Large Plaque Parapsoriasis

A 66-year-old woman is concerned about a gradually increasing rash located on the buttocks, lower back, abdomen, and upper, posterior thighs. The eruption has been present at least 18 months and is mildly pruritic. Her feet and toenails are normal. She has no house pets.

Key point: The annular, scaling erythema should suggest tinea corporis. However, a potassium hydroxide preparation and fungal culture were both negative. A biopsy disclosed dense, dermal infiltrates consisting of lymphocytes, some of which appeared atypical. This picture is diagnostic of a rare entity: large plaque parapsoriasis. This entity is seen mostly in older individuals, and is classically located in the “bathing suit” distribution.

Treatment: Phototherapy is the treatment of choice, utilizing special wavelengths of ultraviolet light. Ultrapotent topical corticosteroid application is used to supplement light treatment.

Note: Parapsoriasis is considered to be a precursor to cutaneous T-cell lymphoma. Long-term follow-up, with repeated biopsies of any recalcitrant areas, is mandatory.