Parents, and especially teachers, should be aware that distance acuity is likely to be poorer than for children who do not have Down's Syndrome. They should provide distance objects (pictures and signs) which are larger than usual.

J. Margaret Woodhouse (UK)

The Down's Syndrome Vision Research Unit
Our research has been running since 1992 and over the years, we have had funding from the Down's Syndrome Association, Medical Research Council, National Lottery Charities Board, Mencap City Foundation, Healthcare Foundation, National Eye Research Centre, Welsh Assembly Government and Action Medical Research. 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, of course, the cohort of children and their families who take part in our studies. Originally, the group consisted of children from South and West Wales but more recently, children from wider afield have joined us and we now have over 250 children ‘on our books’.  Families have welcomed us into their homes, we have spent time with children in schools, and families have travelled (sometimes very long distances) to our clinic and research unit.


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 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 60% 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, and in the past this is what clinicians have assumed. However, we have used EEG techniques to record the brain's responses to visual targets (EEGs measured this way are known as visual evoked potentials or VEPs) and show that children with Down’s syndrome have a genuine deficit in both visual acuity and contrast sensitivity, compared to their peers. This means that children with Down’s syndrome are not seeing as well as other children, at any age. While children are small, their learning and play materials tend to be big and bold, so any visual deficit may not be obvious. But as children progress through school, visual tasks such as print and workbooks become more demanding, and children with Down’s syndrome are at a disadvantage in not seeing as clearly. What is crucial is that all teachers are aware of this problem and compensate for it by enlarging or increasing contrast when necessary. We do not want any difficulties a child has with school work put down to learning disability, if the real reason is that the print is too small or too faint.

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. Our research has shown, 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. Whatever the reason, it is something that we can address very successfully with bifocals. 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 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 some cases, teachers and classroom assistants reported improvement in concentration and quality of work when the children wore bifocals. An independent group in Waterloo, Canada, has also conducted a bifocal trial and shown improvement in the children’s focusing. In addition, and most importantly, the Canadian group demonstrated significant improvement in the children’s performance in standardised tests of literacy and visual skills, when they wore bifocals.

We now prescribe bifocals routinely for children who have poor accommodation and we find that the children wear them very successfully. For school-age children, we usually recommend beginning bifocal wear at school only, but most children make their own choice to wear their bifocals all the time. The youngest child for whom we have prescribed bifocals was two years old. Some children can be very reluctant to wear conventional glasses, but will wear bifocals very happily.

Not all children need bifocals. Our analysis showed that about 25% of children with Down’s syndrome can focus accurately without glasses, or with conventional glasses. But it is not possible to predict which children need bifocals without proper testing, so it is essential that all children with Down’s syndrome have their accommodation measured when they have an eye test and are offered bifocals if they have accommodative problems.

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.

Our most recent finding is that, for some children, bifocal wear is only temporary. About 40% of children with Down’s syndrome who go into bifocals learn to focus accurately without them, and we are able to return them to conventional spectacles. On average, this is after about two years of bifocal wear, but this is very variable, and some children continue to need bifocals even after five years.

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. 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. In some cases, the squint is much worse for close work, and this is another case for the use of bifocals to help straighten the eyes.

Keratoconus

This is a condition in which the cornea, the transparent ‘window’ at the front of the eye, grows into a cone shape, causing distorted vision. Keratoconus can also occur in the general population, but is much more common among young people with Down’s syndrome, and may happen in as many as 10%. The onset is usually at adolescence. The distortion in vision cannot be corrected with spectacles, but can be managed very well with contact lenses and there are many very successful contact lens wearers who have Down’s syndrome.

If keratoconus progresses, it can cause the cornea to become thin and fragile, and scar tissue can further reduce vision. The recent development of a treatment for keratoconus can, however, stop progression of the condition, and prevent sight loss. The treatment is called cross-linkage therapy, and involves drops put onto the cornea, which are then activated by UV light, and act to ‘seal’ the corneal tissue and stop it distorting. It is a highly successful treatment. Cross-linkage is only viable in the early stages of the disease, when the cornea is still reasonably sturdy. It is therefore vital that keratoconus is picked up in an eye examination as soon as it begins, and this is especially important for young people with Down’s syndrome, since the cornea is thinner, even when healthy, than in the general population. This means that there is a shorter time period when cross-linkage is possible, before the cornea thins too much for treatment.

