Pityriasis rosea.

Pityriasis rosea is a common, acute exanthem of uncertain etiology. Viral and bacterial causes have been sought, but convincing answers have not yet been found. Pityriasis rosea typically affects children and young adults. It is characterized by an initial herald patch, followed by the development of a diffuse papulosquamous rash. The herald patch often is misdiagnosed as eczema. Pityriasis rosea is difficult to identify until the appearance of characteristic smaller secondary lesions that follow Langer's lines (cleavage lines). Several medications can cause a rash similar to pityriasis rosea, and several diseases, including secondary syphilis, are included in the differential diagnosis. One small controlled trial reported faster clearing of the exanthem with the use of erythromycin, but the mechanism of effect is unknown. Resolution of the rash may be hastened by ultraviolet light therapy but not without the risk of hyperpigmentation. Topical or systemic steroids and antihistamines often are used to relieve itching.