History

A 12-year-old spayed female mixed-breed dog was referred for evaluation of exophthalmos of the left eye. Ophthalmic examination of the affected eye, including use of slit lamp biomicroscopy and indirect ophthalmoscopy, revealed an intact menace response, a dilated but responsive pupil, a normal lens, conjunctival hyperemia, exophthalmos, indentation of the ventrotemporal portion of the retina, and an intraocular pressure of 19 mm Hg (reference range, 15 to 25 mm Hg). Results of ophthalmic examination of the right eye were within normal limits.

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Images in a computed tomography study of the head of a 12-year-old spayed female mixed-breed dog evaluated for exophthalmos of the left eye. Five-millimeter slices with 50% contrast.

Citation: Journal of the American Veterinary Medical Association 231, 6; 10.2460/javma.231.6.863

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A CBC revealed mild leukopenia (5,500 WBC/μL; reference range, 6,000 to 17,000 WBC/μL) and a mildly high concentration of creatinine (1.9 mg/dL; reference range, 0.5 to 1.8 mg/dL). Thoracic radiography did not reveal abnormalities. Abdominal ultrasonography revealed multiple ill-defined hypoechoic nodules within the liver that were consistent with age-related hepatic nodular hyperplasia. Ocular ultrasonography followed by contrast^a computed tomography of the orbit were performed (Figure 1).

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Diagnostic Imaging Findings and Interpretation

Ocular ultrasonography revealed deformation of the caudal portion of the globe by a homogeneous soft tissue density mass. Contrast computed tomography revealed a round, lobulated soft tissue mass in the left orbit caudal to the eye (Figure 2). The mass deformed the posterior portion of the sclera and the vitreal chamber and appears to follow the optic nerve's path toward the optic foramen. Rostral displacement of the left globe is evident. There is no evidence of abscess, necrosis, orbital bone lysis, or invasion of the brain or sinuses. Because of the apparent involvement of the optic nerve, myositis and cellulitis were deemed less likely causes of the mass lesion than retrobulbar neoplasia.

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Same computed tomography images as Figure 1. A—Notice exophthalmia of the left eye. B—The rostral portion of the mass is visible within the orbit (arrow) and was closely associated with the posterior aspect of the globe. C—Notice the large bilobulated soft tissue density mass in the left retrobulbar space (arrows). The mass obscures the left optic nerve and was intimately attached to tissues of the caudal portion of the orbit.

Citation: Journal of the American Veterinary Medical Association 231, 6; 10.2460/javma.231.6.863

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Comments

Differential diagnoses for exophthalmos include retrobulbar abscess, retrobulbar cellulitis, zygomatic mucocele or sialocele, vascular anomaly, and neoplasia. Primary tumors of the orbit can arise from bone as well as vascular, glandular, epithelial, or connective tissues of the orbit. Clinical signs typically include unilateral exophthalmos, conjunctival hyperemia, protrusion of the nictitating membrane, and exposure keratitis. Retrobulbar or ocular neoplasms account for 4% of tumors seen in dogs and cats.¹ In dogs, most orbital neoplasms arise as primary tumors, whereas orbital neoplasms in cats are most commonly metastatic or locally invasive tumors arising from adjacent tissues.^1,2,3 Differential diagnoses for retrobulbar neoplasms of neural origin include meningioma, astrocytoma, glioma, benign or malignant schwannoma, neurofibroma, or neurofibrosarcoma. Seventy-five percent of retrobulbar tumors in dogs are malignant with a mean survival time of 13 months.¹

Fine-needle aspiration provides a definitive diagnosis in approximately 45% of retrobulbar masses but was not attempted in the dog of this report.^1,2 Complete orbital exenteration was performed. Grossly, a 1.5-cmdiameter firm white mass was intimately attached to the posterior portion of the globe and periocular tissues. The orbit was closed in a routine manner, and recovery was not complicated.

Histologically, the retrobulbar mass was composed of haphazardly interlacing bundles of neoplastic spindle cells supported by variable amounts of collagenous stroma. In 1 region, neoplastic cells had an Antoni type A arrangement with the formation of subtle Verocay bodies. Arrangement of the neoplastic cells as well as immunohistochemical evidence for vimentin, S100, and glial fibrillary acidic protein were consistent with a tumor of nerve sheath origin.⁴ Regions typical of Schwann cells accompanied by regions consistent with other perineurial cells supported classification of the tumor as a neurofibroma or neurofibrosarcoma.⁵ Cellular atypia and a moderate mitotic index (15 mitotic figures/10 random hpfs) indicated malignant potential for the final diagnosis of neurofibrosarcoma. Malignant nerve sheath tumors in dogs commonly recur following surgical excision but rarely metastasize.⁵ Documented cases of retrobulbar neurofibrosarcomas or other tumors of nerve sheath origin are rare in the veterinary literature.

Computed tomography performed 2 weeks after surgery did not reveal evidence of tumor recurrence in the orbit. Because of the risk of local recurrence for this type of neoplasm, radiation therapy was administered at a dose of 3 gray daily for 18 days (total dose, 54 gray). The dog was doing well 5 months after diagnosis and treatment.