9. Vision screening by measurement of visual acuity (from age four years)
Chapter editor: Maria Fronius
Although vision screening is generally recommended for the reasons outlined in part I and part II, there is still no consensus about the timing and frequency of the screening, the tests that should be used, and even the target condition. Prior to setting up any screening programme all these issues should be considered. The EUSCREEN cost-effectiveness model can help decision-makers make appropriate and cost-effective decisions in relation to some of their local, regional or national circumstances.
b. Implementing a new programme
In the interest of equity and ethics it is important that all children have the opportunity to be screened. Due to different local circumstances, for example between urban and rural areas, different solutions may be necessary, even within a country or a region. If a new screening programme is implemented, then it is important to consider how everyone can access the screening. This is particularly important in areas that are difficult to access.
In order to achieve the best coverage it is important to consider where children of the target age are most likely to be encountered. In countries with an early school entry age this may be best achieved if the screening were to take place in school. It could be in preschool in other countries. If coverage is expected not to be high enough to be considered sufficient, combining visual acuity screening with other important medical appointments such as vaccinations should be considered (see also chapter 4b). Some countries have child healthcare centres where physicians and nurses screen all children for general health conditions, including vision disorders. In other countries, paediatricians and nurses/assistants in private practice may be in charge of vision screening. In countries with an appropriately early school entry age (before age six), visual acuity screening may be combined with a general school entry health assessment which exists in some countries.
Once a screening programme is in place, it is important to regularly evaluate it, and be prepared to modify or implement change processes where necessary (see also chapter 11). The EUSCREEN cost-effectiveness model can assist in modelling alternative scenarios.
c. Programme objectives and targets
i. Objective setting
Communities, screeners and professionals receiving referrals need to be clear about the objective of the programme and the target condition(s) in their specific healthcare setting. High income countries, or those with good public awareness of eye care may be prepared to accept and support greater costs for health services, resulting from earlier or multiple testing and many referrals for mild or borderline problems, or for conditions such as intermittent exotropia or defective binocular vision that are not amblyogenic. However, long-term population-level outcomes may not be significantly better than from a single later screen event, so these additional costs should be considered carefully. Removing a screening episode that a community or profession has come to expect can be unpopular.
Lower-income countries may only have the capacity to screen once and may have to prioritise the most at-risk groups and more severe cases. Even high-income countries may prioritise elsewhere. Public acceptability of the screening may be low due to poor health awareness so uptake may be poor. The EUSCREEN cost-effectiveness model can assist decision makers in evaluating different options. Currently there are two main alternatives with published evidence-bases: are a visual acuity test, with or without additional tests such as a cover test or a stereotest or earlier autorefraction or photoscreening, again with or without basic orthoptic assessment1. There are also some automated testing technologies being developed which can detect strabismus or absence of foveal fixation.
Amblyopia is the common target condition for vision screening after the age of three years, but is important to establish whether strabismus, and refractive error without amblyopia such as in myopia are also target conditions. If all potential visual deficits, such as mildly reduced stereopsis or occasionally intermittent exotropia are chosen as the target condition, referral rates, and so costs, will be much higher.note]Nishimura M, Wong A, Dimaras H, Maurer D (2020): Feasibility of a school-based vision screening program to detect undiagnosed visual problems in kindergarten children in Ontario. CMAJ 192(29):E822-E831.[/note].
Even if amblyopia is the target condition, other conditions will be detected during screening, such as pathology, non-amblyogenic refractive errors, and strabismus. Incorporating additional orthoptics tests to detect strabismus or poor binocular vision has a weak evidence base (see chapter 7) and unless carried out by trained orthoptists, is often carried out poorly2.
Hyperopia is a particularly controversial topic. Even moderate hyperopia (+3.00 to +6.00 DS) may only reduce VA slightly 3, and without cycloplegic eye drops, it may evade photoscreening if children accommodate during testing. While it is possible that such hyperopia might interfere with schooling and is a major risk factor for esotropia, the evidence of causal associations is currently weak. Only a dilated eye examination will identify all hyperopic children, but this stops being a screening test and approaches a full eye examination. Whatever the screening test, some hyperopic children may remain undetected, and decision-makers must decide how much effort and resources should be devoted to detecting these children, and whether it is cost-effective.
