Brown Spots (Iris Melanosis) in Cat’s Eye

At a glance

  • Iris melanosis is a benign condition in which brown spots develop in the cat’s iris (the coloured part of the eye) due to hyperpigmentation.
  • While iris melanosis is benign, it should be evaluated by a veterinarian to rule out malignant melanoma, which is cancerous.
  • Iris melanosis should be monitored as some cats can develop glaucoma.
  • There is no treatment for iris melanosis, but it must be monitored to ensure it doesn’t develop into melanoma.

What are brown spots in a cat’s eye?

Medically known as iris melanosis, iris freckle, brown spots appear as areas of darker pigmentation which develop in the anterior surface of the iris (the coloured part of the eye).

Brown spots in the eye of a cat are medically known as iris melanosis, iris freckle or malignant melanosis, and appear as areas of darker pigmentation which develop in the anterior surface of the iris (the coloured part of the eye).

Dark spots can develop in cats of any eye colour and may be cancerous (malignant) or benign. For the sake of simplicity, we will refer to the benign form as iris melanosis and the cancerous form as malignant melaosis.

Iris melanosis is caused by the increased proliferation of melanocytes (cells responsible for the production of pigmentation) across the surface of the iris. In rare cases, benign iris melanosis can transform into malignant melanoma if the cells undergo malignant transformation. More often than developing a malignancy, iris melanomas can cause 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 to the lungs, liver and other parts of the body.

Appearance

Focal or multifocal areas of dark brown pigmentation within the iris which start out pale but progressively darken. Iris melanosis is commonly unilateral, however, both eyes can be affected. Spots may start small and gradually increase in size and pigmentation. Where multiple spots are present, as they grow in size, they may coalesce. 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 itself, if infiltration of the neighbouring drainage angle occurs, glaucoma, an extremely painful increase in intraocular pressure can develop.

When to see a veterinarian

Any changes to a cat’s eye should be evaluated by a veterinarian as soon as possible. The veterinarian will perform a complete physical examination and obtain a medical history from you including how long the spots have been present and any accompanying signs.

Differential diagnosis:

Iris melanosis isn’t the only cause of brown spots in the cat’s eye, and the veterinarian needs to rule out other causes before a definitive diagnosis is made.

Malignant melanoma – A cancer of the melanocytes (pigment-producing cells).

Feline corneal sequestrum – 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.

Diagnostic workup:

  • Slit-lamp examination: An examination of the front of the eye using a slit-lamp, which is an instrument consisting of a high-intensity light, and a biomicroscope. One reader who recently saw a veterinarian for an eye freckle writes ‘She noted the freckle was not veiny, was not raised, didn’t affect the pupil, and it reflected like pigment would reflect. So as a combination, this is all fantastic news. (If one or more of these things were abnormal, further investigation would have been needed).
  • 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.

Treatment

Brown spots (iris melanosis) in the eye of a domestic cat
Brown spots (iris melanosis) in the eye of a domestic cat. Image mamkaklass, Pixabay
  • 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) for cats with malignant melanomas.