Coloboma

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What is a Coloboma?

Image showing a "normal" iris and pupel compared to one with Coloboma

Ocular coloboma is a condition where there is missing normal tissue in or around the eye at birth. The term coloboma stems from the Greek word “curtailed.” 

It occurs when the closure of the embryonic fissure (gap) is not possible. 

During the first three months of the fetus’s growth, the eye begins to take form. A gap (also referred to as the choroidal fissure or optic fissure) rests at the base of the stalks that give shape to the eye. In most cases, this fissure closes before the seventh week of gestation. However, when it does not, a coloboma or space arises. 

The cause of colobomas can vary. The eye condition can happen by chance or because of a gene defect or chromosomal abnormality. Environmental factors, such as alcohol during pregnancy, may also play a role. 

Regardless of the reason, a coloboma can impact one or both eyes. It may also contribute to vision loss or light sensitivity problems. Because colobomata (plural for coloboma) can affect different parts of the eye, symptoms and complications will differ.  

To diagnose a coloboma, an ophthalmologist will perform a comprehensive eye exam using an ophthalmoscope, as well as a visual acuity test

Coloboma of the iris, which gives a “keyhole” appearance of the pupil, does not usually result in vision loss.

How Common is a Coloboma?

Coloboma develops in approximately 1 in 10,000 individuals. However, because some cases of coloboma do not trigger any symptoms like vision loss, the actual figure may be different. 

Uveal coloboma (located in the middle layer of the eyewall) contributes significantly to blindness. Studies suggest an estimated 5 to 10% of blind European children present with uveal coloboma or uveal coloboma-related malformations. 

Types of Coloboma

1. Lens Coloboma

A lens coloboma happens when the structures (ciliary body and zonules) maintain the lens in place have a focal defect. When this occurs, the lens shape could become asymmetric and cause vision problems. For example, an irregular lens can lead to a refractive error and/or amblyopia (lazy eye). 

2. Macular Coloboma

The center of the retina is called the macula. This eye structure determines light, high-resolution, and color vision. When a fetus has a macular coloboma, it is because the macula did not form completely or inflammation of the retina occurred during gestation. A macular coloboma can have a significant impact on vision.

3. Eyelid Coloboma

In cases of eyelid coloboma, the fetus is missing a part of the upper or lower eyelid. This may occur alongside a genetic syndrome or because eyelid development was interrupted. 

4. Uveal Coloboma 

The wall of the eye is composed of the uvea, a middle layer that includes:

  • Iris (colored part of the eye)
  • Choroid
  • Ciliary body 

When a uveal coloboma appears, the baby can have a “keyhole” or “cat-eye” appearance to the iris. It may also extend from front to back (continuous) or have “skip lesions.” 

5. Optic Nerve Coloboma

The optic nerve is an essential structure in the eye. It helps relay visual information to the brain. 

When a fetus suffers an optic nerve coloboma, the defect can impact the retina and optic nerve.

What Causes Coloboma?

Coloboma occurs because of abnormal eye development. During the second gestational month, a seam called the choroidal fissure closes to form the eye’s structures. When this doesn’t happen correctly, the fetus develops a coloboma. 

Because the choroidal fissure is at the eyeball base, coloboma arises in the lower half of the eye. 

The causes of the defective closure of the fissure vary. On the one hand, colobomas can occur sporadically. On the other hand, coloboma can arise due to an abnormality in the chromosome or changes in genes involved in the fetal development of the eye. 

For example, in uveal coloboma, researchers have reported an association of some specific genes with an eye malformation in the microphthalmia, anophthalmia, coloboma (MAC) spectrum. Yet, even then, more remains unknown about genes that cause the majority of cases of coloboma. 

Finally, environmental factors, like alcohol exposure during pregnancy, may contribute to developing a coloboma. 

Can a Coloboma be Inherited?

Genetics can be a leading factor in why some fetuses have coloboma.

At times, a coloboma can form part of a genetic syndrome, e.g., cat-eye syndrome. In this particular example, the rare disorder takes its name from the unique shape found with an iris coloboma caused by a concrete genetic mutation. 

However, it is important to mention that a family history of coloboma or a specific syndrome has not always been found in babies born with a coloboma. This means that the condition can appear by chance.  

Who is at Risk of Developing this Condition?

Coloboma can occur because of changes in genes involved during early fetal development or chromosomal abnormalities. 

However, it is difficult to determine which individuals will have this condition. The majority of genetic changes related to coloboma have been identified only in extremely low numbers of affected individuals. 

