EYE AND VISION PROBLEMS IN CHILDREN WITH DOWN'S SYNDROME
J. Margaret Woodhouse (UK)

• The problems
• The Down's Syndrome Study Group
• Development of long and short sight
• Visual Acuity (detail vision)
• Accommodation (focusing at near)
• Strabismus (squint or eye-turn)
• Guidelines

The problems

Most people are now aware that children with Down's Syndrome are much more at risk of eye disorders than are children who do not have Down's Syndrome. They are much more likely to be born with congenital cataracts, sometimes requiring an eye operation at a very young age, more likely to have nystagmus ('wobbly' eyes, which continually move, usually from side to side), and more likely to suffer from sticky eyes, blepharitis (inflammation of the margins of the eyelids) and conjunctivitis. As they get to their teens and twenties, they are more at risk of developing cataracts and keratoconus (distortion of the cornea).

In addition to the conditions listed above, children and adults with Down's Syndrome are more likely to have long or short sight, requiring spectacle correction, and squints (eye-turns). And they are not alone: children with all forms of intellectual disability (including those with no specific diagnosis) are more likely to have eye problems needing attention. The following table lists the prevalence of eye problems amongst children with different disabling conditions.

The reasons why children with intellectual disabilities should so commonly have eye problems are as yet unknown. Our work in the Cardiff Down's Syndrome Vision Research Unit is beginning to suggest some of the answers.

The Down's Syndrome Vision Research Unit

The group was founded in 1992 by Professor Bill Fraser (Welsh Centre for Learning Disabilities) and myself. With original funding from the Down's Syndrome Association and later from the Medical Research Council, the National Lottery Charities Board, Mencap City Foundation, PPP Healthcare and the National Eye Research Centre, we have supported a succession of research assistants and associates who have made invaluable discoveries about the course of visual development in children with Down's Syndrome. Our greatest asset is the cohort of children and their families from all parts of South and West Wales who have been part of our study for many years. They have welcomed us into their homes, and travelled long distances to take part in studies at the University research clinics. We currently have over 110 children in the study group. Teachers are also playing a role in the research by allowing us to see the children in the school environment and helping us to determine the effects of spectacles on the children's school performance.

Development of long and short sight

In order to appreciate the problems that arise in children with Down's Syndrome, we need first of all to look at development in children who do not have Down's Syndrome. At birth, many babies are long or short sighted; the average is 3 dioptres of long sight but the spread of refractive errors can be very wide. Over the first few years of life, the usual course of events is for children to outgrow these baby errors. The state of 'zero error' is called emmetropia, so the process of eyes growing towards this state is called emmetropisation. The more long or short sighted the eyes are to begin with, the faster the growth. There is plenty of evidence that emmetropisation is an active process, rather than a passive growth, and the mechanisms are not yet understood. By school-age most children have no error or an insignificant amount of long sight. (The 'norm' in the general population is a small amount of long sight, about 1 to 1.5 dioptres; most people who do not wear glasses are, in fact, long sighted).

Our studies have shown that, at a very young age, children with Down's Syndrome have the same spread of refractive errors as do children who do not have Down's Syndrome. However, over the early years, the children tend not to emmetropise, that is, they do not outgrow the baby errors. Some children are lucky: they begin in babyhood with no or a very small error and maintain the state of emmetropia. Others begin with long or short sight and stay that way, while still others become more long or short sighted. The distribution of errors among children with Down's Syndrome widens with age rather than narrows, and by school age as many as 40-45% will need to wear glasses.

Visual Acuity (detail vision)

At birth, detail vision is quite poor for all children, and rapidly develops over the first two to three years. (We cannot be precise about when a child sees as well as an adult, because the detail vision which we record depends a great deal on the test that we use to measure it. The same child tested in three different ways will have three different levels of vision recorded). Children with Down's Syndrome do the same, but it appears that most lag behind their peers at every age. The difference is small, the equivalent of perhaps one or two lines on a conventional letter chart, and is there even when children are wearing glasses that correct any long or short sight.

One reason why we might record a poorer visual acuity for children with Down's Syndrome is that the children might under-perform on the test. Children with intellectual disabilities might simply not try as hard when a test becomes difficult. One of our latest studies, still ongoing, is to measure acuity objectively. We use EEG techniques to record the brain's responses to visual targets (EEG's measured this way are known as visual evoked potentials or VEP's). Our data are showing that even with this technique, visual acuity is poorer in children with Down's Syndrome than it is in children who do not have Down's Syndrome..

Accommodation (focusing at near)

Conventionally, we measure visual acuity and refractive errors for distance targets. However, children spend relatively little of their time looking into the distance. Children's interests are mostly close at hand, and this is where most of children's learning takes place. It is here that we find the greatest differences between vision in children with Down's Syndrome and children who do not have Down's Syndrome..

