The color of the eye is determined by the iris, a thin, circular structure that surrounds the pupil and controls the amount of light reaching the retina. The observation that eye color changes over a lifetime is common, but the underlying causes are a combination of natural aging processes and structural alterations within the eye. Changes to the iris itself or to the structures that surround it can make the eye color appear subtly or dramatically different over decades.
The Biology of Eye Color
Eye color is determined primarily by the concentration of a single pigment, melanin, within the iris stroma. The stroma is the front layer of the iris, and the amount of melanin present largely dictates the perceived hue. Brown eyes, the most common color globally, result from a high concentration of melanin that absorbs most light entering the eye.
Lighter eye colors, such as blue and green, contain much less melanin in the stroma. The appearance of blue is actually a structural color, caused by a phenomenon called the Tyndall effect. In eyes with low melanin, light scatters as it passes through the stroma’s collagen fibers, reflecting shorter, blue wavelengths back to the observer. Green eyes are the result of a moderate amount of yellowish pigment combined with this background blue scattering effect.
Melanin Loss and Pigment Redistribution
The color of adult eyes is not completely static and can change over time due to the gradual alteration of the iris’s pigment cells. The melanocytes, which are the cells responsible for producing melanin, can lose efficiency or stability as a person ages. This natural wear and tear can lead to a slight decrease in the pigment content within the iris stroma.
This loss of pigment is most noticeable in individuals with lighter eyes, such as blue or green, which may appear to lighten, fade, or become slightly duller over many years. The structural nature of blue eyes makes them particularly susceptible to visible fading if the density of the stroma changes. In a smaller number of cases, pigment granules may redistribute or migrate, sometimes causing a very dark brown eye to appear slightly lighter or more mottled.
How Other Aging Conditions Alter Appearance
Beyond the iris itself, age-related conditions that affect the surrounding structures can dramatically change how the eye color is perceived. One of the most common is the development of cataracts, which involves the clouding and yellowing of the lens located directly behind the iris. The natural lens begins to accumulate protein clumps that turn opaque, filtering the light that passes through.
This clouding can make the iris color appear faded, duller, or grayer because the vibrant light reflections are muted before they reach the observer. Another frequent change is the appearance of arcus senilis, a white, gray, or blue ring that forms around the outer edge of the cornea. This visible ring is caused by lipid and cholesterol deposits in the corneal tissue, and it is a common finding in individuals over the age of sixty.
The ring of arcus senilis does not affect the iris pigment, but its opaque border provides a high-contrast frame that can make the central eye color seem less intense. Because this is a change to the cornea, the eye’s outermost clear layer, it alters the visual presentation of the iris color without changing the melanin content.
Eye Color Changes That Require Medical Attention
While subtle fading is a benign part of the aging process, any sudden, rapid, or painful change in eye color warrants an immediate examination by an eye care professional. Pathological conditions can cause dramatic color shifts, often affecting only one eye, a condition known as acquired heterochromia.
For example, pigmentary glaucoma is a condition where pigment flakes off the back of the iris and clogs the eye’s drainage system, which can cause the iris to lighten or show patchy discoloration. Inflammatory conditions, such as uveitis or Fuchs heterochromic iridocyclitis, can also lead to a noticeable loss of pigment in the iris, often resulting in the affected eye becoming lighter.
Certain neurological issues, like Horner’s syndrome, can cause nerve damage that results in the loss of iris pigment and an accompanying difference in pupil size between the eyes. Additionally, some medications, particularly prostaglandin analogs used to treat glaucoma, are known to cause the iris to gradually darken over time.