Laboratory Findings

In all forms of polymyositis there may be elevated serum levels of the enzymes present in skeletal muscle, such as, aldolase, serum glutamic oxaloacetic transaminase, lactic acid dehydrogenase, and serum glutamic pyruvate transaminase. The degree of elevation decreases from the first to the last in this series of enzymes, and the pattern is the reverse of that seen in liver disease. The erythrocyte sedimentation rate is elevated in about two-thirds of cases. Tests for circulating rheumatoid factor are positive in less than one-half and for antinuclear antibodies in about three-quarters of the cases. Most other hematologic indexes are normal. Several autoantibodies seem to be associated with clinically distinct groups of patients. Anti-Jo-1 antibodies are more common in polymyositis, especially in patients with interstitial lung disease, and anti-nRNP antibodies are often associated with polymyositis seen in lupus erythematosus.

Other antibodies seen in patients with dermatomyositis and polymyositis in association with connective tissue diseases include anti-Scl-70 (progressive systemic sclerosis), anti-Sm (lupus erythematosus), anti-Ro and anti-La (Sjögren’s syndrome and lupus erythematosus), and anti-ENA (mixed connective tissue disease). Myoglobin can be found in the urine when muscle destruction is acute and extensive; rarely, acute polymyositis causes the full syndrome of rhabdomyolysis and myoglobinuria. In about 40 percent of cases EMG reveals a markedly increased insertional activity (muscle irritability), together with the typical myopathic triad of motor unit action potentials, which are of low amplitude, are polyphasic, and have an abnormally early recruitment. In a further 40 percent of the patients only myopathic changes are present.

The ECG is abnormal in about 5 to 10 percent of the cases at presentation. Since the pathologic process in myositis is patchy, greater diagnostic yield is accomplished by obtaining a biopsy from two clinically affected muscles and by skip serial sectioning of all specimens. Magnetic resonance imaging may serve to identify sites of muscle involvement. Muscles recently used for EMG or intramuscular injection must be avoided as these procedures can produce inflammatory changes and muscle fiber damage, leading to false-positive results. In about two-thirds of cases, the biopsies will demonstrate the typical pathologic changes of myositis, but despite following the above recommendations, about 10 percent of cases have normal muscle biopsy.