Visual Acuity Testing (Snellen Chart)
Why Use
Provides a portable and quick method to assess visual acuity.
When to Use
To assess binocular and monocular visual acuity in cooperative patients.
Pearls / Pitfalls
Patients over the age of 40 may have presbyopia and often require reading glasses; thus, their near-vision test results may be unreliable if the test is administered without reading lenses. Alternate charts exist for illiterate patients or young children, but have limited studies evaluating their use in smartphones. While the Snellen chart is the most widely used method of testing visual acuity in clinical practice, it is not without its pitfalls, including difficulty with standardization of the chart, as well as test-retest reliability. Instructions for Use: Ensure proper room lighting and set phone brightness to 100%. Hold the screen 4 feet (1.2 m) from the patient (approximately the end of a standard hospital bed if patient is sitting upright). Test each eye independently. The patient should completely cover the opposite eye. If the patient is unable to read any of the letters on the chart, the person checking vision should hold up fingers at varying distances and assess whether the patient can count them; this is recorded as counting fingers (CF) vision (e.g. CF at 4 feet, 1.2 m). If the patient cannot count fingers, the person checking vision should move his/her hand across the patient’s field of vision to assess whether the patient can see this; this is recorded as hand motion (HM) vision. Finally, if the patient is unable to see hand movements, a penlight should be swept across the patient’s eye to assess whether the patient can tell when the light is pointed at the eye. If they are able to identify when the light is pointed at the eye, this is recorded as light perception (LP) vision. If they are unable to identify the light, this is recorded as no light perception (NLP) vision.
Management
Any patient with sudden changes in visual acuity from baseline or new obscurations in vision requires immediate referral to an ophthalmologist for dilated fundus examination and further testing.
Advice
Visual acuity worse than 20/25 should be evaluated by a licensed eye professional to determine whether corrective lenses or other treatments may be necessary. Visual acuity is not a measure of a patient’s prescription. This is a separate value that needs to be measured in an office setting.