The risk of this condition can also increase if the mother consumes alcohol during pregnancy. 

A family history of coloboma is also a risk factor for this condition. 

Symptoms of a Coloboma

The symptoms of a coloboma will vary depending on where it arises.

The most common symptoms will include:

  • A defect in the iris (iris coloboma)
  • Eye misalignment, e.g., strabismus or amblyopia 
  • Poor vision, field defect, or vision loss (macular or optic nerve coloboma)
  • Increased light sensitivity 

Additionally, other eye abnormalities may appear in people with coloboma, such as:

  • Cataracts (clouding of the lens)
  • Glaucoma (damage to the optic nerve due to increased pressure in the eye)
  • Refractive errors, including myopia (nearsightedness), nystagmus (involuntary back-and-forth eye movements), or retinal detachment (retina breaks off from the back of the eye) 

Diseases Associated With Coloboma of the Eye 

Coloboma of the eye can occur for different reasons.

In some cases, a coloboma develops because of specific diseases or chromosomal abnormalities that involve other body structures. 

Here is a list of various diseases and syndromes that are associated with coloboma:

  • Goldenhar syndrome. This rare congenital condition is characterized by irregular development of the eye, ear, and spine.
  • Duane syndrome.  This condition is a congenital (present at birth) and non-progressive type of strabismus caused by abnormal development of the sixth cranial nerve.
  • Treacher Collins syndrome. This is a genetic disorder that results in deformities of the eyes, eyes, cheekbones, and chin. Disease severity will vary. 
  • CHARGE syndrome. People with this condition experience heart defects, choanal atresia, retardation of growth and development, genital or urinary tract anomalies, or ear malformations 
  • Kabuki syndrome. A rare, multisystem disorder, kabuki syndrome causes different abnormalities like distinctive facial features, delayed growth, intellectual disability, skeletal problems, and short stature.
  • 13q deletion syndrome. This rare genetic disease appears when some or all of the large arm of human chromosome 13 is deleted. People born with this condition have low birth weight, head and facial malformations, abnormalities of the eyes, and much more. 
  • Wolf-Hirschhorn syndrome. This genetic disorder can cause a distinctive facial appearance, delayed growth and development, intellectual disability, low muscle tone, and seizures.
  • Microphthalmia. One or both eyes are atypically small. In some cases, the eyeball may seem to be missing entirely. 

At the moment, there is no high yield clinical test for uveal coloboma, with at least 80% of cases still without an explanation. 

Possible Complications

Colobomas can give rise to some complications. 

For example, an eyelid defect can leave part of the cornea exposed and result in excessive dryness. 

Also, in retina and optic disc colobomas, retinal detachment may occur. This condition will require surgical correction. Similarly, blood vessels may grow alongside the coloboma and cause bleeding, which can lead to vision loss in rare cases. 

Treatment Options for a Coloboma 

There is no cure for coloboma, and treatment options will differ according to the type of coloboma. 

For instance, people with an iris coloboma could wear colored contact lenses to give a rounder appearance to the iris. They may also undergo surgery. 

In the case of eyelid colobomas, corrective surgery is an option. 

For other types of coloboma, your ophthalmologist may recommend the use of low-vision devices, glasses, or eye drops. Your ophthalmologist may also provide treatment for other eye conditions that can appear with coloboma, such as cataracts or new blood vessel growth in the back of the eye.

Resources
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“Coloboma of the Iris.” Mount Sinai Health System, www.mountsinai.org/health-library/symptoms/coloboma-of-the-iris.

“Coloboma.” Coloboma - American Association for Pediatric Ophthalmology and Strabismus, aapos.org/glossary/coloboma.

“Coloboma.” National Eye Institute, U.S. Department of Health and Human Services, www.nei.nih.gov/learn-about-eye-health/eye-conditions-and-diseases/coloboma.

“Coloboma: MedlinePlus Genetics.” MedlinePlus, U.S. National Library of Medicine, 18 Aug. 2020, medlineplus.gov/genetics/condition/coloboma/.

Lingam, Gopal, et al. “Ocular Coloboma-a Comprehensive Review for the Clinician.” Nature News, Nature Publishing Group, 21 Mar. 2021, www.nature.com/articles/s41433-021-01501-5.

Porter, Daniel. “What Is a Coloboma?” American Academy of Ophthalmology, 23 Oct. 2020, www.aao.org/eye-health/diseases/what-is-coloboma#:~:text=A%20coloboma%20describes%20conditions%20where,first%20three%20months%20of%20growth.

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