Primary ocular neoplasms are relatively uncommon in dogs and cats. Ocular tumors may be divided into intraocular tumors, orbital tumors, and tumors arising from adnexal structures and the ocular surface. Intraocular melanocytic and orbital tumors will be discussed here; iridociliary tumors and conjunctival and eyelid neoplasms will additionally be visited during the lecture.

In both dogs and cats, the vast majority of primary intraocular tumors are melanocytic in origin. Distinct species differences in tumor location, gross and histologic appearance, and clinical progression exist between dogs and cats. In dogs, the most common intraocular neoplasm is anterior uveal melanocytoma.¹ These tumors are behaviorally benign but may be progressively expansile and are associated with complications such as uveitis and hyphema. Glaucoma may develop if the ciliary cleft is compressed and obstructed. Anterior uveal melanocytomas are distinguished from malignant uveal melanomas based on mitotic activity and the morphology of neoplastic cells. Uveal malignant melanoma is locally aggressive and associated with a poorer prognosis.

The most common feline intraocular neoplasm is diffuse iris melanoma.^2,3 Diffuse iris melanoma is characterized by progressive, asymmetrical darkening and thickening of the iris which may develop rapidly or over many years. Neoplastic melanocytes infiltrate the stroma of the iris and may extend into the ciliary cleft, overlying sclera, peripheral cornea, and ciliary body. Feline diffuse iris melanoma is often slowly progressive; however, biologic behavior is variable, and some tumors may progress rapidly or metastasize to distant sites.

Two unusual entities are peculiar to cats and, although they are rare, warrant discussion. Feline restrictive orbital myofibroblastic sarcoma (FROMS) is a debilitating progressive disease involving the orbit and surrounding connective tissue. The disease name is derived from the myofibroblastic immunohistochemical staining characteristics of neoplastic cells and the tendency of these tumors to restrict mobility of the eyelids and globe.⁴ FROMS was originally termed feline orbital pseudotumor, based on the benign histologic appearance of the lesion and accompanying inflammation. Although biopsy findings are often most consistent with granulation tissue or reactive fibroplasia, the disease is progressive with a poor prognosis. A second feline ocular sarcoma, termed feline ocular post-traumatic sarcoma (FOPTS), follows penetrating damage to the globe with rupture of the lens capsule and chronic ophthalmitis. FOPTS is likewise progressive with local infiltration of the optic nerve and metastatic potential.⁵ Accurate diagnosis of both entities requires consideration of the clinical history (e.g., history of trauma) and recognition of unusual clinical and histologic features such as orbital infiltration, concurrent inflammation, and lens rupture.

Submitting Eyes for Histopathologic Examination

Ocular pathology is a sophisticated subspecialty; both gross and histologic examinations of the eye are complex undertakings that benefit from care during specimen collection and submission. Where possible, avoid rupturing the globe during enucleation and collect the optic nerve head for evaluation. To assist fixation, gently trim adnexal tissue from the globe, unless the lesion of interest arises from the conjunctiva or eyelid. Intraocular structures are particularly vulnerable to anoxia, and the globe should be placed in fixative as soon as possible to avoid artifacts. Many pathologists prefer Davidson's solution or Bouin's solution to formalin for fixation of whole eyes, as formalin may not adequately preserve delicate intraocular structures such as the retina. If specialized fixatives are not available, however, 10% neutral buffered formalin will suffice. Avoid opening the globe and do not inject fixative into the eye, as this may introduce gross and histologic changes that complicate interpretation.

References

1.  Giuliano EA, et al. A matched observational study of canine survival with primary intraocular melanocytic neoplasia. Vet Ophthalmol. 1999;2:185–190.

2.  Dubielzig RR. Tumors of the eye. In: Meuten DJ, ed. Tumors in Domestic Animals. 4th ed. Ames, IA: Iowa State Press; 2002:739–748.

3.  www.vetmed.wisc.edu/pbs/dubielzig/pages/coplow/powerpoints.html

4.  Bell CM, Schwarz T, Bubielzig RR. Diagnostic features of feline restrictive orbital myofibroblastic sarcoma. Vet Pathol. 2011;48(3):742–750.

5.  Dubielzig RR, et al. Clinical and morphologic features of post-traumatic ocular sarcomas in cats. Vet Pathol. 1990;27(1):62–65.