Polymyositis (PM)

Polymyositis is an inflammatory muscle disease in which the immune system is understood to directly target muscle fibers, producing weakness without the skin rash that distinguishes dermatomyositis. Research describes the process as involving immune cells, particularly CD8 T-cells, infiltrating and damaging muscle tissue from within, most often affecting the muscles closest to the trunk.

Polymyositis and dermatomyositis are frequently studied together because they share many features and differ mainly in histopathology and skin involvement, and both are considered rare, serious conditions. Epidemiological studies describe a female predominance, with one Japanese cohort reporting a female-to-male ratio of roughly 2.7 to 1, and onset most often in mid-adulthood, though reported rates vary by population and diagnostic criteria used.

Common signs

Gradual, usually symmetrical weakness in the muscles of the hips, thighs, shoulders, and upper arms, making it harder to climb stairs, lift objects, or rise from sitting. Muscle aching, fatigue, and sometimes swallowing difficulty can occur, and some people have lung involvement.

This is a plain-language summary for orientation, not a diagnosis. Mechanisms are described as current understanding, not settled fact. Work with your own clinician for your care.