Visual Acuity

Question 1

Define VA

Answer 1

= Ability of eye to discriminate fine detail
Gives indication of amount of RE or potential pathology
Legal driving requirement: 6/12
Used to assess adequacy of spectacle correction

Question 2

What are the different types of VA?

Answer 2

1. Detection: threshold of vision in the normal eye (5-14Q)
2. Discrimination: ability to distinguish object from background (line subtending up to 0.5s of arc; disc up to 30s)
3. Resolution: ability to detect gratings or checkerboard patterns (mainly research) / see 2 objects as separate, min angle of resolution ~30-40s of arc
4. Recognition and form vision: ability to detect letters, words, C orientation, MAR 1min arc
5. Vernier acuity: discrimination of small displacements of a target (2-5s of arc)

Question 3

VA limitations

Answer 3

Neural factors: size of foveal cones (predicts 47-49s of arc)
Optical factors: aberrations, diffraction, pupil size (2.0-2.4mm max resolution), luminance

Question 4

Vision vs habitual VA vs VA

Answer 4

Vision: unaided
Habitual VA: with Rx
VA: after RE correction

Question 5

If Px cannot see logMAR chart @ 6m

Answer 5

Move closer up until 1m (1/60)
HM
LP
NLP
*finger counting is not useful because fingers are highly variable

Question 6

VA chart illumination

Answer 6

Externally illuminated chart: 480 lux
Internally illuminated chart: 120Cd/m^s
100% contrast

Question 7

Normative VA values

Answer 7

18-24yo: 6/4.5
>75yo: 6/6+1
Binocular 2-3 letters better
Test-retest reliability 2-3 letters
Near: N5 (equiv. 6/9)

Question 8

Common errors when taking VA

Answer 8

Allowing Px to decide acuity
Permitting screwing eyes up
Looking around occluder
Using low illumination/dirty chart
Wrong WD
Not recording

Question 9

N print standards

Answer 9

N4 Medicine bottles/stocks
N6 Telephone directories
N8 Newsprint
N20 Large print books

Question 10

Advantages of LogMAR near charts

Answer 10

Task equally difficult at all size levels
Consistent with D VA, however not identical
Includes smaller print than other cards
Log scale facilitates prediction of spectacle addition and magnification required for low vision patients
(5, 6, 8, 10)

Standardised chart
Provides no contextual help
Largest print on paragraph chart is N18, on LogMAR is N80.

Question 11

Other VA charts

Answer 11

Illiterate E
Landolt C
Numbers
Pictures
Lea symbols
Sheridan Gardiner
Gratings / FC Preferential looking

Question 12

Normative PD values

Answer 12

D: 55 to 75mm

D & N:
2mm difference if small
3mm diff. if medium
4mm if large