Information for eye care professionals
The need for Optometric services
- Incidence (birth-rate) of children with Down’s syndrome is around 1 per 1000. Around 60% of the children are boys, and over 50% have congenital heart defects (although not all life-threatening). Advances in medical care for heart conditions means many more children survive to adulthood, so the prevalence of the condition is increasing. Many of the children are educated in main-stream schools, with appropriate support.
- Amongst children with Down’s syndrome (and children with other forms of learning disability), ocular and visual defects are much more common than among the general population.
- Our study unit was first set-up in 1992-3 to determine the prevalence and age of onset of the defects that we were seeing clinically. In particular, we were (and remain) interested in the number of children with Down’s syndrome who appeared to under-accommodate for near targets.
- We now have considerable data on the ways in which eyes and vision develop in young children with Down’s syndrome so that eye-care practitioners can be better informed when making clinical decisions about managing children’s eye-care.
- Overall, we find the prevalence of ‘Optometric defects’ to be as follows:
- DefectControlsDown’s syndrome
Myopia/Hypermetropia4.5%42% Astigmatism7.7%28% Poor accommodation0%82% Strabismus4-7.5%23% Below-normal acuity0%
As the table shows, children with Down’s syndrome are much more likely to have eye and vision disorders than do other people. The defects can arise throughout childhood and therefore children with Down's syndrome should be encouraged to have regular eye examinations whatever their age. If any abnormalities are found then subsequent examinations may need to be more frequent.
|Regular eye examinations from an early age|
|< two years old – three to six monthly intervals|
|> two years old – six monthly intervals|
|> five years old – yearly intervals (if refractive error has stabilised)|
Generally, the same techniques as are used with typically developing children are equally applicable to children with Down's syndrome. Because of the learning disability associated with Down's syndrome, techniques designed for a younger age may be more suitable. But all children with Down's syndrome are individuals and many are very able, so avoid automatic assumptions about which tests to employ.
As for all children, have a wide variety of tests available, and adopt a flexible approach to avoid boredom.
Many children with Down’s syndrome find it disturbing to ‘fail’ and are, therefore reluctant to try when tasks become difficult. When doing a visual acuity test, for example, a child may begin enthusiastically but get distracted and lose interest when the smaller targets are presented. You will get the best out of a child with Down’s syndrome if you avoid reinforcing ‘failure’. Try interspersing difficult targets with easy ones, give lots of praise and encouragement, and always end the procedure with a success.
Acuity can be measured by letter charts, letter matching, picture/symbol naming or matching, or by Preferential Looking tests (e.g. Cardiff Acuity Test ), depending on the child’s age and ability. It is important to keep the child’s interest and motivation in order to obtain a valid result.
Since children with Down's syndrome are at relatively high risk of ocular defects (such as cataract, nystagmus), some will have reduced acuity and may even be registered as partially-sighted. However, our research findings show that all children with Down's syndrome have below-normal acuity, even in the absence of ocular anomalies and with refractive errors fully corrected.
Acuity measured by behavioural tests (letter / picture charts or Preferential looking) has been described as below-normal in Down's syndrome by many researchers. However, the lack of motivation when tasks get hard, as described above, might influence acuity scores. Acuity measured by the objective technique of Visual Evoked Potentials is free from motivation bias. Our recent study shows that children with Down's syndrome have VEP acuity below their age-matched peers.
Parents, carers and particularly teachers need to be aware that children with Down's syndrome have reduced acuity, even if refractive errors are fully corrected with spectacles. Some compensation in school (such as enlargement and/or increased contrast) may be needed, and it is our recommendation that all children with Down's syndrome are known to their local educational support services for visual impairment (part of the school pupil support service of every UK Local Education Authority).
Since accommodation is inaccurate in many children with Down's syndrome, resulting in blurred near vision, near acuity may be poorer than distance acuity and should always be tested.
Amongst typically developing children, distribution of refraction is wide at birth and narrows over the first few years of life. In individual children refractive errors (both hypermetropic and myopic) tend to decrease in infancy; this is the process of emmetropisation. By school age, the mean refraction is 1.00-1.50D hypermetropia and very few children have a significant refractive error. Unless an ocular defect is present (such as strabismus or amblyopia) most practitioners would avoid prescribing for refractive errors in early infancy, preferring to monitor the progress of refraction and would consider prescribing only if emmetropisation is clearly not taking place.
In early infancy, the distribution of refraction among children with Down's syndrome is similar to that in typically developing children. However, the distribution widens over time, rather than narrows.
In our study of 55 children with Down's syndrome followed from infancy, only 25% of those who had significant refractive errors at the outset showed emmetropisation. Twenty percent of the children had no significant refractive errors at the outset but went on to develop refractive errors. Thus, failure of emmetropisation is common in children with Down's syndrome. Clinically, this means that examination intervals and prescribing guidelines drawn up for typically developing children are not applicable to children with Down's syndrome. In our clinic, we now tend to prescribe spectacles earlier for children with Down's syndrome, since emmetropisation is unlikely. And we continue to examine children regularly, even if they are within the emmetropic range in infancy, since refractive errors can develop.
