Interstitial granulomatous drug reaction with a histological pattern of interstitial granulomatous dermatitis.

The interstitial granulomatous drug reaction (IGDR) is a novel drug-associated entity, characterized by violaceous plaques with a predilection for skin fold areas. Light microscopically, it resembles the incipient diffuse interstitial phase of granuloma annulare. Differentiating light microscopic features include the absence of complete collagen necrobiosis, the presence of interface dermatitis, and variable lymphoid atypia. The lack of vasculitis rules out the extravascular necrotizing granuloma (Winkelmann granuloma) associated with systemic disease. The differential diagnosis with interstitial granulomatous dermatitis with arthritis as defined by Ackerman et al. has not been studied until now. Our aim was to determine the histologic criteria allowing us to differentiate IGDR without interface dermatitis and lymphoid atypia from interstitial granulomatous dermatitis. We report three patients with IGDR triggered, in two cases by respectively angiotensin convertin enzyme (ACE) inhibitors and furosemide, and in one case by the association of an ACE inhibitor, furosemide, and fluindione. Histologic examination showed a histological pattern of interstitial granulomatous dermatitis. We found a dense, diffuse histiocytic infiltrate distributed interstitially and in palisaded array within the reticular dermis. Eosinophils and some neutrophils were scattered throughout the infiltrate. In some tiny foci, enveloped by histiocytes, thick collagen bundles associated with basophilic nuclear debris or "flame figures" were seen. Vasculitis, interface dermatitis, or lymphoid atypia were absent. Our study allowed us to expand the histological spectrum of IGDR including a histological pattern similar to interstitial granulomatous dermatitis. The lack of degenerated collagen could be a subtle clue in favor of interstitial granulomatous dermatitis triggered by a drug.