Visual Acuity Testing
Visual acuity testing measures the smallest detail that the eye can resolve. It is the most basic test of visual function, and it is the first step in the examination of every eye. A fall in acuity is a sign of disease along the whole visual system, from the tear film and cornea to the lens, retina, optic nerve and brain.
Acuity is tested at distance (usually 6 metres or 20 feet) and at near (about 40 cm), with each eye alone and then together, with and without the patient's glasses. A chart of letters, numbers, or symbols is used, with lines that get smaller.
In fraction notation, the top number is the test distance and the bottom number is the distance at which a normal eye can read the same line (6/6 or 20/20 is normal). Other scales are the logMAR chart, which has equal steps between lines, and the decimal notation.
For young children or people who cannot read, pictures, tumbling E, or Landolt C symbols are used, and preferential looking cards are used in infants. When the chart cannot be read, vision is recorded as counting fingers, hand movements, light perception, or no light perception.
The test takes 2 to 5 minutes. A pinhole is used to check whether poor vision improves, which suggests a refractive error and not disease. About 2.2 billion people have some vision impairment, and in about half of them it could be prevented or has not been treated, with uncorrected refractive error and cataract the leading causes.
Indications.
- Every eye examination, and every patient with an eye complaint (blurred vision, pain, redness, flashes, floaters, trauma).
- Screening of children and school age vision, and of adults for refractive error.
- Fitness for driving, work and occupations that need good vision.
- Monitoring of eye disease (diabetic eye disease, glaucoma, macular disease, uveitis) and response to treatment.
- Neurological assessment (optic neuritis, raised intracranial pressure, stroke).
- Before and after eye surgery, and when taking drugs that affect the eye (hydroxychloroquine, ethambutol).
- Assessment of disability, and of vision in patients with systemic disease.
Not applicable in the unconscious patient, where pupillary and other responses are used.
Contraindications and precautions.
- No contraindications.
- A chemical injury to the eye needs irrigation first, and vision testing follows.
- A suspected open globe injury: the eye is not pressed and the test is done gently and quickly, with a shield, and no further examination until specialist review.
- Patients with poor cooperation, learning disability or dementia: adapted charts and patience.
- Good lighting and the correct distance are needed, and the chart is placed at eye level.
Preparation and technique.
The patient sits at the correct distance from a well lit chart (6 metres, or 3 metres with a reduced chart, or a mirror). The patient's usual distance glasses or contact lenses are worn, and one eye is covered with the palm, an occluder, or a patch without pressure on the eye.
The patient reads down the chart, starting with the largest letters, to the smallest line where more than half of the letters are read correctly. If the top line cannot be read, the patient moves nearer to the chart, or counting fingers, hand movements, and light perception are tested. A pinhole is held in front of the eye when acuity is reduced.
Near vision is tested with a reading card at 40 cm, with the patient's reading glasses. The result for each eye is recorded, with the correction used.
