Test your medicine knowledge with the MKSAP challenge, in partnership with the American College of Physicians.

A 62-year-old woman is evaluated for a “sun allergy,” manifesting as a rash on her scalp, eyelids, upper back, and knuckles. This started 2 years ago, but is less pronounced over the winter months. Medical history is unremarkable. Her only medication is hydrocortisone cream.

On physical examination, vital signs are normal. Pink-violet edematous macules are present on the eyelids. The scalp has pink-violet diffuse scaly macules with some thinning of the hair. There is a poikilodermatous pink scaly plaque on the upper back and posterior neck. The metacarpophalangeal and proximal interphalangeal joints have pink-violet flat-topped papules on their dorsal surface. There are dilated periungual capillary loops on each finger, and all 10 cuticles are dystrophic. There are no swollen joints, no abnormalities on strength testing, and no oral ulcerations.

Laboratory studies show antinuclear antibody titer of 1:640. All other laboratory values, including creatine kinase, aldolase, aspartate aminotransferase, and gamma-glutamyl transferase, are normal.

Which of the following is the most likely diagnosis?

A. Amyopathic dermatomyositis
B. Mixed connective tissue disease
C. Polymorphus light eruption
D. Systemic lupus erythematosus

MKSAP Answer and Critique

The correct answer is A. Amyopathic dermatomyositis.

This patient has amyopathic dermatomyositis, an underrecognized presentation of dermatomyositis that does not include muscle disease. These patients experience characteristic cutaneous features of dermatomyositis, such as the heliotrope sign, shawl sign, and Gottron papules, but muscle enzymes and strength testing are normal. Skin findings in dermatomyositis are photosensitive and tend to flare after sun exposure. Amyopathic dermatomyositis carries similar risk for underlying malignancy and pulmonary fibrosis. There is no strong consensus on how to screen for malignancy.

Mixed connective tissue disease is an overlap syndrome that includes features of systemic lupus erythematosus (SLE), systemic sclerosis, and/or polymyositis in the presence of anti-U1-ribonucleoprotein antibodies. Skin manifestations include sclerodactyly, scleroderma, calcinosis, telangiectasias, photosensitivity, malar rash, and Gottron rash. The absence of findings consistent with SLE and systemic sclerosis make this diagnosis unlikely.

Polymorphous light eruption (PMLE) is the most common idiopathic photosensitivity disorder. PMLE typically manifests before the age of 30 and is most common in fair-skinned women, first appearing in the spring and early summer. The rash will persist for weeks and resolve without scarring, even with continued exposure to the sun. Lesions appear within hours of sun exposure and are found on sun-exposed body parts. Although many different types of eruptions may occur, the most common are pruritic skin-colored or pink papules. PMLE does not explain the positive antinuclear antibody titer or periungual findings.

The most specific skin manifestations of systemic lupus erythematosus are variations on interface dermatitis, with pink-to-violet macules or plaques and varying scale or atrophy. Cutaneous lupus as a rule spares the upper eyelids and the knuckles. Inflammatory joint involvement occurs in 90% of patients with SLE. The absence of joint findings, rash over the knuckles, and periungual findings are not consistent with SLE.

Key Point

• Amyopathic dermatomyositis presents with skin findings characteristic of dermatomyositis, but without clinical or laboratory evidence of muscle disease; it carries risks for underlying malignancy.

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