Eye color can appear to shift over time. While dramatic changes are rare, subtle alterations can occur throughout a person’s life. Understanding the biology of eye color clarifies why these changes happen.
The Science Behind Eye Color
Eye color is primarily determined by the amount and distribution of melanin, a pigment also responsible for skin and hair color, within the iris. The iris is the colored part of the eye that surrounds the pupil and regulates the amount of light entering the eye. Specialized cells called melanocytes produce melanin, which is stored in structures called melanosomes.
There are two main types of melanin affecting eye color: eumelanin, which produces brown and black hues, and pheomelanin, contributing to amber, green, or hazel colors. Brown eyes contain a high concentration of melanin, absorbing most light, while blue eyes have very low melanin content. Green and hazel eyes have intermediate levels of melanin.
The perception of eye color also involves how light interacts with the iris, a phenomenon known as Rayleigh scattering. In eyes with low melanin, such as blue eyes, shorter wavelengths of light (blue and violet) are scattered more, creating the blue appearance, similar to how the sky appears blue. This structural color can make lighter eyes appear to change hue under different lighting conditions.
Age-Related Changes in Eye Color
While often believed to be fixed after infancy, eye color can undergo subtle changes with age due to physiological processes. During the first year of life, many babies, particularly those of Caucasian descent, are born with lighter eyes that darken as melanin production increases with light exposure.
One reason for age-related changes is a gradual alteration in melanin production and distribution within the iris. For instance, in a small percentage of Caucasian individuals, melanin production in the iris may slow down, causing brown pigment to fade slightly and eyes to lighten. Conversely, long-term sun exposure can lead to increased melanin, potentially causing eyes to darken slightly or develop freckles on the iris.
Another common age-related change affecting the appearance of eye color is arcus senilis, also known as corneal arcus. This condition involves lipid (fatty) deposits accumulating around the outer edges of the cornea, forming a blue, white, or gray ring. While arcus senilis does not change the iris’s actual color, the opaque ring can make the eye appear paler or even make brown eyes seem bluer due to the contrast with the corneal whitening. This condition is generally harmless and does not typically affect vision.
Non-Age Related Factors Influencing Eye Color
Beyond the natural aging process, several other factors can influence eye color or its perceived appearance. Certain medications, particularly prostaglandin analog drops used to treat glaucoma, can cause a permanent darkening of the iris. These medications increase melanin in the eye, potentially darkening lighter eyes or even turning them brown.
Eye injuries can also lead to changes in eye color. Trauma to the eye can disrupt iris tissue or blood vessels, altering its appearance. For example, bleeding (hyphema) can darken the eye, or tissue loss can alter its color.
Specific medical conditions can also alter eye color. Fuchs heterochromic iridocyclitis (FHI), an inflammatory condition, can cause iris depigmentation, often making the affected eye lighter. Pigment dispersion syndrome, where pigment flakes off the iris, can result in lighter patches. Horner’s syndrome, a rare neurological condition, can also cause iris depigmentation and uneven pupil size, affecting perceived eye color.
When to Seek Medical Advice
While some eye color changes are normal or benign, certain alterations can signal an underlying medical condition requiring professional attention. It is advisable to consult an eye care professional if you notice a sudden, significant, or lasting shift in eye color. This is especially important if the change occurs in only one eye or is accompanied by other symptoms.
Accompanying symptoms warranting medical evaluation include pain, redness, light sensitivity, or vision changes like blurriness. Conditions like uveitis, an inflammation of the middle layer of the eyeball, can cause eye color changes and require immediate medical attention. If an eye color change follows an injury, a prompt eye exam is crucial to rule out hidden issues like glaucoma or retained foreign bodies.