Booklet  Examining People with Learning Disabilities, Section 3:

This section will continue to concentrate on determining vision and visual skills in a patient with learning disabilities. This section looks at measuring acuity.

Ordering The Test Procedures

As optometrists, we all have our own routine for seeing patients and the order in which we carry out the tests does not differ much from patient to patient. However, when seeing people with learning disabilities, it is important to be flexible. Throw routine out of the window and work in the order that is most appropriate for the patient.

Some patients may have short concentration and be unwilling to sit for a long period of time. Following a discussion of history and symptoms, you should have an idea of the most important issues; it may be best to concentrate on these first. If the patient becomes uneasy half way through the examination and refuses to continue, you will have already gained the most useful information. On the other hand, a very nervous or anxious patient may need plenty of time to settle and feel comfortable. It may be best to begin with fun, less critical procedures and only move on to the more important ones when the patient is ready. Many patients will find repeating tests tedious and may not co-operate well. Measure binocular acuity early in the examination and leave monocular testing for later (or vice versa if a difference in acuity between the two eyes is the important issue) – that way you break up the test procedures and avoid boredom. You may like to consider using different tests of acuity (being cautious in interpretation, see below) if your patient does not want to repeat the same test over again. 

Visual Acuity

There is a wide range of acuity tests designed for children available and many of these are suitable for adults with learning disabilities. It is important to remember, however, that adults with learning disabilities are not children; avoid patronising your patients. It is also important to use standardised tests so that results can be compared. In a domiciliary setting, it is usual to test at 3m with equipment specifically designed for use at this distance, adults with learning disabilities may find taking part easier if the practitioner is not too far away; almost all of our social interactions are within 2-3 metres and we want to make the testing procedure as easy and familiar as possible. Snellen charts and Snellen-based tests, with their arbitrary acuity levels and different number of targets on each line cannot reliably be used at different distances where accuracy is important. The only tests described below are those based on LogMAR principles, which can be used at any distance and the results extrapolated. The tests are also well described in the literature. Many, if not most, hospital eye departments are now using the LogMAR scoring system for acuity. It therefore makes sense for optometrists to record acuity in LogMAR (alongside Snellen if preferred) so that referral letters use appropriate terminology.

The Letter tests

Figure 1

The standard Bailey-Lovie letter chart is generally too cumbersome for domiciliary use. It also can be overwhelming for patients with learning disabilities because the entire chart is visible at the same time. The Keeler LogMAR test (described in the literature as the Glasgow Acuity cards), shown in Figure 1, is a letter test which presents one line of letters at a time, with a matching card. Your patient can name the letters if he/she wishes, or match them. The pack contains three versions, two with ‘crowded’ letters and one with single letter presentations. Acuities are scored in LogMAR and in Snellen equivalent. One word of caution; the test is calibrated for 3 metre viewing distance, so changing the distance (as is often useful with our patients) requires a little thought. The letters designated as 6/24 are only that from 3 metres, they are actually 3/12. Thus if you test from 2 metres, that line becomes 2/12 (and not 2/24).

Picture tests

In the UK, Kay Pictures are a very popular range of test booklets that all orthoptists and many optometrists already use (see Figure 2). All versions come with matching cards and there are two versions of this. One has all eight pictures on one card; the other has four pictures on each side. The latter, because the pictures are widely spaced, is really useful when dealing with a patient with physical limitations, a patient with cerebral palsy for instance who cannot point with accuracy. The pictures were deliberately chosen to be readily identifiable by young children, and co-incidentally but very usefully, the pictures all have easily recognisable Makaton signs and many adults with learning disabilities know these signs. Kay produces LogMAR versions of the tests, both in crowded and singles format; both are useful. A crowded test more faithfully reproduces a real-world situation in which objects are very near to each other. A singles test can be easier for anxious patients and for patients with eye-movement difficulties.



Figure 2

Kay Pictures LogMAR versions are calibrated in Snellen for 6 metres (so the 6/24 line changes to 2/24 when testing at 2 metres). However, the LogMAR value is calibrated for 3 metres. Specifying the LogMAR value from a different distance requires an understanding of logarithms. Alternatively, if you are going to use LogMAR values, remember that reducing the viewing distance by a factor of 2, changes the LogMAR value by +0.3. So if you work at 3 metres and 1.5 metres, the modification becomes easier. 

Lea symbols

These tests (again, there is a range available) use four symbols (see Figure 3). Crowded and singles versions are available, along with a matching card. Spiral booklet forms and wall-type charts are produced. The symbols have a sharper ‘end-point’ than letters or pictures, as once the acuity threshold is reached, guessing is not possible from the overall shape. The acuity values are given for a 3m working distance, in LogMAR, in Snellen (expressed as 3/) and in the European format of decimal Snellen. Once again, apply caution when changing distance!



