The most common primary anterior uveal tumors include melanoma of the iris or ciliary body, and adenomas or adenocarcinomas of the ciliary body.
Melanomas are the most common primary uveal tumor. In dogs, iris melanomas typically appear as dark-brown (pigmented) raised nodules within the iris itself. Ciliary body melanomas may appear as a brown mass within or behind the pupil, or as a brown spot that develops within the white of the eye.
Melanomas may be benign and may remain confined to a localized area of the iris and slowly enlarge. Benign ciliary body melanomas gradually enlarge and disturb nearby structures within the eye. Malignant melanomas tend to spread extensively through the eye. They alter the shape and thickness of the iris, distort the pupil, may grow towards the back of the eye, may partially dislocate the lens, and may extend through the sclera (white covering of the eye). Glaucoma may also develop.
Some melanomas lack the typical brown/black pigmentation and are pink/white in color. These are called amelanotic melanomas. All amelanotic melanomas are considered malignant.
All anterior uveal melanomas have malignant potential. However, in the dog most anterior uveal melanomas are benign when they develop in middle-aged to older dogs (8 to 10 years). In contrast, uveal melanomas that develop in dogs less than four years old tend to be malignant and more likely to expand rapidly within the eye. Darkly pigmented dog breeds
, specifically the German shepherd
, Labrador retriever, Weimaraner and boxer have a higher risk for developing uveal melanomas.
The most common secondary anterior uveal tumor is lymphosarcoma. Usually the lymphosarcoma in the eye is one component of widespread cancer through the body. It is rare for lymphosarcoma to appear only in the eye alone. Other uveal tumors represent the spread of malignant tumors from some other location in the body, and these include metastatic mammary carcinoma, renal carcinoma, hemangiosarcoma, transmissible venereal tumor, and malignant melanoma of the skin or oral cavity. Metastasis to the anterior uvea from any kind of malignant tumor is possible, although most metastatic tumors appear in the choroid rather than in the anterior uvea.
Some other ophthalmic diseases or conditions can mimic the signs that develop with anterior uveal tumors. It is important to exclude these conditions before establishing a conclusive diagnosis. Severe uveitis
Traumatic injuries to the eye
Chronic anterior uveitis with hyperpigmentation of the iris
Ocular melanosis (pigmentary glaucoma) in Cairn terriers, West Highland white terriers and Scottish terriers
Conjunctival and scleral tumors
Conjunctival and scleral inflammatory masses that look like tumors
Old hemorrhage within the eye