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Assessing visual acuity

In his sixth article on common eye complaints, Dr Scott Fraser looks at this important examination technique

The commonest method of visual acuity testing is by using Snellen vision charts.

While there are some disadvantages with this examination technique, the advantages mean that it persists in

primary and secondary care.

If the system is to be used it is important to know how to get a consistently accurate vision from it.

Distance

Classically, Snellen visual acuity is measured with the patient sitting or standing at six metres – this provides the numerator of the Snellen fraction, ie 6/.

The patient is asked to read as far down the chart as they can and the last line that they can read is noted as the denominator of the fraction. For example, if they can only read the top line (the largest letter) the vision is denoted as 6/60, if they can read the bottom line (the smallest letters on the chart) it will be 6/6 (or 6/5 depending on the chart).

The Americans use feet rather than metres so the numerator is 20 and the best vision from the smallest letters would be denoted as 20/20.

Optometrists, because they are measuring the vision more precisely, use a further notation if the patient can only see some of the letters on a line. For example, if on the 6/6 line two letters could not be discerned, this would be written as 6/6 - 2, alternatively this could be written in terms of the line above, ie 6/9 + 3.

Of course not all rooms are six metres in length but there are a number of ways of getting round this:

lUsing a corridor outside the examination room

lIn a 3m room the patient can read the letters in a mirror from a chart placed 3m behind them

lScaled-down Snellen equivalents are available.

Illumination

Snellen acuity is measured in good illumination. This means it is done with the room lights on and preferably with direct illumination on to the chart.

The fact that Snellen acuity is performed in such well-lit conditions suggests why it is felt to be unrepresentative of a patient's vision in everyday conditions.

Glasses and contact lenses

As Snellen testing is a distance measure, patients with distance glasses should wear the pair they usually use for driving or watching TV. If they have bifocals they need to read the chart through the top part of their glasses. Similarly, patients with contact lenses should be tested with the lenses in.

Each eye is tested separately and it is obviously important to make sure the patient is unable to see around the occluder.

Pinhole

Checking the vision through a pinhole is useful for a number of reasons.

lIt negates the refractive error so that if the patient has forgotten their glasses or has an uncorrected refractive error it will overcome this

lIt will reduce the effect corneal and lens abnormalities (ie cataract) have on the visual acuity

lBy removing the refractive element and reducing corneal/lens aberrations it can indicate the cause of a visual reduction. For example, if unaided vision is 6/18 but pinhole is 6/6 this is suggestive that macular function remains good.

The easiest way to use it is via one of the commercial occluders that have a cover on one side and a series of pinholes on the other (see picture on page 45).

These are available from most medical supplies shops. If you do not have one, a pinhole made in a card will suffice. Patients whose vision is reduced but who correct with a pinhole are usually worth referring for an optometric assessment for correction of refractive error.

Unable to see the chart

If the patient is unable to see the chart through their glasses or pinhole they still need to have their vision recorded.

The first step is to move the patient nearer to the chart. If the top letter is then seen at say 4m this would be denoted as 4/60.

Continue this until within 1m and if the patient is still unable to see the chart, hold your fingers in front of their eyes and ask them to count the number. Repeat this two or three times to make sure they are not guessing – this is recorded as count fingers (CF).

If they cannot discern the number of fingers but can see your hand moving this is denoted as HM. If HMs are not seen, shine a bright light into the eye (being careful to cover the fellow again) if this is seen, it is denoted as perception of light (PL). If not, then the eye is no perception of light (NPL).

Driving acuity

This is an area that seems to cause more concern than any other to patients. In fact the visual acuity standard for driving is whether someone can see the letters/numbers on a car numberplate at 25 yards (with or without glasses).

It is up to the DVLA to decide if someone is fit to drive – although you do have a duty to tell the patient to inform the DVLA if you have concerns.

As a very rough guide, if the patient can see the 6/9 line with both eyes open they can usually see a numberplate at 25 yards. However, there are other restrictions such as visual fields and for driving other vehicles, such as HGV.

Advantages to Snellen system

lIt is relatively simple to use

lIt is quick

lIt is a well-recognised and long-used system allowing comparisons between patients, countries and over time

lThe charts are inexpensive and robust

Disadvantages to Snellen system

lIt equates poorly to 'real-life'

conditions such as dim light or glare

and may therefore overestimate the

visual acuity

lIt can be difficult to assess in children, especially those with amblyopic eyes

Alternative methods of vision testing

Near vision testing Near vision charts should be held by the patient at a comfortable distance and in good light. One eye at a time is tested and the patient should be instructed to wear the glasses they would normally use for reading. The patient's vision is recorded depending on the best line they can

read and most patients with normal vision can get to N5 or N6.

Reduced Snellen charts These can be used under the same conditions as above and again the vision read off directly.

If you have no vision testing equipment

A newspaper or book can be used. Again ask the patient to put on their usual reading glasses and get as good a light, such as an Anglepoise, as you can on the page. It is useful to know that standard newspaper print size (ie, not headline) is around N8.

Take-home points

l Vision testing is essential in the assessment of all patients with eye problems

l Snellen testing remains the gold standard as it is quick, cheap and universal

l Always try to ascertain the patient's best vision in each eye by using glasses or pinholes

l The decision if someone is fit to drive is made by the DVLA

l Near vision testing can be used in situations where a Snellen chart is not available

Further information

Further tips on vision testing

www.mdsupport.org/library/acuity.html

Alternative methods of vision testing

www.bradford.ac.uk/acad/lifesci/optometry/

research/projects/ClinVisFunc.htm

GMC

www.gmc-uk.org/standards/default.htm

Scott Fraser is consultant ophthalmologist at Sunderland Eye Infirmary and co-author

of Eye Know How (BMJ Books, 2000)

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