pityriasis

(originally) any of a group of skin diseases typified by the development of fine branlike scales. The term is now used only with a modifying adjective. Pityriasis alba is a common condition in children in which pale scaly patches occur on the face; it may be related to atopic ECZEMA . Pityriasis rosea is a common skin rash, believed to be viral in origin, that starts with a single patch (a herald patch) on the trunk and is followed by an eruption of oval pink scaly MACULE . The spots are often aligned along the ribs. The rash clears completely in about eight weeks. Pityriasis versicolor is a common chronic infection of the skin caused by the fungus PITYROSPORUM , which is a normal inhabitant of the scalp. In susceptible people it changes to a pathogenic form called Malassezia furfur and produces a persistent depigmented scaly rash on the trunk. Treatment with SELENIUM SULPHIDE (Selsun) shampoo or with oral itraconazole readily kills the organism but the skin may take months to regain its normal colour. See also DANDRUFF .