A clear decision needs to be made when vision needs to be assessed – as early as possible, repeatedly, or later, at fewer, or a single, screening event(s). This decision may well depend on capacity, other health intervention timings, and capacity to deal with onward referrals.
ii. Referral threshold
The referral threshold used will depend on age-related normative data for the specific test and age range. Visual acuity of >0.2 logMAR in one or both eyes is a common referral threshold. This may be due to amblyopia, refractive error or pathology, which can only be diagnosed after referral to a specialist. 0.2 logMAR is a common threshold because smaller deficits carry few lifetime adverse consequences and using stricter criteria will include children with no visual abnormalities but who have acuity at the end of the low range of normal vision on validated tests4. The range of normal vision will vary with age, and all screening services should consider these age norms when determining referral criteria. Normal VA for children over 5 years on some tests is 0.1 logMAR, but if worse than 0.1 is chosen as a referral criterion, referrals will increase and it is not clear whether children with 0.2 logMAR vision are disadvantaged compared to those with 0.1.
A standard and valid pass threshold is that three out of five letters on a line must be seen to pass that line. This is scientifically valid and an easy rule for screeners with less specialist training to follow. The VA notation can be either in decimal (most common in Europe), fraction (in feet or metres e.g. 20/20 or 6/6) or logMAR values (for a detailed explanation, see Appendix 1). Using logMAR VA values allows for each letter on a line to have a numerical value e.g. only one letter seen on a 0.2 line of five letters will equate to 0.28 logMAR VA, and 4 out of the 5 letters would be 0.22 VA in logMAR notation, but in a screening context with a pass/fail criterion the three out of five letters to pass a line is recommended. It is easy for confusion to occur if decimal and logMAR notation are used by different professionals, so training and communication must be consistent, and the notation always has to be specified.
The opportunity to re-screen at a later date an unco-operative or a borderline fail child (for example identifying 4 out of 5 letters in one eye and 2 out of 5 in the second eye in a child getting bored) within the screening age ‘window’ will reduce false referrals. It may be more cost-effective to re-screen children with inconclusive results rather than to refer them directly to a specialist5.
Some countries may decide that their secondary referral infrastructure could not cope with many referrals of mild problems e.g. children with equal vision of 0.3 logMAR, or those with only one line difference in vision between the eyes. They may choose to adopt a different referral threshold to prioritise more severe cases, either in the early stages of a new screening programme until secondary services develop, or in the longer term if resources are scarce.
In cultures where spectacle wear is less accepted, parents and children are more likely to notice a difference after treatment if the referral threshold is slightly worse vision, which might help to raise acceptability of the screening service in the community in the longer term. For example a child or parent may notice little functional difference between 0.2 and 0.3 VA, so not feel the screening was worth it, but a noticeable improvement in function might be discussed as a benefit in convrersations among parents.
An alternative strategy is testing for refractive error risk factors which predict low vision and amblyopia. However, the relationship between having a specific refractive error at any age and the chances of being truly amblyopic is currently unknown (for more detailed information on this subject, see chapter 8). More children will have refractive risk factors than will become genuinely amblyopic, but all are likely to be offered treatment or observed over time once referred.
d. Screening locations
It is best to screen where there are most children of the appropriate age. This is often when attendance at preschool or school is advised or mandated by the state, or at clinics or community centres where all children come for health checks or immunisations. This practical consideration to maximize coverage may override testing at the most visually optimal time to start treatment. The EUSCREEN cost-effectiveness model can help inform these decisions. The implementation study in Romania highlighted that different solutions might be necessary in urban compared to rural areas. In densely populated settings many can be tested by a nurse covering a large population in a defined locality, while in a sparse rural population an experienced travelling tester visiting small, widely dispersed settings might be the most effective option.
While preschool screening might be considered preferable due to potentially better treatment outcomes, unless preschool attendance is high and screening can take place there, early testing often relies on parents bringing their children to be screened. This may affect attendance, which in turn will impact upon the efficiency and cost-effectiveness of the screening programme itself. Poor community awareness of the importance of eye care, and transport difficulties can also result in reduced uptake of the screening programme and the children most at risk are the most often missed because they do not attend6. This can lead to significant inequalities in access to care. For example, while one child might have a better outcome to amblyopia treatment if referred a year earlier, 40% of the children may not be screened at all if testing relies on parents bringing their children to a test, so ten children with the target condition may be missed. By screening a year later in school, it may be better to accept the marginally worse outcome for the one child detected later, but screen closer to 100% of a community’s children (eligible population) and pick up more amblyopes overall. Final outcomes can be very similar despite later referral7 if school entry age is sufficiently early.
