1. Central visual acuity. Visometry, examination method.
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Central Visual Acuity — Visometry and Examination Method
1. Definition and Physiological Basis
Central visual acuity (VA) is the ability of the eye to discriminate fine spatial detail, specifically the capacity to resolve two separate points as distinct. It is determined by the resolution of the foveal cone mosaic.
Physiologically, light focused on the retina produces a spot of ~11 µm in diameter due to optical imperfections. Foveal cones average ~1.5 µm in diameter. Because a focused point has a bright center and shaded edges, two points can just be resolved when their retinal images are ~2 µm apart — about the width of a single foveal cone. The normal angular limit of resolution is approximately 25 seconds of arc (minimum angle of resolution, MAR). At 10 metres, this corresponds to two bright pinpoints separated by 1.5–2 mm being just distinguishable as two.
The fovea subtends <2° of visual field. Outside it, VA falls off rapidly — more than 10-fold toward the periphery — because more rods and cones share each optic nerve fiber in the peripheral retina. (Guyton and Hall Textbook of Medical Physiology)
2. Visometry — Notation Systems
"Visometry" refers to the quantitative measurement of VA. Three notation systems are in common use:
a. Snellen Notation (fraction notation)
The standard clinical expression of VA is a non-mathematical fraction:
Distance patient reads the chart / Distance at which a normal observer reads the same line
- 6/6 (metric) = 20/20 (feet) = normal acuity
- 6/12 (20/40) = the patient reads at 6 m what normals read at 12 m
- 6/60 (20/200) = threshold for legal blindness
In youth, VA is often better than 6/6 (e.g., 6/5 or 20/15). (Adams and Victor's Principles of Neurology, 12th ed.)
b. Decimal Notation
VA expressed as a decimal: 6/6 = 1.0, 6/12 = 0.5, 6/60 = 0.1.
c. LogMAR Notation
LogMAR = log₁₀(MAR), where MAR is expressed in minutes of arc.
| Snellen | Decimal | LogMAR |
|---|---|---|
| 6/6 (20/20) | 1.0 | 0.00 |
| 6/12 (20/40) | 0.5 | 0.30 |
| 6/60 (20/200) | 0.1 | 1.00 |
LogMAR charts (Bailey-Lovie, ETDRS) address Snellen deficiencies: equal numbers of letters per line, balanced letter legibility, systematic spacing — making them preferred for research. Each letter = 0.02 log units; testing continues until half a line is missed. (Kanski's Clinical Ophthalmology, 10th ed.)
d. Near VA — Jaeger System
Used when testing at near (standard distance: 16 in / 40 cm, e.g., Rosenbaum card at 36 cm):
- J1 ≈ 20/20
- J7 ≈ 20/50
- J13 ≈ 20/100
- J16 ≈ 20/200
3. Examination Method
Equipment
- Snellen chart — wall-mounted at 6 m (20 ft); illuminated
- Rosenbaum card — portable near card held at 36 cm (14 in); useful at bedside
- Pinhole occluder — opaque disc with ~1 mm hole(s) (Fig. 1.2 in Kanski)
- ETDRS / Bailey-Lovie chart — for research-grade measurement
Step-by-Step Procedure
- Refractive correction: Test first with the patient's habitual correction (glasses or contact lenses). Unaided acuity may also be recorded separately.
- Monocular testing: Test each eye individually — the worse eye first, with the fellow eye fully occluded.
- Distance testing: Patient stands 6 m (20 ft) from the chart; reads the smallest line possible. Push the patient to attempt every letter.
- Record acuity: Note the smallest line where the majority of letters are read correctly.
- Pinhole test: If VA < 6/6, repeat with a pinhole aperture over the tested eye.
- Improvement with pinhole → refractive error or anterior media opacity (cornea, lens, aqueous, vitreous) — optical cause
- No improvement → suggests retinal or neural disease (macular, optic nerve)
- Note: pinhole may worsen acuity in macular disease and posterior lens opacity
- Binocular VA: Usually equals or exceeds the better monocular VA when both eyes are roughly equal.
For Very Poor Vision (Below Chart Level)
| Level | Abbreviation | Test |
|---|---|---|
| Counting fingers | CF | Patient counts fingers at 1 m |
| Hand movements | HM | Patient detects hand motion in front of eye |
| Perception of light | PL | Patient detects pen-torch light only |
| No light perception | NLP | No response to any light |
For light perception, direction testing is also performed: the patient identifies which quadrant (superior, inferior, nasal, temporal) the light comes from. Correct localization in all 4 quadrants suggests intact retinal function — useful when dense media opacity (e.g., mature cataract) prevents fundal view. (Kanski's Clinical Ophthalmology)
Legal and Regulatory Thresholds
- Legal blindness: best-corrected VA ≤ 6/60 (20/200) in the better eye, OR visual field ≤ 20°
- Driving (most jurisdictions): corrected VA ≥ 6/12 (20/40) in at least one eye
- Homonymous hemianopia is an independent contraindication to driving regardless of acuity. (Harrison's Principles of Internal Medicine, 22nd ed.)
Testing in Preverbal Children
- Fixation and following response (face or bright targets)
- Preferential looking (Teller/Keeler acuity cards, Cardiff acuity cards): infants prefer patterned over homogeneous stimuli; finest grating resolved = VA estimate
- 10Δ prism test and rotation test assess inter-eye equality
- Note: grating acuity may overestimate Snellen acuity in amblyopia. (Kanski's Clinical Ophthalmology)
4. Summary Table: VA Notation Equivalents
| LogMAR | Metre | Foot | Decimal |
|---|---|---|---|
| 0.00 | 6/6 | 20/20 | 1.0 |
| 0.30 | 6/12 | 20/40 | 0.5 |
| 0.50 | 6/19 | 20/63 | 0.32 |
| 1.00 | 6/60 | 20/200 | 0.1 |
Sources: Kanski's Clinical Ophthalmology, 10th ed. · Guyton and Hall Textbook of Medical Physiology · Adams and Victor's Principles of Neurology, 12th ed. · Harrison's Principles of Internal Medicine, 22nd ed.
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