Glaucoma 4 - Collating Evidence

Question 1

Visual field sensitivity depends on:

Answer 1

• The sensitivity of the eye is not constant across the whole of the visual field
-eccentricity
-adaptation level
-nature of test stimuli

Question 2

Why assess visual fields?

Answer 2

•Visual fields are direct measure of visual function (compared with IOP or disc)
•They tell us what the patient can see and how much impact the damage that we observe has had on that patient’s visual capacity/ ability
• Field plots aid differential diagnosis (pituitary lesion vs. glaucoma)

Question 3

Visual field measurements:

Answer 3

• Confrontation
• Bjerrum screen
• Goldmann bowl perimeter
• Octopus
• Friedmann
• Henson
• Humphrey - Gold standard
• SWAP / FDT / HRP

Question 4

Visual field by stimulus type?

Answer 4

• Manual vs Automated
• Kinetic vs Static
- kinetic: stimulus moves into VF
- static: stimulus presented at different points and different intensities

Question 5

Kinetic strategies:

Answer 5

• Goldmann and Bjerrum screen (30 deg)
• Centre of the VF is normally more sensitive than the periphery. (a stimulus that is a little too weak to be seen at the edge of the VF becomes visible as it is brought towards the centre)

Question 6

Static strategies

Answer 6

• Stimuli don’t move but varying numbers of stimuli are presented at different locations
- operator independent
- fast
- automated
- numerical analyses
• Supra-threshold test - present stimuli at an intensity calculated to be slightly above the patient’s threshold and record whether or not the stimuli are seen (good for rapid screening)
• Full threshold test - Strategy derives an estimate of the eye’s threshold at a whole
series of different retinal locations = full
threshold test (good for monitoring)

Question 7

What is threshold Testing??

Answer 7

• Involves precise quantification of visual sensitivity at each point tested
- Maximum information gain to quantify defects - slow procedure

Question 8

The Humphrey perimeter

Answer 8

• SITA strategies (Swedish Interactive
Threshold Algorithm), staircase strategies are used to alter stimulus intensities at predetermined test point locations
• VF is tested in the central 30 degrees
30-2 and 24-2 (evaluates the temporal
24 and nasal 30 degrees)

Question 9

Factors affecting Visual Fields: 7 in total?

Answer 9

•Age
-changes in pupil size
-lens changes
-changes in retinal and visual pathway sensitivity
(Rate of sensitivity loss increases with age as does the spread of results)

•Pupil size
-age
-pilocarpine
(A small pupil dims the intensity of the stimulus)

• Lens and media opacities have an effect on retinal sensitivity:
1. Act as a filter reducing amount of light reaching the retina
2. Scatter the incoming light reducing the contrast of any stimuli

•Refractive errors
- uncorrected errors result in VF errors due to defocus especially small and central stimuli
- Refractive errors greater than 1 dioptre should always be corrected (including presbyopia)

• Lens rim artefacts
normally occur at the edge of the central VF at an eccentricity of 25-30 degrees and can mimic the appearance of a nerve fibre layer defect
- Common in elderly, deep set eyes, hyperopia >6dioptres

• Eyelid, eyelashes and eyebrow:
droopy upper lids or prominent eyelashes prominent brows deep set eyes
may all give superior VF defects!

• Patient experience
- the learning effect/ curve
- Short-term and long-term fluctuation
- Patient fatigue manifests as an increase in threshold or an increase in fluctuation and increases as test lengthens (elderly)

Question 10

What are lens rim artefacts?

Answer 10

•Artefact on the visual fields scan:
6mm decentration produces a defect with 43.00D
3mm decentration produces a defect with +10:00D
• Misaligned correction lens

Question 11

Ptosis/brow defect:

Answer 11

Look for ptopsis - ofter obvious on camera allowing fixn to be monitored by field technician

Question 12

Where do most defects occur in glaucoma field test?

Answer 12

• Most of the defects occur within the central 30 degrees although the peripheral VF is frequently involved
• VF loss may be focal or generalized or both
•Retinal ganglion cell axons follow an arcuate path to the optic ve head. Field defects are a direct result of axonal damage at level of the optic nerve head. Axonal damage will always spect the horizontal midline!

Question 13

Bjerrums area defect:

Answer 13

• Bjerrum’s area defects: are frequent in early glaucoma and may account for 70% of early VF defects usually in superior VF between 10-20 deg

Question 14

Arcuate Defects:

Answer 14

• Arcuate defects
are slightly more advanced than paracentral defects and occur in arcuate or Bjerrums area usually sup > inf

Question 15

Para-central defects:

Answer 15

• Para-central defects:
Highly suggestive of glaucomatous aetiology
Defined as within 10 degrees of fixation

Question 16

Nasal Step Defects:

Answer 16

• Nasal step
up to 40% of patients with glaucomatou loss have nasal steps (usually superior)

Question 17

Temporal Wedge:

Answer 17

• Uncommon but correlated with significant nasal cupping

Question 18

Enlargement of blind spot:

Answer 18

• Enlargement of blind spot there are many different causes from neurological to ocular but in glaucoma may manifest as an elongation of blind spot in an arcuate fashion

Question 19

Overall depression of glaucoma:

Answer 19

•Overall depression
-due to reduced sensitivity of the retina secondary to diffuse loss of nerve fibres throughout optic nerve
-accounts for 38% of VF defects
- generalised depression especially nasally
- Reduced mean deviation scores
- Can be due to media opacities

Question 20

Global Indices in VF:

Answer 20

• Mean Deviation
Weighted average of total deviation values.
Values near centre receive more weight than in periphery. MD is a reflection of general sensitivity of VF (-)
• Pattern Standard Deviation
is a measure of the variability of the hill of vision and is high when a localised defect is observed

Question 21

Systematic VF assessment approach:

Answer 21

• Check the patient data (refraction, pupil)
- Check the reliability indices
• Look at grayscale (overall impression)
• Rule out possible artefacts (ptosis, lens, patient)
• Observe numerical graph
• Analyse total deviation probability plot (looking for diffuse loss of sensitivity)
• Look at Pattern deviation plot (localised)
• Analyse the global indices (confirm depth)
• Check GHT (glaucoma hemifield test)
• Compare VF to clinical information

Question 22

Are visual field plots consistent?

Answer 22

• Reality very variable
• Variation greater at edges of defects
• Over 60’s more than others
- similar to other stimuli
• Variation in instrumentation

Question 23

Where do visual fields have a place clinically?

Answer 23

• Visual field abnormalities should be repeatable before being considered real
• Remember that changes at the optic disc will commonly be present before any visual field defect manifests in early glaucoma