What Visual Acuity Measures
Distance visual acuity (VA) measures the eye's ability to resolve fine detail at a standardized distance. It is the most fundamental measurement in ophthalmology, performed at virtually every patient encounter. VA provides a baseline for monitoring disease progression, evaluating treatment outcomes, and determining the need for optical correction.
The Snellen Chart
The Snellen chart is the most widely used visual acuity chart. It consists of rows of letters (optotypes) that decrease in size from top to bottom. Each row corresponds to a specific acuity level.
Testing Distance
- Standard testing distance: 20 feet (6 meters)
- If the exam lane is shorter, a mirror system or projected chart can simulate the 20-foot distance
- Some charts are calibrated for shorter distances (10 feet) with appropriately sized optotypes
Snellen Notation
Visual acuity is expressed as a fraction:
- Numerator (20): The testing distance in feet
- Denominator: The distance at which a person with normal vision could read that line
Examples:
- 20/20: The patient sees at 20 feet what a normal eye sees at 20 feet (normal acuity)
- 20/40: The patient sees at 20 feet what a normal eye sees at 40 feet (reduced acuity)
- 20/200: Legal blindness threshold when best corrected in the better eye
Testing Protocol
- Test each eye individually: Begin with the right eye (OD), then left eye (OS), then both eyes (OU)
- Occlude properly: Use an occluder that blocks vision completely without pressing on the eye
- Start high: Begin with larger letters and progress to smaller lines
- Record the smallest line read: The patient must read more than half the letters on a line to be credited with that line
- Note partial lines: Record as "20/30-2" (missed 2 on the 20/30 line) or "20/40+1" (read 1 extra letter on the next smaller line)
Recording Notation
Always document the testing conditions:
- sc (sine correctione): Without correction (no glasses/contacts)
- cc (cum correctione): With correction (habitual glasses or contacts)
- PH: Through the pinhole occluder
Complete notation example: OD 20/30 cc, 20/20 PH means the right eye sees 20/30 with current glasses and improves to 20/20 through the pinhole.
When VA Is Worse Than 20/400
For patients who cannot read the largest Snellen letter at 20 feet, alternative measurements are used in descending order:
- Count Fingers (CF): Record the maximum distance at which fingers can be counted (e.g., "CF at 3 feet")
- Hand Motion (HM): The patient can detect hand movement but cannot count fingers
- Light Perception (LP): The patient can detect light but cannot perceive hand motion
- No Light Perception (NLP): Total blindness; no detection of light
Key Takeaways
- Distance VA is tested at 20 feet using the Snellen chart
- Snellen notation uses a fraction where the denominator indicates the distance a normal eye could read that line
- Test OD first, then OS, then OU, using proper occlusion technique
- Record acuity as sc (without correction), cc (with correction), and PH (with pinhole)
- For very low vision: use CF, HM, LP, or NLP notation in descending order
- A larger denominator means worse acuity; 20/200 is the legal blindness threshold