Week 3: Perimetry

Question 1

Define visual field and perimetry

Answer 1

• Visual field = the area of space perceived by the eyes sensitivity to light
• Perimetry = plotting the sensitivity of this area in space

Question 2

What are the boundaries of monocular visual field?

Answer 2

• superiorly = 50-60 degrees
• nasally = 60 degrees
• inferiorly = 70-75 degrees
• temporally = 90-100 degrees

Question 3

How are visual fields assessed?

Answer 3

1. Mapping the boundaries of the field
2. Measure the sensitivity of the eye in detecting a stimulus in different regions of visual field

The use of Weber’s Law = ΔL/L = C
- Where; difference in threshold (ΔL) & visible relative to background (L) is constant

Question 4

What is the Weber’s Law dependent on?

Answer 4

1. Background photopic luminance (10 cd/m2)
2. Pupil size > 2mm
3. Clear Ocular media
4. Test conditions – stimulus size, duration etc

Question 5

Explain the ‘normal visual field’

Answer 5

• Images are inverted & flipped as they pass through the lens with a significant amount of overlap
• Images projected onto retina can be divided left and right with FOVEA at the centre

Question 6

What does right Nasal retina view?

Answer 6

Views right half of visual field

Question 7

What does the Right Temporal retina view?

Answer 7

Views left half of visual field

Question 8

What does Homonymous view?

Answer 8

Right nasal retina & left temporal retina project the same area in visual space to visual cortex

Question 9

What is the dimension of degrees for binocular visual field (horizontal & central field)?

Answer 9

• Horizontal visual field = +/- 90 degrees (central 180 degrees visual space)
• Binocular central visual field = +/- 30 degrees (central 60 degrees visual space)

Question 10

What does ‘traquair’ mean?

Answer 10

“an island of vision in a sea of darkness”

Question 11

Define Hill of Vision

Answer 11

Visual function is not uniform across the whole visual field

Question 12

In the hill of vision what is in the central and peripheral retina?

Answer 12

Central Retina:
1. High density cones – good colour vision
2. Low density rods - responsible for good central vision
3. Good visual acuity
4. Peak of ‘hill of vision’

Peripheral Retina:
1. Low density of cones
2. High density of rods
3. Poor visual acuity

Question 13

What is the blind spot in HOV?

Answer 13

-15° temporal to fixation
- 2° below horizontal meridian

Question 14

Define isopters

Answer 14

• Areas of equal sensitivity
• Connection points are joined together

Question 15

What are the testing conditions in HOV?

Answer 15

1. Photopic – Brightly lit condition
2. Mesopic – Dim condition (twilight)
3. Scotopic – Dark condition

Question 16

Why perform perimetry?

Answer 16

1. Diagnosis of Visual pathway pathology such as; glaucoma management, neurological disorders, retinal diseases
2. Monitor change in function
   - Assess baseline severity of vision
   - Measure lesion progression over time

Question 17

List the traditional/objective perimetry

Answer 17

1. Kinetic
2. Static
3. Suprathreshold Static

Question 18

Briefly explain kinetic perimetry

Answer 18

• Detection of moving target
• Stimulus repeated along different meridians
• Different spot intensities used

Question 19

List advantages/disadvantages of kinetic perimetry

Answer 19

Advantages:
- Ability to characterise entire central and peripheral visual field
- Flexibility and interaction between examiner and patient
- Accurately characterise the shape of visual field scotomas

Disadvantages:
- Higher variability than other forms of perimetry
- Difference in clinical performance between examiners

Question 20

Briefly explain static perimetry

Answer 20

• Stationary target with varying brightness
• Brightness is increased until threshold
• Most common method of measuring HOV

Question 21

List advantages/disadvantages of static perimetry

Answer 21

Advantages:
- Standardised test procedure
- Ability to exchange information between devices/clinics
- Age corrected normative databases
- Monitoring response reliability

Disadvantages:
- Time consuming and greater burden on patient attention and performance
- High test-retest variability
- Less flexible

Question 22

Briefly explain suprathreshold static perimetry

Answer 22

• Rapid screening procedure to detect gross defects
• Typically a smaller number of targets used

Question 23

List the new perimetry techniques

Answer 23

1. Short Wavelength Automated Perimetry (SWAP)
2. Frequency Doubling Technology (FDT)
3. Flicker Perimetry
4. Motion Perimetry
5. High Pass Resolution Perimetry

Question 24

Briefly explain short wavelength automated perimetry (SWAP)

Answer 24

• To permit use of short-wavelength-sensitive light (blue) stimuli on a yellow background in a similar duration

Question 25

List advantages/disadvantages of short wavelength automated perimetry (SWAP)

Answer 25

Advantage: - Principle of Redundancy Disadvantages: - Greater variability and reproducibility between tests - Adversely affected by cataracts

Question 26

Briefly explain frequency doubling technology (FDT)

Answer 26

- Tests an illusion - The illusion is that there are twice as many dark and light bars present - Task is to determine the minimum contrast where the stimulus can be detected

Question 27

List advantages/disadvantages of frequency doubling technology (FDT)

Answer 27

Advantages: - Principle of Redundancy - Results are unaffected by refractive errors up to 6D - Very rapid screening test 90 sec with at sensitivity of 94% (true positives) Disadvantage: - Poor spatial assessment of visual field loss

Question 28

Briefly explain flicker perimetry

Answer 28

- Determining highest temporal frequency - Can be an early indicator of functional loss in glaucoma

Question 29

List disadvantages of Flicker Perimetry

Answer 29

- Loss of flicker sensitivity - Is affected by pupil size and other pathological conditions

Question 30

Briefly explain motion perimetry

Answer 30

- Detection of rapid movement of a single stimulus presented at various visual field locations - Tested using computer based programs - Were found to be more sensitive for nerve fibre bundle-shaped defect compared to conventional perimetry

Question 31

List disadvantages of Motion Perimetry

Answer 31

- Motion can be detected by both Parvocellular and Magnocellular cells, therefore this selectivity does notmean exclusivity - Optical defocus does affect foveal motion thresholds

Question 32

What are Factors Affecting Visual Field Sensitivity Profile

Answer 32

Stimulus (sport of light) factors: 1. Size 2. Duration 3. Colour 4. Background Luminance Patient Factors: 1. Dark adaptation level 2. Age 3. Blurred / low vision (uncorrected refractive error)

Question 33

Describe Ricco's Law

Answer 33

- Describe a human’s ability to visually detect targets on a uniform background Luminance Threshold x Area = Constant - Larger the spot size, lower the threshold

Question 34

Describe Bloch's Law

Answer 34

- For relatively short presentations, the product of contrast and duration is constant at the detection threshold Luminance Threshold x Duration = Constant

Question 35

Briefly describe background luminance

Answer 35

- Background luminance = 10 cd/m2 - Contrast needed to just detect stimulus from uniform background - 0 dB sensitivity = no response to brightest stimulus produced by instrument

Question 36

What are the advantages of background luminance?

Answer 36

- Shorter adaptation time - Reduced sensitivity to changes in room luminance during testing

Question 37

What is the effect of blurred vision on visual sensitivity?

Answer 37

- Retinal defocus increases detection threshold and produces flatter HOV - Contrast needed to just detect stimulus from uniform background