How to Measure Distance Visual Acuity

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It only measures one aspect of visual function but, if done consist- ently, it can detect changes in the visual system. VA is dependent on the sharpness of the focus of the target object upon the retina, the sensitivity of the nervous system and the ability of the brain to interpret the information. If any one, or any combination of, these systems are affected, a person’s visual acuity will change.

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CLINICAL SKILLS How to measure distance visual acuity

Janet Marsden with or without correction (spectacles). Nurse Advisor: Community Eye Health For example: Right VA= 6/18 with Journal, London, UK. correction, Left VA= 6/24 with Email: [email protected] Heiko Philippin correction. Sue Stevens • If the patient cannot read the largest Nurse Advisor (retired): Community Eye (top) letter at 6 metres, move him/her Health Journal. closer, 1 metre at a time, until the top letter can be seen – the VA will then be Anne Ebri recorded as 5/60 or 4/60, etc. West Africa Sub-Regional Manager: • If the top letter cannot be read at Brien Holden Vision Institute, Calabar, 1 metre (1/60), hold up your fingers at Nigeria. varying distances of less than 1 metre Visual acuity (VA) is a measure of the and check whether the patient can ability of the eye to distinguish shapes count them. This is recorded as counting and the details of objects at a given distance. fingers (CF): VA = CF It is important to assess VA in a consistent • If the patient cannot count fingers, wave way in order to detect any changes in your hand and check if he/she can see vision. One eye is tested at a time. this. This is recorded as hand Visual acuity should be measured from a movements (HM): VA = HM Indications standard distance, using a standard • If the patient cannot see hand • To provide a baseline recording of VA chart with a white background movements, shine a torch toward the • To aid examination and diagnosis of eye eye and ask if they can see the light. If disease or refractive error out of which eye they see best). This they can, record ‘perception of light’ • To assess any changes in vision ensures that the minimum is read with (VA = PL). If they cannot, record ‘no • To measure the outcomes of cataract or the ‘worse’ eye, and more will be read perception of light’ (VA = NPL). other surgery. with the ‘good’ eye. This means that no • After testing without any correction, test letters are remembered, which could Equipment the patient while wearing any current make the second visual acuity appear • Multi-letter Snellen or E chart distance spectacles and record the VA better than it is. • Plain occluder, card or tissue in each eye separately, with correction. • Ask the patient to cover one eye with a • Pinhole occluder • If 6/6 (normal vision) is not achieved, plain occluder, card or tissue. They • Torch or flashlight test one eye at a time at 6 metres using should not press on the eye; this is not • Patient’s documentation. a pinhole occluder (plus any current good for an eye that has undergone spectacles). The use of the pinhole Procedure surgery. It can also make any reduces the need to focus light entering • Ensure good natural light or illumination subsequent intraocular pressure the eye. on the chart. It is important to ensure reading inaccurate and it will distort • If the vision improves, it indicates the that the person has the best possible vision when the occluded eye is tested. visual impairment is due to irregularities chance of seeing and reading the test • Ask the patient to read from the top of in the cornea, a problem in the lens, or chart as treatment decisions are made the chart and from left to right. If the refractive error, which is correctable with based on the results of VA testing. patient cannot read the letters due to spectacles or a new prescription. • If the test is done outdoors, the chart language difficulties, use an E chart. • Repeat the whole procedure for the should be in bright light and the patient The patient is asked to point in the second eye in the shade, with enough light to illumi- direction the 'legs' of the E are facing. • Summarise the VA of both eyes in the nate the patient’s face during the test. Note: there is a one in four chance that patient's notes, for example: • Explain the procedure to the patient. Tell the patient can guess the direction; Right VA= 6/18 without specs, 6/6 patients that it is not a test that they therefore it is recommended that the with pinhole and Left VA= NPL. have to pass, but a test to help us know patient should correctly indicate the how their eyes are working. Tell them orientation of most letters of the same Children not to guess if they cannot see. size, e.g. four out of five or five out of six. • It is usually possible, with patience, to • Ensure that any equipment that the • The smallest line read is expressed as a measure VA in children from about the patient touches is clean and is cleaned fraction, e.g. 6/18. The upper number age of 5, although this does vary. between patients. Infections can be refers to the distance the chart is from Children need praise and reassurance passed between patients if equipment – the patient (6 metres) and the lower that it does not matter if they are wrong. or the testers’ hands – are not clean. number (usually written next to the line • Ask the parent to cover the eye not • Position the patient, sitting or standing, on the chart) is the distance in metres being tested, so that the child cannot at a distance of 6 metres from the at which a ‘normal’ eye is able to read see around the occluder. chart. The patient can hold one end of a that line of the chart. • Try to show only one line of the chart to cord or rope of 6 metres long to ensure • Incomplete lines can be added to the the child at a time. that the distance is maintained last complete line. e.g. 6/12+3, • Test the eyes one at a time, at first indicating that the patient read the '12' VA testing measures one aspect of visual without any spectacles (if worn). line at 6 metres and gained three of the function, but it is important that it is done Note: Some people prefer to always letters on the '9' line. well and accurately. An incorrect VA can test the right eye first. Others prefer to • Record the VA for each eye in the lead to inappropriate decisions and test the ‘worse’ eye first (ask the patient patient’s notes, stating whether it is management.