What is a melanoma of the eye?
A melanoma of the eye is a type of cancer that develops from the disorganized uncontrolled proliferation of melanocytes. Melanocytes are cells found throughout the body in many tissues (e.g., skin, eye, inner ear, bones, and heart). In the eye, they can be found in the iris (the thin, circular structure in the eye that gives the eye its color and controls the size of the pupil) as well as beneath the retina (the thin layer of tissue that lines the back of the eye). Melanocytes produce melanin. Melanin is a pigment that gives skin, hair, and eyes their color.
Intraocular melanomas are rare, but the diffuse iris (iridal) melanoma – referred to as ‘diffuse’ because it has the ability to spread – is the most common primary intraocular (inside the eye) tumor in cats. Diffuse iris melanomas develop from the melanocytes of the iris. They are malignant (cancerous).
Another type of ocular melanoma in cats is the limbal (sometimes called epibulbar) melanoma. Limbal melanomas develop from the melanocytes found at the limbus, the border of the cornea (transparent front part of the eye), and the sclera (white part of the eye). Limbal melanomas are benign (non-cancerous) and less common than iris melanomas.
What causes this cancer?
The reason why a particular pet may develop this, or any tumor or cancer, is not straightforward. Very few tumors and cancers have a single known cause. Most seem to be caused by a complex mix of risk factors, some environmental and some genetic or hereditary. No breed or genetic predisposition has been identified for ocular melanomas in cats, but light exposure may contribute to the development of limbal melanomas.
What are the signs of these types of tumors?
These tumors, whether benign or malignant, will change the appearance of your cat’s eye.
With diffuse iris melanoma, you may see what appears to be one or more freckles on the iris. They may be round, irregular, or streaky in shape. Initially, the freckles may be very light brown in color, but over time they usually turn very dark brown. They tend to grow slowly, may overlap one another, and can cause a progressive darkening of the color of the iris. The surface of the iris may also develop a thickened or roughened appearance.
As the tumor grows, it may distort the shape of the pupil or cause the pupil to dilate. It may also cause a condition called uveitis (inflammation of the middle layer of the eye) that makes the eye appear cloudy, or glaucoma (increased pressure within the eyeball) that can cause the eye to bulge and lead to blindness. Both uveitis and glaucoma are very painful. Cats usually develop diffuse iris melanoma when they are middle-aged to older, with an average age of 10 years at the time of diagnosis.
With limbal melanoma, you may see a raised, distinct, dark-colored mass arising from the edge of the limbus along the white of the eye (sclera). Most limbal melanomas grow from the upper half of the limbus.
With either tumor, your cat may rub or scratch the affected eye. This could lead to an eye infection or corneal ulceration (an open sore on the cornea), which may cause redness, tearing, discharge, squinting, and closed eyes. Corneal ulceration causes intense pain.
How is this cancer diagnosed?
This cancer is diagnosed in large part by the clinical signs and the appearance of the tumor.
Your veterinarian will perform a full physical examination on your cat, followed by a complete ophthalmic examination. The ophthalmic examination will include inspecting the interior structures of the eyes using an ophthalmoscope and measuring the intraocular pressures using a tonometer. Other diagnostic procedures may be performed, usually by a veterinary ophthalmologist. X-rays and ultrasound may also be helpful to determine the size of tumor and extent of spread within the eye. Ultrasound is also helpful to differentiate tumors from cysts, which are benign and of no concern.
Since diffuse iris melanomas are malignant and spread not just locally (within the eye), but also to other areas of the body (metastasis), your veterinarian may recommend staging. Staging is the process of determining the extent to which a cancer has grown and spread. Staging may include bloodwork, urinalysis, chest and abdominal X-rays or ultrasound, and FNA of the nearby lymph nodes.
How does this cancer typically progress?
Without treatment, diffuse iris melanoma will continue to grow slowly and spread locally within the eye. This can lead to uveitis or glaucoma, both very serious conditions, and potentially blindness. As well as spreading locally, iris melanomas have a high rate of metastasis. Metastasis occurs about 60% of the time, usually to the nearby lymph nodes, the abdominal organs (such as the kidneys and liver), and sometimes the lungs and bones as well. Early diagnosis and treatment is very important, and associated with an increased life expectancy. Without treatment, iris melanoma will progress to cause death in 30-50% of the cases.
Most limbal melanomas grow slowly but may eventually compromise one or more eye structures and lead to problems if left untreated. Evidence of metastasis (spread) with limbal melanomas has not been reported.
What are the treatments for this type of tumor?
The treatment of choice for diffuse iris melanoma will vary according to the initial appearance of the cancer, how it progresses over time, and your cat’s age. When there are only mild to moderate changes in the iris, most veterinary ophthalmologists prefer to monitor the progression of the cancer with periodic examinations, although laser surgery or partial iridectomy (the surgical removal of part of the iris) is possible. The intraocular pressure is also monitored. If the size and number of lesions substantially increase, the lesions become raised, the pupil changes shape, the pigment spreads to other areas of the eye, or the intraocular pressure rises, enucleation (the surgical removal of the eyeball) is recommended. Enucleation is always advised in cases of fast-growing, locally invasive melanoma, even in older cats.
"The treatment of choice for diffuse iris melanoma will vary according to the initial appearance of the cancer, how it progresses over time, and your cat’s age."
Histopathology (the examination of the tissue with a microscope by a pathologist) should always be done in cases of enucleation. This will provide a definitive diagnosis, assess whether the tumor has metastasized, and enable you and your veterinarian to form a long-term plan of care. If there is evidence of metastasis, your veterinarian may recommend periodic imaging (e.g. X-rays or ultrasound) to determine presence and extent of metastatic disease. Unlike many other cancers that metastasize, metastatic disease may not become evident for years.
As limbal melanomas are benign and grow slowly, they are usually monitored. If the tumor grows rapidly or starts to grow into other eye structures, surgery is required. Surgery to remove the tumor is sometimes combined with cryosurgery (using a probe at an extremely cold temperature), laser surgery, or radiation therapy. Most of these treatments are curative, but if these procedures are unsuccessful, or regrowth occurs, or the tumor has grown into the eye, enucleation is recommended.
Is there anything else I should know?
The diagnosis of suspected diffuse iris melanoma relies on close monitoring of the early lesions and detection of changes that suggest progression. Monitoring is seldom, if ever, disadvantageous. Some iris melanomas grow very slowly, and the eye may not have to be removed for years. It is often difficult to think about removing your cat’s eye, even with the diagnosis of a life-threatening cancer, but enucleation can both prevent pain and save your cat’s life. Most cats will quickly adjust to the change in visual capacity and function very well.