The pathophysiology of pityriasis alba: Time-dependent histologic changes.

Although pityriasis alba is a common dermatologic condition, its pathogenesis is poorly understood, and there are many discrepancies in the literature. To assess the effect of the duration of disease on the histologic findings, a search of cases labeled "pityriasis alba" was performed on any cases submitted to our dermatopathology laboratory. Of 179 cases of pityriasis alba, five cases identified the duration of the disease, when the biopsy was taken. A biopsy for a lesion of only 1-month duration demonstrated groups of large, prominent melanocytes heaped up upon one another. Compared with biopsies from patients who had the lesions for increasingly longer periods of time, it was apparent that the melanocytes became progressively less abundant and smaller with less prominent dendritic processes. The time that the biopsy is taken may affect the histologic findings of pityriasis alba. Additionally, an abundance of melanosomes was observed between the melanocytes in all sections examined which may reflect a problem with the transfer of melanosomes into keratinocytes in this condition.