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Dermatomyositis/Polymyositis: Differential Diagnoses & Workup

Author: Sushma Podila, MD, Resident Physician, Department of Neurology, Columbia Presbyterian Medical Center
Coauthor(s): Thomas H Brannagan III, MD, Associate Professor of Clinical Neurology and Director, Peripheral Neuropathy Center, Columbia University, College of Physicians and Surgeons; Co-Director, EMG Laboratory, New York-Presbyterian Hospital, Columbia Campus, New York
Contributor Information and Disclosures

Updated: Aug 28, 2009

Differential Diagnoses

Other Problems to Be Considered

Polymyositis is a diagnosis of exclusion.

Groups of diseases that should be excluded

Familial neuromuscular diseases
Systemic metabolic muscle diseases, eg, endocrinopathies and mitochondriopathies
Systemic medical illness, eg, malabsorption syndromes, alcoholism, cancer, vasculitis, granulomatous disease, sarcoidosis, or treatment with various known myotoxic drugs or toxins
Biochemical muscle diseases (eg, enzyme deficiencies)
Inclusion-body myositis (excluded by histologic findings)

Diseases differentiated by skin changes

The rash and subcutaneous calcifications are so characteristic of dermatomyositis that the diagnosis of dermatomyositis is very rarely in doubt. Some of the diseases from which it must be differentiated on the basis of skin changes are as follows:

SLE: The skin changes in the phalanges must be distinguished from dermatomyositis because, in SLE, the phalanges are involved and the knuckles are spared; in dermatomyositis, the reverse is true.

Eosinophilia myalgia syndrome: This was caused by the ingestion of contaminated L-tryptophan and has not been observed since the contaminated product was identified. The skin can be tight and shiny, but it is not erythematous. Joint contractures are common. Although the inflammatory process is confined mostly to the subcutaneous tissue, inflammation can spread to the muscle and cause myopathic muscle weakness.

Shulman syndrome (eosinophilic fasciitis): Patients with this syndrome present with joint contractures and thickening of the skin and subcutaneous tissue.

Other conditions

Other conditions that must be kept in mind when considering polymyositis and dermatomyositis are as follows:

Overlap syndromes: Polymyositis and, less commonly, dermatomyositis can be associated with other collagen-vascular diseases (ie, overlap syndromes). SLE, systemic sclerosis, rheumatoid arthritis, and Sjögren syndrome all may have weakness as a facet of the disease complex. Muscular weakness and atrophy are greater than what arthritis alone can account for. They are characterized by elevated titers of anti–U1/U2-ribonucleoprotein antibodies, PM-Scl antibodies or SSA antibodies in scleroderma, Sjögren syndrome, SLE, or mixed connective tissue disease. Dermatomyositis is rarely associated with other collagen-vascular diseases except for scleroderma.

Carcinoma with polymyositis or dermatomyositis: Polymyositis and especially dermatomyositis may be a part of paraneoplastic syndromes. About 10-20% of patients with dermatomyositis have neoplasms. Cancers of breast, lung, ovary, and stomach are most commonly implicated.

The risk of cancer is greater in patients with DM than PM. Other risk factors noted in individual studies include:

• Evidence of capillary damage on muscle biopsy
• DM complicated by cutaneous necrosis on the trunk
• Cutaneous leukocytoclastic vasculitis 
• Older age at diagnosis of DM or PM

Age-appropriate cancer screening tests (eg, mammography and colonoscopy) are a valuable part of the workup of a patient with DM or PM. Serum CA125, CA19-9, PSA, and stools for occult blood can be considered as part of workup. Some experts recommend screening female DM patients with a serum CA125 determination at least twice yearly, and the performance of annual pelvic and transvaginal ultrasonography for up to 5 years after the diagnosis of DM.

