Visual Acuity

  • Assessment of high-resolution achromatic vision
  • Learn how to test distance visual acuity
  • If you cannot test visual acuity at distance, test near visual acuity
  • If visual acuity is subnormal, use pinhole visual acuity test to detect uncorrected refractive error or media opacity such as cataract
  • In patients who cannot identify largest letter on Snellen chart, grade visual acuity as follows: counts fingers (can count fingers displayed 1 foot from eyes), hand movements (can distinguish horizontal from vertical hand motions at 1-foot distance), light perception (can tell if bright light is shined on eye), no light perception (cannot tell if bright light is shined on eye)
  • In illiterate patients, use tumbling E's visual acuity test or picture visual acuity test
  • Snellen Distance Acuity test most practical, but...
  • Test is SUBJECTIVE, so results depend on what patients report they see
  • Results will be unreliable if patient does not cooperate or is untruthful
  • Objective tests of visual acuity under study but not yet practical
  • Normal limits of visual acuity by age: up to 3 years—can fix and follow face, toy, light; 3 to 5 years—can reach 20/40 or better with no more than 1 Snellen line difference¬† between eyes; 5 years or older—20/25 or better with no acuity difference between eyes
  • Subnormal visual acuity may result from optical causes (uncorrected refractive error, corneal or lens irregularities) or neural causes (lesions of retina or visual pathway) or psychogenic causes, and...
  • Distinguishing between these causes may require expertise, special instruments, ancillary studies