Examine / Visual acuity
Visual acuity
The first thing measured, and the one everything else is judged against.
Acuity is measured before anything is shone into the eye, one eye at a time, with the glasses the patient wears for distance, and then through a pinhole. It is written down every visit because the number is the record of what the disease and the treatment have done.
How it is measured
Six metres from a Snellen chart (or a logMAR chart, which has the same number of letters on every line and is used for research and increasingly in clinic). Cover one eye properly, with a palm or an occluder, not fingers. Read down until the letters cannot be read; the line where more than half are right is the acuity. Then the pinhole. Then near vision with a reading card at the patient's usual distance.
| Notation | Means | Notes |
|---|---|---|
| 6/6 (20/20) | At 6 metres the patient reads what a normal eye reads at 6 metres | Normal; many young eyes read 6/5 or 6/4 |
| 6/12 | At 6 metres reads what a normal eye reads at 12 | The driving standard is about here, both eyes open |
| 6/60 | The top letter only | Legally defined severe impairment thresholds start around 6/60 in the better eye |
| Counting fingers, hand movements, perception of light | Worse than the chart | Written with the distance; no perception of light is total loss |
| logMAR 0.0 | Equivalent to 6/6 | Each line is 0.1; lower is better; 1.0 is 6/60 |
The pinhole
A pinhole admits only central rays and needs almost no focusing, so if acuity improves through it the blur is refractive and glasses will fix it. If it does not improve, the limit is the media, the retina or the nerve, and the examination has to explain it. Two lines of improvement is a refractive error; no improvement in a 6/60 eye is a disease.
Children and those who cannot read
Matching tests (Kay pictures, the Sheridan Gardiner letters) from about three; preferential looking cards in infancy; and before that, whether each eye fixes and follows and objects to being covered. A child who objects to one eye being covered and not the other has told you which eye sees.
Go and look
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