How to Interpret the Snellen Chart
The Snellen chart measures distance visual acuity by recording the smallest line of letters a patient can read at 20 feet, expressed as a fraction (e.g., 20/40), where the numerator is the testing distance and the denominator is the distance at which a person with normal vision could read that line. 1
Understanding the Snellen Notation
The standard notation (e.g., 20/20/40,20/200) represents a ratio: the first number is the testing distance in feet (typically 20 feet), and the second number indicates the distance at which a person with normal vision could read that same line 2
20/20 vision is considered normal: this means the patient can read at 20 feet what a person with normal vision can read at 20 feet 2
20/40 vision means reduced acuity: the patient must be at 20 feet to read what a normal person could read at 40 feet, indicating the letters appear twice as large as they should need to be 2
Visual impairment is typically defined as best corrected vision worse than 20/40, while legal blindness is defined as vision worse than 20/200 2
Testing Methodology
Test each eye monocularly with the fellow eye covered by an adhesive patch or tape to prevent peeking 1
Always test with best refractive correction in place (glasses or contact lenses) to determine corrected visual acuity 3
Present optotypes in full lines of five whenever possible, as the majority of optotypes must be correctly identified to "pass" a line 1
Testing distance should be standardized at 20 feet (approximately 6 meters), though shorter distances with flip charts can be used for young children 1
Critical Limitations of the Snellen Chart
The Snellen chart is less desirable than modern alternatives because individual letters are not of equal legibility and spacing does not meet World Health Organization standards. 1
Chart design is not standardized: different Snellen charts may have variable letter sizes, spacing, and number of letters per line, leading to inconsistent measurements between testing locations 4, 5
Letters have unequal difficulty: some letters (like E or C) are easier to identify than others (like S or B), making the test less reliable 1
Spacing is non-proportional: horizontal and vertical spacing between letters and lines does not follow recommended standards where spacing should equal the optotype size 1
Testing only at distance with high-contrast letters in low-ambient lighting underestimates functional problems experienced in real-life situations 3
Common Pitfalls to Avoid
Single optotype testing overestimates visual acuity in amblyopia due to the crowding phenomenon—it is easier to discriminate an isolated letter than one presented in a line 1
Do not cover or mask adjacent optotypes when pointing to successive letters, as this eliminates the crowding effect and falsely improves measured acuity 1
Snellen visual acuity alone is insufficient for clinical decision-making: it does not adequately describe functional visual status, particularly for conditions like cataracts where glare, contrast sensitivity, and reading speed are equally important 3
Repeated testing may result in chart memorization, leading to falsely improved measurements 6
Preferred Alternative Testing Methods
For older children and adults, Sloan letters with logMAR progression (as used in ETDRS charts) are preferred over Snellen charts because they have standardized design, equal letter difficulty, and proportional spacing 1
For young children, LEA Symbols or HOTV letters are more appropriate as they include only four possible responses and each optotype blurs similarly 1
logMAR charts provide more reliable test-retest variability (±4 letters) compared to Snellen whole-line method (±5 letters) or Snellen interpolation method (±10 letters) 5
Clinical Context for Interpretation
Visual acuity of 20/40 or worse warrants further evaluation for refractive error, cataracts, or other ocular pathology 2
In cataract evaluation, Snellen acuity is an excellent tool for healthy eyes but should not be the sole criterion for surgical decision-making, as functional impairment from glare and contrast sensitivity may be significant even with preserved Snellen acuity 3
For monitoring ethambutol therapy in tuberculosis patients, baseline Snellen visual acuity testing is recommended before prescribing the medication to detect potential optic neuropathy 7