Widest temporally
Island of vision
The blind spot is 15 degrees temporally from fixation (corresponding to the displacement of the optic nerve from the fovea)
The fovea is the centre of the field
Normal field extends

90-110 degrees temporally
60 degrees nasally
70-80 degrees inferiorly
50-60 degrees superiorly

## 4 main types of visual field testing

Confrontation visual fields: primarily a screening test
Kinetic perimetry
Static perimetry
Amsler grids

## Kinetic perimetry

Usually done manually
A stimulus of fixed size and intensity is moved through the visual field and the area in which it is perceived is called the target’s isopter. Different colours are used to indicate different sizes and intensities of target
“Slices” the island of vision into isopters which can be plotted
Goldmann perimetry

Controller on machine allows selection of stimulus size and illumination
Each Roman numeral (I-V) denotes incremental increases of the test object size. Each increment doubles the radius (so quadruples the area)

Eg. Size II = 1mm2

Arabic numeral (1-4) then denotes the illumination in 5dB steps
Filters a-e are in increments of 1dB each
Armaly Drance technique used to plot:

Concentrates on nasal horizontal
Tests pericentral area with a suprathreshold stimulus to exclude central scotomas

“Art form”: takes experience to become a good perimetrist
Maximum brightness is 1000 apostilbs

Useful if patients cannot do automated fields: advanced disease, very old or very young patients
Spiraling: an isopter of greater luminance overlaps one that is dimmer. Implies functional visual loss.

## Static threshold perimetry

A fixed site within the field is chosen and the stimulus size and intensity varies until it is large/bright enough to be seen
Measures the “thickness” of the island
Lighter grayscale indicates thicker areas of better vision and darker/black areas indicate poor vision
Humphrey, Octopus etc.
Finds the threshold: the intensity of light seen 50% of the time

Measured in decibels of attenuation of the light
Light presented for less than 0.5 second
White target on white background: achromatic

Full threshold: this indicates static perimetry wherein the exact threshold of the eye is measure at every point tested
Suprathreshold: used as a screening test. Objects are presented at a fixed intensity, may miss early defects.
Maximum brightness is 10,000 apostilbs

The range of measurement results is given in decibels between 0 and 50
30dB implies the patient can still detect the light despite 3 orders to magnitude dimming of the 10,000asb (to a stimulus of 10asb)
NPL vision would result in 0dB (ie the patient cannot see the stimulus despite no dimming of the 10,000asb
Target sizes range from I-V, with gross intensity designated 1-4 and fine intensity designated a-e
The standard target has become a III4e

Standardised
Eliminates perimetrist variability: requires less skill to operate
Ideal for glaucoma
30-1 and 24-2 strategies

First number indicates the area of field tested in degrees from fixation
Second number indicates the pattern of targets presented to the patient
NB: the 24 degree strategy present 54 points

Global indices

Summary of results condensed into one figure, mainly used to monitor glaucoma progression

Total deviation

Deviation of patient’s result from age-matched controls
Does not confirm a scotoma but shows a generalised depression

Mean deviation

Average deviation of sensitivity from age adjusted normal population
Similar to total deviation
May decrease after cataract surgery

Pattern standard deviation

Average deviation from the normal hill of vision after correcting for overall sensitivity differences
Can confirm a scotoma
May increase after cataract surgery (as focal glaucomatous defects are revealed)

Short term fluctuation (SF)

A measure of intra-test error in determining thresholds
10 predetermined points are each tested twice
>5dB is considered unreliable or a sign of early glaucoma

Corrected PSD

PSD after accounting for short-term fluctuation
If the SF shows unreliability of the test then the corrected PSD will be better than the PSD. If the SF is due to true pathology, the PSD will be better than the corrected PSD.

Glaucoma hemifield test (GHT)

Divides 24-2 visual field into 10 regions, with 5 inferior regions mirroring 5 corresponding superior regions
Differences between corresponding superior-inferior zones are compared with normal population controls
Defined as ‘outside normal limits’ if differences are greater than those in 99% of normal population
‘Borderline’: abnormal at the 97th percentile
A normal GHT does not rule out glaucoma

Humphrey field other important values

Astigmatism >1.25D needs correcting
Pupils <2mm and >6mm may influence outcomes by introducing artifacts
The background illuminance is set at 31.5 apostilbs which saturates rod photoreceptors to produced photopic conditions to isolate and test cone function

Reliability indicators in Humphrey perimeter

Fixation loss: patient responds to a target presented at their blind spot. Fixation loss >20% makes the field unreliable. Kinetic perimetry may be better
False-negative: patient fails to respond to a suprathreshold target at a location where they have already demonstrated sensitivity. Suggests inattention and the result will appear worse than it really is
False-positive: the machine makes a familiar noise and alters the motorised light but does not present a target and the patient responds as if seeing one (trigger-happy). Field will look better than it is.
A value of 40dB or higher in the numeric decibels graph indicates a possibly trigger-happy patient

