Coloboma

Coloboma is caused by a disruption in the eye's development during pregnancy. The eyes start to develop around day 22 of embryonic development. They first appear as hollow sack-like structures arising from the forebrain called optic vesicles. These vesicles invaginate and develop into double-walled optic cups, which later generate various parts of the eye. As the eye continues to develop, a gap known as the choroidal fissure forms on the underside of the optic cup to allow the hyaloid artery to enter the developing eye. Around the 5^th to 7^th week of gestation, the choroidal fissure closes, giving rise to the pupil and sealing off the eye into a complete sphere. 

Coloboma occurs when the choroidal fissure doesn't close completely or at the right time. This leaves a gap in one or more structures of the eye, depending on where the closure process was incomplete. The exact cause of coloboma is unknown, however, having a genetic predisposition and environmental factors, such as drinking alcohol or smoking tobacco during pregnancy may play a role. While coloboma can occur as an isolated birth defect, it’s often associated with other genetic anomalies, the most common being CHARGE syndrome. CHARGE syndrome is an acronym for coloboma of the eye, heart defects, atresia of nasal choanae, retardation of growth, genitourinary abnormalities, and ear problems.  

Coloboma can affect the eyelids, iris, retina, optic nerve, or other parts of the eye. In mild cases, especially if the coloboma is small and only affects the iris, there may be no noticeable symptoms except for a characteristic keyhole-shaped pupil due to a gap in the iris. Iris coloboma rarely causes vision problems, but it can result in increased sensitivity to light because more light can enter the retina through the pupil. 

Iris coloboma is often associated with lens coloboma, which can be seen as a flattening or indentation in the lens. People with lens coloboma have a higher risk of cataracts, which can make vision blurry, hazy, or less colorful. Iris coloboma can also be associated with eyelid coloboma, which typically looks like a notch or gap in the upper eyelid. 

Coloboma affecting the retina and optic nerve usually results in more severe vision problems, such as vision loss or blind spots in the visual field. This is especially true if coloboma affects the macula, which is the part of the retina responsible for the sharp, central vision needed for activities like reading or driving.  

The main risk of retinal coloboma is retinal detachment, which happens when the neurosensory layer of the retina pulls away from its normal position. Retinal detachment typically presents with light flashes or floaters described as fine dots, veils, cobwebs, clouds, or strings, followed by sudden visual field loss in the affected eye. Vision loss typically begins in the periphery of the visual field and progresses toward the center over hours to weeks like a “curtain being drawn down.” 

Treatment of coloboma depends on the specific type, location, and severity of the coloboma, as well as the associated symptoms and impact on vision. If coloboma affects the shape of the eye or causes refractive errors, like myopia (i.e., nearsightedness), hyperopia (i.e., farsightedness), or astigmatism, glasses or contact lenses can be used to correct these issues. In mild cases of iris coloboma, wearing sunglasses with UV protection can help reduce light sensitivity and colored contact lenses can be used to make the coloboma less visible. Severe cases may require surgery to reconstruct the iris, where microscopic sutures are used to close the gap in the iris, creating a more natural, round pupil.  

Management of choroid and retinal colobomas involves regular eye examinations to assess any changes in vision and detect potential complications like retinal detachment early. Retinal detachment is a medical emergency that requires surgery to restore vision and prevent any further vision loss.