Dermatomyositis and Polymyositis are two forms of inflammatory myopathies – diseases of muscle caused by an immune response. They cause muscle weakness as their major symptom, and dermatomyositis also causes a skin rash. Both are thought to be autoimmune diseases, in which the body's immune system attacks the muscles.
Because the causes, symptoms and treatments are similar, polymyositis and dermatomyositis are often discussed as a pair.
It is estimated that polymyositis and dermatomyositis combined affect some 20,000 people in the United States, with approximately 1,400 new adult cases per year. These conditions are two to three times as common among blacks as whites, and twice as common among women as men. They may appear at any age, but begin most commonly between the ages of 40 and 60. A juvenile form of dermatomyositis also occurs, with approximately 2,000 new cases per year in the United States. It appears most commonly in middle childhood to early adolescence (ages 5-15).
Symptoms of dermatomyositis usually begin gradually, with muscle weakness, joint pain, fever, weight loss, and fatigue. Children may experience a more rapid onset. Muscle weakness usually affects the upper arms, neck, and legs, on both sides of the body. Some people experience joint and muscle pain, and some have difficulty swallowing.
As the disease progresses, the muscles begin to waste away (atrophy) and permanently shorten (contracture). A grayish-red/violet rash appears at the same time as the muscle weakness. The rash commonly appears on the face and includes a purplish swelling, called a heliotrope, around the eyes/eyelids. There may be a shawl-like rash over the neck, shoulders, and upper chest and back. Another rash often appears around the joints, especially around the fingers but also on the knees and elbows. This rash may be smooth or have hard/scaly patches, and is called Gottron's sign. When the rash fades, it leaves behind pigmented patches of skin.
Exposure to sun can worsen existing areas or lead to the development of new ones. Hard nodules of calcium deposits under the skin (calcinosis) are common in juvenile dermatomyositis, but not in the adult form.
Some people with dermatomyositis develop a rash without muscle weakness. This is called dermatomyositis sine myositis. Others may also have Raynaud's phenomenon, in which the small arteries in the fingers and toes constrict, causing the skin to look pale or bluish, usually after exposure to cold.
The muscle weakness may appear suddenly and progress over weeks to months. Patients may have difficulty raising the arms above the shoulders, climbing steps, or arising from a sitting position, and be unable to raise the head from the pillow. Patients may become wheelchair bound or bedridden because of weakness of pelvic and shoulder girdle muscle groups. The flexors of the neck may be severely affected. Weakness of the laryngeal musculature is responsible for dysphonia. Involvement of the striated muscle of the pharynx and upper portion of the esophagus leads to dysphagia and regurgitation. A diminution in peristaltic activity and dilation of the lower esophagus and small intestine may be present. The muscles of the hands, feet and face escape involvement. Contractures of limbs may develop late in the chronic stage.
Dermatomyositis is diagnosed through a thorough physical exam and the characteristic symptoms. A doctor will order diagnostic tests to check levels of certain enzymes in the bloodstream. He or she will also take a small sample of tissue from the affected muscle (a muscle biopsy), to check for abnormalities. A test called electromyography will reveal any abnormal electrical activity in the muscles. Magnetic resonance imaging (MRI) is also helpful in revealing muscle inflammation. The doctor should order blood tests and other cancer-screening tests to check for malignancies because people with dermatomyositis are more likely to develop malignant cancers.
The first line of treatment for dermatomyositis is restricted activity and a daily injection of a corticosteroid, such as prednisone, to reduce inflammation and relieve pain. This dose is gradually decreased as the condition improves. Adults usually need to stay on a maintenance dose for life. However, children may be able to stop steroid treatment after approximately one year. Some patients cannot tolerate or do not respond to corticosteroids – they may instead be treated with immunosuppressive drugs, such as azathioprine or methotrexate. Other patients may respond well to intravenous immune globulin. Dermatomyositis often improves with treatment of malignant tumors.
While there is no known way to prevent dermatomyositis, you can find relief from symptoms and possible remission by following your doctor's treatment recommendations. It is very important to reduce your physical activity until your muscles have had an opportunity to recover.
People with polymyositis or dermatomyositis are more likely to have other autoimmune diseases. Arthritis is common, as is Raynaud's phenomenon; others include systemic lupus erythematosus and Sjögren's syndrome.
Interstitial lung disease occurs in 10-30% of all people with polymyositis or dermatomyositis. This condition causes fibrous scarring of lung tissue, leading to a dry, unproductive cough and shortness of breath with exertion.
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