Black or dark brown masses on a dog's eye may not be readily visible, and dog owners often stumble on them by chance when they are looking at their dog's eye in the sunlight or by a strong light source. As one can imagine, dark masses on a dog's eyes are not normal, but the good news is that most of them are slow growing and benign, but in the long term they can wreck damage inside of the eye. If you notice a dark mass on your dog's eye, it's important to see your vet so so to obtain an insight on whether the mass is something that you can keep an eye on, or if it's something that warrants investigation, and possibly removal. Eye problems often require the attention of an eye specialist, which you can be referred to by your vet.
About Uveal Melanomas
Uveal melanomas are melanomas that may affect the dog's iris, ciliary body, and choroid. The iris is the pigmented area of the dog's eye that surrounds the pupil, the ciliary body is the ring-shaped layer of tissue inside the eye just behind the iris, and the choroid is the vascular layer located in the back of the eye. All these three layers together form what's known as the uvea.
Melanomas are cancers that affect cells that produce a pigment called melanin that are known as melanocytes. Melanomas are known to affect several different parts of the dog's body, and when they affect the layers that form the uvea, they are therefore known as uveal melanomas.
More distinctly, they may be named after the areas they affect, therefore they can be classified as choroidal melanomas (rare in dogs), ciliary body melanomas and iris melanomas.
Uvueal melanomas are common types of tumors affecting the dog's eyeball. The dog's iris is the part that is often most affected (iris melanoma), with the ciliary body and choroid affected secondarily as the tumor extends to these layers of tissues.
What they Look Like
Since melanomas affect pigmented cells, when they affect the dog's eye, they appear as dark pigmented areas within the iris. Pigmented areas on dog's eye may appear as small dark spots (iris freckles) or larger spots (moles) or they can appear as a darkened mass or a raised nodule in the iris. Some dog owners describe the darkened area as if "black paint was dropped on the dog's iris" or "a stain has appeared."
When a roundish protruding, yet smooth, black nodule originates from the limbus, the border of the iris, it's known as a limbal melanocytoma. According to Veterinary Vision, these tumors originating from the rim are benign, but tend to progressively enlarge, and may invade the corner between the iris and the cornea (iridocorneal angle) and cause secondary glaucoma.
Limbal melanocytomas tend to affect heavily pigmented large breeds with golden retrievers, Labrador retrievers and German Shepherds often affected.
Left untreated, certain types of uveal melanomas can cause problems to the dog's eyes and nearby structures. Affected dogs may develop eye pain, squinting, cloudiness, tearing, redness, swelling, abnormal pupil shape, bleeding in the eye, glaucoma, inflammation (uveitis) which can progress to vision loss and damage to structures near the eye.
Local or Invasive?
Generally, uveal melanomas are benign (melanocytomas) and have a very low chance for spreading to other areas (metastasis). When they remain confined to just the iris area, they may slowly enlarge. Usually, uveal melanomas affecting older dogs between 8 and 10 years old are benign, while those affecting dogs aged less than four years old, they have a tendency to be malignant and spread.
If we look at statistics, uveal melanocytomas account for 90 percent of uveal neoplasia in dogs. and almost all of them are benign. Rates of metastasis (spread to other body parts) are very low, comprising less than 5 percent, explains veterinary ophthalmologist Andrew Geller.
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Melanomas that invade the ciliary body can also be benign but they can enlarge and do damage to nearby tissues of the dog's eye. Some iris melanomas affecting the dog's iris and ciliary body can be malignant though and not only spread to other structures of the eye, but also the brain, lymph nodes and liver, explains veterinarian Dr. Noelle McNabb.
Telling whether a uveal melanoma is benign or not can be challenging. The only sure way is unfortunately by removing the whole eye and having it biopsied.
"Melanomas are divided into melanocytic melanomas (80%–90%) and malignant melanomas (10%–15%). Metastasis is infrequent (<5%)." ~Kirk N. Gelatt, veterinary opthamologist
Secondary to Other Tumors
In some cases, uveal melanomas in dogs may occur secondary to other types of cancers that spread and end up affecting the eye. These are called secondary tumors. For example, a common tumor that spreads to the dog's eye is a is a lymphosarcoma. Tumors originating from the mammary glands, kidneys, spleen, skin, nails or oral cavity may also spread to the dog's eye.
When the uveal melanoma presents secondary to other tumors affecting other parts of the dog's body, such as organs, you may see other symptoms, but these symptoms are usually present for some time before spreading to the eye. Symptoms also tend to be more systemic than localized and can include lethargy, loss of appetite, and weight loss. When originating from other distant body parts, the tumor tends to affect the choroid rather than the dog's anterior uvea.
At the Vet's Office
Your vet will carefully assess your dog's eyes with an ophthalmoscope and will check how the pupil responds to light and look at the eye under magnification. Other eye tests that may be carried out are tonometry eye test to check for pressure in the eye, Shirmer tear test to evaluate tear production, and fluorescein staining to check the surface of the cornea. Gonioscopy may be also be done to evaluate the iridocorneal angle.
Other diagnostic tests may include blood work, ultrasound of the eye, chest and abdominal x-rays, and computerized tomography (CT) or magnetic resonance imaging (MRI).
Your vet may palpate your dog's lymph nodes and feel for any abnormal swellings of the dog's organs. He may listen to your dog's heart and lungs. Your vet may be checking your dog's body to rule out cancer that has spread from a distant site. If the vet suspects lymphosarcoma, he may take a bone marrow aspirate.
Treatment of Uveal Melanomas
Treatment is based on the vet's findings and varies considering that tumors tend to behave differently. If the tumor is caught early and is small, there may be chances the affected portion can be removed through a procedure known as iridectomy. Laser treatment using diode lasers is an option when there is early detection with good results and the dog maintains good vision. If the melanoma is secondary to lymphosarcoma, the vet may recommend chemotherapy.
As the cancer enlarges, the vet may recommend removal of the eye, a medical procedure known as enucleation. This is done sooner than later if there are risks for the tumor to spread and if the tumor is particularly large and may damage nearby structures. Since limbal melanocytomas are raised nodules, they may respond well to diode laser and surgical excision without removing the eye.
Considering the potential benign behavior of several uveal melanomas, often vets will take a conservative approach and suggest to observe the mass for signs of progression rather than removing an eye with good vision. However, waiting for the affected eye to develop signs of glaucoma likely means removing the eye, points out veterinarian Dr. Elaine. The eye is therefore often removed when vision is lost or the eye becomes uncomfortable and fails to respond to use of eye drops.
- DVM360, Ocular neoplasia and treatment (Proceedings)
- Merck Veterinary Manual: Ocular Neoplasia in Dogs