Interstitial granulomatous dermatitis? An unusual presentation in the mucosa and periungual skin.

Interstitial granulomatous dermatitis (IGD) is a reactive phenomenon accompanying disorders including autoimmune disease, lymphoproliferative disorders and drug reactions. Histologically, IGD shows a granulomatous infiltrate surrounding piecemeal fragmentation of collagen in the diffuse interstitium. IGD presents with linear cords, papules and plaques mainly on the trunk and extremities. Herein, we report a case with peculiar clinical features that were histopathologically consistent with IGD. A 74-year-old man presented with periungual painful erythema, nail deformity of all fingers and labial, penile and anal erosive erythema. Histopathological examination of the lesions showed interface dermatitis and a diffuse interstitial granulomatous infiltrate mainly composed of CD68-positive histiocytes and lymphocytes. Degenerative collagen bundles were also observed in granulomas. C-reactive protein and the white blood cell count were elevated, but further examinations did not reveal systemic inflammatory disorders such as autoimmune disease, lymphoproliferative disorder, inflammatory bowel disease or drug hypersensitivity. The lesions were successfully treated with oral and topical steroids.