Coloboma

Congenital coloboma is a developmental defect of the eye occurring at embryonic stage. It is caused by defective closure of the embryonic fissure of the optic cup.¹ This failure of closure results in a cleft of absent tissue. Visual prognosis is linked to the severity of ocular malformation. It is an important cause of childhood visual impairment and blindness.

Congenital coloboma can occur as an isolated condition or as part of a number of different syndromes involving neurological, craniofacial and systemic developmental defects.²

It can involve one or more ocular structures including the cornea, iris, ciliary body, lens, retina, choroid and optic disc. It most commonly involves the inferonasal part of the iris¹ and can be unilateral or bilateral.

Coloboma can also be caused by eye surgery or trauma. This article discusses congenital coloboma.

• Most cases are sporadic.
• Autosomal dominant, autosomal recessive and X-linked inheritance patterns have been described.³
• Coloboma can be caused by a mutation in the PAX6 gene.¹
• Environmental factors may also underly its aetiology. This is more likely in sporadic, unilateral coloboma.
• Ingestion of certain drugs (including thalidomide) as well as excessive alcohol consumption during pregnancy have been implicated. Maternal vitamin A deficiency has also been suggested.

• Coloboma accounts for between 3.2 and 11.2% of blind children worldwide.²
• The estimated prevalence of coloboma is 1 in 10,000.⁴

Coloboma can be classified as a disease of increasing severity:

• It can be isolated
• It can be associated with another ocular anomaly (for example microphthalmos or anophthalmia)
• It can be associated with a central nervous system anomaly and with systemic manifestations outside the central nervous system

More common associations include:

• Cleft lip
• Skeletal defects
• Genitourinary abnormalities
• Cat-eye syndrome
• Velo-cardiofacial syndrome
• DiGeorge syndrome
• Noonan syndrome
• Hirschsprung syndrome
• Trisomy 13
• CHARGE syndrome: coloboma, heart defects, choanal atresia, retarded growth, genital abnormalities and ear abnormalities or deafness³

• Iris coloboma can be isolated but is often associated with other colobomata in the eye and can extend to the retina, choroid or optic nerve.
• It can affect one or both eyes.
• It appears visually as a black notch of varying depth at the edge of the pupil and gives the pupil an irregular shape. This appearance is sometimes referred to as a 'keyhole pupil'.
• Investigation includes detailed eye examination and MRI of the eye and optic tract.
• Surgery is sometimes needed either for cosmetic reasons or to treat photophobia.
• A cosmetic contact lens is an alterative to surgery for cosmetic reasons.
• Visual prognosis is linked to the severity of ocular malformation.

• Occur uni-bilaterally, often with profound systemic associations.
• There is decreased visual acuity and the presence of a visual field defect.
• They are associated with colobomata of other parts of the eye.
• Tend to progressively enlarge and are associated with retinal detachments.

• Eyelid coloboma has also been described. This is thought to arise from failure of the mesodermal folds to fuse at about 7 to 8 weeks of gestation.²
• There is a full-thickness defect in the eyelid. The most common position is at the junction of the medial and middle third of the upper lid.⁵
• It can be associated with Treacher Collins syndrome.⁵
• The treatment goal is to protect the cornea.
• This can be achieved using artificial tears and patching but surgery to close the defect is usually performed.