3.1.5 Investigates the visual fields of patients with all standards of acuity and analyses and interprets the results

What are the characteristics of the visual field?

• The binocular field has a zero zone where you achieve stereopsis. Beyond this zone is the extreme periphery, it is only governed by one eye at a time.

• The normal hill of vision

  • Light sensitivity is greatest at the fovea

  • The blind spot is 5.5° wide and 7.5° high

  • The blind spot is located 15° degrees from the fixation point and 1.5° below the horizontal midline.

Decibels (dB)

refers to stimulus intensity

• 0dB = maximum brightness

• 51dB = minimum brightness

Visual Field Index (VFI)

• Quantifies the overall status of the visual field

• Useful for monitoring changes over time but not a useful in classifying the severity of vision field loss

  • E.g. patients could have a dense central scotoma but have a VFI of 99%

Mean Deviation (MD)

• How the average of the whole field differs from agenormal findings

• Approx. 0 dB in normal fields

Pattern Standard Deviation (PSD)

- The SD of the values in the total deviation plot

- An indication of focal loss as it demonstrates whether the total deviation are approximately the same or different from each other

- As focal loss increases, PSD becomes more positive

- Once PSD >12.0dB, it decreases as focal loss is so large that it begins to diffuse and be less anomalous

Glaucoma Hemifield TEST (GHT)

- Pattern deviation in 5 zones of the upper hemifield are compared to mirror-image zones in the lower hemifield (Used in 30-2 and 24-2 tests)

“Outside normal limits” - one zone pair differs by amount of found in fewer than 1% normal subjects

“Borderline” - one zone pair differs by amount found between 1-3% normal subjects

“Abnormally high sensitivity” - when best points are so high that they’re on seen by 0.5% normal subjects

“General reduction of sensitivity” - when best points are so low that they are not seen by fewer than 0.5% normal subjects

The Greyscale

- Visual representation of raw decibel sensitivity

- Not compared to normal data ranges

- Successfully shows profound visual defects or artifactual loss

Total deviation

- Threshold sensitivity is compared with age-corrected normal at each test point to produce numerical maps

- These numerical values are then images with probability agents to clearly show areas outside normal limits

Pattern deviation

• Removes any generalised depression or elevation so vision loss pattern can be distinguished

• Based on a “general height adjustment”

  • The external readings are placed in order form positive to negative

  • The 7th most positive value is applied to all the readings so that the 7th value becomes 0db

• Means that subtle but significant variations are highlighted and easily visible to the interpreter

Stimulus parameters

SITA - Swedish Interactive Threshold Algorithm

-Compares the frequency of seeing curve at each location in a normal eye and also in the glaucomatous eye to the patients frequency of seeing curve

- Allows for short test times as the predicted values are close to the threshold

- SITA STANDARD uses 2-4dB jumps

- SITA FAST uses 4dB jumps

Stimulus parameters

Stimulus colour

- Background is always at 31.5 ASB

- Colour of stimulus is always fixed as ‘Goldman white’

Stimulus parameters

Stimulus size

- All normative data and statistical analysis is based upon the Size III stimulus

Stimulus parameters

False negatives

-Presenting 9dB brighter stimulus on a place where a stimulus was apparently seen

- If exceeds 20% then test deemed unreliable

Stimulus parameters

Stimulus parameters

- When patient clicks the button too quick that there were clearly anticipating the stimulus

- Unreliable test if error exceeds 15%

The “-2” indicates…

the stimulus offset from the horizontal and vertical midline

30-2

- Stimuli are 6º apart; the middle stimuli are 3º off the midline

- The field of examination is 27º wide as there a 9 spaces between stimuli across the wides horizontal line

24-2

- Stimuli are 6º apart; the middle stimuli are 3º off the midline

- The field of examination is 21º wide as there a 7 spaces between stimuli across the wides horizontal line

24-2c

- Modified 24-2 the incorporates 10 additional test points in central 10º of vision

10-2

- Stimuli are 2º apart; the middle stimuli are 1º off the midline

- Field is 9º wide

- Used for advanced glaucoma when patient has small visual field

Fixation stability

- Blind spot monitoring

- The patient shouldn’t see stimuli presented to the blind spot area

Gaze tracking system

Analysis of distance between centre of pupil and the corneal reflex

- Upward markings indicate fixation loss of >10º

- Downward markings indicated blinking

What is Suprathreshold perimetry (C40)

- Presents stimulus at a known luminance higher than expected threshold

- Stimulus is either seen or not seen

- Doesn’t give indication of the depth of the loss so. if the stimulus is not seen then the depth of the loss should be investigated though full threshold testing

Suprathreshold perimetry (C40)

Disadvantages

• Non-quantitative

• Cannot separate optical loss from neural loss

• Insensitive to shallow field loss

- USELESS AT DETECTING EARLY GLAUCOMATOUS FIELD LOSS

Artefacts

Ocular factors

-Increasing cataract or media opacities

- Reduction in pupil size

Artefacts

Upper lid position

- Ptosis often impacts 30-2 tests

- May need to elevate lid with surgical tape to make sure lid doesn’t give superior defect

Artefacts

Untrained patient

- 10% patients show decreases sensitivity on first attempt

Artefacts

Incorrect date of birth

Inability to compare results with correct DoB results in unfavourable total deviation maps

Artefacts

Trial lens power and position

- ~1dB reduction per 1.00D defocus

- High plus sense cause magnification and some points will be obscured by peripheral scotoma

Artefacts

Patient fatigue or inattention

- Peripheral locations exhibit greater reduction

- May form ‘clover leaf field’

Artefacts

Trigger-happy field

Many false positive results will cause algorithm to push threshold to a non-humanly possible level

