Pityriasis alba: a histologic study.

BACKGROUND: Pityriasis alba is a common dermatitis, but only a few histologic studies have been reported and most of these maintain that its microscopic features are those of a chronic nonspecific dermatitis.

METHODS: A retrospective light microscopic study was performed on biopsy specimens taken from 39 patients with localized pityriasis alba on face, arms and forearms without signs of atopy. Thirty-two patients were girls or women and seven were boys or men, with ages ranging from 7 to 37 years. The duration of illness varied from 1 month to 10 years. The clinical picture was characterized by three clinical stages: papular erythematous, papular hypochromic, and smooth hypochromic.

RESULTS: The histologic study revealed features that were suggestive, though not pathognomonic, of the diagnosis. The most remarkable changes involved the pilary apparatus, which showed horny plugs, spongiotic edema, and atrophic sebaceous glands. There was also a disturbance of melanization secondary to epithelial damage. The tissue reaction was more conspicuous in the skin lesions with follicular papules.

CONCLUSIONS: A histopathologic diagnosis of pityriasis alba may be proposed when the following features are seen in a biopsy specimen taken from a skin lesion with follicular papules: (1) irregular pigmentation by melanin of the basal layer; (2) follicular plugging; (3) follicular spongiosis; and (4) atrophic sebaceous glands.