Eye color, widely considered permanent after early childhood, can change or be lost in rare circumstances. The color of the iris is determined by the amount of melanin pigment housed in its cells. While most perceived shifts are optical illusions, a true alteration in pigment is possible and usually signals an underlying biological process. A permanent change involves a physical redistribution, loss, or increase of this melanin within the eye’s structure.
The Biology of Eye Color Stability
The stability of eye color is rooted in the structure of the iris, the muscular diaphragm that controls the pupil’s size. Color is determined by the concentration and distribution of melanin within the iris’s superficial layer, known as the stroma. The more melanin present, the darker the eye color will be, with brown eyes having the highest concentration. Lighter colors like blue and green have less melanin, and their hue results from the scattering of light within the stroma, similar to what makes the sky appear blue.
Melanocytes, the cells that produce melanin, establish pigment activity during infancy; most babies reach their adult eye color by around nine months of age. Once pigment production stabilizes, the color generally remains constant for life because the number of melanocytes does not change. Temporary shifts in perceived color often occur due to changes in lighting or pupil dilation. When the pupil constricts or dilates, the folds of the iris compress or expand, making the eye appear a slightly different shade without actual pigment change.
Primary Causes of True Color Change
True and lasting changes occur when the amount or location of melanin is biologically altered by disease, medication, or physical trauma. Certain medical conditions cause pigment loss, resulting in eye color lightening. Fuchs’ heterochromic iridocyclitis, a form of chronic inflammation, often leads to atrophy and depigmentation of the iris, causing the affected eye to appear lighter than the other. Similarly, pigment dispersion syndrome involves the rubbing off of pigment cells from the back of the iris, which can clog the eye’s drainage angle and cause areas of the iris to lighten.
Conversely, some factors cause gradual darkening through an increase in melanin. The most common cause of acquired darkening is prostaglandin analog eye drops (such as latanoprost and bimatoprost) prescribed to treat glaucoma. These medications stimulate melanogenesis (melanin production), resulting in a permanent shift that typically changes blue or green eyes toward a brown hue. This effect is often more noticeable in patients with mixed-color irides, potentially resulting in two different-colored eyes if the drops are only used in one eye.
Physical trauma can also trigger a pigment change, either localizing pigment or causing its loss. A blunt force injury can damage the iris tissue, causing a loss of tissue that makes the eye appear darker or misshapen. If a metallic foreign body is retained, iron or copper deposits can accumulate in the iris, leading to a localized reddish-brown (siderosis) or greenish-white (chalcosis) discoloration. Aging can bring subtle changes, such as minor lightening due to a slight reduction in melanin production or the appearance of an arcus senilis, a hazy white ring on the cornea that can make the iris look paler.
When Color Changes Signal a Medical Issue
A change in eye color that is sudden, noticeable, or accompanied by other symptoms should be evaluated by an ophthalmologist immediately. An acute, symptom-linked change is far more concerning than a gradual, asymptomatic shift. For example, a sudden onset of different-colored eyes paired with pain, redness, or extreme sensitivity to light may signal uveitis, a serious intraocular inflammation that can lead to vision loss.
Changes in eye color combined with blurred or decreased vision, seeing halos around lights, or new floaters can indicate rising intraocular pressure, which may be a sign of glaucoma caused by pigment dispersion or inflammation. A very rare neurological condition called Horner’s syndrome, caused by nerve damage, can lead to iris depigmentation and a difference in pupil size, making the affected eye appear lighter or darker. Any noticeable and persistent change warrants a professional examination to rule out underlying disease and protect vision health.