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Uveal Melanoma in Dogs

The uvea is the part of the eye that is made up of the iris (the colored part of the eye surrounding the pupil), the ciliary body (which produces the fluid within the eye [aqueous humour] and controls the ciliary muscle contractions that aid in near focus), the choroid (which provides oxygen and nourishment to the retina – the inner surface of the eye), and the pars plana (at the front of the eye, where the iris and sclera [white of the eye] touch). A melanoma is clinically characterized by malignant growth of melanocytes, cells that are dark in appearance due to the inclusion of the melanin pigment.

Uveal melanomas usually arise from the front of the iris’ surface, with extension to the ciliary body and choroid. These tumors tend to be flat and diffuse, not nodular (unlike intraocular melanomas, which are raised masses). Such tumors initially have a benign (non-spreading) clinical and cellular appearance. Uveal melanomas are the most common primary intraocular neoplasm in dogs. They are usually benign and unilateral, affecting the anterior uvea most often. However, uveal melanomas are just as often capable of destroying the eye. Anterior uveal melanomas have a four percent rate of cancerous spread through the bloodstream to the lungs and visceral organs. Choroidal melanomas rarely metastasize.

Symptoms and Types

Anterior Uveal Melanoma

• Pigmented scleral (white part of the eye) or corneal (the transparent front part of the eye) mass
• Pigmented mass visible
• Irregular pupil
• Inflammation of the eye (the uvea)
• Glaucoma (Increased pressure in the eyes)
• Hyphema  (Blood in the eye)
• No vision loss unless mass obstructs the pupil or glaucoma has developed

Choroidal Melanomas

• Often missed because of tumor location
• Mass farther back in the eye
• Very slow-growing; rarely requires removing the eye
• Rare tumor

Causes

• Unknown
• Flat, pigmented iris freckles have potential to transform into melanomas
• There is a presumed autosomal (non-sex-linked) recessive inheritance in Labrador retrievers

Diagnosis

Your veterinarian will perform a thorough physical exam on your dog, including a complete ophthalmic exam (including testing pressure within the eye and proper drainage of the eye’s aqueous humor). A complete blood profile will also be conducted, including a chemical blood profile, a complete blood count, a urinalysis and an electrolyte panel. Evidence of metastasis may may be present in the blood profile, or the blood count may show increased white blood cells, which can be indicative of the body's immune system fighting the malignant cell growth. You will need to give a thorough history of your dog's health and onset of symptoms.

X-rays and an ultrasound may also help to determine the extent of metastatic disease in the eye. During the ophthalmic exam, tonometry will be used to measure the pressure in the eyes, and gonioscopy will be used to see if the melanoma has spread to the drainage angle. Slit-lamp biomicroscopy can be used to gage the size and location of the mass. The mass should be transilluminated, a technique which uses strong light to shine through the eyeball, illuminating it throughout as an aid to diagnosis. Indirect ophthalmoscopy can also be used to investigate the eye, with or without concomitant scleral indentation.

Treatment

Uveal melanomas in dogs are usually non-spreading tumors (benign), so you may opt to monitor the eye for changes every 3–6 months. Young Labrador retrievers are prone to aggressively growing uveal melanomas and will need surgery. Removal of the affected eye (enucleation) is the suggested treatment.

Indications for enucleation: the size of the mass increases rapidly, the eye cannot be salvaged, the mass spreads diffusely within the eye, visual function is significantly impaired, invasion of the tumor outside the eye, and secondary complications (e.g., glaucoma, signs of pain, bleeding).

Living and Management

Removing an eye is one-sided, and is done to spare the fellow eye. One-eyed animals often function very well, adjusting to the change in visual capacity quickly. If your dog develops glaucoma secondary to an uveal melanoma, your dog will very likely suffer a good deal of great pain. The resulting headaches may manifest as head shaking, head pressing, whining, placing the paws on the head, or lethargy and slow movements.

Your veterinarian will schedule follow-up appointments for X-ray and ultrasound imaging at six and twelve months following the initial surgery or treatment. At these appointments, your veterinarian will evaluate the enucleation site as well as check for tumor recurrence or metastasis.