Radiographic patterns of muscle involvement in the idiopathic inflammatory myopathies.

INTRODUCTION: This study assesses the burden, distribution, and evolution of muscle inflammation and damage on MRI among subtypes of idiopathic inflammatory myopathy (IIM).

METHODS: Musculoskeletal MRIs performed in 66 patients with IIM and 10 patients with non-IIM between 2009 and 2016 were retrospectively graded for muscle edema, fatty replacement (FR), and atrophy.

RESULTS: Immune-mediated necrotizing myopathy (IMNM) patients had severe and extensive lower limb muscle edema, FR, and atrophy. The pelvic muscles and adductors were significantly more affected than in patients with dermatomyositis and polymyositis. Inclusion body myositis (IBM) was characterized by marked anterior thigh involvement, which stabilized or progressed at follow-up imaging. Atrophy and FR grades improved over time in some non-IBM IIM patients.

DISCUSSION: Patients with IMNM and IBM have characteristic patterns of muscle MRI abnormalities that may allow them to be differentiated radiologically from other IIM subtypes. Muscle damage in non-IBM IIM may be reversible.