Juvenile dermatomyositis (JDM) is an idiopathic inflammatory myopathy (IMM) of presumed autoimmune dysfunction resulting in muscle weakness among other complications.

Other Idiopathic inflammatory myopathies include; juvenile polymyositis (PM), which is rare and not as common in children as in adults.

The rash often affects the face, eyelids, and hands, and sometimes the skin above joints, including the knuckles, knees, elbows, etc.

On the hands and face, the rash very closely resembles allergies, eczema, fifth disease, or other more common skin condition, but the heliotrope color is unique to the inflammatory process of JDMS.

[2] When a child becomes irritable, fatigued, reluctant to socialize, and the face becomes flushed easily, physicians refer to this constellation of symptoms as "misery.

Sometimes muscle symptoms never appear at all or occur very gradually over the course of months, and sometimes going from normal strength to being unable to walk within days.

Typical magnetic resonance imaging and muscle biopsy changes are considered the next most useful diagnostic criteria, followed by myopathic changes on electromyogram, calcinosis, dysphonia and nailfold capillaroscopy.

Other useful criteria include myositis-specific or -related antibodies, nailfold capillaroscopy, factor VIII-related antigen, muscle ultrasound, calcinosis and neopterin.

[4] Once a diagnosis of JDMS is made, the treatment is often a 3-day course of Intravenous ("pulse") steroids (methylprednisolone, Solu-Medrol), followed by a high dose of oral prednisone (usually 1–2 mg/kg of body weight) for several weeks.

[citation needed] Once the disease process is under control, oral steroids are tapered gradually to minimize their side effects.

Most JDM patients are very sensitive to sun exposure, and sunburn may be a disease activity trigger in some, so daily application of high-SPF sunscreen is often recommended.