The EUSCREEN Country Reports highlight how national compulsory education laws may determine where maximum coverage can be achieved. In Sweden where children may not start formal education until age 7, screening in school may be too late for effective amblyopia treatment. In the UK, children start school aged 4-5 years so if a problem is detected treatment can be initiated much earlier. Different country regions may have different rules when children start school e.g. in India, so a decision in one state may not apply to another. Testing in kindergarten or nursery may only be acceptable if all children attend (e.g. Israel, has free nursery provision with high attendance), or if there is good national health surveillance data which can trace children not in school or nursery and provide an alternative screening option.
The actual screening setting should be a quiet room with good light, but close to where the children are situated, if within a school setting (see Appendix 1 for detailed tips for screeners).
i. High- vs low-risk children
Some children are at more risk of poor vision than others. Children born prematurely have a much higher risk of both retinal pathology, refractive error and strabismus. Many regions screen all premature or low birth weight children soon after birth for retinopathy of prematurity (ROP) and may put in place additional screenings throughout childhood.
Children with disabilities and special educational needs are also at high risk of visual deficits, so may be targeted for full visual assessment rather than screening (see also chapter 7a).
ii. Opt-in vs opt-out consent
Opt-out consent is preferable because the default for parental non-action is that the child is still screened (see chapter 4e for more information).
Coordination and goodwill are vital to the success of any screening programme. The wider community needs to understand the reasons why vision screening is important and parents need to be prepared and able to access treatment if referred. The programme itself must be well organised, and secondary care providers must be able to see and care for referrals. They should also agree to provide feedback on referral and treatment outcomes so that the screening programme can be evaluated (see chapter 11). In some countries or regions, orthoptists or optometrists may be able to deal with some or many referrals, but their relationship to more specialist paediatric ophthalmology services needs to be defined, for example which more complex cases need upward referral.
i. Information for parents
Whole communities including schools, educational boards/authorities, parents, community leaders and ‘influencers’ (such as general practitioners or religious leaders) can help when planning screening programmes. They can provide help and support in many ways, such as promoting the need for the vision screening; identifying ways to better engage with hard-to-reach communities; and help to encourage families to attend the screening appointments. There are many ways in which this can be achieved, including involvement of local media or social networks, or community outreach events. Ideally this should happen before the programme starts (see chapter 10 for further information).
Information for parents should be in a format they find accessible (appropriate level of language, translation available if necessary, infographics or web-based as appropriate). For a new service, specific community efforts may need to be made to make sure parents understand why vision screening is important.
Most amblyopia is unilateral, and most children will be asymptomatic, so parents may not accept their child has a potential vision problem. In areas where trust in healthcare is poor, trust in the screening service may need to be built carefully. In many successful screening services, this has taken many years, and may depend more on acceptance within parent communities, than formal information.
Thought should be given to the amount and type of information given to parents of referred children about why they have been referred, how to encourage them to seek treatment, but without causing excessive anxiety.
Parents need to be able to understand how to, and be able to, access the pathway for treatment locally if their child is referred.
The parents need to understand that the screening is not a complete eye examination, so other (non-target) eye conditions may be present and be missed. Conditions may also develop later, but the screening may have been carried out before those conditions had developed. Parents also need to understand that some children ‘fail’ screening, but are found to have no vision problems at diagnostic examination/testing (i.e. false positives). It may be difficult to convey this information to parents. Information resources may need to be piloted and modified to ensure that they too are easy to understand, and include all necessary information. These too should be monitored and evaluated regularly.
