3.1 Distance & near visual acuity (Snellen, notation, pinhole)

Understanding Visual Acuity

Visual acuity (VA) is the eye's ability to resolve fine detail, and it is the single most-recorded measurement in the ophthalmic clinic. As a COT you will test acuity on almost every patient, so precision and consistent notation matter. Acuity is measured separately for distance and near, with and without the patient's current correction, and always documented for each eye.

The Snellen fraction

Distance acuity is traditionally recorded as a Snellen fraction such as 20/20, 20/40, or 20/200. The top number is the testing distance in feet (20 ft in the United States) and the bottom number is the distance at which a person with normal vision could read that same line. So 20/40 means the patient reads at 20 feet what a normal eye reads at 40 feet, indicating reduced acuity because the patient must be twice as close to identify the letters. 20/20 is considered normal; 20/15 is better than normal. In metric notation the standard distance is 6 meters, so 20/20 = 6/6 and 20/40 = 6/12.

Snellen charts are calibrated for a 20-foot lane. When room length is limited, a mirror system doubles the optical path (a 10-foot room with a mirror yields the required 20 feet), or a projected/LCD chart calibrated to the true distance is used. Each line is labeled with its acuity value, and the patient reads down the chart to the smallest line they can identify.

Recording notation: OD, OS, OU, cc, sc

• OD = right eye (oculus dexter)
• OS = left eye (oculus sinister)
• OU = both eyes (oculus uterque)
• cc = with correction (glasses or contacts)
• sc = without correction

Always test and record each eye separately, occluding the non-tested eye. The standard order is OD first, then OS, then OU. If the patient misses letters, record the partial line: for example, "20/30 -2" means they read the 20/30 line but missed two letters. Note the correction status for every entry.

Low-vision notation

When a patient cannot read the largest chart letter (typically the 20/200 "big E") even after moving closer, you move down a hierarchy of gross acuity measures:

Always record the distance at which counting fingers or hand motion is seen, because that distance quantifies the result.

The Pinhole Test

The pinhole occluder is one of the most useful quick tools you have. When acuity is reduced, viewing through a small pinhole eliminates most peripheral, unfocused light rays and admits only central rays that do not require refraction. If reduced acuity improves with the pinhole, the cause is refractive (uncorrected myopia, hyperopia, or astigmatism) and better glasses will help. If acuity does not improve through the pinhole, the reduction is more likely pathologic, such as cataract, macular disease, or optic nerve problems, and refraction alone will not fix it. Test pinhole acuity whenever uncorrected or corrected VA is worse than about 20/30.

Near Visual Acuity

Near acuity is tested with a handheld card held at a standardized reading distance of 14 to 16 inches (about 35-40 cm) under good illumination, with the patient wearing any reading correction. Two common notations appear on near cards:

• Jaeger (J) notation uses J1, J2, J3, and so on, where J1 is the smallest print. It is widely used but not perfectly standardized between cards.
• M-units (metric) describe a letter size where 1 M subtends 5 minutes of arc at 1 meter; M-units are often paired with a reduced-Snellen (20/xx) equivalent.

Record the near result with the eye tested and the distance if it differs from standard, e.g., "OD J2 at 14 in cc."

Testing Children and Nonverbal Patients

Not every patient can read letters. Adapt the test to the patient's developmental level:

1. Allen figures or LEA symbols are pictures/shapes for preliterate children.
2. HOTV or tumbling-E charts let the patient match or point the direction of the E, requiring no reading.
3. Fixation behavior is used for infants: observe "central, steady, maintained" fixation and objection to occlusion, since covering the better eye upsets a child who has a weak fellow eye.

Always test each eye individually with reliable occlusion, because a cooperative child may peek. Documenting the method used is essential so later comparisons remain valid.

Optotypes and Modern Charts

The letters on an acuity chart are called optotypes. Traditional Snellen charts have unequal numbers of letters per line and irregular spacing, so many clinics now use logMAR charts such as the ETDRS chart, which place five equally legible Sloan letters on every line with proportional spacing. LogMAR charts allow letter-by-letter scoring and are the research standard, but the Snellen fraction remains the everyday clinical notation you will record.

Practical tips for accurate testing

• Ensure the occluder seals fully so the patient cannot peek with the covered eye.
• Encourage the patient to guess; do not stop at the first hesitation, because the smallest line they can partly read defines the acuity.
• Keep the chart at the correct luminance and free of glare.
• Always note whether the result is best-corrected (through current glasses or a trial frame) or uncorrected, since that distinction changes how the doctor interprets the number.