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Dermatomyositis and polymyositis are autoimmune disorders in which the skeletal muscle is damaged by an inflammatory process dominated by lymphocytic infiltration. The term polymyositis is applied when the condition spares the skin, and the term dermatomyositis when polymyositis is associated with a characteristic skin rash. One-third of cases are associated with various connective tissue disorders, such as rheumatoid arthritis, and polymiositis.
The precise cause of these diseases is unknown, but interplay between host genetic factors, viral infection of muscle, and autoimmune mechanisms is probably contributory. Familial occurrence of these diseases, and the increased frequency of HLA-DR3 and -DRw52 antigens in patients.
A lymphocyte-mediated disease resembling polymyositis has been reported in laboratory animals injected with muscle antigens together with Freund's adjuvant (experimental).
A widely used classification of the dermatomyositis-polymyositis group. Other diseases uncommonly associated with polymyositis are sarcoidosis, giant cell myositis with thymoma, and myositis in systemic infections due to viruses, toxoplasma, or parasites.
Current estimates of the annual incidence of the inflammatory myopathies are approximately five cases per million population. These estimates are probably low.
This group comprises about one-third of all cases of inflammatory myopathy. It is usually insidiously progressive over weeks, months, or even years. Patients first become aware of weakness of the proximal limb muscles, especially the hips and thighs, and find difficulty in arising from the squatting or kneeling position and in climbing or descending stairs. When shoulder girdle muscles are involved.
When the patient is first seen, there may be weakness of the muscles of the trunk, the pectoral and pelvic girdles, the upper arms and thighs, the neck, and the pharynx. Ocular muscles are almost never affected except in a rare association with myasthenia gravis.
At presentation about 25 percent of patients have dysphagia, about 5 percent have significant respiratory impairment, and 5 percent are unable to walk. Dysphagia is due to involvement of striated muscles of the pharynx and upper esophagus. At some time in the course of the disease, cardiac abnormalities are observed in about 30 percent of cases; these include electrocardiogram (ECG) changes, arrhythmias, and heart failure.
This syndrome, which comprises about 8 percent of all cases of myositis, is categorized separately.
This "overlap group" of myositis comprises about one-fifth of all cases that occur in association with several connective tissue diseases. Progressive systemic sclerosis, rheumatoid arthritis, mixed connective tissue disease (the rheumatologic overlap disorder), and lupus erythematosus are the most common associated conditions; polyarteritis nodosa and rheumatic fever are more rarely associated.
Patients with dermatomyositis who have the characteristic skin rash, muscle weakness, EMG changes, and elevation of serum CK may not require a muscle biopsy to confirm the diagnosis. In the case of idiopathic polymyositis, however, a firm diagnosis must be based on the presence of a typical clinical picture, a typical EMG, elevation of serum CK.
Glucocorticoids in high dosage is the accepted treatment for severe dermatomyositis-polymyositis, though there is no controlled trial to prove its effectiveness. Prednisone is generally started at a dose of 1 to 2 mg/kg body weight per day (60 to 100 mg/d for adults).
Cytotoxic drugs should be tried when the response to glucocorticoids is inadequate after 1 to 3 months or when relapses are frequent. The combined use of glucocorticoids and a cytotoxic drug.