What does 20/20 vision mean?

20/20 means the patient sees at 20 feet what a person with normal vision sees at 20 feet. It is considered normal visual acuity. The numerator (20) is the testing distance, and the denominator indicates the distance at which the optotype subtends the standard angle.

What do sc and cc mean in visual acuity notation?

sc (sine correctione) means the acuity was measured without any optical correction (no glasses or contacts). cc (cum correctione) means it was measured with the patient's habitual correction in place. Both measurements should be recorded at each visit.

What measurements are used when a patient cannot read the Snellen chart?

In descending order: Count Fingers (CF) with the distance documented, Hand Motion (HM), Light Perception (LP), and No Light Perception (NLP). These are used when the patient's acuity is too reduced for the largest Snellen letter at the testing distance.

Distance Visual Acuity: Snellen Chart, Notation Systems, and Testing Protocols

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

A larger denominator means worse acuity. The denominator tells you how far away a normal eye could read what this patient can only read at 20 feet. 20/200 means the patient must be at 20 feet to see what a normal eye sees from 200 feet.

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

Pressing the occluder against the eye when testing the other eye. This can temporarily blur vision when the occluded eye is subsequently tested due to corneal molding from the pressure. Use an occluder held in front of the eye without contact.

If a patient wears bifocals or progressives, ensure they are looking through the distance portion of their lenses (upper segment) during distance VA testing. Looking through the near add will artificially reduce distance acuity readings.

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