Visual Acuity Examination
- Assessment of high-resolution central (foveal) achromatic vision
- Complements visual field testing, which assesses low-resolution peripheral (non-foveal) achromatic vision
- Step 1: measure distance visual acuity in one eye at a time, at a test distance of 20 feet (6 meters), with the patient’s customary distance glasses or contact lenses in place
- Step 2: if distance visual acuity cannot be tested, assess near visual acuity with the patient’s customary near correction in place
- Step 3: Grade Snellen acuity with the numerator designating testing distance and the denominator designating the smallest line on which the patient can identify most letters
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Step 4:
if the patient cannot identify the largest letter on the Snellen chart, grade visual acuity as follows
- Counts fingers: identifies the number of fingers displayed at 1 foot from the eye
- Hand movements: distinguishes horizontal from vertical hand movements displayed at 1 foot from the eye
- Light perception: identifies a bright light you have shined on the eye
- No light perception: does not identify a bright light you have shined on the eye
- Step 5: in pre-literate or other language-challenged patients, test visual acuity with
- Step 6: if visual acuity is subnormal, use the pinhole test to detect uncorrected refractive errors or media opacities such as cataract
- Step 7: perform a refraction (or get someone skilled to do it!)
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Tip: designate “best-corrected visual acuity” as visual acuity assessed with the optimal refractive correction in place
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Snellen distance acuity test is the international standard
- For patients aged 3 to 5 years, normal visual acuity is at least 20/40 and at most 1 Snellen line difference in visual acuity between eyes
- For patients aged more than 5 years, normal visual acuity is 20/25 and no difference in acuity between eyes
- For patients aged 3 years or less, fixing and following a face, toy, or light is an acceptable screening standard
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Trap: results of visual acuity testing are subjective--patients report what they say they see. If they are not cooperative or truthful, watch out!
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Tip: there are 4 reasons for reduced visual acuity
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Optical: uncorrected refractive error, corneal or lens irregularities
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Amblyopic: monocular reduction in visual acuity based on lack of a clearly focused image presented to the fovea within the first 5 years of life (ocular misalignment, anisometropia, ptosis, opaque ocular media)
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Neural: lesions of the retina, optic nerves, optic chiasm, or bilateral retrochiasmal pathway
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Psychogenic: poor cooperation, malingering, conversion, or somatoform disorder
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Trap: errors in distinguishing between these 4 reasons are common and lead to misdirected evaluations and diagnoses!