PC - Visual Acuity - Week 2

Question 1

Define visual acuity.

Answer 1

The ability of the eye to discriminate fine detail.

Question 2

Define detection, and the number of photons of light needed to reach threshold.

Answer 2

Detection is the threshold of vision.

5-14 photons of light needed.

Question 3

Define discrimination. Describe the minimum level of discrimination in a normal eye.

Answer 3

Ability of a visual system to distinguish an object from its background.
Light subtending 0.5 seconds of arc can be seen.

Question 4

Define resolution. Describe the minimum angle of resolution.

Answer 4

See as seperate two objects or point sources of light.

30-40 seconds of arc.

Question 5

Define recognition, and the minimum angle needed for recognition.

Answer 5

Ability to detect letters, words, orientation, and shapes.

Minimum angle is 1 minute of arc.

Question 6

Define vernier acuity. What else is it known as? How many seconds of arc is it accurate until?

Answer 6

Discrimination of of small displacements in a target. Also known as hyperacuity.
Accurate to 10 seconds of arc.

Question 7

What limits visual acuity?

Answer 7

Neural factors like foveal cone size, and optical factors such as aberrations and diffraction.

Question 8

What is the receptor theory based on, what resolution does it predict?

Answer 8

Based on the diameter and seperation of foveal cones.

Predicts resolution to be limited to 49 seconds of arc.

Question 9

What is wave theory (for visual acuity) based on, and what resolution does it predict?

Answer 9

Based on hethe wave-like nature of light.

Predicts resolution to be limited to 47 seconds of arc.

Question 10

How does pupil size affect visual acuity? How do they relate in theory vs reality? What is the optimum pupil size?

Answer 10

In theory, visual acuity should increase as pupil diameter increases.
Due to abberations, visual acuity actually gets worse.
Optimum pupil size is 2.0-2.4mm under photopic conditions.

Question 11

Describe the relationship between visual acuity and illumination.

Answer 11

Visual acuity increases greatly with illumination, but only to a point.

Question 12

Define the following:
Vision (unaided vision)
Habitual vision (aided vision)
Visual acuity

Answer 12

Vision - size of the smallest line of letters on a test chart that can be read by the patient unaided.
Habitual vision - size of the smallest line of letters on a test chart that can be read by the patient with their existing aid.
Visual acuity - size of the smallest line of letters on a test chart that can be read by the patient after their refractive error is corrected.

Question 13

In what order are the eyes tested for visual acuity in clinical practice?

Answer 13

Right eye, left eye, both eyes.

Question 14

What is the standard testing distance for visual acuity, and why?
What can be done if there isnt enough room for this distance?

Answer 14

6m is standard, far enough not to stimulate accommodation. 20ft in the US, equivalent.
If there isnt enough room, a mirror can be used to double the distance.

Question 15

What is the standard non-computerised visual acuity chart called?

Answer 15

Snellen chart.

Question 16

Define the snellen notation: D’/D

Answer 16

D’ is the viewing distance (6m standard)

D is the distance at which the stroke width subtends 1 minute of arc.

Question 17

If a patient is unable to read an entire line, how is their visual acuity designated?

Answer 17

Consider the limit to be 6/12 - read entirely
Patient reads 4 letters of 6/6
Visual acuity would be written as 6/6 -1
If they only read 2 letters on 6/6, written as 6/6 +2

Question 18

What is a LogMAR chart?

Answer 18

Has a british standard of letters of almost equal legibility.
Follows a base 10 of the minimum angle of resolution, so each line is 0.1 log units or 1.26 times smaller than the line above.

Question 19

Which chart is the gold standard for visual acuity?

Answer 19

LogMAR

Question 20

Are all letters equally legible?

Answer 20

No

Question 21

What is the illumination of externally illuminated charts?

What about internal?

Answer 21

External - 480 lux

Internal - 120 candelas

Question 22

Describe the clinical procedure for testing visual acuity.

Answer 22

Measure monocularly first, right eye, then left.
The eye not being tested should be occluded.
Ask the patient to read the chart.
Smallest line of letters that can be read completely is recorded.
Per letter method used for fine-tuning recording.

Question 23

6/60 is typically the largest size on a chart. What if the patient cannot read this?

Answer 23

Move the chart closer to the patient until they can.
If they cant see the chart even at close distance (<50cm), then assess using hand movements.
If they cannot detect hand movements, then shine a pentorch, on/off.
If they cant, then they have no light perception.

Question 24

Is it advisable to use finger counting to assess visual acuity?

Answer 24

No, not consistent enough, due to varying sizes and colours.

Question 25

When monocular visual acuity values are equal, by how many additional letters is binocular vision better/worse?

Answer 25

Typically 2-3 letters better.

Question 26

Describe 2 clinical tips that can help when testing for visual acuity?

Answer 26

• Get a crude idea of the patient’s VA from their history, then begin by asking them to read the line slightly larger than estimates.
• Ask the patient to read the first letter of each line, until it becomes difficult, then read across.

Question 27

Name 6 common clinical erros when measuring VA.

Answer 27

• Allowing the patient to decide acuity
• Patient screwing eyes up (permitting them to)
• Looking around the occluder
• Using low lux or dirty chart
• Wrong working distance
• Not recording

Question 28

What is the normal VA for ages 18-24, and >75?

Answer 28

18-24 - 6/4.5

>75 - 6/6 +1

Question 29

How can illiterates be tested?

Answer 29

Tumbling E chart
Shapes
Pictures

Question 30

Is it advised to push patients to threshold in distance VA testing? What about near VA?

Answer 30

Distance - yes, patients should be pushed to threshold, ask them to guess if they cant see.
Near - rarely measured, not pushed to threshold.

Question 31

Describe a near chart. What is the smallest font equivalent to in a distance chart?

Answer 31

Series of paragraphs used, designated N5 to N20, N5 is the smallest font.
N5 is equivalent to 6/9.

Question 32

Describe how near visual acuity is measured.

Answer 32

Ask patient to assume their preferred working distance, and measure it.
Aim for N5, unless they have poor distance VA.

Question 33

Convert metres to feet.

Answer 33

Feet = metres x 10/3