Post SRS Reversal of Masticator Muscles Atrophy in Idiopathic Trigeminal Neuralgia.

Purpose/Objective(s) The trigeminal nerve conducts both sensory and motor impulses through sensory root (50 fascicles) and motor root of wrisberg (about 6-7 fascicles). Separate superior and inferior motor roots typically emerge from the pons just antero-superomedial to the entry point of the sensory root. Motor nerve root of trigeminal nerve receives considerable radiation dose during SRS. Purpose of the study is to evaluate MRI the changes in the muscles of mastication before and after SRS in patients with idiopathic Trigeminal neuralgia Materials/Methods Data analysis was limited to 23 patients over the age of 65 years who had undergone a first-time frameless SRS for the treatment of idiopathic TN with at least 1 year of follow-up between 2015 and 2019. Standardized collection of preoperative demographics, radiosurgical dosimetric data, and postoperative outcomes was performed. Detailed CISS / 3D TOF sequences were acquired to study the neurovascular conflict zones. In addition, MRI the thicknesses of the temporalis, pterygoid, and masseter muscles were measured on both pre- and post-SRS MRI in a blinded fashion. Changes in the muscles like fatty infiltration, MRI signal, or atrophy were noted. Logistic regression analysis was used to define the parameters like site of contact of offending vessels, number of motor roots involved, pain distribution zones and preop BNI score which mostly influenced masticator muscle atrophy. Results The median follow-up was 26 months (range 13– 60 months). On MR imaging while sensory roots were identified in 100% pts, in 13 pts (56%) single motor root, 8 pts (35%) had two motor roots, and remaining pts (9%) had three motor roots identified by neuroradiologist. Among the 23 patients eligible for inclusion in the study, 46 temporalis muscles, 46 medial pterygoid muscles, 46 lateral pterygoid muscles, and 46 masseter muscles were assessed. Pre-SRS atrophy of the masseter, medial pterygoid, lateral pterygoid, and temporalis muscles was seen in 7 (31%), 6 (26%), 4 (17%), and 6 (26%) patients, respectively. Logistic regression analysis showed that distribution of pain in the involvement of at least 2 motor nerve roots (P = 0.01, OR 5.34, 95% CI 1.8 – 19.12), superior cerebellar artery contact (P = 0.01, OR 5.43, 95% CI 1.46–20.12), V3 territory (P = 0.01, OR 5.27, 95% CI 1.21–20.67) and significant pain on chewing (P = 0.02, OR 5.32, 95% CI 1.25–22.48) were predictive of pre-SRS atrophy. Reversal of atrophy of these muscles occurred after SRS in a 22 of 23 patients post radiosurgery as evident in 1 year follow-up scan. The incidence of new-onset permanent post-SRS muscle atrophy correlated with one patient who had 3 roots and received marginal dose of 82gy. Conclusion While a subset of patients with TN with significant pain on chewing have pre-SRS disuse atrophy of the muscles of mastication, a reversal of the atrophy occurs in a majority of the patients following SRS. New-onset motor neuropathy post-SRS was rare suggesting to fact that motor roots are very sturdy to surgical doses of radiation.