Current and Past Research
The Cardiff Down’s syndrome cohort was first set up in 1992 by Bill Fraser and Maggie Woodhouse. Currently we have over 160 families involved in our research – some of which have been with us since the beginning! Consequently we have children ranging from 3 months to 16 years old in the cohort, the majority of whom live in South and West Wales.
As you can imagine, over the years we have amassed a wealth of important information regarding visual development in children with Down’s syndrome, which helps us understand how their vision can be optimised. It is important that we relay this information to others who work closely with people with Down’s syndrome, for example parents and families, teachers and relevant professionals. At present we have 6 ongoing studies being conducted by the research group, which are explained in detail below.
THE LONGITUDINAL STUDY
Since the cohort was established, the children involved have been seen regularly either at home, school or here in the School of Optometry and Vision Sciences, in order to monitor both their visual and cognitive development. The vision data obtained from these visits over the years show that many of the children have refractive errors (long or short-sight). Refractive errors are quite common in early infancy in all children. The usual course of events is for children to grow out of their errors over the first four years of life. We have found that children with Down’s syndrome begin with the same range of errors as other children, but instead of growing out of them, many of the children develop larger refractive errors as they get older. This means that many children with Down’s syndrome need to wear glasses by the time they get to school age. Most of the children who wear glasses are long sighted, but some children are short sighted.
The longitudinal also shows that children with Down’s syndrome have a higher prevalence of oculomotor problems, for example strabismus (squint) (24%), and nystagmus (wobbly eyes) (14%). The data have also highlighted a relationship between accurate focusing, strabismus and significant long-sight. Those children who can focus accurately are less likely to have a squint, and less likely to be long-sighted.
Another major finding that emerged from the longitudinal data is that most children (73%) with Down’s syndrome under-accommodate (inaccurately focus) at near. Those individuals who under-accommodate persist to do so, even when their refractive errors are corrected with spectacles. Thus, most children with Down’s syndrome have a constant retinal blur for all near work, which may have an adverse effect on the children’s ability to discriminate fine detail at near. Since our ‘Bifocal Study’ was completed, we now routinely prescribe bifocals for children who under-accommodate. In the study, we are now monitoring the longer-term effects of bifocals, and the numbers of children who are able to dispense with them over time and return to ‘ordinary’ glasses.
LONG-TERM EFFECTS OF BIFOCALS (Mohammad Al-Bagdady)
Mohammad is finding that most children learn to use their own focusing ability better once they have worn bifocals for a while, and some are even able to give up bifocals and return to wearing ordinary glasses. It is important, therefore, that we keep monitoring the children’s focusing once they are in bifocals as, for some at least, they are a temporary measure.
COLOUR VISION (Mohammad Al-Bagdady)
A few studies, most now quite old, have looked at colour vision in children with Down’s syndrome and found colour defects occurring more commonly. We are quite sceptical about these studies simply because most have not controlled for the difficulty of testing colour vision in children with learning disabilities.
Until we know that children can perform the test easily, we cannot presume that any mistakes they make are due to colour vision defects rather than problems in understanding the test. Mohammad’s latest venture is to assess which of the available colour tests the children can perform most reliably. Once we have chosen the most valid test, then we can go on to see whether children with Down’s syndrome do have more problems with colour than other children.
BINOCULAR VISION AND EYE ALIGNMENT (Mark Deacon)
Mark Deacon is a qualified Orthoptist working within the NHS, and collaborating with us. Mark has been looking in detail at the prevalence and types of squint (eye-turn) in children with Down’s syndrome and the ways in which the two eyes work together. Through Mark’s study, we have already shown that alternating squints (in which the child can use either eye) are much more common in children with Down’s syndrome, whereas most ordinary children who squint have a strong preference for using one eye. We don’t yet understand why this difference should arise. Bifocals, when they are needed to aid focusing, can also help reduce the turn in children with squints, but overall, the type of squint (fully accommodative, partially accommodative or non-accommodative) are similar in children with and without Down’s syndrome.
DIMENSIONS OF EYES (Ping Ji)
From previous published reports, we knew that the retina (the layer at the back of the eye) looks different in people with Down’s syndrome and ordinary people. In Down’s syndrome, there appear to be more blood vessels on the surface of the retina, and the optic disc (where the nerves leave the eye to travel to the brain) has been described as ‘rosier’. Ping Ji decided to measure and catalogue these differences by taking fundus photographs in children with Down’s syndrome and typical children, and analyse the size and shape of the optic disc and the numbers and arrangement of blood vessels. Because children with Down’s syndrome have slightly poorer vision than typical children, we expected to find that the optic disc is smaller in Down’s syndrome. Forty-five children with Down’s syndrome and 44 typical children took part. Ping found, quite unexpectedly, that children with Down’s syndrome have LARGER optic discs. At the moment, we have no explanation for why this should be so. Ping’s photographs show that children with Down’s syndrome do indeed have more blood vessels, and this is because the same number of vessels enters the eye, but they then branch more often as they spread over the retina than do typical children’s.