Figure 3 

Preferential Looking Tests

Although designed for young children, preferential looking (PL) tests can be useful for adults who are not able to name, sign or match letters or pictures. The principle of the tests relies on the fact that young children, especially babies, ‘prefer’ to look at a patterned target rather than a blank. The examiner simply observes the child’s eye movements to determine whether the child can resolve a target.



Figure 4

Acuity Test a) the children’s version


and b) the adult version


The Cardiff Acuity Test (Figure 4) uses pictures and an adult version is now available. This uses adult-appropriate pictures (see Figure 4) to make the test more interesting and to avoid patronising your patient. The Teller cards, designed for babies, uses gratings.

For infants, the response to fixate a target is virtually automatic and the tests work very well. Adults have much more of a choice about whether to look at a target or not, and we often need to adapt the procedure. Asking your patient to point to the target or even to tell you where it is (top or bottom) can be more successful because it involves the patient in the procedure more than simply looking. The disadvantage of these modified procedures is that they remove the objectivity that is a crucial part of PL testing. You therefore need to take care to avoid bias and leading the patient.

The following example of unintentionally leading the patient is very common. If you present a card from the Cardiff Test to the patient and ask him/her to point to the picture and he/she does so correctly, you say something like ‘fine, that’s right’ and proceed to the next card. If the next time, the patient points to the wrong end of the card, it is very tempting to say something like ‘are you sure?’ The patient then quickly learns that your prompting means ‘move my finger to the opposite end of the card’ and you are in danger of grossly overestimating acuity.

It is far better to end the test at the last card of which you are sure of your patient’s success.

Choosing your tests

In choosing what tests to have available, an important consideration is the domiciliary situation; tests need to be readily portable and if you can find the minimum number of tests that allow you to be successful with the maximum number of patients, your life will be easier.

Figure 5 shows the visual acuity tests that proved successful in a study of 154 adults with learning disabilities attending day centres.


Figure 5. Tests successfully yielding visual acuity measures with adults with learning disabilities

‘NP’ means not possible because of lack of co-operation and ‘N/A’ means not applicable because vision was too poor to be recordable with a formal test. Note how infrequently acuity cannot be measured; most adults with learning disabilities can take part in test procedures providing the tests are appropriately presented. The need for grating acuity (Teller) is rare. A letter test (with matching card if needed) was the single most successful method, followed by the Cardiff Test, which uses pictures and a preferential looking technique. The Kay range of tests uses pictures designed for children. Lea symbols would be an appropriate substitute if you wanted to avoid obviously childish pictures.

These three tests, Cardiff, Kay/Lea and Keeler LogMAR would allow a practitioner to test the vast majority of adults with learning disabilities.

Comparison Of Acuity

Different acuity tests yield different results, even in the same patient. While having a variety of tests available is crucial for successful testing of patients with learning disabilities, the practitioner does need to be aware of the differences. We are all familiar with the finding that crowding tests give a poorer acuity result than single presentations. In addition, the higher the cognitive demand of the test, the poorer the acuity appears to be. So a patient may perform more poorly by naming a letter test than by matching. PL tests over-estimate acuity compared to conventional tests.

At the first examination, the choice of test will probably be determined by the patient’s ability level of interest and by their choice. At subsequent examinations, use the same test if possible so that deterioration or improvement in vision can be more reliably determined. If using different tests during the examination, in order to maintain interest, try to use identical procedures.

When comparing acuity from occasion to occasion, with and without correction and so on, we need to be aware of the repeatability of the test, that is, how much variation we can expect on repeated testing. Only when a second examination yields a difference greater than the repeatability can you be sure that a change has occurred. Figure 6 gives the repeatability values of the tests but note that these have been reported for typical patients, not for patients with learning disabilities, who might be expected to exhibit slightly more variation.

Figure 6. Published repeatability values for acuity tests

Interpreting Acuity Scores

We have learned that people with learning disabilities are at high risk of ocular and visual defects, including uncorrected refractive errors. We expect, therefore, to find reduced acuity due to pathology in a number of our patients. In addition, studies have shown that people with Down’s syndrome have poorer than normal acuity, even in the absence of pathology. Another factor comes into play; without wishing to generalise too much (since patients with learning disabilities are as individual as you and me), many patients lack the competitive edge that our other patients exhibit. That is, a patient with learning disabilities is less likely to try hard to reach the difficult stages of an acuity test. It can, therefore, be difficult to decide whether a slightly reduced acuity is due to poor motivation or a genuine deficit. However, remember one of the major purposes of measuring visual acuity is to inform the carers about visual abilities and provide advice on modifications to the environment. Under-estimating a visual acuity is probably of little consequence to the patient, since the carers making things more visible is unlikely to be detrimental. Over-estimating acuity, on the other hand, can be highly detrimental, with carers assuming visibility and putting failure to do tasks down to the learning disability rather than sight.

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