Understanding Visual Fields
Presented on Friday 2 November 2007
The Visual Pathway and Methods to measure Visual Fields
Dr Jennifer Skillen, Specialist Orthoptist, NHS Fife
What are Visual Fields?
The Visul Fields are a measure of the area from which you are able to perceive visual signals, when your eyes are in a stationary position and looking straight ahead.
Normal Adult dimensions
50-60 deg superiorly
70-75 deg inferiorly
60 deg nasally
90-100 deg temporally
Central and Peripheral components
Division of Labour - Central Field
- highly developed area of the retina responsible for detailed vision,
- read, recognise faces, detect colours.
Peripheral Field
- specialised in the detection of motion signals,
- enables safe navigation around our environment.
Schematic of the Retina
Why measure fields?
- Understanding of functional abilities.
- Help diagnose a vision/brain condition.
- Monitor treatment/progression of condition.
What are we looking for?
Relative field defect/scotoma - a light target can be seen but only when it is made brighter or larger (that is, reduced sensitivity).
Absolute defect/scotoma - No response to a stimulus regardless of brightness or size (definitive absence of processing from this area).
Perimetry - measure of the visual field
Two ways to measure the visual field.
Similarities are
- a) patient keeps eyes fixed on a target straight ahead
- b) presented with light stimulus
- c) they indicate by pressing a buzzer that the image is visible
Differences are
- Static testing
- light source is flashed from a stationary point within th visual field
- light source can vary in brightness (luminance level)
- computerised
Normal development of visual field
- Size of field dependent on a lot of variables.
- From 2 months to 8 months there is rapid development of the visual field.
- Slows down but continues until 12 months.
Goldmann Perimetry - Kinetic testing
- Light stimulus is moved from outside of bowl towards the centre.
- Manually operated.
- Light source can be varied in size and brightness.
- Start with I4E brightest and smallest light
- And then reduced brightness of target to I3E and then I4E – checking sensitivity
- If peripheral field reduced – increase size of target III4E until using the largest target V4E
- Result is isopters
Humphrey -Static Perimetry
- Computerised/automated test
- Light stimulus is randomly flashed within a number of ‘static’ positions
- If light is not seen – it will be presented again at a higher intensity
- Light source can vary in brightness
- Calculation on age matched normal data is carried out
Sample field plots - static
- High number = good ( good sensitivity to seeing light) - Dark/grey area corresponds to area with reduced sensitivity to light target
- Darker area = absolute scotoma/Lighter area = relative scotoma
- Related to age related norm – in terms of SD
Testing in Children under 10 - Goldmann
- Quick
- Ensure good fixation
- More interactive
- Need good technician
- Better for peripheral defects
Example of a normal Peripheral field in 10 year old
- Central field – central 30 deg
- Filled in circles – show blind spot
- Test carried out using two separate visual stimuli for each eye which differ in brightness
Confrontation
- This is a quick and easy test which we will be demonstrating in the afternoon session.
- Does not require any equipment
- Good for picking up large defects
What are we measuring?
- The integrity of the visual information – starting at the retina and reaching the visual cortex (located at the back of the brain within the occipital lobe)
- Looking at the pattern of field loss – we can get an idea of where a problem may be occurring within the eyes and/or the brain.
The visual pathway
Vertical perspective
- Anything above the midline superior – represented by the inferior retinal areas
- Any object below the midline will be represented by the superior retinal areas
The visual pathway
Different patterns of field loss
1. Central or peripheral
2. Ocular level
3. Brain Level
a) Early visual pathway
b) Late in the visual pathway
The visual pathway
Field Loss relates to Anatomy of the Visual Pathway
- Pre chiasm lesion cause field defects in the ipsilateral eye only (so if a lesion in the left optic nerve – field defect in left eye only)
- Right visual field will remain unaffected
- Field Defect links with the blind spot
Optic Nerve involvement
- Field defects tend to start from ON – blind spot on the field plot – rather than the central fixation point
- They tend to follow the shape of the nerve fibre bundles
Optic Nerve Field Defects -retinal
- Observe vertical meridians
- Not horizontal
- Can get bilateral defects but will often look asymmetrical
Optic nerve defects
- Complete loss of the LE central visual field
- Compression of the optic nerve – affecting central field only
- RE field remains full
Post Chiasm Lesion Optic Tract
Optic Radiations
Optic Radiations – Clear Segregation Superior and Inferior retinal signals
- Two routes from each LGN
- Superior field information carried via parietal lobe (SHORTER ROUTE)
- Inferior field via the temporal lobe
- MYERS LOOP
Temporal involvement-Myers Loop
- Long way round to the visual cortex
- If damage at Myers Loop – this results in a peripheral field loss – restricted to the inferior field fibres (left field if right cortex and right field if left cortex
- Will give a congrous/symmetrical appearance
Myers Loop involvement
Extensive damage to optic radiations
Inferior defect – quadrantopia damage to the superior fibres passing through parietal lobe – affecting peripheral and central area of field.
Visual Cortex
- The visual cortex is within the occipital cortex – the brain area corresponding to the fovea takes up a larger percentage than the peripheral field.
- If the visual cortex is damaged on one side the field will be absent to the opposite side in both eyes – homonymous hemianopia.
Homonymous Hemianopia
Left side of brain – occipital lobe.
Visual Cortex
Lesion affecting both visual lobes will be devastating to vision – result in cortical blindness.
2 separate blood supplies to occipital cortex
Two main types of Occipital Lesions
Posterior
Cerebral Artery
Middle
Cerebral Artery
Pointers when reviewing fields
- Ocular (retinal/optic nerve) –
- Asymmetrical field loss
- Or only one eye affected
- (unless retinal/ocular condition affecting both eyes to the same degree)
- Generally will involve/ originate from the blind spot if ON/retinal involvement (not always though)
- Lesion will be on ipsilateral side
Pointers
- Brain
- Symmetry of field loss increases through the visual pathway
- Observe vertical and horizontal meridians
- Involvement of the fixation point (as opposed to the blind spot)
- Lesion on contralateral side
Visual Field
Common behaviors observed in children with field defects
- Tilting and turning head
- Bumping into objects
- Losing items easily
- Clumsy
- Hesitates when using stairs and tips head down
- Difficulty locating food on a plate
Field Projection Summary
Adding insult to injury
- Lesions within the visual pathway can lead to a series of different field defects – some which will have more catastrophic effects than others.
- Effect on functioning can be predicted by measuring fields.
- In some children (that is, children with PVL or hypoxia at birth) this can account for only one half of the story.
Attention and Visual Neglect
- Higher visual areas use the information that reaches the visual cortex which is delivered through the visual pathway.
- Objects in our environment fight for attention and we cant process everything to a high degree. Instead we are selective in what we look at – not all under conscious control.
Visual Attention Network
- Bottom up – executive functioning – selection of tasks – frontal eye fields frontal cortex
- Top-down – visual attention – parietal lobe – maintaining interest when information is competing from each visual field.
The prognosis for rehab of field defects which exist in addition to attentional difficulties is much poorer and their visual experience different to children with isolated field defects.