Week 3: Perimetry Flashcards
Define visual field and perimetry
- Visual field = the area of space perceived by the eyes sensitivity to light
- Perimetry = plotting the sensitivity of this area in space
What are the boundaries of monocular visual field?
- superiorly = 50-60 degrees
- nasally = 60 degrees
- inferiorly = 70-75 degrees
- temporally = 90-100 degrees
How are visual fields assessed?
- Mapping the boundaries of the field
- 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
What is the Weber’s Law dependent on?
- Background photopic luminance (10 cd/m2)
- Pupil size > 2mm
- Clear Ocular media
- Test conditions – stimulus size, duration etc
Explain the ‘normal visual field’
- 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
What does right Nasal retina view?
Views right half of visual field
What does the Right Temporal retina view?
Views left half of visual field
What does Homonymous view?
Right nasal retina & left temporal retina project the same area in visual space to visual cortex
What is the dimension of degrees for binocular visual field (horizontal & central field)?
- Horizontal visual field = +/- 90 degrees (central 180 degrees visual space)
- Binocular central visual field = +/- 30 degrees (central 60 degrees visual space)
What does ‘traquair’ mean?
“an island of vision in a sea of darkness”
Define Hill of Vision
Visual function is not uniform across the whole visual field
In the hill of vision what is in the central and peripheral retina?
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
What is the blind spot in HOV?
-15° temporal to fixation
- 2° below horizontal meridian
Define isopters
- Areas of equal sensitivity
- Connection points are joined together
What are the testing conditions in HOV?
- Photopic – Brightly lit condition
- Mesopic – Dim condition (twilight)
- Scotopic – Dark condition
Why perform perimetry?
- Diagnosis of Visual pathway pathology such as; glaucoma management, neurological disorders, retinal diseases
- Monitor change in function
- Assess baseline severity of vision
- Measure lesion progression over time
List the traditional/objective perimetry
- Kinetic
- Static
- Suprathreshold Static
Briefly explain kinetic perimetry
- Detection of moving target
- Stimulus repeated along different meridians
- Different spot intensities used
List advantages/disadvantages of kinetic perimetry
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
Briefly explain static perimetry
- Stationary target with varying brightness
- Brightness is increased until threshold
- Most common method of measuring HOV
List advantages/disadvantages of static perimetry
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
Briefly explain suprathreshold static perimetry
- Rapid screening procedure to detect gross defects
- Typically a smaller number of targets used
List the new perimetry techniques
- Short Wavelength Automated Perimetry (SWAP)
- Frequency Doubling Technology (FDT)
- Flicker Perimetry
- Motion Perimetry
- High Pass Resolution Perimetry
Briefly explain short wavelength automated perimetry (SWAP)
- To permit use of short-wavelength-sensitive light (blue) stimuli on a yellow background in a similar duration
List advantages/disadvantages of short wavelength automated perimetry (SWAP)
Advantage:
- Principle of Redundancy
Disadvantages:
- Greater variability and reproducibility between tests
- Adversely affected by cataracts
Briefly explain frequency doubling technology (FDT)
- 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
List advantages/disadvantages of frequency doubling technology (FDT)
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
Briefly explain flicker perimetry
- Determining highest temporal frequency
- Can be an early indicator of functional loss in glaucoma
List disadvantages of Flicker Perimetry
- Loss of flicker sensitivity
- Is affected by pupil size and other pathological conditions
Briefly explain motion perimetry
- 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
List disadvantages of Motion Perimetry
- Motion can be detected by both Parvocellular and Magnocellular cells, therefore this selectivity does notmean exclusivity
- Optical defocus does affect foveal motion thresholds
What are Factors Affecting Visual Field Sensitivity Profile
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)
Describe Ricco’s Law
- 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
Describe Bloch’s Law
- For relatively short presentations, the product of contrast and duration is constant at the detection threshold
Luminance Threshold x Duration = Constant
Briefly describe background luminance
- 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
What are the advantages of background luminance?
- Shorter adaptation time
- Reduced sensitivity to changes in room luminance during testing
What is the effect of blurred vision on visual sensitivity?
- Retinal defocus increases detection threshold and produces flatter HOV
- Contrast needed to just detect stimulus from uniform background
