Preliminary Testing

• Visual Acuity

  • Purposes:

    • To establish a baseline

    • Legal reasons (driver’s licenses, insurance claim, pension, and disability based on legal blindness)

      • Legal blindness: 20/200 or worse in better eye OR less than 30 of visual field in the widest meridian of the better eye

    • Monitor progression or improvement of eye disease

    • Guide for the rest of the exam (prediction of refractive error, correlation of data, determines additional testing)

  • Always the first test performed after history

    • Exception: chemical burn/spill; irrigate first, then check VA

  • Definition: the resolving power of the eye, or the ability to see two objects as separate

    • “Normal” resolving power is defined as the ability to detect a gap with a width of 1 min of arc

  • VA is highest at the fovea and decreases with increasing retinal eccentricity

  • VA best if pupil is 2-5 mm

  • Types

    • Minimum Detectable Resolution (minimum visible): the ability to distinguish an object from its background

    • Minimum Separable Resolution (minimum resolvable): the ability to resolve two or more spatially separated targets

      • Gratings, bars, Vernier acuity, preferential looking

    • Recognition Resolution (minimum legible): the ability to recognize letters, numbers, and geometric forms

      • Snellen, Landolt C, Tumbling E

  • Snellen acuity chart is the universal method of measuring VA

    • Snellen optotypes: width of each stroke is equal to the width of a gap on that line

      • “Best” letter is E (3 strokes and 2 gaps)

      • “Not as good” letters are T and L ( no gaps)

    • At 20 feet, a 20 foot optotype subtends 5 min of arc

      • Its details each subtend 1 min of arc

    • 20/20 letter is defined as a letter that has a height (x) such that it subtends 5’ arc at 20 ft

      • Letter height can be altered with test distance

      • Details (gaps and strokes) each subtend 1’ arc at 20 ft

    • Utilizes a “folded” room/operatory system

      • Projector with letters, 2 mirrors and a screen

      • Test distance= patient to mirror + mirror to screen

  • MAR (minimum angle of resolution) in minutes of arc is equal to the reciprocal of the decimal acuity value or the Snellen fraction

    • Snellen fraction = 1/MAR

    • 20 ft (6m) is considered optical infinity, which is defined as the distance at which no accommodation is being used

    • Snellen fraction = testing distance

distance at which the smallest letter read subtends an angle of 5’ of arc