Radiological Characteristics of Extraocular Muscles in Myasthenia Gravis Patients with Ocular Manifestations: A Case–Control Study

PURPOSE: To analyze radiological characteristics of the extraocular muscles (EOMs) in myasthenia gravis (MG) patients with ocular manifestations. PATIENTS AND METHODS: This retrospective case–control study included all MG cases with ocular manifestations, who attended a neuro-ophthalmology clinic at a university hospital, Bangkok, from April 2009 to June 2018. They experienced double vision and ophthalmoplegia. Control participants had normal eye movements. Orbital scans were thoroughly reviewed. We measured muscle thickness (MT) of the superior rectus, inferior rectus, medial rectus and lateral rectus muscles in both eyes using magnetic resonance imaging or computed tomography scan. The sum of the muscle thickness at all sites was calculated (MTsum). Comparisons of the mean MT of each muscle type and the mean MTsum between the MG and control groups were performed by using Student’s t-test. MRI signal intensities of the EOMs were also recorded. RESULTS: Twenty MG cases and 20 controls were included in the study. The mean MTsum was 23.7 (standard deviation 4.8) mm in the MG group and 32.6 (3.5) mm in the controls. There were statistically significant differences between the two groups with respect to the mean MT and mean MTsum (p <0.001). In the MG group, there was a negative correlation between the MTsum and disease duration (p= 0.03). By using coronal T2-weighted orbital MRI with fat suppression (T2W/FS), the most frequent finding was isointensity with central hypointensity of the EOMs in the MG group. CONCLUSION: Atrophic EOMs were frequently found in the MG group, particularly in chronic cases. Isointensity with central hypointensity of EOMs on T2W/FS was also common in the MG group. These findings highlight the importance of muscle involvement in MG and may be helpful for clinical decision-making.