Calcium oxalate crystal deposition in epithelioid histiocytes of granulomatous lymphadenitis: analysis by light and electronmicroscopy

In this study of 55 cases of granulomatous lymphadenitis of various aetiologies, both haematoxylin and eosin stained and unstained sections were examined by light and polarizing light microscopy for crystals within epithelioid histiocytes. This investigation was prompted by a case of granulomatous lymphadenitis in which the identification of ovoid birefringent structures within epithelioid histiocytes led to an initial false suggestion of foreign body reaction. Identical single, small, ovoid or biconvex, intensely birefringent crystals, invisible by ordinary light microscopy, were found within the cytoplasm of mononuclear epithelioid histiocytes in 37 cases. There was only minor crystal loss with routine H & E staining. The sections were also examined by scanning electronmicroscopy in secondary and backscattered electron imaging modes and 27 cases were found to contain biconvex crystals which were shown on X-ray energy dispersive spectroscopic microanalysis to be consistent with calcium oxalate. There was a good quantitative correlation between light and scanning electron microscopy. Tissue from the index case was also examined by transmission electronmicroscopy, and electron dense crystals were identified. We confirm the previous findings of ovoid oxalate crystals in a variety of granulomatous conditions including Mycobacterial infection, sarcoidosis and Crohn's disease. Ovoid crystals of calcium oxalate, a common finding in granulomatous lymphadenitis, appear to be endogenously derived, and should be more widely recognized in order to avoid misdiagnosis of foreign body reaction.