Magnetic resonance imaging in idiopathic inflammatory myopathies: deciphering the pattern of muscle involvement

Idiopathic inflammatory myopathies (IIMs) constitute a group of immune-mediated disorders, affecting muscles. Our study aims to investigate the specific patterns of muscle involvement in subgroups of IIM. An ambispective and observational study was conducted. The evaluation encompassed clinical characterization, scales of MMT8, MDAAT, assessment of severity of edema/ fatty replacement using Modified Stramare (grades 1-5) & Modified Goutallier-Lamminen-Mercuri (GLM) scores, respectively on the muscle MRI of 125 patients with IIM. A total of 125 patients were categorized into four subgroups: dermatomyositis (DM), immune- mediated necrotizing myopathy (IMNM), overlap myositis (OM), & seronegative groups. Median age at presentation was 35 years. Median duration of illness was 8 months. In DM, 53.12% of patients showed grade 3 to grade 5 edema in adductor group. In the leg, 46.8% showed grade 3-5 edema in the deep flexors of leg & peronei, and 65.6% had significant fasciitis. In IMNM, 52.1% displayed grade 4-5 edema in rectus femoris & semimembranosus. Interestingly, in the leg, 78.2% of IMNM patients lacked edema in peroneii. For OM subgroup, there was no specific pattern of involvement. In seronegative subset, vastus lateralis showed grade 4-5 edema, while in the leg, deep posterior compartment and peroneal muscle group exhibited subtle to no edema. The involvement of peroneus muscles in leg emerged as a characteristic feature of DM, they were conspicuously spared in other subgroups. Mild muscle fatty replacement was observed in all groups except for IMNM, where it was more pronounced. The research broadens the imaging spectrum of inflammatory myositis & introduces innovative concepts of selective involvement of muscles.