Current estimates of the annual incidence of the inflammatory myopathies are approximately five cases per million population. These estimates are probably low, however; the true incidence may be as high as two to three per hundred thousand.
Group I: Primary Idiopathic Polymyositis
This group comprises about one-third of all cases of inflammatory myopathy. It is usually insidiously progressive over weeks, months, or even years. Rarely the disease is acute, producing severe muscle weakness in a matter of days or even rhabdomyolysis. The disease may develop at any age. Affected females outnumber males 2:1.
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, placing an object on a high shelf or combing the hair becomes difficult. Occasionally the disease is more restricted, affecting only the neck, the shoulder, or the quadriceps muscles. Aching pain in the buttocks, thighs, and calves is experienced in about 10 percent of the cases, and tenderness on palpation in another 20 percent. In the majority of patients the disorder is painless. Early symptoms of dysphagia and weakness of flexor muscles of the neck in a patient with a chronic myopathy suggest the diagnosis of polymyositis.
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. The distal muscles are spared in about 75 percent of cases. Muscle atrophy, contractures, and diminished tendon reflexes are rare in early myositis and never as pronounced as in muscular dystrophies and denervating conditions. When the reflexes are disproportionately reduced, carcinoma with polymyositis and polyneuropathy or the Lambert-Eaton syndrome should be considered. Occasionally, the reflexes may be paradoxically brisk in dermatomyositis-polymyositis, perhaps due to irritation of muscle spindle receptors by the inflammation.
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 secondary to myocarditis. About half of the fatal cases have pathologic evidence of cardiac disease with necrosis of myocardial fibers, usually with only modest inflammatory reaction. The frequency of myocardial infarction may be increased in those treated for long periods with glucocorticoids. In a few cases there is dyspnea due to lymphocytic pneumonitis, obliterating bronchiolitis, pulmonary edema, or pulmonary fibrosis. Arthralgia, Raynaud’s phenomenon, and, rarely, low-grade fever may also be present.
Group II: Primary Idiopathic Dermatomyositis
This group comprises just over one-third of all cases of myositis. The skin changes may precede or follow the muscle syndrome and include a localized or diffuse erythema, maculopapular eruption, scaling eczematoid dermatitis, or, rarely, an exfoliative dermatitis. The classic lilac-colored (heliotrope) rash is on the eyelids, bridge of the nose, cheeks (butterfly distribution), forehead, chest, elbows, knees and knuckles, and around the nail beds. Itching may be troublesome in some cases. The skin lesions may be subtle and easily overlooked. Periorbital edema is frequent, particularly in acute cases. The skin lesions may occasionally ulcerate. Subcutaneous calcification may occur, especially in children.
The typical rash and myositis allow a diagnosis of dermatomyositis, and such cases may be placed in this category if idiopathic and into groups III, IV, and V if there are other features, namely malignancy, vasculitis in children, and an established collagen vascular disease. There should be concern about an underlying malignancy in patients over the age of 40 with dermatomyositis.
Group III: Polymyositis Or Dermatomyositis With Neoplasia
This syndrome, which comprises about 8 percent of all cases of myositis, is categorized separately, although muscle and skin changes are indistinguishable from those in the other groups. Malignancy, however, is uncommon in myositis seen in children and in association with a connective tissue disorder. The malignancy may antedate or postdate the onset of the myositis by up to 2 years. The incidence of neoplasia is higher in patients over the age of 40 and is particularly high in patients over age 60; therefore, in such patients a thorough history and clinical examination (including breast, gynecologic, and rectal) should be supplemented by complete blood count, biochemical profile, serum protein electrophoresis and immunofixation, screening for carcinoembryonic antigen, urine analysis for blood and cytology, stool samples for occult blood, chest x-ray, sputum for cytology, and bone scan seeking clues for an underlying malignancy. This relatively inexpensive search uncovers most malignancies; undirected radiologic screening procedures are costly and unhelpful in improving the yield. The most common malignancies are lung, ovary, breast, gastrointestinal tract, and lymphoproliferative disorders. The myositis is a paraneoplastic syndrome, the cause of which may lie in an altered immune status, cross-reactive antigens between tumor and muscle, or an occult viral infection of the muscle.
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