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.
one will see−0.1,−0.2 and−0.3, which is equivalent to 6/4.8, 6/3.8 and 6/3. LogMAR measures visual acuity loss; positive values indicates vision loss while negative values denote normal or better visual acuity.
Each letter on the chart, regardless of the line concerned, has a score of 0.02. So if a patient reads the top line accurately, which is 1.0, and goes on to read all the letters accurately up to the 0.1 line but can cannot get any further and makes one mistake on the 0.0 line, than the LogMAR acuity is 0.02. If the patient gets two letters wrong on the 0.0 line, than the LogMAR acuity will be 0.04; three letters wrong would be 0.06 and so on. If a patient gets all the letters wrong on the 0.0 row, one is then going onto the row above, which is the 0.1 row. If a patient gets one letter wrong on the 0.1 row, the patient’s LogMAR reading will be 0.12.
Observe the patient during the test, noting characteristics such as eccentric viewing which could be indicative of vision problems such as macular degeneration, for example.
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Near vision
Near vision is tested by cards consisting of different sizes of ordinary printer’s type, each card being numbered. The eyes are tested and recorded separately and, if the patient uses reading glasses, these should be worn during the test.
The card is held at a comfortable distance (approximately 25 cm) and should be well illuminated by a light from behind the patient’s shoulder. The near vision is recorded as the card number of the smallest type size he can most easily read.
Utilising information gained from visual acuity testing
The measurement of accurate visual acuity is an important and vital part of any ophthalmological examination. It not only provides the healthcare professional with information on a patient’s visual system, but also allows us to gain a realistic insight into a patient’s ability to function as a whole. The information gained from measuring a patient’s visual acuity allows us to:
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Provide a baseline for monitoring of treatment including medical, laser and surgical.r
Monitor the progression or otherwise of the disease.r
Assess a patient’s ability to cope and also enable us to provide the most appropriate form of support.r
Provide a measurement tool to assess a patient’s vulnerability to trips and falls and to take appropriate measure for their prevention.r
Plan an appropriate form of communication with the patient.r
Prioritise a patient’s needs and provide appropriate intervention where nec- essary, depending on the wishes of a patient – such as social services or district nurses.The measurement of a patient’s visual acuity must never be regarded as a mundane task, and time should be allocated to this important skill. This is an ideal opportunity for the healthcare worker to explore any issues of concern for the patient such as worsening of the ophthalmic condition, inability to cope, issues of compliance with medication and the impact of the eye disease on themselves and his carers. It is also important to document any findings and action taken in the patient’s notes.
Special consideration when measuring visual acuity in children
Assessing and measuring visual acuity accurately in children requires a special skill and takes time, patience and understanding. Young children, especially the very young, often find it difficult to articulate their eye symptoms or their visual disturbance. Depending on the age of the child, various methods of measuring visual acuity should be adapted to suit the child’s age, abilities, knowledge, understanding and experience. For example, the measurement of visual acuity in infants, pre-school children and children with special needs is not possible with a letter chart. Rather, the Kay picture test chart should be utilised.
Before the commencement of visual acuity testing, it is worthwhile for the ophthalmic nurse to spend a few moments of her time making the child and parents comfortable and for them to become familiar with the visual acuity testing environment. The nurse should create a rapport with the child and parents, talking to the child and finding out his age and interests in order to gain his trust and co-operation. It goes without saying that if a child is crying from fear, discomfort or hunger, the test should be postponed. If required, a demonstration of what the test involves can be helpful so that the parents and child are aware of what is expected in the test.
Depending on the age of the child, the child may feel happier sitting on the parent’s lap. Children who normally wear glasses should have their vision tested with their appropriate distance glasses on. If the child does not wear glasses, consideration must be given to ensure that child does not ‘peek’ out of the eye that is not being tested. Consider purchasing occluder glasses, which are available in bright colours and are available for both left and right eyes.
It is often difficult to achieve a complete occlusion with a hand or piece of cardboard.
The child should be asked to keep both eyes open during the test. In order to get the child familiar with the test, one should start off with a 6/60 picture and gradually work down the line. During visual acuity testing using the picture chart, it is important to ensure that the child is familiar with the pictures and is able to articulate. If a child gets a picture wrong, that line should be re-tested with a different picture. One should ensure that the parents do not inadvertently try to ‘assist’ the child. At the end of the session, and again depending on the age of the child, coloured stickers and a bravery certificate can be given to the child as a reward and to encourage further co-operating during other examinations that may be necessary.