Common Glaucomatous Field Defects

Nasal Step

• Widespread involvement of fibres in all parts of the optic disc are effected

• Light sensitivity in opposing hemifield may not be equal

• Doesn’t cross horizontal meridian

Common Glaucomatous Field Defects

Arcuate Defect

• Deep focal notch at optic disc leads to loss of all RNFL in the area corresponding to the notch

• Visual loss circumnavigates around the point of fixation abiding horizontal raphe

Common Glaucomatous Field Defects

Ring scotoma

When two separate arcuate defects occur in each hemifield and potentially meet at the horizontal raphe

Common Glaucomatous Field Defects

Central scotoma

Can arise from superior and inferior fixation splitting defects are both present

Label the defect:

Label the defect:

Label the defects:

Neurological Field Defects

The Optic Chiasm

• Nasal macula fibres cross over from one eye to the other through the centre of the chiasm.

• Temporal fibres do not cross over

Neurological Field Defects

The Optic Chiasm

Superior / inferior temporal fibres don’t cross over, they come from the right optic nerve and carry on into the right optic tract

Neurological Field Defects

The Optic Chiasm

Even though nasal fibres cross, the path isnt direct.

Superior nasal fibres cross over at the chiasm but pass into the optic tract of the same eye before coming back and crossing over to the contralateral optic tract. “Knee of Willbrand"

Neurological Field Defects

The Optic Chiasm

Inferior nasal fibres go back into the opposite side of the optic nerve head before heading to the contralateral optic tract

Neurological Field Defects

Retinal induced defects

• Retinal toxicity

• Vascular damage

• Age-related Macular Degeneration

• Retinitis pigmentosa

Retinal induced defects

Retinal toxicity

Drugs for systemic conditions can be toxic to the retina causing photoreceptor damage

Vigabatrin is an anti-epilepsy drug where 1/3 patients give rise to permanent bilateral nasal defects.

Retinal induced defects

Vascular damage

BRVO obscures light from reaching the photoreceptors and can give rise to an arcuate defect if severe

Retinal induced defects

Age-related Macular Degeneration

Visual field loss mimics damage to the photoreceptors, resulting in pronounced and severe central field loss

Retinal induced defects

Retinitis pigmentosa

Tunnel vision due to the degeneration of photoreceptors starting at in the peripheral retina

What are the characteristics of optic nerve disease?

• Only affects the affected eye

• Most optic nerve diseases cause central scotoma

• Early optic disc oedema may only enlarge the blind spot

What optic nerve diseases can produce visual field defects, and what are these defects?

• Optic disc drusen or optic nerve head coloboma can produce arcuate defects that may be indistinguishable from glaucomatous defects

• Optic neuritis can cause defects from complete inferior field loss to a small paracentral defect

• Meningioma tumour can compress the optic nerve but this doesn’t have a characteristic defect

What happens to the optic nerve fibres as they pass down towards the chiasm?

- As the optic nerve fibres pass down there is a reorganisation of the fibres so by the time it reaches the chiasm, all macula fibres are in the centre of the optic nerve and there is a concentration of different nerve fibres with different functions in different aspects of the nerve.

Optic chiasm lesions

View of the chiasm

There are a variety of ways in which we can damage the optic chiasm:

1. Compressive injury (tumour)

2. Vascular origin

What are the effects of meningioma compression on the anterior optic chiasm?

• Compresses the left optic nerve and the anterior knee of Willbrand.

• This will cause complete loss of vision in one eye, and a scotoma in the other eye.

What are the effects of pituitary gland tumours on the visual field?

• Fairly common. Tumour grows upwards, causing pressure on the chiasm. Over a period of time the pressure starts to damage fibres and kill them, leading to visual field loss.

• Pituitary adenoma is the most common lesion and it is a bitemporal defect, which leads to reduced visual acuity.

What are the effects of subarachnoid haemorrhage?

This will damage the chiasm but more importantly, the brain, this is life threatening.

What causes damage to the circle of Willis, and what are the effects?

• Due to atherosclerosis.

• This is plaque forming, containing cholesterol, which stick to blood vessels in the aorta and in the internal carotid artery in the neck.

• VF defects from this are in the form of an altitudinal defect or sometimes a diffuse visual field defect.

What are the effects of lateral geniculate nucleus (LGN) damage?

• The LGN blood supply is often thin vessels so vascular damage is the most likely cause of LGN damage.

• Damage to the ventral boundary of the LGN leads to an hourglass defect.

What are the effects of optic tract lesions?

• Tumours in the visual pathway become rarer the further back in the pathway you are.

• The most common of the rare tumours is the glioglioma.

• Results in a homonymous hemianopia. There may be low symmetry as the overlap between the two eyes is poor.

What are the effects of optic radiation damage?

• Compressive tumours or vascular damage can damage the optic radiations.

• The types of tumours are very rare.

What are the effects of a stroke on the visual field?

• Most common cause of death

• Obstructing blood supply in the brain leads to oxygen starvation, giving rise to vision damage.

• The most common visual field presentation is a homonymous hemianopia.

• If recovery occurs it will tend to happen within 1 month of onset.

What are the effects of optic chiasm lesions on the visual field?

• Damage to the optic chiasm can cause visual field defects affecting both eyes.

• Pituitary adenoma is the most common lesion and it causes a bitemporal defect, which leads to reduced visual acuity.

What are the characteristics of the striate cortex?

• The central 30º visual field is represented by approximately 80º of the striate cortex

• Macular sparring is difficult to demonstrate clinically.

• Hard to image on a 30-2 or 24-2 as the stimuli are 6º apart. However a 10-2 grid (2º apart) and Amsler grid have enough resolution to demonstrate macular sparing