I. Serum chemistries. As a result of the association of hypokalemia, hyperkalemia, hypocalcemia, hypercalcemia, hypomagnesemia, and hypophosphatemia with muscle weakness, tetany, and sometimes cramps, it is prudent to obtain serum chemistry values if clinical suspicion so indicates.
Muscle enzymes commonly measured are CK, aldolase, lactic dehydrogenase (LDH), and AST. CK is probably the most reliable indicator of muscle damage because skeletal muscle, compared with other tissues, contains relatively more of it. However, heart, brain, and smooth muscle also contain CK. Strenuous physical exertion as well as intramuscular injections may increase CK and cause elevated values for a week. Serum myoglobin is elevated in polymyositis in essentially all patients with active disease. However, this test is only performed in the setting of red urine or renal compromise. An immunoassay should be performed to detect myoglobinemia. Plasma cortisol, growth hormone, thyroid-stimulating hormone (TSH), and thyroxine values are indicated if one of the endocrine myopathies is suspected. CK may also be elevated in amyotrophic lateral sclerosis.
II. Hematologic studies. The erythrocyte sedimentation rate (ESR) may be elevated in inflammatory myopathy. It is useful for following patients but is not a specific test for any myopathy. Atypical lymphocytosis may occur in viral illness or toxoplasmosis. Eosinophilia may occur in infections and polymyositis. Cold agglutinins may indicate Mycoplasma infection.
III. Urinary studies. Steroid myopathy and some muscular dystrophies may be associated with elevated creatine-creatinine urinary excretion ratios with normal muscle enzymes:
% creatinuria = creatine (mg/24 hr)/creatine (mg/24 hr) + creatine (mg/24 hr) * 100
This ratio should be calculated on at least two 24-hour urine collections, as day-to-day variability may occur. A ratio above 6% in adults is elevated. Urinary studies for heavy metals may be useful if a peripheral neuropathy is suspected. Myoglobinuria occurs in several diseases of muscle. Both hemoglobin and myoglobin give positive reactions to orthotolidine and benzidine. Immunoassay and electrophoresis techniques can measure myoglobin. The ammonium sulfate test has often proved unreliable or difficult to interpret. Excretion of acid maltase is decreased in acid maltase deficiency.
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