Magnetic resonance imaging patterns revealing muscle pathology and clinical features in idiopathic inflammatory myopathies

Objective: Idiopathic inflammatory myopathies (IIMs) are autoimmune disorders significantly impacting skeletal muscles; however, the precise correlation between muscle magnetic resonance imaging (MRI) findings, muscle pathology, disease subtypes and clinical characteristics remains uncertain. Thus, we investigated the association of muscle MRI findings in IIMs with muscle pathology and clinical features. Methods: New-onset IIM patients underwent proximal upper and/or lower limb muscle MRI. Patterns of muscle oedema on MRI were categorised into fascial, honeycomb, peripheral, foggy, dense, or coarse dot patterns and compared with inflammatory cell infiltration sites in corresponding muscle biopsies. The incidence of MRI patterns was examined in patient subgroups using myositis-specific antibodies (MSAs) and 2017 EULAR/ACR classification criteria. Univariate and multivariate analyses were conducted to determine the odds ratios (ORs) of MRI findings for clinical characteristics. Results: Fifty-six of 85 patients underwent muscle biopsy. Foggy, honeycomb and fascial patterns at biopsy sites correlated with inflammatory cell infiltration in the endomysium (OR 11.9, P = 0.005), perimysium (OR 6.0, P = 0.014) and fascia (OR 16.9, P < 0.001), respectively. Honeycomb and foggy patterns were characteristic of patients with anti-TIF1 gamma or anti-Mi2 antibodies and MSA-negative dermatomyositis, and those with anti-SRP or anti-HMGCR antibodies and MSA-negative polymyositis (PM), respectively. The honeycomb pattern positively correlated with malignancy (OR 6.87, P < 0.001) and Gottron sign (OR 8.05, P = 0.002); the foggy pattern correlated with muscle weakness (OR 11.24, P = 0.005). The dense dot pattern was associated with dysphagia (OR 6.27, P = 0.006) and malignancy (OR 8.49, P = 0.002). Conclusion: Muscle MRI holds promise in predicting muscle pathology, disease subtypes and clinical manifestations of IIMs.