Infection-induced panniculitis.

We evaluated the histopathologic findings in 15 cases of panniculitis due to infection. Organisms were identified by special staining in 14 cases, and in 6 of these the etiologic agent was confirmed by culture studies. In one additional case, lesional culture was positive despite negative special staining. Eight cases were caused by gram positive or gram negative bacteria, 2 by atypical mycobacteria, 2 by Nocardia, and 3 by fungi (Candida and Fusarium species). Most patients had conditions or were receiving therapies predisposing to immunosuppression, but at least 2 individuals had no other major medical problems. There were some variability in microscopic findings and overlap with established forms of panniculitis; 3 cases (2 due to bacteria and 1 to Fusarium) resembled acute, "neutrophilic" erythema nodosum, and evidence of vasculitis was seen in septicemias due to Pseudomonas, Nocardia, and Fusarium. Nevertheless, several distinctive features associated with infectious panniculitis of diverse etiologies included: epidermal alterations such as acanthosis and parakeratosis; dermal edema with a diffuse or perivascular neutrophilic infiltrate; and mixed septal-lobular panniculitis with neutrophilic infiltration, vascular proliferation and hemorrhage, and necrosis. Infection should be suspected in virtually any case of panniculitis, particularly when occurring in an immunosuppressed patients, but the microscopic features enumerated above should arouse particular suspicion of an infectious process.