patient’s personal details or taking telephone messages in a crowded waiting room can also be problematic. It may not always be feasible to take patients away to a quieter and more private area, but privacy and dignity must be maintained in all consultations and treatments, and should be built into the care delivered to patients as well as in the environment in which care is being delivered.
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It is important that the appropriate testing chart, such as the Sheridan Gardner test chart, Kay picture chart or the tumbling ‘E’ chart (see below for explanations), is used on patients with any learning disabilities and language difficulties. Good communication skills and patience are needed in these circumstances.r
The measurement of visual acuity in children also requires special skill and patience, and it is important that an appropriate chart is used on those who are unable to recognise the alphabet.Common charts used in the measurement of distance visual acuity The most common chart for measuring distance visual acuity in a literate adult is the Snellen chart (Figure 3.1).
Distance vision is tested at 6 m, as rays of light from this distance are nearly parallel. If the patient wears glasses constantly, vision may be recorded with and without glasses, but this must be noted on the record. Each eye is tested and recorded separately, the other being covered with a card held by the examiner.
Snellen’s test type
Heavy block letters, numbers or symbols printed in black on a white back- ground, are arranged on a chart in nine rows of graded size, diminishing from the top downwards. The top letter can be read by the normal eye at a distance of 60 m, and the following rows should be read at 36, 24, 18, 12, 9, 6, 5 and 4 m, respectively.
(Car number plate at 23 metres)
(Normal) 6
12 6 6
6 metres
A
D F
H Z P
T X U D
Z A D N H P N Y V M I
Figure 3.1 Testing distance visual acuity.
The patient is seated 6 m from the chart, which must be adequately lit, and is asked to read down to the smallest letter he can distinguish, using one eye at a time.
Visual acuity is expressed as a fraction and abbreviated as VA. The numera- tor is the distance in metres at which a person can read a given line of letters.
The denominator is the distance at which a person with normal average vision can read the same line, e.g. if the seventh line is read at a distance of 6 m, this is VA 6/6. If some letters in the line are read but not all, it is expressed as, for example, VA 6/6−2, or VA 6/9+2.
For vision less than 6/60, the distance between the patient and the chart is reduced by a metre at a time and the vision is recorded accordingly as, for example, 5/60, 4/60, 3/60, 2/60, 1/60.
If the patient cannot read the top letter at a distance of 1 m, the examiner’s hand is held at 0.9 m, 0.6 m or 0.3 m away against a dark background and the patient is asked to count the number of fingers held up. If he answers correctly, record VA=CF (count fingers). For less vision, the hand is moved in front of the eye at 0.3 m, record VA=HM (hand movement).
In the case of less vision, test for projection of light by shining a torch into the eye from different directions to see if the patient can tell from which direction it comes. If he sees the light but not the direction, it is noted as VA=PL (perception of light). This test is performed in a dark room. If no light is seen, record no PL, which is total blindness.
Using the pinhole in the measurement of visual acuity
Occasionally, a patient’s visual acuity may be below average, which could be a result of a refractive error not corrected by glasses, or due to the patient wearing an old pair of prescription glasses. One effective, but very simple, way to see if distance visual acuity can be improved through spectacles or a change of prescription is a pinhole. A pinhole disc only allows central rays of light to fall onto the macula and does not need to be refracted by the cornea or lens. A ‘pinhole disc’ is used if the VA is less than 6/6 or 6/9, which may improve VA. If considerable increase in vision is obtained, it may usually be assumed that there is no gross abnormality, but a rather a refractive error.
Sheridan Gardner test chart
The Sheridan Gardner test chart can be used for children and patients who are illiterate. This test type has a single reversible letter on each line. For example, A, V, N. The child holds the card with these letters printed on and is asked to point to the letter on his card which corresponds to the letter on the test type.
This test can also be used for very young children as they do not have to name a letter.
Kay picture chart
The Kay picture chart is again used with patients who are illiterate or with children. Instead of letters, the book contains pictures, which are also of vary- ing sizes. The patient is asked what the picture represents. In order to avoid
any misunderstanding amongst patients with language difficulties, it is good practice to ask the hospital’s official interpreter to translate for patients.
Tumbling E chart
The tumbling E chart again is mainly used for patients who are illiterate. In the chart, the Es face in different directions. The patient is asked to hold a wooden E in his hand and to turn it the same way as the one the examiner is pointing to on the test chart.
It is important to remember to identify in the patient’s notes which chart system has been used to test the patient’s visual acuity; for example, if the Kay picture chart is used, this must be indicated in the notes.
The LogMAR chart was designed by Bailie and Lovie and was originally used in the Early Treatment Diabetic Retinopathy Study. The LogMAR chart (Figure 3.2) is expressed as the logarithm of the minimum angle of resolution. LogMAR scale converts the geometric sequence of a traditional chart to a linear scale.
It measures visual acuity loss so that positive values indicate vision loss, while negative values denote better or normal vision. It is therefore more accurate than the Snellen’s chart and it is gaining more popularity clinically. The chart
Figure 3.2 LogMAR chart. From Olver and Cassidy (2005), Ophthalmology at a Glance, Blackwell Publishing, reproduced with permission.
is designed to be used at various distances such as 4, 3 or 2 metres. Unlike the Snellen chart which has 11 lines of block letters and subsequent rows of increasing numbers of letters in decreasing size, the LogMAR uses a special font in which all the lines are of equal thickness and the letter size increases in equal steps of 0.1 LogMAR per line. Bailey and Lovie also advocated that the visual acuity test chart should essentially be the same at each size level on the chart, so that there are the same number of letters on each line and the task of testing for visual acuity is equivalent for each line. The LogMAR chart has five letters of ‘almost equal legibility’ on each of the rows. Spacing between letters on each row is equal to one letter width, and spacing between rows is equal to the height of the letters on the smaller row. The LogMAR chart thus uses letters of equal legibility, the same number of letters in each row and uniform between-letter and between-row spacing to overcome some of the limitations of the Snellen’s chart in which the letters are fairly large and in which there are uneven jumps in the acuity level between the rows. In addition, the crowding of letters on the Snellen’s chart also inherently makes it more difficult to read.
LogMAR charts come in 2, 3 and 4-metre types. It should be ensured that the correct chart is selected relative to patient distance from the chart. Different charts should be used for each eye – usually chart 1 for the right eye and chart 2 for the left. LogMAR is also available as a computerised system.