What does 20/40 vision mean?

A Snellen acuity of 20/40 means the patient can read at 20 feet what a person with normal vision can read at 40 feet. In other words, the patient's vision is reduced compared to normal, requiring them to be twice as close as a normal-sighted person to resolve the same letter. The larger the denominator, the more reduced the vision.

What is the difference between cc and sc acuity?

cc stands for cum correctione, meaning the acuity was measured with the patient's glasses or contact lenses in place. sc stands for sine correctione, meaning the acuity was measured without any correction (bare eye). Both measurements are important: sc acuity reflects natural vision, while cc acuity shows best-corrected performance. Recording both helps identify uncorrected refractive error.

What do you record when a patient cannot see the Snellen chart at all?

If a patient cannot read any letter at the standard 20-foot distance, move them closer (10 feet, 5 feet) and record the equivalent. If they cannot read letters at any distance, test for finger counting (CF at X feet), hand motion (HM), light perception (LP), or no light perception (NLP). These designations describe the most basic levels of functional vision.

Distance Visual Acuity Testing: CPO Exam Guide

What Is Distance Visual Acuity?

Distance visual acuity (DVA) measures the clarity and sharpness of a patient's central vision at far distance, standardized at 20 feet (6 meters). It is the most fundamental measurement in ophthalmic pre-testing and is performed at every comprehensive eye examination. As a CPO, you will perform this test multiple times daily.

The Snellen Chart

The Snellen chart is the standard tool for measuring distance visual acuity. It consists of rows of letters (optotypes) decreasing in size from top to bottom. Each row is calibrated to correspond to the distance at which a person with normal vision can resolve those letters.

The chart is used at a testing distance of 20 feet (or 6 meters in metric notation). In smaller exam rooms, a mirror system or projected chart at 10 feet with optical reversal is used to simulate the 20-foot testing distance.

When using a mirror system to project the chart, make sure the patient understands they are reading the reflection and that the chart is effectively at 20 feet. Occasional patients become confused and try to read the actual projector rather than the mirror image.

Understanding 20/xx Notation

Visual acuity is recorded in Snellen fraction format:

The top number (numerator) is the testing distance: always 20 (feet)
The bottom number (denominator) is the distance at which a person with normal vision could read that same line

Examples:

20/20: patient reads at 20 feet what a normal eye reads at 20 feet. Normal acuity.
20/40: patient reads at 20 feet what a normal eye reads at 40 feet. Reduced acuity.
20/200: legal blindness threshold in the better-seeing eye
20/10: better than normal acuity; patient reads at 20 feet what a normal eye must be at 10 feet to resolve

A larger denominator means worse vision. 20/200 is significantly worse than 20/40, even though 200 is a bigger number. Think of it as: the bigger the bottom number, the farther away a normal eye needs to be to read that same letter, meaning the patient's vision is more impaired.

cc vs. sc Notation

Always specify whether the acuity was measured with or without correction:

cc (cum correctione = "with correction"): patient is wearing their glasses or contact lenses
sc (sine correctione = "without correction"): patient's uncorrected, naked eye acuity

Standard practice records both sc and cc acuity at the new patient exam to establish baseline uncorrected vision and best-corrected vision. At follow-up visits, the required measurements depend on the visit type.

Testing Protocol

Seat the patient at the correct testing distance (20 feet or equivalent)
Test the right eye (OD) first by occluding the left eye
Test the left eye (OS) next
Test both eyes (OU) together
Have the patient read from the top, identifying the smallest line they can read accurately
If the patient misses letters on a line, record the acuity with a notation: e.g., 20/40 -2 (missed 2 letters on the 20/40 line)
If a patient cannot read any letters, test at closer distances (10 feet, 5 feet) or use finger counting (CF), hand motion (HM), or light perception (LP)

Allowing the patient to squint during testing without noting it. Squinting creates a pinhole effect and can improve acuity artificially. Instruct patients not to squint. If they do, note it as the patient may be masking a refractive error.

Documentation Format

A complete acuity entry looks like: OD: 20/40 sc, 20/20 cc; OS: 20/50 sc, 20/25 cc; OU: 20/20 cc

Key Takeaways

Test at 20 feet; Snellen fraction compares patient's vision to normal
Larger denominator = worse vision (20/200 is worse than 20/40)
Always record cc (with correction) and sc (without correction) separately
Test OD, then OS, then OU; occlude the non-tested eye
Record missed letters as +/- from the line (20/40 -2)
For very poor vision, document CF, HM, LP, or NLP as appropriate