Dermatomyositis is an inflammatory skin and muscle disease that is related to polymyositis; in fact, both conditions are considered to be the same disease except for the presence or absence of any skin findings. Cutaneous manifestations without detectable muscle disease are known as amyopathic dermatomyositis.

The female-to-male ratio is 2:1.

An autoimmune origin, which may be initiated by a virus in genetically susceptible people, has been proposed as a possible cause of dermatomyositis. As a result, antibodies that attack the skin and muscle are produced.

An overlap syndrome with scleroderma or lupus (mixed connective tissue disease) is characterized by the presence of antiribonucleoprotein (anti-RNP) antibodies.

Adults with dermatomyositis appear to have an increased risk of certain malignant diseases and the skin disease often follows the clinical course of exacerbations and remissions of the cancer. Most malignant diseases are the common cancers (e.g., colon, ovarian, and breast cancer). Usually dermatomyositis antedates tumor by 1 to 2 years.

The heliotrope rash consists of red or violaceous coloration around the eyes and is associated with periorbital edema (Fig. 34.25). The change in color may be a subtle clinical finding, particularly in dark-skinned patients.

Gottron papules consist of erythematous or violaceous, flat-topped papules on the dorsa of the hands (Fig. 34.26). Lesions are located on the joints of the fingers; they begin as papules and subsequently become atrophic and hypopigmented.

Poikiloderma is a characteristic eruption of dermatomyositis, consisting of telangiectasia, atrophy, hyperpigmentation, and hypopigmentation. Poikiloderma occurs on the extensor aspects of the body, upper back (shawl sign), forearms, and “V” of the neck (Figs. 34.27 and 34.28). Atrophic lesions may occur particularly on the knees and elbows.

Periungual telangiectasias (Fig. 34.29) and nail dystrophy occur (see Fig. 34.17). Nail fold changes also may display a characteristic hypertrophy of the cuticles (ragged cuticles [Fig. 34.30]).

“Mechanics hands” describe a roughening and fissuring of the tips and sides of the fingers, resulting in irregular, dirty-appearing lines that resemble those of a manual laborer.

Scalp involvement in dermatomyositis is relatively common and manifests as an erythematous to violaceous, psoriasiform dermatitis that can be very itchy. Clinical distinction from seborrheic dermatitis or psoriasis is occasionally difficult.

Progressive, bilateral, symmetric, proximal muscle weakness develops, as suggested by difficulty with brushing or combing hair and standing from a seated position.

Muscle tenderness or pain is usually not a complaint.

Photosensitivity is evidenced in areas of poikiloderma.

Arthralgias occur in one third of patients.

Pulmonary fibrosis affects 10% of patients, particularly in the presence of anti-Jo 1 (histidine) or anti-PL 12 (alanine) antibodies.

Evidence of vasculitis (e.g., palpable purpura or ulcers) may be present.

Calcinosis cutis is seen in the juvenile form of dermatomyositis (see Chapter 10: Cutaneous Manifestations of Systemic Disease).

Myocardial disease may be an associated finding.

Dysphagia may occur.

There may be features of an overlap syndrome or a mixed connective tissue disease.

The diagnosis is usually made on clinical grounds

A magnetic resonance imaging (MRI) is the most useful study for the assessment of a myositis.

Elevation of creatine phosphokinase levels is often a reliable indicator of muscle involvement.

Aldolase levels may be increased.

Electromyography (abnormal in about 90% of active cases).

A muscle biopsy may aid in the diagnosis.

Findings on skin biopsy are often nonspecific but are generally suggestive of a connective tissue disease resembling systemic lupus erythematosus.

The presence of autoantibodies, such as anti-DNA, anti-RNP, and anti-Ro, may be found. Anti-M-1 antibody is highly specific for dermatomyositis, but it is present in only 25% of patients.