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Iris Melanoma in Cats


synonyms: melanosis, iris freckles, iris nevi, melanocytoma, iris hyperpigmentation syndrome

Iris Melanoma in Cats


Older cats are sometimes presented to veterinarians and veterinary ophthalmologists with a brown or black spot on the iris (coloured portion) of the eye. The spot may be single and discrete, multiple or diffusely affecting the iris. These dark spots are associated with an abnormal invasion and local proliferation of pigmented cells called melanocytes. By definition, a tumour is an abnormal growth or proliferation of cells. Tumours may be benign, or malignant (cancerous).

Iris Melanoma

The veterinary ophthalmologist will carefully examine these pigmented areas with a tool called a slit-lamp biomicroscope. When the pigmented area is flat we tend to use the term iris melanosis. If the spot becomes raised, or velvety or distorts the iris or pupil shape then progression is evident and we call it iris melanoma. A pathologist can differentiate between benign and malignant melanoma. If benign, the term melanocytoma is often used. If malignant, the term "melanoma" is used. Based upon slit lamp examination it is possible for the ophthalmologist to detect progression from melanosis to the more serious forms of this condition, but it is not possible for the ophthalmologist to differentiate between melanocytoma and melanoma based upon routine examination.

This disease is enigmatic. We know that an iris tumour freely exfoliates cells into the aqueous fluid of the eye and that the aqueous fluid freely circulates out of the eye and into the bloodstream. This would suggest that a malignancy in the eye would have a high rate of metastasis (spreading) to the body.

Luckily, most cases of iris melanosis do not undergo malignant change to melanoma. In fact, a cat with progressive iris melanosis is more likely to develop glaucoma from infiltration and obstruction of the drainage angle (the area in the eye where the intraocular fluid drains) than he is to develop distant metastasis.

However if clinical observation suggests progression, malignant transformation is a possibility and the risk of metastasis is recognized and the prognosis becomes guarded.

Although opinions vary, the usual approach to cats with iris melanosis is to follow the progress by semi-frequent (2 times per year) re-examination. If glaucoma develops, and the eye is painful, enucleation (removal) of the eye is recommended. If any of the pigmented lesions become raised, velvety or if pupillary distortion is evident this would suggest progression and the risk of malignant transformation.

It is possible to do a fine needle aspiration biopsy of the mass and collect enough cells for a definitive diagnosis by the pathologist. This is a tricky technique which should only be done by a veterinary ophthalmologist.

Two possible treatment options exist for iris melanoma. The first is a laser treatment using a diode laser to destroy the tumour cells. Laser treatment is confined to small masses. The second treatment option is enucleation (removal of the eye). In either case, presurgical blood work and chest x-rays should be done to look for any obvious signs of pre-existing metastasic disease. Since metastasis may occur occultly (without obvious changes in the blood or on x-rays) some cats will succumb to metastatic disease despite laser or enucleation surgery. There is no evidence that doing laser or enucleation surgery will precipitate metastasis.