What are the steps for performing a Snellen chart visual‑acuity test?

Steps for Performing Snellen Chart Visual Acuity Testing

Perform Snellen chart testing at a standardized distance of 20 feet (6 meters) in a well-lit, distraction-free environment, testing each eye separately with the fellow eye occluded by an adhesive patch, using high-contrast black letters on a white background. 1, 2, 1

Pre-Test Preparation

Environment Setup

• Ensure the testing room is quiet and free of distractions to reduce errors and optimize patient performance 1
• Use standardized lighting conditions with adequate illumination of the chart 1
• Position the chart at exactly 20 feet (6 meters) from where the patient will sit 1, 2
• Verify the chart uses high-contrast optotypes with black letters on a white background 1

Patient Preparation

• Have the patient wear their best refractive correction (glasses or contact lenses) during testing 1
• Establish rapport with the patient before beginning, especially important with children or anxious patients 1
• For younger or cognitively impaired patients, consider a practice session with optotypes at near distance first 1

Testing Procedure

Monocular Testing Sequence

• Test each eye separately, beginning with the right eye, then the left eye 2, 3
• Occlude the fellow (non-tested) eye with an adhesive patch or tape to prevent peeking 1, 2, 1
• If adhesive occlusion is not tolerated, use an occluder held by the examiner while ensuring the patient cannot peek around it 1, 2, 1
• The examiner must actively verify that the patient is not using the covered eye 2, 1

Letter Identification Process

• Direct the patient to read the smallest line they can see clearly 2
• Start with larger letters and progress to smaller lines 2
• Have the patient read from left to right across each line 2
• Record visual acuity as the smallest line where the patient correctly identifies at least 4 of 6 optotypes (for children ages 3-5) or the standard threshold for adults 2

Special Considerations for Specific Populations

• For patients with nystagmus or latent nystagmus, blur the fellow eye with high plus lenses or use a translucent (not opaque) occluder rather than complete occlusion 1
• If the patient will not tolerate any monocular occlusion, measure binocular visual acuity and document this limitation 1
• For children who cannot name letters, allow them to match optotypes on the chart to those on a hand-held card 1

Recording Results

Documentation Standards

• Record the visual acuity for each eye separately (e.g., right eye 20/40, left eye 20/30) 2, 3
• Document whether testing was performed with or without correction 1
• Note any special testing conditions (binocular testing, modified occlusion technique, etc.) 1
• Referral is indicated if visual acuity is less than 20/40 (or 10/20 at 10 feet for children) or if there is a two-line difference between eyes 2

Important Limitations and Caveats

Chart Design Issues

• Snellen charts are less desirable than ETDRS or logMAR charts because individual letters are not of equal legibility and spacing does not meet WHO standards 1
• Research demonstrates that visual acuity scores are significantly better (mean 6.5 letters) on ETDRS charts compared to Snellen charts, with the greatest discrepancy in patients with poor vision (<20/200) 4
• The lack of standardization in Snellen chart design can lead to variability in measurements between different testing locations 5

Testing Accuracy Concerns

• Repeated testing with the same chart may result in memorization and falsely elevated scores 6
• Heterogeneity exists in visual acuity evaluation across different centers, and the same patient may obtain different measurements at different locations 5
• Patient performance depends heavily on examiner skill, rapport, and cooperation level 1

When to Use Alternative Methods

• For children unable to perform Snellen testing reliably, use age-appropriate alternatives such as LEA Symbols, HOTV letters, or Sloan letters with logMAR progression 1
• Consider computerized methods that maintain proper optotype proportionality and can be adjusted for different distances 5