VOL: 103, ISSUE: 31, PAGE NO: 26
Marion Richardson, BD, DipN, CertEd, RNT, RN, is principal lecturer, University of Hertfordshire
The first two articles in this series examined the physiological process involved in vision. This final article exp…
The first two articles in this series examined the physiological process involved in vision. This final article explains some of the simple tests used to assess aspects of vision. Many other aspects are tested in a full eye examination; more detailed explanations can be found in Fraser et al (2001).
Testing visual acuity
Visual acuity is the ability to discriminate between two separate points of light. Acuity is most acute when the image falls on the fovea (see NT Clinical, 24 July, p24-25) and Fraser et al (2001) list a number of causes of problems with visual acuity. These include:
- Refractive errors (myopia, hypermetropia, astigmatism);
- Obstruction to the passage of light, such as corneal scarring or cataract formation;
- Retinal abnormalities (detachment or macular degeneration);
- Optic nerve abnormalities such as optic neuritis;
- Higher visual centre abnormalities such as a pituitary tumour pressing on the optic chiasm or infarctions in the occipital lobe.
Visual acuity may be tested for a number of reasons including to determine the need to correct vision with spectacles or contact lenses, to monitor an eye problem such a diabetic retinopathy or for some types of employment (the armed forces, for example). In a nursing context, visual acuity is most commonly tested following a head injury or trauma to the eye.
It is important to test visual acuity in a well-lit area. Distance visual acuity is usually tested using a Snellen chart or one of its modifications (Fraser et al, 2001), although there has been some criticism of their accuracy (McGraw et al, 1995). The charts typically consist of seven rows of letters or pictures of different sizes, which should be readable by someone with normal vision at 60, 36, 24,18, nine, six and five metres (these are printed on the chart).
In the UK, visual acuity is usually tested from a distance of six metres, so the top row of letters - called the 6m row - should be legible at that distance. Each eye is tested separately and the visual acuity for each is expressed as 6 over the number of the line of smallest letters that can be accurately recognised. So ‘normal’ visual acuity is 6/6 (metres over line readable). In the US, the distance used is 20 feet and so a ‘normal’ result would be 20/20, hence the term ‘20:20 vision’.
If a patient standing 6m away can only read letters that should be legible from a greater distance this is recorded, for example 6/18 or 6/60 (Clancy and McVicar, 2001). The value of the smallest fully readable row should be recorded.
If a patient is unable to read the top row from 6m (6/60), move them closer to the chart until they can read it, then record the distance, for example 4/60 (can read the top row at 4m). If they cannot see the top row at 1m, hold up a random number of fingers at a distance of 1m - if the patient can distinguish them, this is recorded as ‘counts fingers’ (Fraser et al, 2001).
It is important to record whether visual acuity was tested ‘unaided’ (without glasses or contact lenses), ‘aided’ (with glasses or lenses) or ‘through a pinhole’ (this would be done if the patient cannot see unaided or aided). Reduced vision through a pinhole is unlikely to be caused by errors of refraction (Fraser et al, 2001).
If a patient reports distortion, or ‘kinks’ in their vision, this must be assessed. Fraser et al (2001) describe a simple way of testing this by asking the patient to look at a vertical window or door frame with each eye in turn and to report any distortion, disruption or interruption in the continuity of the line. Distortion usually indicates a problem with the macula and the patient needs to be referred to the ophthalmologist at once.
Testing colour vision
Colour vision depends on the normal functioning of the cones (see NT Clinical, 24 July, p24-25) and defects can be congenital or acquired. Almost all congenital defects are ‘red-green’ (8% of men and 0.4% of women). They are bilateral and usually severe (Fraser et al, 2001). Acquired colour vision defects, such as optic neuritis, usually affect both ‘red-green’ and ‘blue-yellow’ colour vision.
Colour vision is normally only tested to screen for or to diagnose colour blindness or when applying for jobs where colour perception is important (electronics or the armed forces, for example). It is best if the person conducting the test has normal colour vision and the tests should be conducted in a well-lit room.
The most commonly used test is the Ishihara plates - circles of coloured dots containing a number in different coloured dots. The patient should wear glasses or lenses as usual and the plates should be held at a comfortable reading distance and each eye tested in turn. The patient is shown a selection of the plates (for example, 10-15) and the number of plates read accurately is recorded, for example 4/10.
Testing the visual field
The visual field is the total area in which objects can be seen in the peripheral vision while the eye is focused on a central point (Fig 1). While gross defects in the visual field can be tested fairly simply, sophisticated equipment is needed to test for glaucoma or macular degeneration. Nurses may wish to test the visual field of a patient who has suffered a stroke, head injury or other condition that reduces cerebral blood flow.
Sit facing the patient about one metre away with your head and eyes at roughly the same height as the patient’s. Cover your right eye with your hand and ask the patient to cover their left eye with their left hand. (Alternate this for the other eye.) Ask the patient to look directly into your uncovered eye, then present a random number of fingers on your free hand to each of the four quadrants of the patient’s visual field (Figs 2 and 3) and ask the patient to count them (CF).
Ensure the patient does not move their eyes while you are testing the visual fields and remember that if you cannot see your fingers, nor can the patient and you need to bring your fingers closer to the centre.
Lesions from the optic chiasm to the visual cortex produce bilateral visual field loss (Fraser et al, 2001). Optic nerve lesions in one eye only produce central loss of the affected eye. Retinal lesions and some optic nerve lesions affect various amounts of visual field of the same eye. Glaucoma field loss can occur from one or both eyes but requires sophisticated testing if it is to be identified in the early stages.
In general, testing for double vision (diplopia) should be done by a specialist but it is relatively easy to test whether it is monocular (one eye) or binocular (both eyes).
Ask the patient to fix on a distant object such as the top letter of the Snellen chart, then occlude each eye in turn. If the diplopia is still present, it is monocular in the relevant eye. If it disappears when one eye is covered, it is true binocular diplopia and the patient needs referral to an ophthalmologist within 24 hours (Fraser et al, 2001).
Correct testing of the various aspects of vision ensures problems are noted early and appropriate specialist referral can be made.This article has been double-blind peer-reviewed