Iris Melanosis / Melanoma

The iris is the colored part of the eye. In cats, an abnormally pigmented area of the iris may represent either a benign or a malignant lesion. There are two similar-sounding diagnoses that describe iris pigmentation in cats: iris melanosis and iris melanoma. These two terms are described below:

  • Iris melanosis is a general term meaning “discoloration of the iris.” The pigmented area may be a small freckle (nevus), or the iris may have many diffusely discolored areas. Iris melanosis usually occurs in only one eye, but may occur in both eyes. Over time, the pigmented area often spreads to other parts of the iris. Iris melanosis may be a benign (harmless) condition, or it may cause secondary glaucoma if the pigmented area spreads and blocks intraocular fluid drainage. Severe lesions can also prevent the iris muscles from functioning properly, thereby making it difficult for the cat to constrict the pupil in response to bright light.
  • In contrast to iris melanosis, iris melanoma refers to a malignant lesion of the iris. Melanoma lesions are more likely to be nodular and lumpy rather than diffuse and flat (this can be distinguished via the aid of a hand-held microscope). If left untreated, the melanoma may remain unchanged, it may spread to other parts of the iris, or it may metastisize to other parts of the body and lead to fatal respiratory disease or liver failure. Treatment options (i.e. conservative management/therapy vs. surgical intervention) will be discussed at length during an examination. Treatment recommendations will depend on the severity of the disease as well as the health of the eye(s) otherwise.