Ping also took measures of the dimensions of the cornea and front of the eye (anterior chamber) in the children The cornea is much steeper in children with Down’s syndrome, and therefore it is more powerful. The length of the eye is very similar in children with and without Down’s syndrome. This means that the lens within the eye is weaker in children with Down’s syndrome, and it is tempting to suggest a weaker lens as an explanation for the poor focusing ability that many children show. But as yet we have no additional evidence for this new hypothesis.
THE GLAUCOMA STUDY (Ping Ji)
Glaucoma is a group of eye conditions causing progressive loss of vision, and as we grow older, there is an increased risk of us developing glaucoma. These are usually symptom-free conditions and result in damage to the optic nerve at the back of the eye. One of the things optometrists do when carrying out an eye test is check the general health of the eyes, to make sure all is well. When testing for glaucoma, optometrists look for changes in the appearance of the back of the eye, particularly the optic disc and measure eye pressure and visual fields. The current criteria for detection of glaucoma in the general population are unlikely to apply to people with Down’s syndrome for a number of reasons. Firstly, if the optic disc looks different in people with Down’s syndrome, changes signalling early glaucoma may be more difficult to spot, especially for optometrists not familiar with the usual appearance in Down’s syndrome. Secondly, people with Down’s syndrome often have thinner corneas than other people, and this means that the measurement of eye pressure (which assumes a ‘standard’ cornea) may be unreliable. Thirdly, assessing visual fields can be difficult for people with Down’s syndrome, and are usually not attempted. We therefore want to establish clinical ‘norms’ for people with Down’s syndrome with and without glaucoma. These guidelines will help optometrists to develop the skills to detect glaucoma in people with Down’s syndrome. Having said that, we do not know at the moment, how common glaucoma is in adults with Down’s syndrome. It is even possible that it is much more rare than in the general population.
Ping has taken measurements of corneal thickness and dimensions in adults with Down’s syndrome, along with intra-ocular pressure, It turns out that the relationship between corneal thickness and pressure measurement is different in people with Down’s syndrome, and this adds to the complications surrounding glaucoma detection in our patients. We need to do more work in the future to solve yet another puzzle!
The Keratoconus Study (Jack Sheppard and Marcela Votruba)
Keratoconus (KC) Is a condition in which the cornea becomes conical in shape because of the thinning of the cornea. This impairs the vision at both near and distance. It usually starts in late childhood. It can be corrected up to a point with contact lenses, although when KC is more advanced only a corneal graft will restore sight. Up to 15% of people with Down’s syndrome have KC, which is far more than the general population, but we do not know why this is so. One hypothesis is that environmental factors, such as eye rubbing, may contribute, whilst other hypotheses involve the role of key genes that may be involved together with environmental factors. The genetic causes of KC are being investigated internationally, but so far only a small number of genes have been directly implicated. One of these genes is called VSX1. So far it has only been studied and been found to be abnormal in some people with KC who do not have Down’s syndrome. So we are looking for the gene in people with Down’s syndrome, including those without keratoconus as well as those with it. We are inviting people with Down’s syndrome to provide a DNA sample (by mouthwash, buccal swab or by blood if they prefer).So far, Jack has looked at the DNA of 10 patients with Downs Syndrome to see if they had a change in the VSX1 gene; unfortunately he has yet to find any changes that we can consider to be disease-causing. We are continuing this study to involve more patients in the hope that a change can be found.
THE BIFOCAL STUDY: funded by The Health Foundation
The Bifocal study looked at the effects of wearing bifocal spectacles on near visual functions, such as accommodation (focusing). Any ageing adult who under-accommodates when approaching middle age is familiar with the concept of bifocal spectacles to aid focusing at near, so why not offer the same aid to children with Down’s syndrome who don’t focus accurately? Some clinicians routinely prescribe bifocal spectacles for children with Down’s syndrome, namely in Scandinavia and Canada, and find that the children comply well with the wearing of these glasses, as well as benefiting visually from wearing them. However, bifocals are not prescribed commonly in the U.K, and there have been no controlled trials examining the effects of these bifocal corrections on near functions or educational performance.
