Polymyositis

A systemic autoimmune cellularly mediated inflammatory myopathy characterized by cytotoxic T-cell-mediated attack against muscle fibers, leading to symmetric weakness and some muscle atrophy.

Prevalence is 6 to 8 per 100,000 persons; incidence peaks between 45 and 65 years. Polymyositis is most common in blacks and women.

Although considered an autoimmune connective tissue disease, polymyositis may result from disorders that cause slowly progressive myopathy with muscle fiber damage from cytotoxic lymphocytes (eg, unidentified foreign proteins, viruses, or other illnesses). Polymyositis is cellularly mediated, unlike dermatomyositis, which is humorally mediated. Muscle pathology differs from that of inclusion body myositis and dermatomyositis.

Symptoms, Signs, and Diagnosis

Muscle weakness is generalized but is more prominent in the hips and thighs; it begins as aching pain in about 10% of patients and as tenderness on palpation in 20%. Painless progressive weakness affecting the legs and arms is the presenting complaint in the remaining 70% of patients. Muscle atrophy and loss of tendon reflexes are rare in the early stages. Dysphagia is common. Cardiac involvement (eg, atrioventricular conduction defects, bundle branch block) is relatively common; about 33% of patients have ECG abnormalities. Interstitial lung disease occurs in about 10% of patients.

Polymyositis that occurs with another connective tissue disorder is called an overlap syndrome; as many as 10 to 20% of cases of polymyositis and dermatomyositis can present as overlap syndromes. One example is mixed connective tissue disease, which has elements of scleroderma, lupus, and polymyositis (or dermatomyositis) and is associated with antibodies to ribonucleoprotein.

The diagnosis is based on typical symptoms and signs, a myopathic pattern on electromyography, typical biopsy findings, and elevated CK levels. Anti-Jo-1 antibodies, which are antibodies to histidyl-t-RNA synthetase, are present in about 20% of patients with polymyositis and are positive in about 50% of patients with interstitial lung disease. Arthralgia, fever, an elevated ESR, and a polyclonal hypergammaglobulinemia on electrophoresis may occur.

Prognosis and Treatment

The overall mortality is about four times that of the general population; death is usually due to pulmonary or cardiac complications. Blacks and women have a less favorable prognosis.

Treatment is the same as for dermatomyositis. About 50% of patients recover, and therapy can be discontinued within 5 years of the onset of symptoms. About 20% of patients have persistent active disease requiring continued therapy. The remaining 30% develop inactive disease with permanent residual muscle weakness.