Unique histopathologic findings in a patient with adult-onset Still disease.

Adult-onset Still disease (AOSD) is an uncommon disorder characterized by fever, polyarthralgia, elevated white blood cell count, and a maculopapular rash, the histologic features of which have not been well-known. A 55-year-old Asian woman presented initially with a "burning" and severely pruritic eruption on her face, hands, and arms, thought clinically to be urticaria. Within 1 month, she began spiking high fevers, developed diffuse joint pain, and had marked elevations of ferritin, C-reactive protein, and erythrocyte sedimentation rate, characteristic of AOSD. The cutaneous eruption became more widespread, involving the trunk, scalp, and remainder of the extremities, with diffuse thickening of the skin with papular and linear hyperpigmentation and accentuation. Biopsies from several locations showed focal hyperkeratosis associated with dyskeratotic keratinocytes with a peculiar, distinctive distribution in the upper epidermis and cornified layers. In addition, increased dermal mucin was present, with minimal fibroblast proliferation and inflammation. This unusual combination of diffuse dermal mucinosis and a unique pattern of dyskeratosis can present a challenge in generating an accurate differential diagnosis, and may represent an unusual response to chronic scratching or be a distinctive histologic manifestation of AOSD.