Coloboma is an abnormality in the development of the eyeball during embryonic life. It can involve different structures of the eyeball such as the iris, the choroid, the retina, the lens, the eyelid or the optic nerve.

Definition of coloboma

Coloboma is a congenital eye defect, that is, an abnormality in the development of the eyeball that is present from birth. This malformation results in the presence of a slit or gap in normal tissue.

Coloboma can affect different structures of the eyeball. It is possible to distinguish several forms depending on the location of the anomaly:

• the Iranian coloboma which results in a slit in the iris (colored part of the eye);
• macular coloboma which results in a lesion at the level of the macula (anatomical structure of the retina contributing to vision);
• chorioretinal coloboma which affects the choroid (one of the layers of the wall of the eyeball) and the retina (membrane lining the bottom of the eyeball and constituting the main site of vision);
• lens coloboma, which affects the lens (a lens-like structure at the back of the iris);
• eyelid coloboma that affects the upper eyelid;
• optic nerve coloboma (cranial nerve that transmits visual information from the eyes to the brain).

These different ocular colobomas can occur in isolation or occur together. They can affect only one eye (unilateral coloboma) or both eyes (bilateral coloboma).

Cause du colobome

Coloboma is a developmental anomaly occurring during embryonic life (the first few weeks after the union of an oocyte and a spermatozoon), and more exactly between the 5th and the 7th week of intrauterine life.

Studies show that coloboma can be inherited (familial). Nevertheless, research continues to better understand the genetic implications. It is found that coloboma can be associated with different genetic diseases like:

• cat-eye syndrome, or so-called cat eye syndrome, which is characterized by a variable association of birth defects including an iris coloboma (iris coloboma);
• CHARGE syndrome, a rare disease characterized by the occurrence of coloboma, heart defects, developmental abnormalities of the nasal cavity, developmental delay, developmental abnormality of the genital tract, ear abnormalities and / or deafness.

Diagnostic of colobome

The diagnosis of coloboma depends on its location. Some colobomas, such as iris coloboma or eyelid coloboma, are visible and can be diagnosed by physical examination. Others are not directly observable and require in-depth ophthalmologic examinations including a fundus examination.

Colobomas vary in severity: some do not cause any disturbance while others can strongly impact visual capacity. The degree of severity depends on the location of the coloboma but also varies if both are affected and if other abnormalities are present.

Sometimes visible malformations

Coloboma results in the presence of a slit or gap in normal tissue. The malformation may or may not be visible. For example, iris coloboma results in a visible black slit in the iris. In the case of eyelid coloboma, there is a split in the upper eyelid or the complete absence of the eyelid.

Possible visual disturbances

Some forms do not cause any disturbance while others can affect vision. The risk of complications is particularly greater in the case of chorioretinal coloboma or macular coloboma.

A coloboma can cause:

• decreased visual acuity;
• loss of vision which may be total or partial;
• microphthalmia, which is a decrease in the size of the eyeball;
• anophthalmia, or the absence of an eyeball;
• retinal detachment;
• a cataract which corresponds to the partial or total opacification of the lens;
• a corneal ulcer which is an infection of the eye affecting the cornea (the transparent part of the eye located in the front).

The management of colobomas involves treating the associated symptoms. The affected person may wear glasses or contact lenses to correct a vision disorder or for cosmetic purposes. Eye covers can also be used. In case of cataract or retinal detachment, surgery is necessary and followed by ophthalmologist rehabilitation.

There is no definitive treatment for colobomas, with the exception of eyelid coloboma for which surgery is possible to correct the malformation.

Research continues to better understand the genetic implications of colobomas. Their results could perhaps allow genetic testing to assess a couple’s risk of giving birth to a child with coloboma.