2024-07-25 01:01:29
The color of the iris can be used to determine risk factors for certain eye diseases. Which risk is increased for whom.
90 percent of all people have brown eyes, the rest are blue, green and gray. A person’s eye color depends on the concentration of melanin in their iris – the pigment that determines not only the color of the eyes but also the color of the skin and hair, as the German Ophthalmological Society (DOG, Society for Ophthalmology) explains in a press release.
“The melanin always has the same brownish color – green and blue eyes also have no other pigments,” explains Professor Dr. med. Claus Cursiefen, Director of the Center for Ophthalmology at the University Hospital of Cologne and Secretary General of the DOG. The other color shades are based on light refraction effects that come into play with different melanin contents.
The pigment melanin not only determines the color of the skin and hair, but also protects against the influence of sunlight because it filters both the visible part of the light spectrum and its UV component.
People with very light eyes have low melanin levels and are therefore particularly sensitive to strong light. This apparently increases their risk of developing age-related macular degeneration (AMD).
This chronic disease, which usually affects both eyes, is caused by a metabolic disorder. It occurs in the macula, the part of the retina that is particularly important for sharp vision, and leads to a gradual loss of vision in this area. You can find out more about this here.
“Free radicals, oxidative stress and the accumulation of waste products in the retina play a role in the development of AMD – processes that are intensified by UV light,” explains Cursiefen. An extensive meta-analysis with almost 130,000 participants was able to prove that at least the wet form of AMD is significantly more common in people of European origin than in people with Asian or African roots.
However, it is still unclear whether this is mainly due to eye color or whether other genetic factors also play a role.
People with light eyes also have an increased risk of developing so-called uveal melanoma, an aggressive tumor of the choroid.
“Although this type of cancer is very rare, it is found 20 to 30 times more frequently in people of European descent than in people of Asian or African descent,” explains Professor Dr. med. Nikolaos Bechrakis, President of the DOG and Director of the University Eye Clinic in Essen.
People with dark eyes, on the other hand, are apparently more prone to cataracts, a clouding of the lens. This eye disease develops two to four times more often in people with brown eyes than in blue-eyed people – an effect that has also been demonstrated within the white population and thus appears to be independent of ethnicity.
“One theory is that the more light is absorbed by the iris, the higher the temperature in the anterior chamber of the eye,” says Cursiefen. With a dark iris, a slightly higher temperature stress can therefore be expected, which in turn is a known risk factor for the development of cataracts.
The outcome of eye surgery can also depend on eye color. In corneal transplants, rejection reactions and other complications are more common when the iris is dark. “It is suspected that melanin influences the immune system in the anterior chamber of the eye,” explains Cursiefen. The pigment may intensify inflammatory processes.
“The examples show that seemingly insignificant factors such as eye color could be quite relevant in everyday clinical practice,” the DOG experts concluded. Now it is important to further define these complex relationships, take them into account in treatment and, wherever possible, compensate for increased risks and disadvantages.