Pruritus often produces distinctive skin lesions, characterized by angulated borders. Endogenous diseases (in common parlance, an “inside job”) tend to produce lesions that are rounded in character, whereas exogenous caustic agents, scratching, and other forms of external trauma tend to produce angulated, linear, or geometric shapes (signs of an “outside job”).
Pruritus can also result in the isomorphic (Koebner) phenomenon, where lesions of an endogenous disease localize in areas of trauma. These lesions often demonstrate a hybrid morphology, suggesting both endogenous origin and external trauma.
Lichenification results from chronic scratching or rubbing and is characterized by hyperkeratosis and papillary dermal fibrosis. The resulting clinical morphology includes slight induration and accentuation of skin markings. Excoriation results in eosinophilic necrosis of the granular layer. The corneum can remain intact, but more severe excoriation results in loss of the corneum and viable epidermis. More pronounced trauma can result in ulceration, in which the injury extends to the level of the dermis.
Broad areas of lichenification are characteristic of lichen simplex chronicus. Discrete papules with focal evidence of excoriation are typical of prurigo nodularis and arthropod bites, including those caused by bedbugs. This portion of the atlas will guide you through the clinical manifestations related to pruritic disorders.