5.1 Measuring Acuity at Every Level & in Preliterate, Illiterate and Non-Verbal Patients
Key Takeaways
- Snellen acuity is a ratio of test distance to the distance at which the letter subtends 5 minutes of arc.
- Counting fingers must be recorded with the distance at which fingers were correctly counted.
- Light perception is recorded as with or without projection, tested in four quadrants in a darkened room with the fellow eye fully occluded.
- Preferential looking (Teller cards) estimates infant acuity in cycles per degree and is converted to a Snellen equivalent.
- Pinhole acuity improves with uncorrected refractive error or irregular media and does not improve with macular or optic nerve disease.
What a Snellen fraction means
Snellen acuity is a ratio: the numerator is the test distance and the denominator is the distance at which the smallest letter read would subtend 5 minutes of arc overall, with each stroke and gap subtending 1 minute of arc. A patient reading 20/40 at 20 feet sees at 20 feet what a standard observer sees at 40 feet.
logMAR notation expresses the same thing as the logarithm of the minimum angle of resolution: 20/20 is logMAR 0.00, 20/40 is 0.30, 20/200 is 1.00. LogMAR charts (ETDRS) have five letters per line with equal log spacing, which makes them the research standard and lets you score letter by letter — each letter is worth 0.02 logMAR.
| Snellen (ft) | Snellen (m) | logMAR | Decimal |
|---|---|---|---|
| 20/20 | 6/6 | 0.00 | 1.00 |
| 20/25 | 6/7.5 | 0.10 | 0.80 |
| 20/40 | 6/12 | 0.30 | 0.50 |
| 20/60 | 6/18 | 0.48 | 0.33 |
| 20/100 | 6/30 | 0.70 | 0.20 |
| 20/200 | 6/60 | 1.00 | 0.10 |
| 20/400 | 6/120 | 1.30 | 0.05 |
Optotypes for different patients
The blueprint names Allen figures, Tumbling E, numbers and Snellen letters. Choose by the patient's ability, not their age.
| Optotype | Best for | Notes |
|---|---|---|
| Snellen letters | Literate patients | Unequal letter legibility across lines is its main weakness |
| Numbers | Patients who read numerals but not the Latin alphabet | Useful for many non-English speakers |
| Tumbling E | Illiterate, non-verbal, and many non-English speakers | Patient points in the direction of the legs; demonstrate with a handheld E first |
| Landolt C | Same indications as Tumbling E | International standard; patient indicates the gap |
| HOTV / Lea symbols | Children roughly 2.5 to 5 years | Matching card avoids the need to name |
| Allen figures | Young children | Familiar pictures; tends to overestimate acuity because the figures are recognisable when blurred |
| Teller / preferential looking cards | Infants and non-verbal patients | Reported in cycles per degree, convertible to Snellen equivalent |
For children, test each eye separately with an adhesive occluder patch, not a hand or a paper occluder they can peep around. Test the worse eye first if known, because attention fades. For preferential looking, a trained observer judges which side of the card the infant fixates while blind to the grating's position.
Low vision acuity sequence
When the largest optotype cannot be read at the standard distance, do not jump straight to counting fingers — first bring the patient closer. A patient who reads the 20/200 line at 5 feet has an acuity of 5/200, which is far better documentation than "counts fingers."
Then proceed in order:
- Counting fingers (CF) — hold up fingers against a plain dark background and record the distance: "CF at 3 ft OD."
- Hand motion (HM) — record the distance: "HM at 2 ft OS."
- Light perception with projection (LP with projection) — in a darkened room, fellow eye completely occluded, shine a bright light from each of four quadrants and ask the patient to point toward it. Record which quadrants are perceived.
- Light perception without projection (LP) — light is seen but cannot be localised.
- No light perception (NLP) — confirm meticulously. NLP has major medico-legal and prognostic weight.
Pinhole testing
A pinhole of about 1.2 mm restricts the beam to the central, least-aberrated part of the optical system, increasing depth of focus and neutralising most refractive blur.
- Improves with uncorrected refractive error, irregular astigmatism, early cataract or corneal irregularity.
- Does not improve, or worsens, with macular disease, optic neuropathy, dense central media opacity or amblyopia.
- A multiple pinhole is easier for patients with poor fixation than a single aperture.
- Pinhole rarely improves acuity beyond about 20/25 to 20/30 because diffraction begins to limit resolution.
Record pinhole whenever unaided or habitual acuity is worse than expected: "20/60 cc, PH 20/25 OD."
Special populations in practice
Non-verbal or cognitively impaired adults. Use Tumbling E with pointing, or matching with a handheld card. If no response is obtainable, document fixation behaviour: CSM notation records whether fixation is Central, Steady and Maintained through a blink with the fellow eye occluded.
Non-English-speaking patients. Numbers or Tumbling E remove the language barrier entirely. Use a professional interpreter for instructions and document that one was used.
Hearing-impaired patients. Face the patient, do not cover your mouth, demonstrate the task rather than explaining it, and use written instructions.
Nystagmus. Test binocularly as well as monocularly and record both, because occlusion often worsens nystagmus and underestimates functional acuity. Use a +5.00 D fogging lens over the fellow eye rather than full occlusion where possible, and allow the patient to adopt their null-point head posture.
Malingering and functional loss. Suspect it when acuity is inconsistent with pupil responses, with the ability to navigate the room, or with the optokinetic response. Testing with progressively less obvious cues, using a fogging lens, or checking stereoacuity (good stereoacuity is incompatible with severe monocular loss) helps sort it out. The technologist documents observations and defers conclusions to the provider.
Near acuity
Near acuity is recorded with the test distance and the notation system used — Jaeger (J1 to J16), point size, reduced Snellen or M units. J notation is not standardised across cards, so the card used should be recorded. Presbyopic patients must wear their near correction, and low vision patients may be tested at a shortened distance with the distance explicitly recorded, for example "1.6 M at 10 cm."
A patient cannot read the 20/400 optotype at 20 feet but reads it at 5 feet. How should this be recorded?
A patient has 20/80 acuity that improves to 20/25 with a pinhole. What does this suggest?
Which optotype set tends to overestimate a young child's true acuity?
How is light perception with projection correctly tested?
A patient with nystagmus is being tested monocularly and reads only 20/100, but reports better everyday vision. What technique may give a more representative result?