Visual Acuity

• Objects in the visual field subtend the “visual angle” at the retina
• Arcminute: 1/60th of a degree
• Arcsecond: 1/60th of an arcminute

• Minimum resolvable task (ie. identification of small objects): possible at 30-60 seconds of arc

• Neonatal VA approximated to about  <6/60
• Neural pathways to the visual cortex continue to develop to age 8 years
• Angle alpha: angle between the optical and visual axes
• Describes the degree of "offset" of the eye compared to the optical axis
• Usually around 5.2 degrees horizontally

• Angle kappa: angle between the pupil axis and visual axis (ie. the angle between the centre of the pupil and the visual axis)

• Usually around 2.6 degrees horizontally
• Less reliable due to decentering of the pupil

• Visual axis: line from image to the fovea
• Pupillary axis: line through centre of pupil

Optotype acuity (subjective measurements)

• Optotype tests are based on the Landholt C test: the ‘gap’ in a C or E subtend 1 arcminute while the whole letter subtends 5 arcminutes

• Based on the idea that the ‘average’ person can resolve two points separated by 1 arcminute

• Snellen VA = 1/minimum angle of resolution
• Therefore:
• 6/60 = 1/MAR
• MAR = 10 arcminutes

• The reciprocal of the VA fraction is the angle of resolution of a single element of the optotype (eg. for 6/6, each element of the optotypes subtends 1 arcminute; and for 6/60, each element subtends 10 arcminutes)

• Snellen fraction:

• Visual acuity = Distance at which test is readDistance at which optotype substend 5 arcminutes at eye

• LogMAR: five letters per line, spacing proportional to letter size
• Reduces ‘crowding’ phenomenon
• Read from 4m
• Potential acuity meter: projects a Snellen type chart on the fovea
• Laser interferometry: forms diffraction grating on retina

• Sheridan-Gardiner and STYCAR charts use optotypes but are limited to letters that children recognise

• Kay’s and Cardiff tests use pictures for preschool children
• Catford drum uses oscillating dots and is used for babies

• Sonksen-Silver use crowded optotypes so are useful for detecting reduced visual acuity in amblyopia

Objective measures of VA

• Visual evoked potentials as above can approximate to VA
• Size of the checkerboard patterns and luminance can be adjusted
• Flash VEP is best for infants/uncooperative patients
• Oculomotor responses: useful for infants
• Requires functioning oculomotor system
• Sinusoidal grating target commonly used