How is it assessed?
Visual acuity
Electrophysiological investigations
Screening
All new born babies are screened for congenital eye abnormalities by medical staff prior to discharge from hospital. The red reflex test (to identify any luminous red appearance seen on the retina) is mandatory.
Premature infants at risk of retinopathy of prematurity (a condition causing marked increase of the retinal blood vessels) are regularly screened for this by an ophthalmologist.
Routine checks of visual behaviour are carried out in the community at about 8 weeks of age.
Formal testing of visual acuity usually takes place at primary school entry. Some areas also offer this at nursery age and additional checks for junior school age children but this is not universal.
All children with Juvenile Immune Arthritis (Juvenile Idiopathic Arthritis (JIA) should be screened for uveitis according to guidelines.
Assessment of vision
Best corrected vision is the vision in the better of the two eyes assessed using spectacles, contact lenses etc.
Vision is assessed by testing visual acuity for both distance and near vision and the visual fields. Binocular vision is vision using both eyes and monocular vision is vision using one eye. The method of assessment depends upon the age of the child.
Visual acuity
Over three years of age
Lea Symbol Chart
In children over the age of three years, distance visual acuity is usually checked using Lea Symbols. The test is back illuminated and carried out at 3 metres.
Four symbols are repeated and progression is in a logarithmic manner. The child is asked to identify or match the symbols. The central symbol of each line is identified till an incorrect answer is obtained. The child then must identify four of the five symbols to accept the level of visual acuity. Numerical values denoting LogMar and Snellen are printed on the chart.
Snellen Chart
Snellen’s chart with equivalent LogMar measurements
LogMar is a scale that expresses visual acuity as a decimal. It is usually used for statistical purposes and is rarely used in clinical practice.
Snellen 6 metres |
Snellen 3 metres |
LogMar |
6/60 |
3/30 |
1.0 |
6/48 |
3/24 |
0.9 |
6/38 |
3/19 |
0.8 |
6/30 |
3/15 |
0.7 |
6/24 |
3/12 |
0.6 |
6/19 |
3/9.5 |
0.5 |
6/15 |
3/7.5 |
0.4 |
6/12 |
3/6 |
0.3 |
6/9.5 |
3/4.8 |
0.2 |
6/7.5 |
3/3.8 |
0.1 |
6/6 |
3/3 |
0.0 |
The Snellen Visual Acuity Chart is designed to be read at a distance of 6 metres and is for older children. Its use is declining and being replaced by a LogMar letter chart.
Each row on the chart has a number, 60 on the top and 5 on the bottom. A child who can only read the top row has a visual acuity of 6/60 which means that they can read at 6 metres what a normal sighted child could read at 60 metres. A child with normal vision would have a visual acuity of 6/6.
If the child cannot see the numbers on the chart, they are moved to 3 or 4 metres from the chart and tested. If this is not possible, counting fingers and hand movements (at 30cm) or light perception are recorded.
2 - 3 years of age
In children over the age of two years who cannot read letters, vision can be formally tested using the Crowded Kay Picture Chart. The instructions are similar to the Lea Symbol Chart.
Under two years of age
The vision of infants and preverbal children is assessed by -:
- Asking the parents about visual behaviour,
- Observing whether the child can fixate and follow a small target (they should be able to do this from about two to three weeks after birth,
- Forced preferential looking (FPL) in which the child should look preferentially at a pattern stimulus rather than a plain one. A series of cards with a pattern (black and white stripes on one side of a plain card, i.e. gratings) are displayed sequentially through a puppet screen. The finest pattern that elicits preferential looking towards the stripes is taken as the vision. Norms for grating acuity have been established and at present is the most reliable test within this age group. A difference between the two eyes is more reliable than actual grating acuity levels.
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None of these can be precisely correlated with Snellen acuity.
LogMar
LogMar is a scale that expresses visual acuity as a decimal. Visual Acuity Charts with LogMar scale are now commonly used in clinical practice. However, Snellen notation is still used for communicating visual acuity levels. These are going to replace Snellen chart. The values for conversion to a Snellen notation are on the chart.
Near visual acuity
This can be tested using the reduced Snellen tests read at 0.3 metres or the Maclure type test. Reliable picture or symbol tests for near vision are not available in routine clinical practice though Lea Symbols are available as a near card test.
Visual field assessment
This is a “map” of the entire area that the retina normally sees, even at the edges of vision, (known as “peripheral vision”).
The fields of the two eyes overlap giving an overall horizontal field of 160 to 170 degrees for binocular vision in adults. The infant’s visual field enlarges to reach normal adult values at 12 – 15 months of age.
Visual field assessment is usually carried out by either -:
- Confrontation where the examiner moves their hand in front of the child’s eyes. This can provide an estimate of visual loss in children of any age.
- Perimetry where a machine or computer is used. This provides an accurate assessment of visual field loss but can only be carried out in children of school age. Perimetry may be kinetic where points of light are moved in until the child sees them or static where points of light are flashed onto a white screen and the child is asked to press a button in response. Assessment of the binocular visual field can be determined by either combining the results obtained from testing the visual field of each eye separately or by using a binocular test, for example, the Esterman binocular visual field test.
- Visual field assessment in routine clinical practice is only possible in children over 7 years of age.
It would be reasonable to assume that central 10 degrees or less of a residual visual field would be considered as severe visual impairment. If the visual field is more than 10 degrees then it would depend on the pattern of the visual field defect. Inferior visual field defects, temporal visual field defects and involvement of the central visual field are all serious visual field defects.
Refraction testing
The refraction test measures the extent of any refractive error present and allows for appropriate spectacles or contact lenses to be prescribed. In older children, subjective refraction may be performed where the child reads a chart with lenses of varying power in front of the eyes. In infants and younger children who cannot cooperate, cycloplegic refraction is carried out, when the pupils are dilated and the retina is viewed by an ophthalmoscope using lenses of varying power.
Electrophysiological investigations
There are a number of investigations that can be carried out. Some of them may be used to confirm the diagnosis, but the Visual Evoked Potential (VEP) test can provide supplementary information about the likely degree of visual function.
In the VEP, electrodes are attached at the back of the head over the area of the visual cortex. The response to stimulation of the eyes by a flash of light (in infants) or a patterned chart (in older children) is analysed by computer. This test provides an estimate of Snellen acuity.
The Electroretinogram (ERG) is obtained by electrodes on the cheek or lower eyelid or placed as a contact lens o the cornea. This tests the retina for retinal problems such as inherited retinal dystrophies.
There are other electrophysiological investigations, which are beyond the scope of this guidance.
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