Poly-/dermatomyositis (PM/DM) is an acute or chronic inflammatory disease of muscle and skin that may occur at any age. Women are affected twice as commonly as men are. The prevalence of the disease is one per 200,000 population.

PM/DM can be subclassified into six categories:
(1) idiopathic polymyositis
(2) idiopathic dermatomyositis
(3) PM/DM associated with cancer
(4) childhood PM/DM
(5) PM/DM associated with other rheumatic diseases (SS, SLE, SSc, MCTD)
(6) inclusion body myositis.

The most common manifestation is weakness of involved striated muscle. The proximal muscles of the extremities are most often affected, usually progressing from the lower to the upper limbs. The distal musculature is involved in only 25% of patients. Weakness of the cervical muscles with inability to raise the head and weakness of the posterior pharyngeal muscles with dysphagia and dysphonia are also seen. Muscle pain, tenderness, and oedema also occur.

The characteristic rash of dermatomyositis, present in approximately 40% of patients, consists of raised, smooth or scaling, dusky red plaques over bony prominences of the hands, elbows, knees and ankles. An erythematosus telangiectatic rash may appear over the face and areas exposed to the sun.