Symptoms of weakness, stiffness, and pain in the muscles usually follow but may precede the skin manifestations. The weakness is generalized but always more severe in the muscles of the shoulders and hips and proximal portions of the limbs. A tiptoe gait, the result of fibrous contractures of flexors of the ankles, is a common late abnormality. Tendon reflexes are depressed or abolished, but only commensurate with the degree of muscle weakness. Intermittent low-grade fever, substernal and abdominal pain (like that of peptic ulcer), melena, and hematemesis from bowel infarction may occur, the result of an accompanying systemic vasculitis.
The mode of progression of DM of childhood, like that of the adult form, is variable. In fulminant cases, the weakness appears rapidly, involving all the muscles including those of chewing, swallowing, talking, and breathing and leading to total incapacitation. Perforation of the gastrointestinal tract from bowel infarction may be the immediate cause of death, as it has been in two of our patients. In others, there is slow progression or arrest of the disease and, in a small number, there is a remission of weakness. Flexion contractures at the elbows, hips, knees, and ankles and subcutaneous calcification and ulceration of the overlying skin, with extrusion of calcific debris are manifestations in the late, untreated stages of the disease.
In both PM and DM, the inflammatory changes are often not confined to muscle but are associated with systemic autoimmune diseases such as rheumatoid arthritis, scleroderma, lupus erythematosus, or combinations thereof (mixed connective tissue disease); the same muscle changes are associated less often with the Sjögren syndrome. Conversely, in the aforementioned immune diseases, inflammatory muscle changes are frequently found but in only a limited number of muscles and often asymptomatically. The incidence of these “crossover” or overlap cases cannot be stated with certainty. A true necrotizing–inflammatory myopathy has been reported in up to 8 percent of cases of lupus erythematosus (far higher than in our experience), and an even smaller proportion of cases of systemic sclerosis, rheumatoid arthritis, and Sjögren syndrome. The treatment of rheumatoid arthritis with D-penicillamine increases the incidence of, or perhaps independently precipitates, a myositis.
Also notable is the sporadic concurrence of myositis with other autoimmune diseases such as myasthenia gravis and Hashimoto thyroiditis and less often, with a monoclonal paraprotein in the blood; it is not clear whether these are coincidental, but it is likely that they reflect an underlying genetic propensity to autoimmune disease.
In the overlap syndromes that incorporate autoimmune disease and myositis, there is usually greater muscular weakness and atrophy than can be accounted for by the muscle changes alone. Inasmuch as arthritis or periarticular inflammation may limit motion because of pain, result in disuse atrophy, and also at times cause a vasculitic polyneuropathy, the interpretation of diminished strength in these autoimmune diseases is not simple. Malaise, aches, and pains are common and attributable mostly to the systemic disease. Sometimes the diagnosis of myositis must depend on muscle biopsy, EMG findings, and measurements of muscle enzymes in the serum. In these complicated cases, myositis may accompany the connective tissue disease or occur many years later.
It is worth noting that PM may occur during pregnancy and that rarely the fetus is affected (most often the fetus and neonate are normal) with elevated CK levels for months postpartum (Messina et al).
Laboratory Diagnosis of PM and DM