Workup

Laboratory Studies

• Polymyositis
  • The diagnosis of polymyositis is established by using serum enzyme levels and electromyography (EMG) studies and confirmed by means of diagnostic muscle biopsy.
  • The most sensitive enzyme is creatine kinase (CK). In the presence of disease, levels can be elevated as much as 50 times the reference level. The CK level usually parallels disease activity. In active polymyositis, it is rarely in the reference range. The level may also be in the reference range in some patients with polymyositis associated with connective-tissue disease.
  • Anti-Jo-1 antibodies are present in one fifth of patients.
  • Along with CK, levels of aldolase, serum aspartate aminotransferase (AST, or glutamic-oxaloacetic transaminase [SGOT]), serum alanine aminotransferase (ALT, or glutamic-pyruvic transaminase [SGPT]), and lactate dehydrogenase (LDH) may also be elevated. If AST levels are higher than ALT levels, a myogenic cause should be suspected.
• Dermatomyositis
  • Levels of serum CK are usually, though not always, elevated. Levels may be as high as several 100s to 1000 U/mL.
  • AST, ALT, LDH, and aldolase levels may also be elevated.
  • Cancer antigen 125 (CA125) and cancer antigen 19-9 (CA19-9) may be useful markers of the risk of tumors in patients with dermatomyositis and polymyositis. Therefore, tests of these markers should be included in the search for cancer in patients with dermatomyositis/polymyositis, especially those without interstitial lung disease

Imaging Studies

• MRI may show increased signal intensity in the affected muscles and surrounding tissues.
• Because of the lack of sensitivity and specificity, MRI is not helpful in diagnosing the disease; however, it can help in monitoring its progress, and it may be used to guide decisions regarding muscle biopsy.

Procedures

• EMG may be helpful in diagnosis, though findings can be normal in 15% of patients. It shows a typical myopathic pattern with irritability, although it is not specific for the condition. These findings are most consistently observed in weak proximal muscles. EMG also is helpful in selecting a muscle for biopsy.
• Specific findings are as follows:
  • In polymyositis, needle EMG shows myopathic potentials characterized by short-duration, low-amplitude polyphasic units on voluntary activation and increased spontaneous activity with fibrillations, complex and repetitive discharges, and positive and sharp waves, along with early recruitment.
  • In dermatomyositis, needle EMG shows myopathic features with indications of muscle irritability. The myopathic potentials are characterized by short-duration, low-amplitude, polyphasic units and increased spontaneous activity with fibrillations, complex repetitive discharges, and positive sharp waves. Recruitment pattern shows early recruitment.
• Single-fiber EMG (SFEMG)
  • In 1 study, SFEMG were recorded from the extensor digitorum communis of 34 patients with polymyositis or dermatomyositis and compared with findings on routine EMG, serum CK determination, and muscle biopsy.
  • SFEMG recordings were abnormal in all 34 patients. The prominent feature was markedly increased fiber density with normally or mildly increased jitter.
  • SFEMG is of great value in diagnosis and in evaluating the disease process to understand inflammatory myopathies in patients with clinically suspected disease but normal findings on routine EMG, CK determination, and muscle biopsy.

Histologic Findings

Polymyositis

Muscle biopsy is the definitive test not only for establishing the diagnosis of polymyositis but also for excluding other neuromuscular diseases. In polymyositis, inflammation is the histologic hallmark of the disease. The endomysial infiltrates are mostly in foci in the fascicles, initially surrounding healthy muscle fibers and finally invading these cells and resulting in phagocytosis and necrosis. Because the inflammatory infiltrates can be small and multifocal, they can be missed in a small muscle-biopsy specimen. Perifascicular atrophy or prominent perivascular infiltrates are not present, and the blood vessels are normal. When the disease becomes chronic, the connective tissue increases. The diagnosis of polymyositis is definite when a patient has subacute elevated levels of serum CK and findings on muscle biopsy consistent with the histologic features of polymyositis (see Images 1-4).

Findings on muscle biopsy can be diagnostic. Although inflammation is the histologic hallmark of dermatomyositis, polymyositis, and inclusion-body myositis, dermatomyositis is the only disease 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 Images 5-7).

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