Subcutaneous Nodules at Injection Sites: Histopathological Pitfalls in Diagnosis

Abstract

Subcutaneous swellings uncommonly occur at vaccination or desensitization injection sites and may present long after the procedure. Such lesions can show a range of histopathological appearances that are not well recognized, often leading to erroneous diagnoses. We describe the clinicopathological features of 13 cases (12 adults, 1 child) with subcutaneous swellings presenting at injection sites (upper arm n12, thigh n1). In the cases studied the histological appearances were varied, although predominantly the subcutis was involved. Six cases showed a mixed panniculitis with focal fat necrosis and fibrosis. Four cases demonstrated a pseudolymphomatous pattern, mimicking a B cell lymphoma. One case was reminiscent of lupus profundus and 3 other cases resembled deep granuloma annulare. Importantly, all cases contained histiocytes with violaceous granular cytoplasm, representing ingested aluminium salts employed for adsorption of antigens in the injection preparations. Energy dispersive x‐ray microanalysis in 8 cases confirmed a positive aluminium peak. Electron microscopy in 3 cases showed crystalline material in the histiocytes. In conclusion, the spectrum of histological appearances in so called aluminium granuloma can be misleading and it is important to identify the characteristic histiocytes of this condition, for otherwise erroneous diagnoses, with serious clinical implications, can follow.