: 1.46-20.12) and significant pain on chewing (p=0.02, OR 5.32, 95%CI: 1.25-22.48) were found to predict pre-GKRS atrophy. Reversal of atrophy of these muscles occurred after GKRS in up to 2/3rds of the patients.

In TN, muscle proprioceptive impulses could trigger pain by ephatic transmission. Patients tend to not use those muscles leading to disuse atrophy over time. This atrophy is reversible following  relief of pain. There is differential atrophy in the involved muscles with masseter being most affected and lateral pterygoids least affected.