Eye color is primarily determined by the amount and distribution of melanin, a pigment, within the iris stroma. The iris is the colored, muscular part of the eye that controls pupil size and light entry. High melanin concentration results in darker colors like brown, while lower concentrations cause light scattering, resulting in lighter shades such as blue or green. For most adults, eye color remains stable, but a true, lasting shift in hue can sometimes occur. Though rare, these changes may signal an underlying health issue, a reaction to medication, or a natural developmental process.
Natural Shifts in Eye Pigmentation
The most common change in eye color occurs during infancy. Many babies are born with lighter eyes, often appearing blue or gray, because the specialized cells that produce melanin, called melanocytes, have not yet fully activated. As the infant is exposed to light, these melanocytes begin producing melanin, causing the eye color to gradually darken to its genetically determined shade, usually stabilizing between six months and three years of age. Beyond childhood, perceived changes are frequently mistaken for actual shifts in iris pigmentation.
The color of the eye can appear to change dramatically based on surrounding light, clothing colors, or even mood. When the pupil dilates due to low light or strong emotion, the darker inner iris is exposed, creating the illusion of a deeper or different color. A benign condition called heterochromia, where an individual has two different-colored eyes or multiple colors within one iris, is usually present from birth and does not represent an acquired change.
Conditions That Cause Color Alterations
A variety of medical conditions can cause a true physical change in the iris color or structure, often affecting only one eye. One such condition is Fuchs’ Heterochromic Iridocyclitis (FHI), a chronic, low-grade inflammation of the front part of the eye. FHI often leads to iris atrophy and depigmentation, causing the affected eye to appear noticeably lighter than the unaffected eye.
Another cause of iris lightening is Horner’s Syndrome, which results from a disruption of the sympathetic nerve pathway from the brain to the eye and face. If this nerve damage occurs in a child under the age of two, it interferes with the development of melanin pigmentation, resulting in the affected eye being lighter. This syndrome is also characterized by a constricted pupil, a drooping eyelid, and decreased sweating on the affected side of the face.
Pigment Dispersion Syndrome (PDS) involves the release of pigment granules from the back of the iris, which then float freely within the eye. This occurs when the iris structure is concave, causing it to rub against the lens fibers, particularly during movement or exercise. While PDS typically does not cause a dramatic color change, the shedding of pigment can make the iris appear lighter or speckled. Crucially, the pigment can clog the eye’s drainage system, potentially causing pigmentary glaucoma.
In rare and serious cases, a growing dark spot on the iris may be a sign of ocular melanoma, a type of eye cancer that develops from pigment-producing cells. Iris melanoma often presents as a new or enlarging dark spot, which can alter the shape of the pupil. This cancer requires immediate professional evaluation.
Changes Induced by Trauma or Medication
External factors, including physical injury and certain medications, can induce changes in eye color. Trauma to the eye, such as a blunt impact, can damage the delicate iris tissue, leading to tissue loss or bleeding that temporarily or permanently alters the eye’s appearance. A severe injury can also cause iron deposits to accumulate in the iris, a condition called siderosis, which may give the eye a reddish-brown appearance.
Certain prescription eye drops used to manage glaucoma, specifically prostaglandin analogs such as latanoprost and bimatoprost, are known to cause increased iris pigmentation. These medications stimulate melanogenesis, the process of melanin production, in the iris melanocytes. This side effect results in a gradual, irreversible darkening or browning of the iris, and is more common in individuals with mixed-color eyes, like hazel or green.
Cosmetic procedures, such as iris implant surgery, are another external cause of color alteration. These procedures involve placing an artificial colored disc over the natural iris, and they carry a high risk of serious complications. These risks include glaucoma, cataracts, and chronic inflammation.
Knowing When to Consult a Specialist
While many perceived color changes are harmless and transient, any true or lasting shift in iris color warrants a professional eye examination. It is particularly important to consult an ophthalmologist if the change is sudden, noticeable, or affects only one eye. Unilateral changes, where one eye darkens or lightens compared to the other, often indicate an acquired medical condition requiring diagnosis.
Medical attention is also necessary if the color change is accompanied by other symptoms:
- Eye pain
- Persistent redness
- Blurred or distorted vision
- The appearance of a growing dark spot on the iris
- Unequal pupil size
Seeking a specialist’s evaluation ensures that any underlying disease is detected and managed early to preserve vision and overall eye health.