Approach Considerations
Workup for dermatomyositis may include selected laboratory tests, diagnostic imaging (eg, magnetic resonance imaging [MRI], chest radiography, ultrasonography, electromyography [EMG], or computed tomography [CT]), as well as muscle and skin biopsy and other tests as appropriate.
In older patients with dermatomyositis, the frequency of an associated malignancy increases. Assessment for malignancy should be performed upon initial diagnosis and repeated at least annually for 3 years. The exact testing order should be based on the patient’s sex, age, and race.
Laboratory Studies
Muscle enzyme levels are often abnormal during the course of dermatomyositis, except in patients with amyopathic dermatomyositis (ADM). The most sensitive/specific enzyme is elevated creatine kinase (CK), but aldolase studies and other tests (eg, for aspartate aminotransferase [AST] or lactic dehydrogenase [LDH]) may also yield abnormal results.
At times, the elevation of the enzymes precedes the appearance of clinical evidence of myositis. Thus, if a patient who is presumably stable develops an elevation of an enzyme that was previously within the reference range, the clinician should assess the possibility of a flare of the muscle disease.
Several serologic abnormalities have been identified and may be helpful in the classification of subtypes for prognosis, but they are not used for routine diagnosis. As a group, these antibodies have been termed myositis-specific antibodies (MSAs). These autoantibodies occur in about 30% of all patients with dermatomyositis or polymyositis.
A positive antinuclear antibody (ANA) finding is common in patients with dermatomyositis.
Anti–Mi-2 antibodies are highly specific for dermatomyositis, but their sensitivity is low, because only 25% of patients with dermatomyositis demonstrate them. These autoantibodies are associated with acute-onset classic dermatomyositis with the V-shaped and shawl rash (poikiloderma) and a relatively good prognosis.
Anti–Jo-1 (antihistidyl transfer RNA [t-RNA] synthetase) antibodies are more common in patients with polymyositis than in patients with dermatomyositis. They are associated with pulmonary involvement (interstitial lung disease), Raynaud phenomenon, arthritis, and mechanic’s hands.
Other MSAs include antisignal recognition protein (anti-SRP), associated with severe polymyositis, and anti–PM-Scl and anti-Ku, which are associated with overlapping features of myositis and scleroderma.
One study found that autoantibody against p155 was highly related to cancer-associated myositis and could be a reliable marker of cancer in patients with dermatomyositis.[25]
Imaging Studies
MRI may be useful in assessing for the presence of an inflammatory myopathy in patients without weakness. It can assist in differentiating steroid myopathy from continued inflammation and may serve as a guide in selecting a muscle biopsy site.
Chest radiography should be obtained at the time of diagnosis and when symptoms develop.
A barium swallow allows evaluation of esophageal dysmotility.
Ultrasonography of the muscles has been suggested for evaluation but has not been widely accepted.
EMG is a means of detecting muscle inflammation and damage and has, at times, been useful in selecting a muscle biopsy site. Since the introduction of muscle MRI, EMG has been obtained less commonly in this setting.
CT scanning is useful in the evaluation of potential malignancy that might be associated with inflammatory myopathy.
Other Studies
Other tests may include the following:
- Pulmonary function studies
- Electrocardiography (ECG)
- Esophageal manometry (in selected patients)
Muscle biopsy, either open or via a needle, may enhance the clinician’s ability to diagnose dermatomyositis. The biopsy results may be useful in differentiating steroid myopathy from active inflammatory myopathy when patients have been on corticosteroid therapy but are still weak.
Histologic Findings
Skin biopsy reveals an interface dermatitis that is difficult to differentiate from lupus erythematosus (see the image below).[26] Vacuolar changes of the columnar epithelium and lymphocytic inflammatory infiltrates at the dermal-epidermal junction basement membrane can occur.
Histopathology of dermatomyositis is interface dermatitis. Findings on muscle biopsy can be diagnostic. Muscle biopsy in patients with dermatomyositis reveals perivascular and interfascicular inflammatory infiltrates with adjoining groups of muscle fiber degeneration/regeneration (see the image below). This contrasts with polymyositis infiltrates, which are mainly intrafascicular (endomysial inflammation) with scattered individual muscle fiber necrosis.
Histopathology of dermatomyositis showing inflammatory myopathic changes with a predominantly perivascular chronic inflammatory infiltrate. Although inflammation is the histologic hallmark of dermatomyositis, polymyositis, and inclusion-body myositis, dermatomyositis is the only 1 of the 3 that shows perifascicular atrophy. In addition, many fibers undergo degeneration and necrosis that cause them to lose their staining ability; therefore, they are termed ghost fibers. When these changes are associated with collections of inflammatory cells around the blood vessels, the diagnosis of dermatomyositis is certain (see the images below).
Hematoxylin and eosin paraffin shows dermatomyositis. In dermatomyositis, inflammation is characteristically perivascular and perimysial. Vessel oriented approximately vertically in center has mild perivascular chronic inflammatory infiltrate. Endothelium is plump; wall is not necrotic. A few lymphocytes in wall of vessel are probably in transit from lumen to external aspect of vessel. Some observers may interpret this finding as vasculitis, but it is certainly neither necrotizing vasculitis nor arteritis. Image courtesy of Roberta J. Seidman, MD.
Hematoxylin and eosin frozen section shows perifascicular atrophy in dermatomyositis. Fascicles in this sample show atrophy, predominantly at periphery, along connective-tissue border. Ischemia is considered to cause perifascicular atrophy. This finding is characteristic of dermatomyositis, mostly associated with juvenile form but also observed in adult form. Image courtesy of Roberta J Seidman, MD.
Immunofluorescence for membrane attack complex of complement (MAC) in dermatomyositis. Bright ring of yellow-green fluorescence at center represents MAC in wall of microvessel. Finding was not present after treatment with steroids. Image courtesy of Roberta J Seidman, MD. Callen JP. Dermatomyositis. Lancet. Jan 1 2000;355(9197):53-7. [Medline].
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