Dermatomyositis (DM) is a type of idiopathic inflammatory myopathy characterized by evocative skin lesions and symmetrical proximal muscle weakness.
Annual incidence is estimated between 1 to 10 new cases/million population/year, and prevalence between 1/50,000 and 1/10,000. DM is more common in women than in men (2:1). In the United States, a Black-to-Caucasian ratio of 3:1 has been reported.
Onset is generally in adulthood, in some cases earlier (Juvenile DM, see this term). Patients typically present a heliotrope rash (erythema of eyelids with or without edema) and Gottron's papules (lichenoid papules over knuckles and sometimes knees, elbows), violaceous erythema (on extensor surfaces and face), poikiloderma (photoexposed areas), and periungual telangiectasias. In rare cases, cutaneous vasculitis, ulcerations, and calcinosis are observed. Subsequently, over weeks or months, they develop symmetrical proximal muscle weakness with a variable impact on physical capacities. Other systems may then become involved (vascular, pulmonary, gastrointestinal, and cardiac). Pulmonary manifestations range from aspiration pneumonia to interstitial lung disease (ILD), sometimes with complications such as pulmonary arterial hypertension (see these terms). Other features may include dysphagia, sinus tachycardia, diastolic dysfunction and myocarditis (often asymptomatic). About one third of patients develop malignancy, often within 0 to 3 years before or after disease onset (breast and ovarian cancers (see this term) in women, and lung and prostate cancer in men). Other less frequently reported neoplasms include colorectal cancer, or non-Hodgkin lymphoma, pancreatic, gastric and bladder cancer (see these terms).
The exact pathogenesis has not yet been elucidated. DM is thought to be related to complement-mediated changes in small vessels in muscle tissue leading to vascular damage. Viruses and Toxoplasma and Borrelia species, have been suggested as possible triggers.
Diagnosis is based on the characteristic skin findings with development of proximal muscle weakness, elevated muscle enzymes (serum creatine kinase, aldolasa) and myopathic findings on electromyography (EMG). It is generally confirmed by muscle biopsy showing inflammatory infiltrates around blood vessels and perifascicular atrophy. Patients frequently have circulating autoantibodies antinuclear antibodies (ANAs) and more specific antibodies may be present (e.g. anti-Mi2, anti-Tiff1gamma, anti-MDA5).
The differential diagnoses include muscular dystrophies of late onset, as well as adult-onset nemaline myopathy, proximal myotonic myopathies and systemic lupus erythematosus, pityriasis rubra pilaris, lichen planus (see these terms), and polymorphous light eruption.
The aim of treatment is to eliminate inflammation and restore muscle performance. Initial treatment includes high-dose corticosteroids. Dosage is then tapered to reach an appropriate maintenance dose. Immunosuppressive agents are also frequently used in combination, typically methotrexate, azathioprine, and mycophenolatemofetil. Intravenous immunoglobulin (IVIg) or intravenous methylprednisolone (IVMP) may be used in severe cases. Physical therapy is also recommended. Topical corticosteroid and tacrolimus have been used to treat skin manifestations. Patients should avoid direct UV light and use high-factor sunscreen. Monitoring for extramuscular involvement should include chest X-ray and pulmonary function testing. If cardiac involvement is suspected, echocardiography is recommended. Age-appropriate cancer screening is also recommended.
Prognosis is sometimes poor and depends on patient response to treatment, severity of disease manifestations and comorbidities (notably associated cancer). Long-term corticosteroids may be a source of morbidity.
Last update: May 2014