Eye tests for children with Down's syndrome

Frequency of eye tests

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.

Eye test techniques

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.

Communication

Good communication is essential, as it is for all patients. On the whole, children with Down's syndrome do better at understanding the spoken word than they do at speaking themselves. So don't under-estimate a child's ability if he or she cannot talk to you. 

On the other hand, many children have hearing deficits and their auditory processing is slow, so you will need to speak a little more slowly than usual and in shorter sentences. Give any patient with Down's syndrome longer to respond to your question or instruction. If you try to rush your patient you will only end up confusing or frightening them.

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.

Measuring visual acuity

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.

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.

Advising parents on acuity

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).

Refraction

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. 20% 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 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

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'.

There are published norms for children of different ages, but a useful rule of thumb is that 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. 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. Conversely a young child with accurate accommodation is less likely to go on to develop hypermetropia and/or strabismus, so measuring accommodation can help practitioners to decide on how frequent eye examinations should be for very young children with Down; syndrome.

Our Bifocal Trial has shown that children who under-accommodate once their distance refractive error is fully corrected, 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.

At present, it appears that 30-40% of children who wear bifocals learn to accommodate accurately over the top of their bifocal and are able to return to single vision lenses. The children 'tell' us when they are ready to give up bifocals, by pushing their glasses down their nose and deliberately looking through the distance portion of the lens. Dynamic retinoscopy can then be used to confirm accurate accommodation.

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.