Semi-quantitative analyses of muscle magnetic resonance imaging for pattern recognition in early idiopathic...

Idiopathic inflammatory myopathies (IIM) are a group of autoimmune diseases characterized by skeletal muscle inflammation. Magnetic resonance imaging (MRI) is increasingly used in IIM diagnosis, yet standardized protocols and comprehensive datasets remain limited. This study aimed to characterize MRI findings across IIM subtypes, identify muscle involvement patterns, and assess correlations between imaging abnormalities and clinical severity.

In this retrospective cohort study, 34 patients with IIM evaluated between 2012 and 2020 in Düsseldorf and Halle (Germany) were included: Inclusion Body Myositis (IBM, n = 9), Dermatomyositis (DM, n = 5), Polymyositis (PM, n = 6), Anti-Synthetase Syndrome (ASYS, n = 5), and Immune-mediated Necrotizing Myopathy (IMNM, n = 9). Clinical data (autoantibody status, creatine kinase [CK] levels, and muscle strength via Manual-Muscle-Testing-8 [MMT-8]) and muscle MRI findings were analysed. Muscle involvement patterns were characterized, and associations with clinical severity were evaluated using univariate linear regression.

MRI abnormalities were evident even in early disease stages (median disease duration: 7 months [IQR: 2-18]). Patterns of muscle involvement differed markedly across IIM subtypes; notably, ASYS and IBM showed minimal involvement of upper extremities. A significant correlation was found between MRI-visible pathology and MMT-8 scores (p = 0.027) as well as CK levels (p = 0.028).

MRI reveals subclinical muscle involvement in early IIM and uncovers subtype-specific patterns. Given the heterogeneity of MRI findings, individualized imaging approaches are warranted. While MRI serves as a valuable adjunct in diagnosis and management of IIM, its utility depends on standardized protocols and integration with clinical and laboratory data to enable accurate interpretation and informed decision-making in clinical practice.