Diagnostic tests: Muscle enzymes, EMG, biopsy
The diagnosis of dermatomyositis is confirmed by 3 laboratory tests: elevated muscle enzyme levels, electromyography, and muscle biopsy. A punch biopsy is helpful in differentiating dermatomyositis from other papulosquamous diseases such as lichen planus and psoriasis, but be careful as the histology of dermatomyositis is indistinguishable from cutaneous lupus erythematosus.
During the acute active phase, the following serum muscle enzymes may be elevated: creatine kinase (CK), lactate dehydrogenase (LDH), alanine aminotransferase (ALT or SGPT), aspartate aminotransferase (AST or SGOT), and aldolase. CK is elevated among 65% of patients and is most specific for muscle disease.3 Only one of the aforementioned enzymes may be elevated, so it is necessary to measure them all.
Measuring antibodies such as antinuclear antibody (ANA), Jo-1, SSA (Ro), SSB (La) supports the diagnosis if positive but dermatomyositis cannot be diagnosed solely on positive titers. It is not necessary to obtain an electromyograph or muscle biopsy for a patient with the characteristic skin findings and evidence of elevated muscle enzymes, as the diagnosis of dermatomyositis can be made with confidence. For a patient in whom the presentation is not as straightforward, it may be useful to obtain the electromyograph and muscle biopsy.
Management: Corticosteroids, watch for malignancy
Oral corticosteroids are the treatment of choice (strength of recommendation [SOR]: B).1,4 Prednisone 0.5 to 1.0 mg/kg body weight per day has been recommended until muscle enzyme levels trend toward normal limits, at which time you can taper the dose. Steroid myopathy is a potential side effect of this treatment regimen; it may occur 4 to 6 weeks after therapy starts.