Visual Fields

• 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

• 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

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