Parents need to be informed how their data will be handled, stored and accessed (see chapter 4e).
ii. Information for follow-up care providers
In preparation of a new screening programme follow-up care providers should be informed about its objectives and their future role. Ophthalmologists or others receiving referrals should be given clear details of each referral made: name, contact details, date of birth, reason for referral, any information they may need for triage of appointments (for example mild versus severe defect). Their involvement is vital to successful evaluation, so referral and feedback mechanisms should be as minimal and efficient as possible to prevent loss of data. For example an existing database could be used for data reporting or sending reminders, or just use simple return postcards. In regions where private providers offer care, without any mandate to report back, feedback issues need to be considered carefully. The threshold for glasses prescription may vary between ophthalmologists. Prescription guidelines should be agreed if possible, for example to prevent visually trivial or unnecessary prescriptions).
h. Screening personnel
There may be an existing pool of suitably trained professionals who could screen, such as orthoptists (who consistently have been shown to screen with a true positive rate of over 90%8) or optometrists, but for maximum efficiency of resources less skilled personnel may need to be trained. For example, an orthoptist travelling between schools may be able to screen all children in a large school intake in one morning, but for many reasons this may not be logistically possible in some regions, so people with a basic medical background and good communication skills with children make good screeners, for example school nurses. It is desirable that they are managed or supervised by someone already skilled in the practicalities of vision screening, and in some regions this might mean importing such personnel from other areas to set up and monitor a new screening programme.
Think box: who is going to screen?
Travelling screener covering many sites
+ Quickly becomes expert because screens hundreds of children
+ Consistent screening
+ May be able to act as vision educator for community
– Does not know local circumstances as well
– Advance preparation/information and post screening information on results would have to be done by somebody (less experienced) else or multiple visits would be necessary
– Increased travelling time and expense in rural areas
– May need dedicated funding for a post
+ Increased responsibility for success of the screening service
Local screener e.g. school or GP practice nurse
– May not see many children per year so much less expert
+ Local knowledge of social, economic, logistical considerations
+ Increased trust if embedded in the community
+ VA screening can be combined with other health interactions (vaccinations, weight)
+ Fewer travelling expenses
+ VA screening may be only a small part of their role, so easier to fund within existing posts and budgets
– Less commitment from the screeners to the screening service being successful
– Less consistent performance between local screeners
i. Training screening personnel
i. Minimum theoretical/practical competencies after training
All screeners should have an understanding of the target conditions and their management after referral. They are likely to be acting as ambassadors for good eye care, so need to be enthusiastic about their role. Parents and teachers may look to them for basic advice. Good training and ongoing feedback about their performance is vital to develop and maintain skills and to keep their enthusiasm about doing a good job.
Training should include both theoretical and practical teaching, as well as formal and certified assessment of competence at the end of training. Screeners should be trained to use the equipment accurately to gold standard levels. This includes the use of logarithmic progression charts, ensuring correct testing distance, effective monocular occlusion, as well as being able to spot when a child may be struggling with the test (for more practical advice see Appendix 1).
Training should include experience of interacting with children of the age they will be screening. Successful screening often depends on good communication skills with children as much as being able to administer any particular test. If online resources are available, they are very useful for training and future reference.
ii. Alternative training plans
At the outset of a new screening programme, many people may need to be trained at once, which may be more efficiently delivered via training days and group teaching. Once a programme is established, regular update/refresher days need to be planned to maintain quality standards, team building and motivation. New staff may need to be trained individually. It is important that if trained in the field by another screener, high standards of that trainer are assured and bad habits are not handed down to trainees.
iii. Resources for training materials
As well as face-to-face training, written and online teaching materials should be provided and regularly updated. Every new screener should have a supervisor or mentor for day-to-day advice if necessary. Many screeners will work in isolation from other screeners, so regular opportunities to meet or share experiences are recommended. In remote areas, this might need to be online.
iv. Assessing screeners’ performance
Any test is only as good as its testers. Visual acuity testing on young children is a very skilled procedure and needs well-trained and experienced testers. The referrals made by each screener should be evaluated and underperforming screeners re-trained. To achieve a complete evaluation of results, clear (and preferably mandatory) reporting back from secondary care-providers is vital. Giving screeners feedback about the accuracy of their referrals, and if possible, outcome of treatment, can help maintain motivation and quality.
Experience of the tester is key to accurate results so each tester needs to have tested many hundreds of children to become fully proficient. Locally sited screeners e.g. school nurses in small rural communities may find it difficult to build up such experience.
