Coloboma is a gap or missing piece in one of the structures of the eye, present from birth. It occurs when part of the eye doesn’t fully form during early pregnancy, leaving a notch or hole that can affect the iris, lens, retina, or optic nerve. Coloboma occurs in roughly 1 in every 2,000 live births and can range from a barely noticeable cosmetic difference to a significant cause of vision loss.
How Coloboma Forms Before Birth
During the fifth week of pregnancy, when an embryo is only 7 to 14 millimeters long, a temporary opening called the optic fissure needs to close as the eye takes shape. This closure starts in the center of the developing eye and moves forward and backward from there. If the fissure doesn’t seal completely, whatever tissue was forming along that gap is left incomplete. The result is a coloboma.
Because of where this fissure sits on the developing eye, colobomas almost always appear in the lower inner portion of the eye. The size depends on how much of the fissure failed to close: a small section might leave a tiny notch in the iris, while a larger failure can affect deeper structures like the retina or optic nerve.
Types of Coloboma
Coloboma is named for the specific part of the eye that’s affected, and more than one type can occur in the same eye.
- Iris coloboma is the most recognizable form. The iris, the colored ring that controls how much light enters the eye, has a gap that gives the pupil a keyhole or cat-eye shape instead of a round one.
- Chorio-retinal coloboma involves missing tissue in the retina, the light-sensitive layer at the back of the eye. This type can have a more serious effect on vision because the retina is responsible for capturing the images your brain interprets as sight.
- Macular coloboma means part of the macula is absent. The macula handles sharp central vision, the kind you rely on for reading, recognizing faces, and driving. Losing part of it directly impacts those tasks.
- Lens coloboma involves a missing section of the lens, the clear structure that focuses light onto the retina.
- Optic nerve coloboma affects the nerve that carries visual information from the eye to the brain.
How Coloboma Affects Vision
The impact on vision varies enormously depending on the type and size. Some people with a small iris coloboma see perfectly well. Others, particularly those with optic nerve or retinal involvement, can have vision ranging from normal (20/20) all the way to no light perception at all. The key factor for children born with colobomas involving the optic nerve is whether the fovea, the tiny central point of sharpest vision, develops normally. If it does, useful central vision is often preserved regardless of the coloboma’s size.
Iris colobomas cause the most day-to-day discomfort in the form of light sensitivity, since the misshapen pupil can’t properly regulate how much light enters the eye. Some people also experience visual distortion or double vision. Deeper colobomas affecting the retina or optic nerve carry a higher risk of retinal detachment and severe visual impairment, particularly when structural abnormalities or cysts are present behind the eye.
Genetic Causes
Coloboma can occur on its own or as part of a broader genetic syndrome. At least 27 genes have been linked to syndromes that include coloboma, and a smaller group of genes are known to cause coloboma in isolation, without other birth differences. Coloboma can be inherited from a parent or arise from a new genetic change. In many cases, no specific genetic cause is identified.
The most well-known syndrome associated with coloboma is CHARGE syndrome, where coloboma is one of the defining features alongside blocked nasal passages (choanal atresia), abnormal inner ear structures, and cranial nerve problems. A clinical diagnosis of CHARGE requires a combination of these major features along with several possible minor ones, including heart defects, cleft lip or palate, genital abnormalities, and kidney problems.
How It’s Diagnosed
Iris coloboma is often spotted at birth or during a newborn exam because of the pupil’s distinctive keyhole shape. Deeper colobomas affecting the retina, macula, or optic nerve may not be visible without specialized eye exams. A full evaluation typically includes checking visual acuity, pupil responses, eye pressure, and a dilated exam to see the back of the eye. Additional tests like optical coherence tomography (a detailed scan of the retina) or visual field testing help map the extent of any damage.
Because coloboma frequently occurs alongside other birth differences, a diagnosis usually triggers a broader workup. This commonly includes a hearing test, kidney ultrasound, spine X-rays, blood tests for kidney and liver function, and sometimes an echocardiogram or brain MRI. These screenings check for the kinds of systemic issues seen in CHARGE and related syndromes, even when the coloboma appears to be the only visible finding.
Treatment and Management
There is currently no way to repair the missing tissue in retinal, macular, or optic nerve colobomas. Management for these types focuses on monitoring vision, treating complications like retinal detachment if they arise, and supporting visual development in children through early intervention.
Iris coloboma, on the other hand, has several practical options. Many people manage light sensitivity with tinted glasses or sun hats. A cosmetic contact lens can be fitted over the affected eye to make the pupil appear round and reduce the amount of light getting in. In some cases, surgery can close the gap in the iris by stitching the edges together, correcting both the appearance and the light sensitivity. Not everyone with iris coloboma needs any treatment at all, particularly if the cosmetic difference doesn’t bother them and their vision is good.
For children, one of the most important concerns is amblyopia, sometimes called lazy eye, which can develop if one eye has significantly worse vision than the other. Treating amblyopia early, often with patching or corrective lenses, gives the weaker eye the best chance of developing useful vision.