Distance Visual Acuity for the CPOA Exam

Visual AssessmentClinical SkillsCPOA

Distance visual acuity (VA) is the most fundamental measurement in ophthalmic practice. It quantifies how clearly a patient can see at a standard distance and serves as the baseline against which all other findings are compared. As a CPOA, measuring and accurately recording visual acuity is one of your most frequently performed tasks.

The Snellen Chart and Notation

The standard instrument for measuring distance VA is the Snellen chart, which displays rows of letters (optotypes) in progressively smaller sizes. The standard testing distance is 20 feet (or 6 meters) from the chart.

Visual acuity is expressed as a fraction: 20/x, where:

The numerator (20) is the test distance in feet.
The denominator (x) is the distance at which a person with normal vision can read that size letter.

Common acuity values and their meaning:

20/20: Normal distance acuity (the patient reads at 20 feet what a normal observer reads at 20 feet).
20/40: The patient must be 20 feet away to see what a normal observer sees at 40 feet. This is generally the minimum for driving in most states.
20/200: The legal blindness threshold in the United States.

Testing Conditions and Preparation

Before measuring VA, ensure:

The chart is at the correct testing distance (20 feet or the calibrated mirror system equivalent).
Chart illumination is consistent and adequate.
The patient has their current glasses or contact lenses available.
The room is not brightly lit behind the patient (back-lighting causes squinting).

Testing Sequence

Standard VA testing follows a specific sequence:

Test the right eye first (by convention), then the left eye, then both eyes together.
Occlude one eye with an occluder paddle or tissue; instruct the patient to keep the covered eye open and relaxed.
Instruct the patient to read the smallest line they can see clearly. Start with a larger line and work down.
If the patient misses more than half the letters on a line, record the line above as their acuity with the number of letters missed noted (e.g., "20/40 -2" means 2 letters missed on the 20/40 line).
Record whether VA was measured with or without correction: sc (without correction) and cc (with correction).

Squinting narrows the palpebral fissure, which acts like a pinhole and improves vision artificially. Instruct patients not to squint, or the recorded acuity will be better than it actually is without correction. Observe for squinting during testing.

When VA Cannot Be Read from the Chart

When a patient cannot read even the largest line (typically 20/400) at 20 feet, use these alternative measures:

Bring the patient closer: If they read the 20/400 line at 10 feet, record as "10/400".
Counting fingers (CF): Hold fingers at a specific distance; record as "CF at 3 feet."
Hand motion (HM): Wave hand in front of the patient; record as "HM at 2 feet."
Light perception (LP): Patient can perceive light but cannot see movement.
No light perception (NLP): No vision at all.

Key Takeaways

Snellen notation 20/x: the numerator is the test distance; the denominator is the equivalent normal vision distance.
Test right eye first, then left, then both; record sc (without glasses) and cc (with glasses).
20/40 is the minimum legal driving acuity in most states; 20/200 is the legal blindness threshold.
Instruct patients not to squint; squinting artificially improves acuity.
When the chart cannot be read, use alternative measures: bring closer, count fingers, hand motion, light perception, or NLP.

Frequently Asked Questions

What does 20/40 visual acuity mean?

A visual acuity of 20/40 means the patient can read at 20 feet what a person with normal vision reads at 40 feet. In other words, the patient needs to be twice as close to a target as a normal observer to see it with the same clarity. This level of vision, though impaired, is still functional for many tasks. Most U.S. states require at least 20/40 best-corrected visual acuity for an unrestricted driver's license. In clinical practice, 20/40 acuity is often used as the threshold below which a new glasses prescription or further evaluation is indicated.

Why is visual acuity measured with and without glasses?

Measuring VA with correction (cc) tells you how well the patient sees with their best available correction, indicating functional visual status. Measuring without correction (sc) tells you how much the patient depends on their glasses or contact lenses. Comparing sc to cc also helps identify whether reduced VA is likely due to an uncorrected refractive error (sc poor but cc good, suggesting the prescription needs updating) versus a pathological cause (both sc and cc reduced, suggesting disease rather than just uncorrected refraction). Always document both sc and cc, and specify whether the cc measurement was with glasses, contact lenses, or both.

What is the standard testing distance for distance visual acuity and why?

The standard testing distance is 20 feet (6 meters). At this distance, light rays from a target are nearly parallel (optically considered at optical infinity), so the eye does not need to accommodate to focus. This eliminates accommodative effort from the measurement, providing a true measure of the eye's distance refractive state. Closer distances would require accommodation, introducing variable focusing effort that could confound the result. When space is limited, mirror systems are used to project an optically equivalent distance within a shorter room.

How is counting fingers (CF) acuity recorded and what does it indicate?

Counting fingers (CF) acuity is recorded by documenting the distance at which the patient can count fingers held in front of them, such as 'CF at 3 feet.' It indicates vision severe enough that the patient cannot read any letters on the Snellen chart at any distance but retains enough vision to count fingers at close range. CF vision is roughly equivalent to 20/1200 to 20/2000, well below the legal blindness threshold of 20/200. This level of vision significantly impairs daily function and may qualify the patient for low vision services or disability benefits.

What does 'cc' and 'sc' mean in visual acuity documentation?

In visual acuity documentation, 'cc' (from the Latin 'cum correctione,' meaning 'with correction') indicates the acuity was measured while the patient was wearing their glasses or contact lenses. 'sc' (from 'sine correctione,' meaning 'without correction') indicates the measurement was taken without any corrective lenses. The distinction is clinically critical: 20/200 sc with 20/20 cc means the patient simply needs an updated prescription. 20/200 cc means there is a pathological cause of vision loss that correction alone cannot fix and requires further investigation.