An orbital paraganglioma as the cause of unilateral exophthalmus in a horse. [German]Original Title Ein orbitales Paragangliom als Ursache eines unilateralen Exophthalmus bei einem Pferd.

摘要

A fifteen-year-old Oldenburger warmblood gelding was presented because of unilateral exophthalmia. There had been chronic epiphora since the previous year and a lack of response to conventional topical treatment. By clinical examination and ultrasonography, a retrobulbar mass was found to be the cause of the observed clinical signs. The intraocular pressure and the vision of both eyes were not altered. On the request of the owner, euthanasia of the horse was elected and no further diagnostic workup or treatment was performed. After the gelding had been humanely destroyed, representative samples of the retrobulbar mass were collected for histological examination and processed routinely. The examined tissue samples contained a neoplastic cell population besides portions of striated muscle and parts of the optic nerve. By the use of immunohistochemistry and electron microscopy, the neuroendocrine origin of the neoplasia was confirmed, and the tumour was diagnosed as an equine orbital paraganglioma (EOP) according to the WHO histological classification of endocrine neoplasms. In general, the neoplastic cell population shows principally a benign histological appearance. The most striking and limiting factors regarding a successful healing or remission, however, are the infiltrative growth of the tumour and its localisation inside of the orbita, where it is located in close proximity to the optic nerve and is surrounded by strongly vascularised tissue. Only a few case reports refer to successful treatment via excision of the tumour mass, even in parts. Further differential diagnoses of unilateral exophthalmia include other retrobulbar neoplasms, orbital trauma and haemorrhage, abscesses, cellulitis, cysts or myositis.