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What are dark spots in a cat’s eye?
Brown spots in the eye of a cat are medically known as iris freckles, iris freckles, iris hyperpigmentation, melanoma and iris melanosis. Areas of darker pigmentation develop in the iris (the coloured part of the eye). The incidence of iris melanosis is reasonably common and typically affects middle-aged to senior cats.
Iris melanosis are caused by the proliferation of melanocytes, the cells responsible for the production of pigmentation. For the sake of simplicity, we will refer to the benign form as iris melanosis and the cancerous form as malignant melanomas.
Dark spots can develop in cats of any eye colour and may be cancerous (malignant) or benign.
Benign iris melanosis can sometimes progress into malignant melanoma when the cells undergo malignant transformation. More often than developing a malignancy, iris melanomas can lead to glaucoma, a condition caused by increased pressure within the eyeball. Glaucoma causes distorted vision and can be extremely painful. Malignant melanomas have the potential to metastasise (spread) to other parts of the body, especially the lungs and the liver.
Areas of dark brown pigmentation within the iris. There may be one or multiple areas of pigmentation. Spots usually only occur in one eye, but in some cases, both eyes can be affected. Spots may start small and gradually increase in size and become darker. Where multiple spots are present, as they grow in size, they may merge.
Iris melanosis is flat and should not protrude above the surface of the iris. Malignant melanomas are lumpy, raised and can cause distortion to the surface of the eye.
While iris melanosis is painless, if infiltration of the neighbouring drainage angle occurs, glaucoma, an extremely painful increase in intraocular pressure can develop.
When to see a veterinarian
A veterinarian should always evaluate pigmented spots in your cat’s eye to determine if they are benign iris melanosis, malignant melanoma, a cancer of the melanocytes or feline corneal sequestrum which are brown or black plaques which develop on the eye due to parts of the cornea dying off. The most common causes are prolonged trauma due to eyelid abnormalities or cat flu. Unlike iris melanosis, feline corneal sequestrums are painful and can affect vision. There is a higher incidence in Burmese, Persians and British Shorthairs.
- A slit-lamp examination to check the eye under magnification.
- Fine needle biopsy to determine if the lesion is benign or cancerous.
The veterinarian may refer your cat to a specialist eye veterinarian (ophthalmologist) to confirm the diagnosis.
- A wait and see approach recommended for cats with iris melanosis. Veterinary follow-up every six months are indicated to make sure the iris melanosis isn’t transforming into malignant melanoma. The veterinarian will take comparison photographs to check the size of the melanoma.
- Destruction of the cells with a laser or removal of the entire eye (enucleation) are the treatment options for malignant melanomas.