The cause of pityriasis rosea is not known. An infec-tious agent has always seemed likely but has not yet been proven: human herpesvirus 7 is the latest sus-pect. The disorder seems not to be contagious.
Pityriasis rosea is common, particularly during the winter. It mainly affects children and young adults, and second attacks are rare.
Most patients develop one plaque (the ‘herald’ or ‘mother’ plaque) before the others (Fig. 6.1). It is larger (2–5 cm in diameter) than later lesions, and is rounder, redder and more scaly. After several days many smaller plaques appear, mainly on the trunk, but some also on the neck and extremities. About half of the patients complain of itching. An individual plaque is oval, salmon pink and shows a delicate scaling, adherent peripherally as a collarette. The configuration of such plaques is often characteristic. Their longitudinal axes run down and out from the spine (Fig. 6.2), along the lines of the ribs. Purpuric lesions are rare.
The herald plaque precedes the generalized eruption by several days. Subsequent lesions enlarge over the first week or two. A minority of patients have systemic symptoms such as aching and tiredness. The erup-tion lasts between 2 and 10 weeks and then resolves spontaneously, sometimes leaving hyperpigmented patches that fade more slowly.
Although herald plaques are often mistaken for ringworm, the two disorders most likely to be mis-diagnosed early in the general eruption are guttate psoriasis and secondary syphilis. Tinea corporis and pityriasis versicolor can be distinguished by the micro-scopical examination of scales, and secondary syphilis by its other features (mouth lesions, palmar lesions, condyloma lata, lymphadenopathy, alopecia) and by serology. Gold and captopril are the drugs most likely to cause a pityriasis rosea-like drug reaction, but barbiturates, penicillamine, some antibiotics and other drugs can also do so.
Because secondary syphilis can mimic pityriasis rosea so closely, testing for syphilis is usually wise.
No treatment is curative, and active treatment is seldom needed. A moderately potent topical steroid or calamine lotion will help the itching. One per cent salicylic acid in soft white paraffin or emulsifying oint-ment reduces scaling. Sunlight or artificial UVB often relieves pruritus and may hasten resolution.