Canine orbital pseudotumors : a review of 9 cases

The term orbital pseudotumor was introduced by Birch-Hirschefeld (1905) to describe a space-occupying orbital mass grossly consistent with a neoplasia but histologically composed of a mixed inflammatory infiltration. More recently the definition idiopathic orbital inflammation -(IOI) hasbeen proposed for this entity. Aim of this study is to review and immunohistochemically characterize 9 cases of canine orbital lesions previously diagnosed as orbital pseudotumors. Six cases were from Veterinary Pathology Archive (University of Milan), 4 cases were provided by Dr Dubielzig (COPLOW, US). Microtomic section were Hematoxylin and eosin stained and immunolabelled with antibodies anti vimentin, alpha smooth muscle actin (-SMA), MHCII, lysozyme, CD3 and CD20. Affected dog were aged 4.5 to 13 years (mean 7.7); 5 males/4 females. Different breeds were represented. Three pseudotumors affected the dorso-lateral portion of the orbita with lacrimal gland infiltration in 2/3. Six pseudotumors were located deep in the orbita without connection with any specific structures. A nodular mass, with focal infiltration of surrounding tissues, was surgically removed preserving the eye in 6/9 cases. In 3/9 cases the mass required orbital exenteration. Histologically, all lesions were composed of a mixed inflammatory cell population: in 6/9 macrophages/histiocytes predominate, with a variable number of lymphocytes, scattered plasmacells, neutrophils, large fibroblasts and occasional eosinophils (granulomatous pattern). In 3/9 cases, macrophages/ histiocytes, lymphocytes and plasmacells were almost equally represented, with fewer granulocytes. In 2/3 cases, affecting the lacrimal gland, large bundles of dense, collagen rich fibrous tissue, were evident. These cases were histologically consistent with the sclerosing form of human IOI. Immunohistochemically, histiocytes were consistently MHCII stained, scattered macrophages were lysozyme positive . Lymphocytes T (CD3) and B (CD20) were always present, CD3+ T cells predominating in 5/9 cases. In 2 cases sclerosing IOI-like pseudotumors, fibroblasts were -SMA and vimentin stained (myofibroblast), fibroblast in granulomatous pseudotumors were -SMA negative/vimentin positive. In the present review of canine orbital pseudotumors, all lesions were consistent with an idiopathic inflammation of orbital soft tissues. The authors propose that, consistently with human medicine, the definition "idiopathic orbital inflammation (IOI)" is adopted. Granulomatous inflammation was the most common histological-type in dogs. This pattern was histologically and immunohistochemically strikingly similar to another idiopathic orbital condition: canine nodular granulomatous episcleritis (NGE). Further study could elucidate if NGE and granulomatous-IOI are actually the same disease in different locations. In 2 cases pseudotumors affected the lacrimal and were associated with prominent fibroblastc/myofibroblastic proliferation. In men it has been suggested that these cases could represent chronic dacryoadenitis and that the release of lacrimal secretion could induce fibroblast/myofibroblast proliferation.