Polymyositis

Polymyositis is an autoimmune inflammatory myopathy characterized by symmetric proximal muscle weakness, with T-cell-mediated cytotoxic reaction against muscle fibers. It generally starts after age 18 and is more common in women. Unlike dermatomyositis, there are no typical skin findings.

Pathologically, endomysial CD8+ T lymphocyte infiltration, increased MHC-I expression, and direct muscle fiber damage are seen. Serum creatine kinase (CK), LDH, AST, ALT, and aldolase levels are markedly elevated; myopathic pattern on EMG and muscle biopsy are essential for diagnosis.

Anti-synthetase syndrome (antibodies such as anti-Jo-1) constitutes an important subgroup with interstitial lung disease, mechanic's hands, arthritis, and fever. In treatment-resistant cases, inclusion body myositis and immune-mediated necrotizing myopathy should be considered in differential diagnosis.