Iris transillumination describes a condition where light passes abnormally through the iris rather than being blocked. The iris is the colored part of the eye, a ring-shaped structure that controls the size of the pupil to regulate how much light reaches the retina. It acts like a camera’s diaphragm, adjusting for different light conditions. A component of this function is the iris pigment epithelium, a layer of cells at the back of the iris rich in melanin. This pigmented layer is designed to absorb stray light, preventing it from scattering within the eye and ensuring clear vision.
The Diagnostic Examination
An ophthalmologist discovers iris transillumination during an eye exam using a slit-lamp microscope, which provides a magnified view of the eye’s structures. The technique used to reveal these defects is called retroillumination. During this procedure, the doctor directs a narrow beam of light through the pupil, causing it to reflect off the retina at the back of the eye.
This reflected light illuminates the iris from behind, silhouetting it against a red-orange glow from the fundus. In a healthy eye, the dense pigment epithelium would block this light completely, but if transillumination is present, the light will pass through weakened areas. The examiner will see distinct patterns, such as spoke-like radial slits, patchy “moth-eaten” spots, or a diffuse glow shining through the iris.
Causes of Iris Pigment Loss
Iris transillumination is caused by a defect in the iris pigment epithelium, which occurs in two primary ways. The first is a congenital deficiency, where an individual is born with an insufficient amount of melanin. In these cases, the pigment epithelium never fully develops its light-blocking capability, leaving the iris translucent from birth.
The second cause is acquired pigment loss, or depigmentation, where the pigment epithelium loses its melanin over time. In these situations, pigment granules are shed from the epithelial cells and dispersed into other parts of the eye.
Conditions Associated with Transillumination
Genetic disorders are a primary cause, with oculocutaneous albinism affecting pigment in the skin, hair, and eyes, and ocular albinism affecting only the eyes. In both forms of albinism, the iris has minimal pigment from birth, resulting in significant transillumination.
Pigment Dispersion Syndrome (PDS) is characterized by the chafing of the posterior iris surface against the lens zonules, the tiny fibers that hold the lens in place. This friction scrapes pigment off the epithelium, creating mid-peripheral, spoke-like defects. Pseudoexfoliation Syndrome involves the buildup of a white, flaky material throughout the eye, which can destabilize the iris and contribute to pigment shedding.
Other causes of acquired pigment loss include:
- Inflammatory conditions (uveitis), such as Fuchs’ heterochromic iridocyclitis, which leads to a gradual loss of pigment in one eye.
- Viral infections, like herpetic uveitis, which can lead to patches of iris atrophy and pigment loss.
- Direct trauma to the eye.
- Complications from intraocular surgery that damage the iris and dislodge pigment.
Impact on Vision and Daily Life
The consequence of a transilluminable iris is an increase in the amount of light entering the eye. This excess light is not properly focused by the pupil, leading to symptoms of photophobia, or severe light sensitivity. Individuals may experience discomfort or even pain in bright sunlight or under harsh indoor lighting. The scattering of this extra light within the eye also causes glare, which can degrade the quality of vision.
These symptoms can have an impact on daily activities. Glare can make driving difficult, especially at night when facing the headlights of oncoming traffic, or during the day when sunlight reflects off wet roads or other surfaces. Simple activities like walking outside on a sunny day may require dark, wrap-around sunglasses for comfort. The constant adjustment to different lighting environments can be straining and disruptive, affecting everything from reading to navigating a brightly lit store.