Prurigo pigmentosa, also known as Nagashima disease, is a rare inflammatory dermatosis characterized by pruritic erythematous papules, papulovesicles, and vesicles and prominent postinflammatory hyperpigmentation. Lesions at varying stages may be present. They form a reticular configuration in a symmetric distribution on the trunk or neck. The disease is found predominantly in Japan, but reports from other countries are increasing. The disease is seen most commonly in girls and women (primarily adolescents and young adults), but it is also seen in men.

The etiology of prurigo pigmentosa remains unknown. Endogenous and exogenous factors have been proposed. Potential endogenous factors include ketosis (such as from dieting or fasting), diabetes mellitus, and pregnancy. Exogenous factors may include friction (eg, from clothing) and contact allergic reactions (eg, to chrome or nickel, trichlorophenol, or para-amino compounds). Associations with other conditions have been reported, including adult-onset Still disease, Sjögren syndrome, atopy, and Helicobacter pylori infection.

The exact pathogenesis of prurigo pigmentosa is not clear. Neutrophil-mediated inflammation induced by ketosis has been suggested.