Eye color is generally stable and fully set shortly after infancy. Once the eye reaches mature pigmentation, any noticeable or permanent shift is unusual. A change in adult eye color does not happen spontaneously; it often signals a deeper structural modification within the ocular tissue. When a true change occurs, it requires attention to determine if an underlying medical cause is responsible.
How Eye Color is Determined in Adults
Eye color is determined by the concentration and distribution of melanin, the pigment that also colors skin and hair. The amount of melanin in the front layer of the iris, called the stroma, dictates the final color. Brown eyes have high concentrations of melanin that absorb most light.
Lighter eyes, such as blue or green, contain less melanin in the stroma. This lower pigment level allows light to be scattered, making the iris appear blue or green. Since the amount of melanin in the adult iris is genetically fixed, the eye’s structural color is permanent. Any significant alteration suggests a biological change is taking place within the eye’s tissue.
Factors That Alter Eye Color Perception
Many people report a perceived change in eye color that is not a true structural alteration of the iris. These temporary shifts result from how light interacts with the eye’s surface. Ambient lighting, whether natural or artificial, can significantly affect the eye’s apparent hue.
For individuals with lighter or mixed-color irides, the surrounding environment plays a large role. The colors of clothing, makeup, or reflected objects can influence the light bouncing off the iris, causing the eye to appear slightly lighter or darker.
The size of the pupil also influences color perception. When the pupil dilates, the dark muscle layer at the back of the iris is more exposed, which can make the surrounding iris appear darker by contrast.
Underlying Medical Conditions Causing Pigment Change
True, permanent changes in eye color are often linked to acquired heterochromia, where one eye changes color later in life due to disease or injury. Several medical conditions affect the balance of pigment or tissue within the iris.
Acquired Horner’s Syndrome
This syndrome results from damage to the sympathetic nerve pathway controlling the eye. This nerve disruption decreases the stimulation of melanocytes (pigment-producing cells), causing the affected iris to become lighter, a condition called hypochromia.
Uveitis and Iritis
Inflammation of the middle layer of the eye (uveitis or iritis) can lead to color shifts. Chronic inflammation, such as Fuchs’ heterochromic iridocyclitis, may cause the affected iris to lighten or become faded and hazy. The inflammation alters the structure of the iris and can lead to a loss of pigment or the accumulation of inflammatory cells.
Pigment Dispersion Syndrome
This involves the back surface of the iris rubbing against the eye’s lens, causing pigment granules to shed into the eye fluid. This release of pigment can cause the iris to appear lighter over time due to the loss of pigment from its surface. The freed pigment can sometimes clog the eye’s drainage system, potentially leading to glaucoma.
Iris Tumors
In rare instances, iris tumors (often benign but occasionally malignant melanoma) can appear as a localized dark spot or freckle. These growths represent an excessive proliferation of pigment-producing cells, causing a distinct, localized darkening that alters the eye’s appearance.
Medications and Physical Trauma
Certain pharmacological agents are well-documented causes of permanent eye color change, particularly a class of drugs known as prostaglandin analogs. These are commonly used in eye drops, such as latanoprost, to manage glaucoma by lowering intraocular pressure.
Prostaglandin Analogs
These medications stimulate the melanocytes in the iris, which increases the production and storage of melanin. The result is a gradual and permanent darkening of the iris, most often seen in individuals with mixed-color eyes where the pigment shifts toward brown. The darkening typically progresses slowly and is more pronounced in the treated eye if the medication is applied unilaterally, often resulting in acquired heterochromia.
Physical Trauma
Physical trauma to the eye can also induce a permanent color change. A blunt force injury or a penetrating wound can cause direct damage to the iris tissue. This damage may lead to localized pigment loss, resulting in a lighter area (hypochromia) or a sectorial heterochromia. Trauma can also cause internal bleeding or scarring within the eye. Iron deposits from bleeding, known as siderosis, can cause a progressive darkening of the iris over time.