Dermatomyositis is one of a group of acquired muscle diseases called inflammatory myopathies. Symptoms in children and adults are similar, the only distinction being that childhood onset is more likely to be very acute, and adult onset more gradual. Females are more often affected than males. An acute infection may precede or incite the initial symptoms. Dermatomyositis is characterized by a rash accompanying, or more often, preceding muscle weakness. The rash is described as patchy, bluish-purple discolorations on the face, neck, shoulders, upper chest, elbows, knees, knuckles, and back. Some patients may also develop hardened bumps of calcium deposits under the skin.

The most common symptom is muscle weakness, usually affecting those muscles that are closest to the trunk of the body (proximal). Eventually, patients have difficulty rising from a sitting position, climbing stairs, lifting objects, or reaching overhead. In some cases, distal muscles (those not close to the trunk of the body) may be affected later in the course of the disease. Trouble with swallowing (dysphagia) may occur. Occasionally, the muscles ache and are tender to touch. Patients may also feel fatigue and discomfort and have weight loss or a low-grade fever.

Conventional treatment for dermatomyositis generally consists of a steroid drug called prednisone. For patients in whom prednisone is not effective, other immunosuppressants such as azathioprine and methotrexate may be prescribed. Recently, a drug called intravenous immunoglobulin was shown to be effective and safe in the treatment of the disease. Physical therapy is usually recommended to preserve muscle function and avoid muscle atrophy.