Developmental eye disorders
Deviations from normal development can lead to a wide variety of eye defects that range from anophthalmia (absent eye) to irregularly shaped pupils.
Developmental disorders of the eye can occur in isolation or as part of a syndrome.
Failure of fusion of the optic cup, usually inferiorly, may result in a coloboma, a cleft like defect in the eye. The effect of a coloboma varies depending upon the location and extent of the defect. For example, a coloboma in the pupil may only result in a cosmetic defect whereas a coloboma of the optic nerve and retina may cause severe visual impairment. A coloboma can be an isolated defect or may be part of a syndrome. Between 15 - 30% of children with small eyes (microphthalmia) and coloboma have the CHARGE association (coloboma of the eye, congenital heart disease, choanal atresia (a congenital blockage of the passageway between the nose and pharynx), retarded growth and development, genital abnormalities and ear malformations with or without hearing impairment).
Albinism
This is a group of inherited disorders caused by a deficiency of the pigment melanin. It is characterised by:
- A lack of pigmentation of the skin and hair
- Abnormalities of the eye due to absence of melanin which is normally found in the iris and retina
The eye abnormalities include nystagmus (jerking eye movements), reduced visual acuity, iris translucency, lack of pigmentation of the retina, underdevelopment of the fovea (central retina) and abnormalities of the visual pathways to the brain. Squint and refractive errors, especially astigmatism (irregular curving of the cornea), are common. Examination of parents, siblings and other family members, if possible, is essential to determine the inheritance pattern. Infants with albinism are slow to see and may appear to be blind immediately after birth, with irregular (roving) eye movements that change to nystagmus in the first year. Visual acuity for distance vision is usually reduced to between 6/24 to 6/60. Near vision tends to be better and many young children can read small print unaided. Children may adopt a compensatory head posture in the form of a face turn and head tilt to dampen their nystagmus to obtain better vision. Parents may be reassured that vision usually gets better with time as there is delayed visual development and consequently nystagmus may also decrease with time.
Management involves correction of refractive errors, tinted lenses to reduce light sensitivity and low visual aids. Most children are able to be educated in a normal school environment. However, adequate support within the school should be available and tailored to the child. Surgery to improve the cosmetic appearance of squint or significant head posture may be considered. Referral of the family to a geneticist is recommended to help determine the possible risk to future siblings. It is also advisable to rule out other associations of albinism such as the rare bleeding disorders.
Congenital infections
A number of maternal infections can affect the eye, in addition to the other clinical features seen in these infants. Infections that may affect the eye include rubella, toxoplasmosis, cytomegalovirus, chickenpox, herpes virus, syphilis and HIV.
These infections may affect many different parts of the eye. The degree of visual impairment depends upon the extent to which the various parts of the eye are affected and varies from child to child.
Specific conditions:
These include:
- Abnormalities of the whole eye, that includes:
- Anophthalmos when there is absence of the whole eye or the eye is replaced by a tiny cystic remnant. Treatment is by insertion of serial orbital prostheses to encourage orbital growth and later an artificial eye.
- Microphthalmos when the eye is smaller than usual. It may be associated with other abnormalities of the eye. The smaller the eye or the more severe the associated abnormalities of the eye, the worse the resulting visual impairment.
- Abnormalities of the anterior segment, that includes:
- Abnormalities of the cornea, for example, megalocornea, when the diameter of the cornea is greater than normal, microcornea when it smaller than normal or cornea plana when the cornea is flattened.
- Abnormalities of the iris, for example aniridia in which there is almost complete absence of the iris. In some children aniridia may be associated with a kidney tumour (Wilms tumour) and may need regular ultrasound scanning of the abdomen.
- Abnormalities of the posterior segment, that includes:
- Abnormalities of the vitreous, for example persistent hyperplastic primary vitreous when the vitreous is replaced by a thick mass behind the lens.
- Abnormalities of the retina, for example colobomas (cleft-like defects) of the retina.
- Abnormalities of the optic nerve, for example, optic nerve hypoplasia, an uncommon condition where there is underdevelopment of the optic nerve. Bilateral cases may result in severe visual impairment and nystagmus (jerky eye movements) in infancy. Unilateral cases may be less severe and result in visual field defects and squint.
Prognosis: The above conditions are usually associated with moderate to severe visual impairment. Treatment, wherever possible, is essential to realise the child’s visual potential. Long-term follow up is necessary.
