Eye color is a highly visible human trait, and its appearance can sometimes shift or seem to shift throughout a person’s lifetime. While the underlying pigment in the eye remains largely stable in adulthood, several factors can cause both permanent physical alterations and temporary optical illusions of color change. These shifts are rooted in biological development, the physics of light, and sometimes, underlying health conditions.
The Mechanism of Eye Color
The visible color of the eye is determined by the iris, a muscular diaphragm that controls the size of the pupil. Eye color results from a combination of the concentration of the pigment melanin within the iris and the way light interacts with the iris’s fibrous structure, known as the stroma. Brown eyes have the highest concentration of melanin in the stroma, which absorbs most light entering the eye, resulting in a dark appearance.
Lighter eye colors, such as blue and green, contain less melanin in the stroma. Blue eyes contain almost no melanin in this layer, meaning the color is not a result of a blue pigment. Instead, the appearance of blue is an effect of light scattering, specifically Rayleigh and Tyndall scattering, which preferentially reflects shorter, blue wavelengths of light back out. Green and hazel eyes represent an intermediate stage, featuring a lower to moderate melanin level combined with this same light-scattering phenomenon.
Developmental and Environmental Influences on Color
The most common instance of permanent eye color change occurs during infancy due to biological development. Many babies of European descent are born with blue or gray eyes because the melanocytes, the cells that produce melanin, have not yet fully activated in the low-light environment of the womb. As the infant is exposed to light after birth, these cells begin to produce and deposit melanin into the iris. This developmental process can cause a baby’s eye color to darken and stabilize, typically between six and twelve months of age, though subtle shifts can continue until three years old.
Color changes observed in a healthy adult are usually not true pigment shifts but optical illusions related to environmental factors. For example, the presence of different ambient lighting conditions can alter how light reflects off the stroma, making eyes appear to change hue. The size of the pupil, controlled by the iris muscle, also plays a significant role in perceived color shifts. When the pupil constricts in bright light, the color of the iris appears more concentrated and vibrant. Conversely, when the pupil dilates due to low light or strong emotions, the expanded pupil covers more of the iris, often making the eye appear darker overall.
Medical Conditions Causing Permanent Change
A lasting or dramatic change in eye color in adulthood, particularly in only one eye, is known as acquired heterochromia and is often a sign of an underlying medical issue. Inflammation of the iris, such as that caused by chronic uveitis or Fuch’s heterochromic iridocyclitis, can lead to a loss of pigment, causing the affected eye to lighten. In these cases, a naturally darker eye may appear blue or green over time.
Certain medications are also known to cause permanent color alterations. Prostaglandin analogs, a class of drugs commonly prescribed for treating glaucoma, can stimulate melanocytes to produce more pigment. This results in a gradual and irreversible darkening of the iris, most often seen in patients with lighter eye colors. Physical trauma or injury to the eye can also disrupt the iris tissue, leading to localized or complete pigment change.
Other conditions, like pigment dispersion syndrome, cause pigment granules to slough off the back of the iris and float within the eye’s fluid, which can also lead to a gradual lightening of the iris. Rarely, a dark spot or noticeable color change can be a symptom of ocular melanoma, a form of eye cancer that affects the melanin-producing cells.
When to Seek Professional Consultation
While most perceived eye color shifts are harmless optical effects, any sudden or dramatic color change warrants a consultation with an eye care professional. This is especially important if the change occurs only in one eye (unilateral change). A unilateral change can be an early indicator of a serious condition like Fuch’s heterochromic iridocyclitis, pigmentary glaucoma, or a tumor. Seeking immediate attention is also necessary if the color change is accompanied by other symptoms, including eye pain, persistent redness, excessive light sensitivity, or any sudden blurring or loss of vision.