Booklet  Section three

Contrast Sensitivity

Measuring contrast sensitivity (CS) is a vital procedure in defining visual difficulties, as contrast sensitivity better predicts mobility and daily living skills than does visual acuity. It is also essential in proving advice for carers. If a patient has reduced acuity with good contrast sensitivity, then simple enlargement of tasks will help the patient. If both acuity and CS are reduced then enlargement is insufficient; this patient does not see large objects well and contrast enhancement is needed.

There are far fewer tests available for CS than tests for acuity but with the domiciliary situation in mind, two are recommended. The ‘gold standard’ is the Pelli-Robson letter chart, but this is not readily portable. The Mars test (Figure 7) is an appropriate substitute and measures contrast at near with letters of decreasing contrast on a plain white background.



Figure 7. The Mars Letter Contrast Test



Figure 8. The Adult Cardiff Contrast Test

For patients unable to read a letter chart, the Cardiff Contrast Test (with an adult version available as well as a children’s test, see Figure 8) can be used as a PL test, or as a naming or pointing test.

Interpreting CS Scores

Unless monitoring deterioration in CS with a progressive condition (useful in determining the severity of cataract) the absolute value of CS is probably less important than its classification into good, reduced etc. The only guidelines for classifying CS have been produced for the Pelli-Robson chart but since studies have shown that the X and the Cardiff Contrast Tests yield results comparable to the Pelli-Robson, the data in figure 9 are valid.

Visual Fields

Visual field measurement, problematic enough in fully competent and able patients is arguably the most difficult aspect of testing a patient with learning disabilities. Most patients will find maintaining fixation very difficult and a lack of understanding of the procedure and purpose of the test means that our usual procedures are unsuccessful. In a domiciliary setting, the practitioner will almost certainly have to rely on a modified confrontation technique and only gross and asymmetrical defects are likely to be picked up.

Seated directly in front of the patient, ask him/her to look at you and extend both of your arms so that your hands are beyond his/her visual field. Slowly (people with learning disabilities can be expected to have longer reaction times than our usual patients) bring both arms forward. One hand may have a small object in it, or you may be ‘waggling’ your fingers of one hand. The patient’s task is to tell you as soon as possible which hand that is, or to look at or even make a grab for that hand. It is obvious that you will need to go through the procedure beforehand to check whether the patient understands the task.

An alternative is to ask a carer to stand behind the patient and extend one arm forward into the visual field. You will need to ensure that the carer is silent and there are no noise clues from rattling jewellery etc.

In either case, the practitioner notes the relative position of the patient’s eyes and the target when it becomes obvious that the patient can see the target. This may be a facial expression, or a glance, some time before the patient makes the verbal response.

Because of long reaction times and the patient’s relative interest in the practitioner’s face compared with the target approaching from behind, some adults with learning disabilities will appear to have restricted visual fields. Almost always the field measured in this way seems smaller than that of the conventional patient. If a gross visual field restriction is suspected, this can be confirmed or excluded by general observation of the way the patient moves around or reacts to people and objects approaching from one or both sides.

Be cautious in dealing with people with cerebral palsy who may be able to see a target but may have asymmetry in their ability to respond physically. On the other hand, since cerebral palsy is due to brain injury, visual field defects are relatively common. Advice from a carer can be useful in this situation. He/she may be able to detect the patient’s subtle attempts to respond in a particular direction when the practitioner, unfamiliar with the patient, may not.

Binocular Status

Measurement of binocularity can be carried out in a fairly straightforward manner, such as is used for children, with the cover test being the most important procedure. As with a child, ensuring fixation is probably the greatest challenge and once again, a carer who knows the patient well can help here. Often the carer’s face will be the most attention-grabbing fixation target.

Refraction

As the first section in this article showed, refractive errors have a considerably higher prevalence amongst adults with learning disabilities than in the general population. Not only are refractive errors more common but they can also be of far greater magnitude. Further, adults with learning disabilities are much less likely to access eye care and many have uncorrected refractive errors. An accurate refraction is therefore an essential part of an eye examination.

Some patients will be able to participate in a subjective routine but many will not, so a practitioner needs to be competent at retinoscopy. In a domiciliary setting, finding a suitable distance fixation target may be a problem; try the TV in the corner of the room, or a carer standing across the room. The target will probably need to be animated to hold a patient’s attention. The Mohindra technique (which allows the patient to look directly at the retinoscope light) is worth becoming skilled at, but it requires total darkness which will be difficult to achieve in many situations.

Don’t expect your subject to keep fixation for long. It is as easy to carry out retinoscopy in short bursts as it is to do it in the usual way. Ask your patient to look at the target while you count to three, allowing three (or fewer) sweeps of the beam across the pupil. Then your patient can relax and look away while you reach for the next lens. Use lenses held up in front of the patient’s eye until you approximate neutral and use a trial frame only for the final refinement. This limits the length of time that the patient has to wear an uncomfortable trial frame. Be prepared to take a while to complete retinoscopy and even consider breaks if the patient finds the procedure stressful. 

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