A developmental defect that occurs at the embryonic stage of the eye is termed as coloboma. Any ocular structure, like cornea, iris, lens, retina, optic disc, ciliary body or choroid can be involved with coloboma. Coloboma can be present in one or both eyes.
Symptoms of coloboma are classified based on the location of its occurrence. Iris coloboma will be visible as a keyhole and allows the formation of a second image at the back of the eye leading to double vision. Iris coloboma reduces acuity and creates ghost images. Eyelid coloboma is also visible clearly. Other types of coloboma can hinder the vision based on their size. Children not having a part of retina will not be able to see a specific location of their vision field. It is not possible to identify vision impairment at the time of birth. Chorioretinal and optic nerve coloboma leads to visual loss in adults due to retinal detachment .
Diagnosis of coloboma in children is performed by looking for any associated syndrome or linked anomaly in the eye appearance. Eye of the parents are examined for any heritable cause and chromosomal analysis is performed. Ophthalmoscopy is performed to diagnose chorioretinal and optic nerve involvement. The patient is checked for accurate refraction and visual field. Slit-lamp examination is done for observing anterior eye structures. Computerized tomography and magnetic resonance imaging are done for detecting associated central nervous system defects and microphthalmia. Pictures of optic pits can be taken by optical coherence tomography. Fluorescein angiography is another technique for assessing coloboma .
No treatment is present for coloboma. Children with coloboma have the risk of developing retinal detachment and glaucoma, and the treatment for these conditions are available in the hospitals. If the health of the affected child is stable, regular eye checkup is conducted up to 7 years and then annual checkup is suggested. Children with coloboma will be prescribed with glasses, although glasses cannot solve the problem. Glasses help to correct the sight of the vision field of the child. Sunglasses are also advised to manage light sensitivity. Surgery is suggested, if necessary for eyelid coloboma .
Prognosis of coloboma will be serious leading to macular detachments, if the disease is isolated to the optic disc involving macula or microphthalmia. Another condition called CHARGE syndrome results from optic disc coloboma. Other prognostic defect of coloboma associated with mental retardation, growth failure and multiple congenital anomalies are chromosomal abnormalities. Central nervous system abnormalities like basal encephalocele can result from coloboma .
Genetic or environmental factors might be involved in causing coloboma. Genetic factors as cause of coloboma are explained by Mendelian pattern of inheritance or by chromosomal abnormalities. Inheritance pattern is not yet clear, while certain genetic factors are likely to exist. There are certain gene mutations found associated with the heritable forms of coloboma, anophthalmia and microphthalmia. Inherited coloboma might have some variations in the severity of this condition in different individuals. This variation might be due to variable gene expression and incomplete penetrance. Environmental factors causing congenital coloboma include deficiency of vitamin A, teratogenic drugs like thalidomide and mycophenolate mofetil, fetal alcohol syndrome and infections of cytomegalovirus and toxoplasmosis .
Coloboma occurs in usually 1 in 10,000 people. It is estimated that the disease accounts for 3 to 11 percent of blindness in children, all over the world. In a review of the medical records of children below the age of 19 years, living in Olmsted County and diagnosed with coloboma from 1968 till 2007, the results were reported as given below. There was annual incidence of 2.4 residents (aged <19 years) among 100,000 people. The median age of newly diagnosed 33 children was 3.9 months. Among 33 children, 12 had involvement with anterior segment, 13 with posterior segment and 8 with both the segments. In a follow-up study of 9.2 years, 11 children had amblyopia and 10 had strabismus along with ocular coloboma. In another follow-up study of 16.8 years, non-ocular disorders like abnormal development were seen in 12 children and CHARGE syndrome in 4. CHARGE stands for coloboma, heart anomaly, nasal atresia, restriction of growth or development, genital and ear abnormalities .
The failure of embryonic fissure to close properly will lead to ocular coloboma. Embryonic fissure is the opening along the inferior aspect of the developing eye by which mesodermal tissue enters the eye ball. Closure usually starts from the middle of the fissure during 5 to 6 weeks of gestation and lengthens distally and proximally. Multiple secondary ocular malformations might accompany colobomas due to neuroectodermal induction anomalies. Failing to close at the proximal end leads to optic disc coloboma .
No specific steps are identified for preventing coloboma. Optic nerve coloboma can be prevented by collie eye anomaly testing for all the susceptible individuals in the breeding pool. Parents with genes related to genetic disease collie eye anomaly can be given genetic counseling and warning before child birth .
During the growth of the fetus in the first three months, a gap or choroidal fissure occurs at the bottom of the stalk, which eventually converts into an eye. This fissure usually closes by the seventh week of gestation. If it does not close, it results in coloboma. Depending on the part of the eye that is affected, coloboma is of different types. They are eyelid coloboma, lens, macular, optic nerve, uveal and chorioretinal coloboma . Coloboma occurs both spontaneously or by inheritance. The defect can vary from a tiny notch to a complete disappearance of the eyelid.