A coloboma refers to a hole or absent tissue in any one of the organelles of the eye – the iris, choroid, retina, optic nerve, macula, eye lens. The term “Coloboma” comes from the Greek language and means a defect, mutilation or curtailed structure. Instances of coloboma occur right from the time the baby is born, being caused by genetic aberrations which result in structural faults in the eyes of the growing foetus, such as in retinitis pigmentosa.
A coloboma can develop in just one eye or both eyes and gives rise to symptoms of obstructed vision, strabismus or heightened sensitivity to light, depending upon the location of the eye defect and severity of the condition. In some cases, people with coloboma have yet another visual ailment known as microphthalmia, wherein the size of either one or both eyeballs is smaller than usual. Moreover, on certain occasions, coloboma is accompanied by other visual irregularities such as glaucoma, cataract and myopia.
Types Of Coloboma:
Based on the region of the eye that contains gaps in the tissue structures, there are different types of coloboma, including:
Lens Coloboma: Here, a segment of tissue from the eye lens is absent.
Chorio-Retinal Coloboma: This form of coloboma presents with a hole or gap in a portion of the retina.
Uveal Coloboma: This is the most common type of coloboma which arises from missing tissues in the iris, which is situated in the middle section of the eye known as the uvea. Since the iris is the coloured part of the eye, this kind of coloboma exhibits a characteristic keyhole shape or a cat-eye manifestation.
Optic Nerve Coloboma: Here, some of the tissues in the optic nerve are lacking, leading to a vacuum and decreasing clarity of vision.
Macular Coloboma: This kind of coloboma arises from some tissues in the macula not developing optimally and leading to gaps in its structure.
Coloboma is a very rare genetic eye condition, like Stargardt’s disease, that arises from gene defects or chromosomal aberrations in the developing foetus and is present in the child right from birth. Most instances of coloboma are isolated coloboma and are not passed on from parents to the newborn, but few cases of coloboma are inherited coloboma wherein the genetic faults are transferred from the parents to the child.
Mutations in specific genes such as the PAX2 gene and others are considered to be the reason for the gene defects leading to incomplete eye structures in coloboma. This occurs during eye development in the foetus, which takes place swiftly in the first three months of foetal growth. Initially, a gap called choroidal fissure or optic fissure arises in the bottom of the stalks and this closes in the second month to create the various structures of the eye. When this choroidal fissure fails to seal shut completely, it induces a coloboma.
The distinguishing signs of coloboma depend on which region of the eye is affected and comprise:
- Distinctive keyhole appearance or cat eyes, in cases of uveal coloboma
- In retinal coloboma, vision loss occurs in particular portions of the visual field, known as field defects
- Coloboma afflicting the macula and optic nerve prompts poor and reduced vision which often cannot be fixed by glasses, contact lenses
- Associated visual disorders of glaucoma, cataract
- Increased sensitivity of the eyes to light, especially bright beams
An ophthalmologist or eye specialist confirms the instance of coloboma in the newborn by means of an external eye exam. This involves shining a bright wave of light from an instrument known as an ophthalmoscope, to study the interior portions of the baby’s eyes. The response of the pupil and capability of eye movement is examined.
Once the child grows older, a visual acuity test is also performed, to gauge difficulties in seeing farther objects, problems in side vision or reading small text and determine the extent of vision loss.
There exists no cure for any type of coloboma. However, it can be treated using different strategies based on which part of the eye is affected.
Individuals diagnosed with uveal coloboma have the option of cosmetic contact lenses, which present a round shape of the iris of the eyes and thereby making the keyhole form not noticeable. Another remedial measure to rectify iris irregularities in coloboma is surgery. In cases of retinal, macular or optic nerve coloboma, though affected individuals may still face challenges in viewing clearly, eyeglasses and contact lenses can help improve visual accuracy to some extent.