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)
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