Accommodation is most usefully measured in young children by dynamic retinoscopy. A target that interests the child is mounted on a rule (or held freely) and set at a fixed distance. Retinoscopy is carried out initially from alongside the target. Accurate accommodation means that the retinoscope reflex at this distance will be neutral. If the child is under-accommodating, a ‘with’ movement will be seen and the neutral point will be further away from the child’s eyes. The dioptric distance between the target position and the neutral point is the amount of under-accommodation or ‘lag’.
A ‘lag’ of more than 0.75D is considered outside the normal range.
Our studies have repeatedly shown that the majority of children with Down's syndrome under-accommodate for near targets by a substantial amount. The mean lag for a target at 25cm (requiring 4.00D of accommodation) is 2.20D. This means that near tasks are optically blurred, putting an additional burden on the children’s learning at near. If a child under-accommodates then any significant hypermetropia should probably be fully corrected.
Click below for examples of blurring of different text sizes.
Our Bifocal Trial has shown that children who under-accommodate gain considerable benefit from bifocals. We now recommend bifocals for all children with Down’s syndrome who under-accommodate. It is therefore essential that accommodation and near functions are measured at EVERY eye examination and children offered remediation in the form of bifocals (or varifocals) for under-accommodation.
Click below for downloadable guidelines on measuring accommodation and prescribing bifocals
At present, 73% of children in our study cohort exhibit under-accommodation. Interestingly, the prevalence of under-accommodation is decreasing as the children get older – it appears that some children are learning to accommodate accurately. We do not yet understand the mechanism of the under-accommodation, but have found an association between under-accommodation and increased risk of significant hypermetropia and strabismus. The observation that some children learn to accommodate accurately does suggest that remedial interventions (by early bifocal use?) may be possible in the future.
Strabismus is common amongst children with Down's syndrome; in our cohort the prevalence is 29%. As might be expected, the prevalence increases with age in childhood, since the onset is usually after the age of 2 years. Unlike for typically developing children, in whom there is a strong association between strabismus and hypermetropia, strabismus in Down's syndrome is not linked to any particular refractive error. We find strabismus in children who are myopic and emmetropic as well as in children who are hypermetropic. Most cases are of esotropia, and an alternating strabismus is more common than amongst typically developing children. Clearly the risk factors for strabismus are not the same as for typically developing children, and all children with Down's syndrome should be monitored for the onset of strabismus, whatever their refractive status.
The management of strabismus will be the same for children with Down's syndrome as for typically developing children. Full correction of any hypermetropia is probably indicated.
For any child, the appearance and fit of a spectacle frame is the most important factor in wearing glasses. Invest in a selection of children’s frames in a variety of sizes and be prepared to make modifications and adjustments for each individual child. Small ‘button’ pads or strap bridges can help the fit with small noses. Curl sides or sports bands to keep frames in place are rarely needed, and often make a child more reluctant to wear glasses. If a child wears hearing aids, straight sides that don’t interfere with aids are more suitable. A child will not benefit from a correction unless the frames are comfortable and the lenses are positioned correctly, so be prepared to spend time. Offer appropriate advice to parents on when the spectacles will be needed and what benefit you envisage for the child and consider a written report to the school with the same information. Provide advice on encouraging the child to wear glasses.
Click Below to download our leaflet for parents
Nystagmus occurs in about 15% of children with Down's syndrome. Occasionally it is first seen soon after birth, and in other cases appears during the first few months of life. In many cases the intensity of the nystagmus improves as the child gets older and in some cases it disappears under most conditions and becomes a latent form. Children with nystagmus should be offered additional support since it constitutes a visual impairment, especially if it worsens under stress. More information on the condition and support for parents can be obtained from Nystagmus Network.
Blepharitis is very common in people of all ages with Down's syndrome. As with the general population, the condition responds well to lid hygiene measures.
Eyelash anomalies also occur commonly, especially in adults with Down's syndrome. Ingrowing eyelashes can cause considerable discomfort and are sometimes missed by a general practitioner if a patient presents with a red eye.
Cataract can be congenital and a child with Down's syndrome and congenital cataract will be or have been under the care of an Ophthalmologist. Developmental cataracts are also much more common in people with Down's syndrome than in the general population and appear at a much younger age. Older children and adults should be monitored regularly. Cataract extractions and implants can be very successful; don’t preclude referral just because your patient has Down's syndrome. The RNIB produces useful leaflets for people with learning disabilities and for their carers on cataract and on preparing for surgery.
Keratoconus is much more common in people with Down's syndrome than it is in the general population and can appear in late childhood / early adulthood. Contact lenses may be feasible for some people, but carers may need to be confident with the lenses as well as the patient. The condition may progress to late stages and corneal transplant eventually offered.