Seventy-five percent of eye tumors in the cat are malignant, aggressive forms of cancer (in dogs the ratio is reversed, 75 percent of tumors are benign and do not spread or become life-threatening). An Iris melanoma is the most common type of tumor affecting cat’s eyes. Older cats may develop a visible brown or black spot on the iris of their eye. The Iris is the colored part of the eye. The spots may be single or multiple and are pigmented cells called Melanocytes that have overgrown. Examination involves the use of a slit-lamp biomicroscope, which determines whether or not the tumor is cancerous. If the pigmented area on the iris is flat, the lesion is referred to as an Ins Melanosis. This is considered to be pre-cancerous and is rechecked at six-month intervals.
IRIS MELANOMA / GLAUCOMA
Iris melanoma is also called Melanosis and/or Melanocytoma. Iris melanoma is the term used if the pigmented area on the iris is raised. This is a cancerous lesion. Treatment involves using a diode laser to destroy the cancer cells. Without treatment, Iris melanoma is generally result in Glaucoma. Glaucoma occurs because the cancer cells spread and obstruct the drainage angle of the eye, which is where the fluid within the eye normally drains. Since fluid can not drain from the eye, it builds up within the eye which increases pressure within the eye and causes Glaucoma. In dogs, Glaucoma rapidly results in blindness due to irreversible damage to the retina.
The only sign of Glaucoma in cats may be a slight difference in the size of the pupils which are the dark central area of the eye. Surgical removal of the eye, which is called Enucleation, is often necessary in advanced cases of Ins Melanoma. This alleviates the pain of Glaucoma and removes the cancerous mass.
Retinal Degeneration is an inherited condition in Abyssinian cats. It occurs between four to six years of age and results in blindness. A dietary deficiency of the amino acid Taurine can also occur in cats fed primarily dog food which results in Nutritional Retinal Degeneration. This condition was first described in 1975 and was called Feline Central Retinal Degeneration (FCRD). Commercial cat foods are now fortified with Taurine to prevent this problem.