Just like you can develop a mole or nevus on your skin, you can develop a nevus in the back of the eye in the choroid.
Choroidal Nevi are common and occur in about 5 to 10 percent of the population.
A nevus or freckle or mole can occur in the eye just like one can occur on the skin. Like the skin, the eye has melanocytes. Those are the cells that give the skin its pigment. The eye has a layer of melanocytes behind the retina in the choroid. There are also melanocytes in the iris which give the iris a brown or blue color. If a number of melanocytes grow they can form a nevus in the eye.
The history in a patient with a nevus is mostly aimed at finding out how long the nevus has been present. Mostly patients are asked about previous eye examinations and the findings.
A nevus inside the eye can only be seen by an physician who has the skill and equipment necessary to view the back of the eye. A dilated examination is almost always necessary.
Most eyes with a nevus have fundus photography, fluorescein angiography, and ophthalmic ultrasound a-scan and b-scan. Fundus photography is helpful in looking for change in lesion size over time.
The risk of a choroidal nevus transforming into a choroidal melanoma is about 1 in 20,000. If a nevus is in the macula, it can sometimes cause vision loss through irritation of the overlying tissues. Some patients with a nevus under or near the center of the macula can develop vision loss if blood vessels or scar tissue grow over the nevus. Some nevi near the center of the macula can cause vision loss if the blood vessels in the around the nevus become leaky and fluid develops under the macula. These problems can be treated in the same way leaky or new blood vessel growths can be treated in a patient with age-related macular degeneration.
Most new choroidal nevi are photographed and followed regularly so they can be watched for growth. The chance of a nevus converting into a melanoma is extremely low (1/20,000).