Detecting early keratoconus may not be straightforward. Young people with Down’s syndrome are less likely to complain of distorted vision, and, since they have poorer sight to begin with, the condition may not be picked up by a simple vision test. It is therefore important that optometrists are aware of the likelihood of keratoconus and are suitably equipped to detect it.

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 to 4 years, as refractive errors can develop quite quickly. Thereafter annual examinations are recommended, and these must continue throughout teenage years.
  • Every eye-test should include an examination of accommodation for near tasks. If a child has poor accommodation, then bifocals should be considered.
  • Every eye test for older children and teenagers should include a check for keratoconus.
  • 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.

Collaborators

Mohammad Al-Bagdady, Nathan Bromham, Stephanie Campbell, Rowan Candy, Mary Cregg, Bill Fraser, Helen Gunter, Ping Ji,  Ffion John, Julie-Anne Little, Paul Murphy, Val Pakeman, Margaret Parker, Kathryn Saunders, Ruth Stewart, Lidia Trojanowska, Valldeflors Vinuela-Navarro, Patrick Watts, Cathy Williams, Asma Zahidi.

References

Al-Bagdady M., Murphy, PJ., Woodhouse, JM. (2011)  Development and distribution of refractive error in children with Down’s syndrome. British Journal of Ophthalmology 95: 1091-1097

Nandakumar, K., Leat, S.J. (2010) Bifocals in Down’s syndrome (BiDS) Visual acuity, accommodation and early literacy skills. Acta Ophthalmologica, 88: 196-204

Al-Bagdady, M., Stewart, R.E., Watts, P., Murphy, P. J., Woodhouse, J. M. (2009) Bifocals and Down's syndrome: correction or treatment? Ophthalmic & Physiological Optics, 29, 416-217

Nandakumar, K., Leat, S.J. (2009) Bifocals and Down’s syndrome study (BiDS) Design and baseline visual function. Optometry & Vision Science, 86: 196-207

Stewart, R.E, Woodhouse, J.M., Cregg, M., Pakeman, V.H. (2007) The association between accommodative accuracy, hypermetropia and strabismus in children with Down’s syndrome. Optometry & Vision Sciences, 84:149-55

Little, J-A., Woodhouse, J.M., Lauritzen, J.S., Saunders, K.J. (2007) The impact of optical factors on resolution acuity in children with Down’s syndrome. Investigative Ophthalmology & Visual Science. 48:3995-4001

Stewart, R.E., Woodhouse, J.M., Trojanowska, L.D. (2005) In Focus: The Use of Bifocal Spectacles with Children with Down’s Syndrome. Ophthalmic and Physiological Optics. 25: 514-522

John, F.M., Bromham, N.R., Woodhouse, J.M., Candy, T.R. (2004). Spatial vision deficits in infants and children with Down’s syndrome. Investigative Ophthalmology & Visual Science. 45: 1566-1572

Cregg M., Woodhouse J.M., Stewart R.E., Pakeman V.H., Bromham N.R., Gunter H.L., Trojanowska L.D, Parker M., Fraser W.I. (2003) Development of Refractive Error and Strabismus in Children with Down’s Syndrome. Investigative Ophthalmology & Visual Science 44: 1023-1030.

Cregg, M., Woodhouse, J.M., Pakeman, V.H., Saunders, K.J., Gunter, H.L., Parker, M., Fraser, W.I., Sastry P. (2001) Accommodation and refractive error in children with Down syndrome: cross sectional and longitudinal studies. Investigative Ophthalmology & Visual Science, 42: 55-63

Woodhouse, J.M., Cregg, M., Gunter, H.L., Sanders, D.P., Saunders, K.J., Pakeman, V.H., Parker, M., Fraser, W.I., Sastry P. (2000) The effect of age, size of target and cognitive factors on accommodative responses of children with Down’s syndrome. Investigative Ophthalmology & Visual Science, 41: 2479-2485

Woodhouse, J.M., Pakeman, V. H., Cregg, M., Saunders, K.J., Parker, M., Fraser, W.I., Lobo, S. and Sastry, P., (1997)  Refractive Errors in Young Children with Down’s Syndrome. Optometry and Vision Science 10: 845-851.

Woodhouse, J.M., Pakeman, V.H., Saunders, K.J., Parker, M., Fraser, W.I. Lobo, S. and Sastry, P. (1996) Visual Acuity and Accommodation in Infants and Young Children with Down’s Syndrome. Journal of Intellectual Disability Research. 40: 49-55

This article was first published on the site in 2002. It was updated by the author in February 2011, and subsequently in July 2017.