[Granulomatous reaction pattern of the skin : Interstitial granulomatous dermatitis - lymphoma - vasculitis].

A particular diagnostic situation in the classification of a granulomatous dermatitis results when no circumscribed granulomas but instead a diffuse interstitial histiocytic inflammatory infiltrate, a granulomatous vasculitis or a neoplastic lymphocytic infiltrate is found. Interstitial granulomatous dermatitis was originally described in patients with arthritis. Later, it was recognized that there are also associations with other usually immunological diseases. Differentiating between interstitial granulomatous dermatitis and the interstitial form of granuloma annulare, early morphea and variants of borreliosis or scleromyxedema as well as interstitial granulomatous drug reaction can be very difficult. In long-standing cutaneous granulomatous infiltrates, T‑cell lymphoma should be excluded. Occasionally only a small number of atypical lymphocytes can be found. The detection of a monoclonal T‑cell expansion is then particularly helpful. Only recently, a CD8-positive granulomatous cutaneous T‑cell lymphoma was described which occurred in patients with immunodeficiencies. A granulomatous vasculitis in the skin is extremely rare. According to the Chapel Hill classification from 2012, systemic granulomatous vasculitis is equated with giant cell arteritis. Extracutaneous large arteries are involved. On rare occasions, involvement of the temporal artery can result in skin necrosis. More commonly granulomatous infiltrates in combination with vasculitis can be observed, whereby various infectious diseases, sarcoidosis and nodular vasculitis should be considered. Granulomatosis with polyangiitis (formerly Wegener's granulomatosis), presents in the skin as leukocytoclastic vasculitis. Here granulomas are extremely rare.