Coloboma: what it is, symptoms, causes, treatment

By Cristiano Antonino On Mar 9, 2023

Coloboma is a rare condition affecting the eye: it is a malformation that can affect various eye structures such as the iris, choroid, retina and optic nerve

Some studies estimate an incidence of around 10-20 cases per 100,000 infants.

Coloboma can occur in isolation or be associated with various genetic diseases that also affect other parts of the body, causing multisystem malformations.

What it is: types of coloboma

Coloboma is caused by a defective closure of the optic fissure, a temporary structure that appears during prenatal ocular development, beginning in the fifth to seventh week of embryonic life.

This causes a tissue malformation of one or more ocular structures, which may affect one eye (unilateral) or both eyes (bilateral).

Coloboma of the eyelids can be generated by several factors unrelated to abnormalities of the globe of the eye

The disease develops due to a defective fusion of the eyelid folds at approximately 7-8 weeks of gestation.

The consequences of a coloboma on visual capacity are closely linked to the location and extent of the eye defect.

For example, an iris coloboma, i.e. one that only affects the iris, does not cause any vision problems, only increased sensitivity to light.

In contrast, a malformation extending to the optic nerve or retina can impair vision, even seriously.

Depending on the ocular structure affected by the defect, and thus on the extent of the defect, we can identify different types of coloboma

chorio-retinal, characterised by the partial absence of the pigmented epithelium of the retina and the choroid. Patients may have reduced vision and are at risk of retinal detachment. Chorioretinal coloboma may be isolated, but is usually associated with other eye defects (iris coloboma, microcornea, nystagmus, strabismus, microphthalmos)
of the crystalline lens, characterised by mono- or bilateral morphological abnormalities of the crystalline lens (contraction of the lens with a schisis). It may occur together with other ocular abnormalities, such as colobomas of the iris, choroid or optic disc, cataracts and retinal detachment
iris, associated with the mono- or bilateral presence of a schisis, a gap or hole, usually located in the inferonasal quadrant of the eye, affecting only the pigment epithelium or the iris stroma (incomplete coloboma), or both (complete coloboma). It arises with morphological abnormalities of the iris (e.g. ‘keyhole’ or oval pupil) and/or photophobia. It may be associated with colobomas of other areas of the eye (ciliary body, zonule, choroid, retina, optic nerve) and with complex malformative diseases (e.g. CHARGE syndrome)
macular, characterised by atrophic, usually unilateral, well-circumscribed, oval or round lesions of varying size; the retina, choroid and sclera are rudimentary or absent; the lesions are located on the macula, generating decreased vision and, rarely, other symptoms (e.g. strabismus). Usually the coloboma is isolated, but it may be associated with Down syndrome, skeletal or renal disease
of the optic disc (or optic papilla), characterised by the presence of a shiny white cavity located on the optic disc, mono- or bilateral, sharply outlined, usually located towards the lower portion. Patients usually present with a reduction in visual acuity of varying degrees. The defect may be isolated or associated with other ocular (retinal detachment) or systemic (renal) abnormalities.

Symptomatology

The symptomatological picture associated with a coloboma is variable: from simple division of the inferior iris to the absence of retinal tissue in the lower part of the retina.

Coloboma may also be accompanied by other changes to the eye such as:

anophthalmia, a condition that occurs with the complete absence of the eye
microphthalmia, when the eyes are smaller than normal
cataracts, a visual defect characterised by the presence of an opaque lens
nystagmus, i.e. rapid and repetitive involuntary movements of the eyes
glaucoma, a pathology of the optic nerve that is characterised by increased pressure inside the eye and can lead to total loss of vision.

When the malformation involves the retina and optic nerve, visual acuity is inevitably impaired, to a more or less severe degree.

The diagnosis of coloboma is made by an ophthalmologist, following a thorough ophthalmological test and taking into account the history of the child, pregnancy and family.

Causes: genetic and environmental factors

Approximately two thirds of coloboma cases are thought to have a genetic matrix.

A number of genes have been identified that – if altered – can induce the disease.

These include the genes PAX2, CHX10, MAF, OTX2, SHH and CHD7 (the gene responsible for CHARGE syndrome).

Depending on the gene concerned, the mode of transmission may be autosomal dominant: a parent with the mutation has a 50% chance, at each pregnancy, of transmitting the disease to their child.

Or autosomal recessive: parents are healthy carriers of the mutation and, at each pregnancy, have a 25% chance of transmitting the disease to their child.

Some cases occur randomly, i.e. with no apparent genetic cause.

Some environmental factors in pregnancy have also been identified that could contribute to the development of coloboma in the unborn child, such as:

alcohol use
vitamin A deficiency
intake of teratogenic drugs (e.g. thalidomide, mycophenolate mofetil)
the contraction of infections such as toxoplasmosis or cytomegalovirus.

A non-congenital coloboma, on the other hand, may occur later in life following injury or surgery to the eye. In this case, a unilateral coloboma is almost always present.

The coloboma of any eye structure may present as an isolated anomaly

Or alternatively manifest as part of genetic syndromes that also affect other parts of the body and cause multisystem malformations.

These include:

CHARGE syndrome, characterised by eye malformation, cardiac abnormalities, atresia of the coanae (nasal), developmental delay, ear malformations, genital or urinary tract abnormalities.
epidermal nevus syndrome (ENS)
cat-eye syndrome (Cat-eye syndrome)
velo-cardio-facial syndrome and DiGeorg syndrome
Kabuki syndrome (KS).

Coloboma of the eyelids may be associated with other syndromes that cause abnormal facial development, including Treacher Collins syndrome.

Treatment of Coloboma

Unfortunately, there are no curative pharmacological remedies for the treatment of this malformation.

Depending on the underlying cause of the pathology and the ocular structures affected, however, it is possible to identify an individual therapy or, in some cases, to resort to surgery.

In the case of coloboma of the iris, to correct the appearance of the iris, patients can wear coloured contact lenses, which will also have the effect of reducing the high incidence of light.

In addition, the high glare sensitivity can also be influenced by wearing spectacles with special filter lenses.

In the presence of coloboma of the crystalline lens, one can proceed with surgical removal of the latter and its replacement with an artificial lens.

Coloboma of the eyelids leaves part of the cornea uncovered.

This can cause excessive dryness of the eye, due to the evaporation of tears.

Generally, in this case the eye requires an additional source of lubrication and restorative surgery.

In the case of chorioretinal coloboma, surgery may be necessary to treat or prevent retinal detachment.

Finally, in the case of severe microphthalmia (one or both eyeballs are abnormally small), prostheses may be used.

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