Iris spots in cats: melanosis or diffuse iris melanoma
9-year-old domestic shorthair cat with flat brown spots on the iris of the left eye that the owner has seen spreading over the last year. There are no signs of pain.
Which finding would indicate the lesion is progressing to diffuse iris melanoma?
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A. The spots being dark brown
Colour alone does not distinguish melanosis from melanoma; both can be dark brown.
B. The lesion being unilateral
Both melanosis and melanoma are usually unilateral, so it does not help to differentiate them.
C. Thickening or a velvety surface of the iris, dyscoria, pigment in the iridocorneal angle or raised intraocular pressure
Correct. Melanosis is flat and does not alter the iris architecture. Thickening, dyscoria, angle infiltration and glaucoma indicate diffuse melanoma, which has metastatic potential.
D. The cat being over 8 years old
Melanoma is more common in older cats, but age alone does not indicate transformation of a particular lesion.
lightbulb Key takeaway
Photograph iris spots in cats and recheck them regularly. If texture changes, dyscoria, raised pressure or pigment in the angle appear, refer for assessment of iris biopsy or enucleation with histopathology.
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Updated: October 31, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.