Occasionally, only the pigment epithelium is involved and the defect is not obvious, but can be seen on transillumination.Partial iris coloboma involves the pupillary margin, giving an oval pupil.Complete iris coloboma results in a 'keyhole-shaped' pupil.This may be limited to the iris, but can involve other parts of the eye. MIDAS syndrome ( Microphthalmia, D ermal Aplasia, and Sclerocornea), an X-linked dominant disorder lethal in boys.Īn extensive list of syndromes and genetic conditions associated with coloboma can be found in the review by Chang et al.Coloboma in association with renal anomalies.CHARGE syndrome - Coloboma, Heart anomaly, choanal (nasal) Atresia, Restriction (of growth and/or development), Genital and Ear abnormalities.ĭescribed syndromes involving coloboma together with multisystem malformations coloboma include:
Optic cysts are also part of this spectrum. Often there is no recognised genetic syndromic aetiology. Microphthalmia, anophthalmia and coloboma are an inter-related group of congenital ocular abnormalities which can co-exist. There is good evidence for the role of some environmental exposures on the development of coloboma, including vitamin A deficiency, folate deficiency, maternal hypothyroidism, maternal alcohol use, fetal mycophenolate mofetil exposure, and congenital zika virus infection. Where the coloboma is inherited, there may be variation in severity between individuals, probably due to incomplete penetrance and variable expressivity of the gene. There are recognised gene mutations involved in the heritable forms of coloboma, microphthalmia and anophthalmia. In other cases, the inheritance pattern is less clear, but genetic factors are likely. Genetic factors are sometimes clear, where there is a Mendelian pattern of inheritance or a chromosomal abnormality. There may be genetic and/or environmental factors involved in causation. Colobomata also occur in other quadrants of the eye, but the embryological basis of these colobomata is unknown, and they are sometimes termed 'atypical'. Failure of this fusion leads to a gap in ocular tissue, known as a coloboma, typically located in the inferonasal quadrant of the eye. The optic fissure fuses at 5-7 weeks of development. The eye develops in the embryo, from the optic cup and optic fissure.
Coloboma is estimated to account for 3-11% of blindness in children worldwide. The estimated incidence of coloboma is about 1 in 10,000 births. Although the basic birth anomaly cannot be corrected, most of the complications listed above are correctable to a great extent. Visual acuity is affected when coloboma involves the disc and fovea, or is complicated by occurrence of retinal detachment, choroidal neovascular membrane, cataract, or by amblyopia due to uncorrected refractive errors. At one extreme, the eye is hardly recognisable and non-functional - having been compressed by an orbital cyst, while at the other, one finds minimalistic involvement that hardly affects the structure and function of the eye. Ocular colobomata are more often associated with systemic abnormalities when caused by chromosomal abnormalities. The occurrence of coloboma can be sporadic, hereditary (known or unknown gene defects) or associated with chromosomal abnormalities. It most commonly involves the inferonasal quadrant of the eye, and can be unilateral or bilateral, and is often associated with microphthalmia. It can involve one or more ocular structures, including the cornea, iris, ciliary body, lens, retina, choroid and optic disc. It is used to describe a developmental defect of the eye occurring at embryonic stage. Coloboma comes from the Greek word koloboma, meaning curtailed.
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