The iris is the part of the eye that has color. With iris hyperpigmentation, spots in the eye begin to change color, becoming especially noticeable when reflecting light. There are two causes of iris hyperpigmentation in felines--iris melanosis and iris melanoma.
Iris melanosis is usually a benign condition that can cause spots of discoloration throughout the iris of one eye, though it sometimes affects both eyes.
Iris melanoma is caused by a malignant lesion that grows on the eye and blocks light diffusion.
Iris melanosis usually doesn't distort the surface of the eye, but iris melanoma tends to form a lump that bulges out from the under the surface of the eye.
Iris melanosis is usually left untreated unless it causes secondary glaucoma, which is an increase in eye pressure. The glaucoma would be treated with monitoring and medication. Iris melanoma usually requires removal of the tumor and sometimes the eye, if the cancer is aggressive.
Uveitis is inflammation in the blood supply of the eye, and chronic uveitis can sometimes mimic the signs of iris hyperpigmentation.