In the training phases, screeners should be monitored more closely and appropriate feedback given.
v. Follow-up of screening personnel and training
Regular evaluation will help determine the communication needs and the interval between training and re-validation of individual screener’s competency.
i. Test choice
The choice of test is determined by the target condition (amblyopia), reduced vision from any cause including amblyopia, or risk factors for amblyopia (see Appendix 1 on VA testing). The two main alternatives with the clearest published evidence-base are a visual acuity test (with or without additional tests such as a cover test or a stereotest), or earlier autorefraction or photoscreening.
Visual acuity tests should be as close as possible to the gold standard Landolt C test, but suitable for the targeted age-group. There are many different tests designed to approach these standards and some are designed for digital displays. See Appendix 1 for detailed discussion of test choice but essential criteria are a linear logarithmic size progression test, with rows of letters or symbols with proportional spacing between letters and rows and validated, published age-related norms. These tests allow each letter to have equal value and relationship to each other. There are various commercially available tests with these characteristics9. It is recommended to choose a test for which normative data are available for the envisaged age range of children to be screened. Snellen charts, which have different numbers of symbols per row and inconsistent progression of letter size between rows, are not advised. Single letter or symbol tests (especially if the letters are not surrounded by ‘crowding bars’), and unvalidated picture tests should also not be used because results from these tests are less precise (see Appendix 1 for more detailed information).
Each eye should be tested separately, ensuring effective occlusion of the other eye with an adhesive patch or well-fitting occluded glasses that a child cannot peep over. The child should be carefully watched throughout the test because a child who cannot see will naturally try to peep.
The EUSCREEN Country Reports show that many current vision screening programmes, however, do not use a single test, or a single test event. Many screening programmes use a combination of different tests, at different ages, tested by people with different levels of expertise, with different referral criteria and in many different combinations. Some countries screen annually throughout childhood and adolescence, while others only twice. The EUSCREEN study has highlighted that poor follow-up, audit and reporting of referrals, uptake and outcomes make it extremely difficult to assess the efficacy of comparative schemes and leads to wide disparities in the costs per case detected. Most screening services develop by adding tests to existing batteries and once screening has been set up, removing tests or screening episodes of limited value can be very challenging.
Some screening services specifically target strabismus, but large angle strabismus usually presents before screening due to parental concern. Small angle strabismus, which is just as amblyogenic as large angles can be easily missed by lower skilled testers. So few additional amblyopes are detected by screening for strabismus that would not be detected by a VA test. Binocular tests such as ocular motility assessment, stereotests and prism tests have low specificity for amblyopia10. If a child has good visual acuity, conditions such as asymptomatic convergence insufficiency, small angle strabismus and ocular motility defects not already noticed by parents, are unlikely to result in treatment.
Screening programmes need to be clear whether amblyopia is the prime target condition for the screening, or whether they also want to detect conditions such as refractive error or strabismus which are not necessarily amblyogenic.
ii. Rescreening steps
The opportunity to re-screen a child who struggles with the test will reduce false positive referrals considerably and so save costs post-screening, although a special visit to re-screen one child in a school may not be justified and direct referral be more efficient use of resources. Shy children, those very new to the classroom situation or those with poor attention by the time the second eye is tested may fail the screening without having a visual problem. A repeat screen a few weeks later (or even later in the day) may be more successful. Experienced screeners learn to be able to differentiate an apparently unco-operative child who genuinely cannot see, from one just getting bored, but the option of a re-test is valuable. Conversely, some children clearly fail the first time by saying “I can’t see that” so would not benefit from a repeat screen. Children unable to be tested on a second visit should be referred.
iii. Pass/refer criteria
The pass criterion depends on the test used and the age of the child and needs to be related to normative data for the particular test used. A common pass/refer criterion is >0.2 logMAR in 4-5 years old in each eye with no more than one line difference between the eyes, but it also needs to be decided whether children who do not pass are to be referred straight away or whether there will be an option of a repeat screen.
A decision must also be made as to whether visual acuity should be tested further (to a child’s individual threshold) once it reaches the pass threshold of for example 0.2 logMAR. Further testing of children who see better than the threshold will take longer and also lead to other complications.
Think box: test to pass threshold or test to individual threshold?