Cloverleaf field pattern: normal or near normal central field with dark periphery

Patient has responded well to initial stimuli but then poorly to later stimuli
Suggests: poor attention, malingering, and/or high false negatives

Protocol types:

SITA (swedish interactive threshold algorithm)
Esterman grid
Short wavelength automated perimetry: uses blue stimuli on yellow background rather than white-on-white and may be more sensitive to glaucomatous damage
Frequency doubling perimetry: uses a rapid reversal of broad black and white bars to preferentially test large magnocellular M-cell fibres (may be useful in early glaucoma screening)

M-cell fibres are large diameter and comprise 25% of retinal ganglion cell population. They are believed to die earlier in glaucoma.
FDT has low test-retest variability
Not affected by high refractive errors, variable pupil sizes or colour blindness

## Amsler grid

Assesses the central 20 degrees of the VF
Used to detect central or paracentral scotomas
Held at 1/3rd of a metre, each small square subtends 1 degree of visual field

False field defects

Overlooked physical factors which mimic field defects

Ptosis
Dermatochalasis
Tilted optic disc can cause variations in retinal topography. Bilateral tilted discs can produce a bitemporal visual field defect
Small pupil: a pupil <2.5mm will cause generalised depression
The rim of a trial lens: patients will tend to pull head back from machine
Media opacities

## Hemianopias

Homonymous, total

Implies damage to the whole visual pathway beyond the chiasm unilaterally
Non-localising

Homonymous, partial

Implies injury to post-chiasmal pathway
Non-localising in the absence of other features eg macular sparing implies a lesion in the occipital lobe, incongruous implies optic tract, congruous defects imply optic radiations or occipital lobe
“Optic tract syndrome”: mass lesions of the optic tract lead to incongruous homonymous hemianopias with bilateral optic atrophy in a ‘bow-tie’ pattern and an RAPD on the side opposite the lesion (ie. the eye with temporal field loss)

Bitemporal: signifies chiasmal injury

Pituitary tumours
Pituitary apoplexia
Meningiomas
Aneurysms
Infections
Craniopharyngiomas
Gliomas
Trauma
Demyelination
Inflammatory disease eg sarcoid
Ischaemia

Binasal: signifies damage to uncrossed fibres in the lateral aspects of the chiasm bilaterally, both optic nerves or retinas

Bilateral arcuate scotomas in glaucoma
True binasal defects are rare: aneurysms, pituitary adenomas compressing the temporal optic nerve and the anterior angle of the chiasm

Homonymous quadrantanopia

Pie-in-the-sky: temporal lobe lesions

May coexist with seizures and hallucinations

Pie-on-the-floor: parietal lobe lesions

Coexist with spasticity of conjugate gaze (tonic deviation of the eyes towards the site of the lesion on attempting Bell’s phenomenon) and optokinetic asymmetry (diminished or absent response with rotation of optokinetic objects towards the side of the lesion)

Crossed quadrantanopia: rare

Upper quadrant in one eye and lower quadrant in fellow eye
May occur if the chiasm is compressed from below and pushed upwards against an arterial structure leading to pressure from above and below

Altitudinal:

Unilateral: ischaemic optic neuropathy
Bilateral: may be caused by lesions again pressing the chiasm upwards and wedge the optic nerves eg an olfactory groove menigioma. Also chronic papilloedema
May be mimicked in BRVO, BRAO

Double homonymous hemianopia with macular sparing

Implies gross injury to the occipital lobe eg trauma, anoxia, carbon monoxide, cardiac arrest, exsanguination

Junctional scotoma: unilateral central scotoma with a contralateral supero-temporal field defect

Lesion of the optic nerve just anterior to the chiasm therefore also affecting knee of Willebrand (contralateral infero-nasal fibres)

Glaucomatous defects

Nasal step
Arcuate defects within 15 degrees of fixation (Bjerrum defects). Can appear like altitudinal defects
Comma-shaped enlargement of the blind-spot (Siedel scotoma)

Ring scotoma

Severe glaucoma
Retinitis pigmentosa
PRP
Vitamin A deficiency
Aphakia
Functional visual loss or malingering

Generalized depression of the field without local defect

Media opacities
Glaucoma
Small pupil
Refractive error
inattentive/inexperienced patient

## Centrocecal defects

A defect in the central field extending from fixation to the blindspot
Four classic primary causes

Dominant optic atrophy
Leber’s optic atrophy
Toxic/nutritional optic neuropathy

Tobacco, alcohol, lead, medications

Congenital optic nerve pit with serous retinal detachment

Optic neuritis may also cause