Non-inflammatory mimickers of myositis: a guide for rheumatologists

Muscle dysfunction presenting with weakness and elevated muscle enzymes poses a significant diagnostic challenge for rheumatologists, particularly in differentiating idiopathic inflammatory myopathies (IIM) from other mimicking conditions. This review systematically categorises non-inflammatory muscle diseases, including drug-induced myopathies, endocrine myopathies, genetic muscular dystrophies, metabolic and mitochondrial myopathies, and neuromuscular junction disorders, that can clinically and histologically resemble myositis. We emphasise the importance of a detailed clinical evaluation, including history, pattern of muscle involvement, extramuscular features, and comprehensive laboratory and biopsy investigations, to avoid misdiagnosis. Awareness of these mimickers is crucial for guiding appropriate diagnostic workup and management, given the distinct therapeutic approaches required for each condition. This framework aims to assist rheumatologists in improving diagnostic accuracy, optimising patient management, and enhancing referral decisions in patients presenting with muscle weakness.