This chapter includes granulomatous and histiocytic conditions that are either localized to the skin or can involve the skin as part of a systemic disease. Many will require a skin biopsy to confirm the diagnosis and direct further workup and treatment.
One common granulomatous condition included here is granuloma annulare (GA), a skin condition that typically presents on frictional sites, such as the dorsal feet, forearms, or legs, with skin-colored papules or nodules that form a ring. Other forms of GA can be more difficult to diagnosis, such as the perforating or deep (subcutaneous) forms. Sarcoidosis is another granulomatous condition with skin manifestations in the setting of what is a chronic systemic disease. Skin findings in sarcoidosis can vary and include papules, annular GA-like plaques, ichthyosiform patches, and nodules at site of tattoos. Screening for systemic involvement is essential when sarcoidosis is suspected because it can affect almost every organ in the body.
Histiocytic conditions are featured as well, including juvenile xanthogranulomas (JXG), Langerhans cell histiocytosis (LCH), and rarer conditions such as multicentric reticulohistiocytosis. JXG is a common pediatric condition most commonly limited to the skin in the form of a yellow/orange, smooth papule or nodule. LCH has more varied clinical presentations depending on the age and degree of systemic involvement. Common skin manifestations include erosive, red patches in the folds with petechiae; red-brown papules that can become crusted; xanthomatous nodules; and in adults acneiform eruption on the chest and back.