Usually, children focus very easily and very accurately on near targets and it is only as we approach middle age that we expect to experience difficulty. We find, however, that most children with Down's Syndrome focus very poorly- they tend to under-accommodate by quite a large amount, whatever the distance of the target. This is consistent for any individual child, and persists even when the children wear their glasses to correct long sight. This means that close work, especially in school, must be more difficult for these children because it is out-of-focus.

We do not yet know the reason for the poor focusing. One explanation might be that the children's visual system does not recognise blur as easily as the visual systems of people who do not have Down's Syndrome. An alternative explanation is that the co-ordination between the two eyes is weak and might mean that the focusing mechanism does not get the usual feedback from the alignment of the two eyes. We have studies underway to examine these possibilities.

Whatever the reason behind this poor focusing, there may be a link with the other problems that the children develop. Emmetropisation is not well understood, but seems usually to occur because the eyes recognise that the image is out of focus, and that can signal to the growth mechanisms that adjustment in size and shape of the eyes is needed. It may be that inadequate recognition of blur causes poor accommodation in people with Down's Syndrome and the same fault prevents the eyes from achieving proper growth towards emmetropia. Similarly, the poor focusing means that the eyes have an out-of-focus image whenever a child looks at near objects, which is most of the time in young children. This may mean that fine discrimination mechanisms do not have the opportunity to develop, and that visual acuity is always poorer than it might otherwise be. The above links are, for the present, speculative, but we do have some evidence from our study to back up these possibilities. Some of the children in our study focus accurately, and have done consistently since an early age. Some other children are improving their focusing as they get older. These children and young people are much less likely to have refractive errors and are much more likely to have good visual acuity, than the children who focus poorly.

Our latest study shows that it is possible to dramatically improve the children's focusing with bifocal spectacles. In a controlled trial, we supplied bifocals to a group of 17 primary school children with Down's syndrome, and conventional spectacles to a second group (the control group). The two groups were matched for all of the factors that might influence spectacle use or near work, such as age, cognitive ability, school placement, etc. Over a 20 week trial, the children in the bifocal group consistently focused more accurately on near work than did the children in the control group.

In the trial, and now that we prescribe bifocals clinically, we find that children with Down's syndrome wear bifocals very successfully. None, so far, have encountered any problems, and several of the children prefer to wear their bifocals all of the time rather than keep the for school use. Two children were very reluctant to wear glasses when they had conventional ones, but now wear bifocals very happily. In some cases, teachers and classroom assistants have reported improvement in concentration and quality of work when the children wear bifocals. We are now, therefore, recommending that all children with Down's syndrome who show poor focusing are prescribed bifocals.

The positioning of the bifocal is very important. The top of the bifocal should lie across the child's pupil (this is a much higher position than usual for bifocals) so that the child can look down through the bifocal without effort.

Strabismus (squint or eye-turn)

Amongst children who do not have Down's Syndrome, there is a strong association between squint and long sight. Most children who have a squint are appreciably long sighted. The conventional explanation for the association is that long sight can be overcome by excessive accommodation. Since accommodation (focusing on near objects) is linked to convergence (turning the eyes inwards to look at near objects), excessive accommodation has a tendency to pull one eye inwards, creating a squint.

Squints are much more common amongst children with Down's Syndrome, and are not associated with long sight. In our study group, children who are short sighted, or who have no refractive error are just as likely to develop a squint as children who are long sighted. The mechanism for squint may, therefore, be different. Our study also shows that children who have a squint tend to have poorer focusing than children without squint. None of the children who accommodate accurately have a squint. We have already mentioned that one of the mechanisms for poor focusing might be a lack of co-ordination between the two eyes. Children with the poorest co-ordination would then display both the poorest accommodation and the highest tendency to squint. Alternatively, the out of focus image resulting from poor focusing might disrupt the development of finely tuned binocular mechanisms in the visual system and increase the tendency to squint.

Guidelines

Our work will continue, to uncover the mechanisms that give rise to the problems which occur in children with Down's Syndrome, with our ultimate aim the amelioration of such problems in future. In the meantime, there is much that parents, teachers and other professionals can do to minimise the impact of the visual problems:

• Children with Down's Syndrome should have regular eye-tests. Following an initial examination soon after birth for congenital problems such as cataract, our recommendation would be six monthly checks from the age of 12 months as refractive errors can develop quite quickly.
• Every eye-test should include an examination of accommodation for near tasks. If a child has poor accommodation, then bifocals should be considered.
• At the end of the eye test, a written report should be given to parents, the school and any other professionals involved in the child's care. The report should specify whether glasses are being prescribed and, if so, when they should be worn and what benefit is likely; what distance visual acuity has been recorded and how this differs from the expected value for the child's age; whether the child focuses accurately close up, and if this is being treated with bifocals, and the implications of any other problems such as squint or nystagmus, for example.
• Parents, and especially teachers, should be aware that distance and near acuity is likely to be poorer than for children who do not have Down's Syndrome, even if he or she wears glasses. They should provide tasks (print, pictures and signs) which are large enough for the child to see easily.

This article was first published on the site in 2002.

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