For the benefit of this study, 34 children participated, some were already part of the existing cohort, and others were new recruits. The children were matched into 17 pairs, based on comparable visual problems, similar ages, intellectual ability and educational setting. Once a pair had been matched, one child was randomly assigned to the treatment group (prescribed bifocal corrections) and the other assigned to the control group (given new single vision spectacles).
Two researchers conducted the study, one an optometrist (Ruth Stewart) who assessed each child’s visual functions, and the other a psychologist (Lidia Trojanowska) who assessed each child’s cognitive ability using standardised tests. Both Ruth and Lidia obtained a baseline measurement of each child’s visual and cognitive ability before the study began, and tested all the children again (after prescribing the new glasses) after 1 week, 2 (vision only), 5 and 8 months. The child’s teacher also evaluated his/her progress in school tasks, as well as his/her social adjustment within a school setting at the beginning and end of the study.
Ruth’s optometric and visual data has been analysed, and the results show a significant difference between the bifocal and control groups. The children accommodate much more accurately through their bifocals than do the control children through their ordinary lenses. Surprisingly, the children with bifocals also accommodate more accurately when looking over the top of their bifocals through the ‘distance’ part of the lens. This suggests that the bifocals are ‘teaching’ the children to use their own focusing ability.
None of the children experienced any adverse effects of bifocals. The bifocals were prescribed for school use only, but several of the children chose to wear them all of the time, preferring them to their conventional glasses.
We now prescribe bifocals routinely for children with Down’s syndrome who under-accommodate. A new and exciting part of the longitudinal study is taking place – a follow-up of the use of bifocals and their impact on the children’s ‘natural’ accommodative ability.
A very important aspect of prescribing bifocals for children with Down’s syndrome is the fit of the segment. It is critical that the top of the bifocals is in the correct position.
THE V.E.P STUDY: funded by NERC and Mencap
This study was conducted by Ffion John and Nathan Bromham, in collaboration with Rowan Candy from Indiana University School of Optometry. Visual evoked potentials, V.E.Ps for short, measure the response of the brain to visual stimulation. Visual information flows from the eyes along the nerve pathways to the brain. These nerve pathways are connected to the brain at the back of the head. By placing sensors on the scalp on this part of the head it is possible to record this flow of visual information to the brain. Black and white stripes are used, that flicker at a known rate and we record the strength of the corresponding activity in the brain at that rate.
The more visible a pattern is, the stronger the signal we record from the brain. This relationship is used to work out which patterns are visible to the child, as each individual is shown a succession of patterns on a computer monitor. A pattern with large high contrast stripes elicits a strong signal to the brain, whereas a pattern with very fine (or faint) stripes will give a weaker signal.
Many things can hinder the flow of information from the eye to the brain; therefore the purpose of this study is to discover whether anything is disrupting this flow of visual information in children with Down’s syndrome.
When tested with traditional tests for visual ability, children with Down’s syndrome often appear to be less sensitive to fine detail (reduced visual acuity and reduced contrast sensitivity). It has been suggested that people with learning disabilities have difficulty understanding the demands of traditional vision assessment, and that this could influence their results. Therefore, the V.E.P technique is potentially a more objective way of testing a child’s detail vision, as it doesn’t require any cognitive response from the child beyond that of looking at a target on a screen. However, this study incorporates both methods of vision testing, in order to obtain a full measurement of each child’s vision. This is the first study to date that has measured vision in children with Down’s syndrome using the V.E.P technique.
The results obtained are in agreement with findings from previous studies that have used conventional tests. Children with Down’s syndrome have reduced detail vision (visual acuity), and reduced contrast sensitivity when compared to children without Down’s syndrome. The difference between the two groups of children is less pronounced during infancy, this implying that the development of vision during the first few months of life is as expected. However, as children with Down’s syndrome develop, their fine detail vision and contrast discrimination doesn’t reach the same level as their peers. This could be due to a number of contributing factors, for example uncorrected long or short-sight, reduced accommodation (poor focusing), and conditions like strabismus / squint and nystagmus / wobbly eyes, that are far more common in people with Down’s syndrome than in the average population. However, children with Down’s syndrome who have no known eye problem also tend to have reduced vision. This is most likely due to differences in the visual pathways beyond the eyes.
It is important to note that the correction of any visual problem (where possible) will improve detail vision. It is therefore important that people with Down’s syndrome have regular eye checks from infancy, and that people are made aware of possible visual restrictions in order to optimise visual input, be it in an educational, social or home environment.