Suppose that children are tested to their individual threshold (option A) and a child passes the test with VA of 0.0 logMAR (excellent) in one eye and 0.2 (just pass) in the other. The child still has a difference in visual acuity between both eyes that would be defined as amblyopia.
If the same child had been tested to the pass threshold only (option B), they would have passed the screening. If option A is chosen, should that child with good vision in one eye and just sufficient vision in the other eye be referred and treated?
If option A is chosen, the consequence will be that the programme will refer more children than if option B is chosen, when only children who do not pass the test are referred. Some of the children additionally referred in option A will be treated when they would not be in option B. Option A will therefore be more expensive.
Neither option is right or wrong, but which option is chosen does affect the costs and most likely also the cost-effectiveness of the screening programme. This should be taken into account when deciding which option to choose.
If photoscreening is used in addition to a VA test, clear rules should be outlined in the protocol for every plausible scenario. If every child is also photoscreened, will children with amblyopia risk factors but adequate VA still be referred? Will the photoscreen only be used for borderline cases, or will VA only be tested on children who fail photoscreening? Referral rates may differ widely as a consequence of the criteria chosen, with potentially very different cost implications. The EUSCREEN model may help with these decisions.
After screening the results should be recorded and passed on appropriately in the upward referral and evaluation chain. Referral data should be entered on an appropriate database, checked and audited.
Part of the screening evaluation trail should include following up children who failed screening to establish how many sought referral, obtained a definite diagnosis, received treatment, how long they had to wait for a diagnostic appointment, and if possible, what the outcomes of the treatment were.
k. Communicating results to parents
After screening, the outcome of the screening should be reported back to the parents if they are not present at the test. If the child fails the screening test, the parents should be advised to take the child for diagnosis and advised how to proceed. If the child passes the screening test, the parents should be made aware that this does not rule out current minor deficits or future eye problems (see also chapter 9-g-i). If the parents are not present during screening, for example when screening takes place at school, they have to be informed of the result by letter. Additional communications such as reminder letters or telephone calls are likely to have a positive effect on follow-up rate11. Additional measures to increase follow-up may be necessary in low
The evaluation process should consider whether parents actually received and understood the importance of the initial letter to say their child has failed the screen.. In communities where screening has not previously taken place, some mechanism for encouraging uptake of referrals may be necessary. Good links with local nurses, teachers and GPs may facilitate this (but ensuring that data protection issues have been considered). Parents must be informed about (and consent to) where their child’s data will be held and shared with others.
A decision must be made about what to do about children who are not screened (e.g. travelling or home-schooled families), or who do not make or attend the diagnostic appointment following referral, and how much effort will be made to make sure they access treatment. Local systems should be agreed about how to follow up these families. A primary principle of screening is equity of access for all, and in practice this may mean that the most effort needs to be targeted to disadvantaged groups who are least able or willing to access care. In countries with poorer health infrastructure, low public health awareness and large remote rural populations, this can be a significant problem that should be considered from the outset.
Efficient monitoring of a screening programme is vital to be able to carry out effective quality assurance, evaluation and reporting. Regular, appropriately-funded local and central evaluation should take place, including assessment of coverage, training of screeners, performance of screeners, method of screening, referral criteria, diagnostic uptake and long and short-term outcomes. More detailed information on monitoring can be found in chapter 11. Note that any data registry should comply with applicable legislation (see chapter 4e).
Those evaluating screeners should be aware that children who are the youngest of their age cohort may find the tests more difficult, so, for example, more borderline or referred children might arise from a visit early in a school year than later, however good the screener.
n. Adapting an existing programme
Most high- and middle-income countries already have some vision screening recommendations and there may be many different local schemes already in place. Low-income countries may have such schemes provided by outreach charitable organisations. Therefore, an existing scheme, whose proponents may already have invested heavily in it, may need to be adapted and its stakeholders be persuaded that change is required. Disinvestment in one scheme in order to develop another can be very challenging. The EUSCREEN model may help provide evidence of relative cost-effectiveness of different alternatives, but accurate data and cost-efficiency predictions are the key to driving change13. Once a decision has been made that change is desirable it is important to retain communication, goodwill and motivation between all involved. Small, incremental changes with careful audit can lead to highly efficient services.
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