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Juvenile dermatomyositis (JDM) is an autoimmune disease causing vasculitis that manifests itself in children; it is the pediatric counterpart of dermatomyositis. In JDM, the body’s immune system attacks blood vessels throughout the body, causing inflammation called vasculitis. In the United States, the incidence rate of JDM is approximately 3 cases per million children per year, leading to 300 to 500 new cases annually and affecting an estimated 3,000 to 5,000 children. Other forms of juvenile myositis are juvenile polymyositis (JPM) and juvenile inclusion-body myositis (JIBM), which are extremely rare and are not as common in children as in adults.
The underlying cause of JDM is unknown. It most likely has a genetic component, as other auto-immune disease tend to run in the families of patients. The disease is usually triggered by a condition that causes immune system activity that does not stop as it should, but the trigger is almost certainly not the cause in most cases. Common triggers include immunizations, infections, injuries, and sunburn.
The vasculitis caused by JDM manifests itself predominantly in two ways:
One is a distinctive rash. The rash often affects the face, eyelids, and hands, and sometimes the skin above joints, including the knuckles, knees, elbows, etc. The color of the rash is a pinkish purple, and is called heliotrope (after a flower of the same name with approximately this color). 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. Some children develop calcinosis, which are calcium deposits under the skin. The rash is the source of the “dermato-” part of the name of the disease.
The second symptom caused by vasculitis is muscle inflammation. This symptom is the source of the “-myositis” part of the name of the disease (“myo” = muscle, “-itis” = inflammation of). Muscle Inflammation causes muscle weakness, which can cause fatigue, clumsiness, not keeping up physically with peers, and eventually inability to perform tasks like climbing stairs, lifting objects, and performing other manual tasks. Other signs may include falling, dysphonia, or dysphagia. The muscle weakness often causes a medical misdiagnosis of muscular dystrophy or other muscle disease. Some patients develop contractures, when the muscle shortens and causes joints to stay bent; exercise can prevent this. The muscles first affected tend to be proximal (i.e., neck, shoulders, back, and abdominal). About half of children with JDM also have pain in their muscles.
The speed of the progression of JDM is highly variable. Nearly all JDM patients have some skin involvement. The JDM rash usually occurs as the initial symptom. Sometimes it is so slight as not to be recognized for what it is until muscle symptoms appear. Sometimes muscle symptoms never appearing at all or occur very gradually over the course of months, and sometimes going from normal strength to being unable to walk within days. Usually, muscle symptoms appear weeks to months after the onset of the rash.
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