Overview of Canine Tumors of the Anterior Uvea
The uvea in the eye consists of three parts: the iris, which is the colored portion of the eye behind the cornea; the ciliary body, which is the tissue immediately behind the iris lying between the iris and the choroid; and the choroid, which is the vascular layer in the back of the eye that supplies nutrition to the dog’s retina. The iris and ciliary body make up the anterior uvea and the choroid is called the posterior uvea.
Tumors occurring in the anterior uvea involve the iris, ciliary body, or both tissues. They may originate from cells within these tissues (primary tumors) or they may spread to the eye from other sites (secondary tumors).
Melanomas are the most common primary uveal tumor. In dogs, melanomas usually appear as dark brown (pigmented), raised nodules within the iris and ciliary body. Some melanomas of the iris are benign and remain confined to an area of the iris and slowly enlarge. Some melanomas of the ciliary body are benign, but gradually enlarge and do damage to nearby tissues in the eye. Other melanomas of the iris and ciliary body are malignant and spread not only within the eye, but may also extend into the tissues around the eye, to the brain and to other organs such as lymph nodes and the liver.
Other primary tumors include the a benign tumor of the ciliary body (ciliary body adenoma), a malignant tumor of the ciliary body (adenocarcinoma), and a tumor that arises from embryonic tissues of the eye (medulloepithelioma).
The most common secondary anterior uveal tumor is lymphosarcoma. Metastasis to the anterior uvea from malignant tumors of other body organs is also possible.
Advanced anterior uveal tumors in many animals cause eye pain and blindness from inflammation (uveitis), bleeding within the eye (hyphema), elevations in pressure within the eye (glaucoma), and damage to nearby structures of the eye.
What to Watch For
Physical symptoms of illness may be seen with secondary (metastatic) uveal tumors because these tumors originate in other body organs and are usually present in that organ for some time before they spread to the eye. Systemic signs that may be seen include:
Diagnosis of Anterior Uvea Tumors in Dogs
Diagnostic tests for anterior uveal tumors often include:
If a tumor is suspected or confirmed on the eye examination, then further testing may be warranted and may include the following:
Treatment of Anterior Uvea Tumors in Dogs
Medical treatment alone is not enough to treat most anterior uveal tumors. Chemotherapy may be used for lymphosarcoma. Response to medical treatment for all primary and other secondary uveal tumors is generally poor.
The treatment of choice for most uveal tumors is removal of the eye (enucleation). Malignant tumors must be removed as soon as possible to prevent their spread to other tissues. Large benign tumors often cause damage to nearby structures, as well as glaucoma, and the eye generally needs to be removed.
Occasionally some benign iris tumors are discovered while they are still small. Surgical removal of the mass can be attempted by surgical excision of the affected portion of the iris (iridectomy). Laser treatment using Nd-YAG or diode lasers may also be performed on very small anterior uveal tumors that are dark in color. Lasering is less effective when the tumor is white, cream-colored or pink.
One complicating factor is that it is not often possible to determine if the uveal tumor is benign or malignant just by its appearance. There are also no effective biopsy techniques that can be used for these tumors. The tumor may only be determined to be benign/malignant once the eye is removed and a thorough pathological examination has been performed. For this reason, it is often necessary to remove the eye just to play it safe in case the tumor is malignant.
Prior to removal of the eye, it is important to determine that other organs in the body are free of tumor. With the exception of lymphosarcoma, if there is evidence of tumor(s) in other areas of the body, then no treatment may be effective and euthanasia may need to be considered.