Tuesday, 29 May 2012

Could my child have cerebral visual impairment?



Professor Gordon N Dutton MD, FRCS Ed Hon, FRCOphth
Children’s Vision Consultant
Emeritus Professor of Vision Science, Glasgow Caledonian University
Honorary Senior Research Fellow, University of Glasgow


Cerebral visual impairment (CVI) is the commonest cause of visual impairment in the developed world. A large part of the brain is devoted to seeing, so it is not surprising that conditions that affect the structure or function of the brain can interfere with vision [1].

The front of the brain ‘thinks’ ‘understands’ and ‘behaves’, the middle ‘feels and moves the body as well as hearing and understanding language’, and the back ‘sees’.

The picture made by the eyes is sent to the back of the brain where it is sorted out in different ways:

1. Processing the picture
First, at the back of the brain (the occipital lobes) the detail of the picture is sorted (measured as visual acuity). Seeing colour, handling the number of shades of grey that can be seen (contrast sensitivity), knowing the extent of the picture, (the visual fields), and dealing with the speed and direction of moving images are all processed in this area.

2. Guiding our movement and seeing lots of things at once
Second, at the top of the brain at the back (the posterior parietal lobes), there is an amazing system. The visual information is carried there from the occipital lobes by a connecting pathway, called the dorsal stream. For every moment of our waking lives, this area of the brain uses the incoming moving picture to guide our movements.

Reach out and pick something up. You ‘know’ that the picture is out in front of you, but it isn’t really is it? It’s inside your brain! What is amazing is that the picture out there in front is so compelling that you believe that it is there, but you are actually moving through the picture in your brain, fully expecting that it will coincide with the reality of the object you are about to pick up, which of course it is. This can take a while to get one’s head round, but it gets even more complicated. The whole process takes place completely unconsciously!

Some people who have gone blind in both eyes because of damage to the back of the brain, which spares the movement seeing centre, are able to accurately move through the world that they do not consciously see. Functional scans of the brain show that they do this with their intact posterior parietal lobes. “But wait a minute” you may be thinking, “I know what I can see; and I chose and knew what I picked up.” Yes, but the conscious seeing and knowing is performed separately by the closely interlinked bottom of the brain at the back, the temporal lobes, which are connected to the occipital lobes by another pathway called the ventral stream.

3. Recognition
This is the third part of the brain’s conscious visual processing system. The temporal lobes serve conscious vision and recognition.

You recognise a friend in a group of other people you don’t know. To do this, you have a memory store of all the people you know. In came the picture of the group of people. The pictures of all their faces were compared with your memory store and there was one match, - your friend. What an amazing bit of computing! This part of the brain also allows you to find your way around without getting lost.

4. Dealing with the cluttered scene
To be able to move through the visual scene, a number of things need to happen. The unconscious picture needs to be processed very quickly, it then needs to be held in automatic unconscious working memory for a short time, after which it needs to be shed. Otherwise the brain would quickly fill up with useless out-of-date information.

As many items as possible in the visual scene need to be available at the same time to let us move through a cluttered environment, and to let us choose where we go. This is done by the visual reference library in the temporal lobes giving conscious knowledge of what we are seeing, while the dorsal stream pathways are providing the RAM in the computer of the brain so that we can attend to a number of things at once, by linking to the front of the brain which makes the conscious choices of where to go and what to focus on.

The two visual analysis systems, the dorsal and ventral streams, are blended so well that they work together in harmony, but of course we are unaware of all the unconscious visual workings of the brain, and therefore believe, wrongly, that we have conscious awareness of everything that our brains are seeing and handling.

So how does this information help in finding out whether a child has cerebral visual impairment?

Any element of what has been described may not work so well. Even with the right glasses, children with cerebral visual impairment can have a range of visual difficulties, unique to themselves, due to any of the processes outlined not working so well, in any combination or severity. The table below gives a list of some of the difficulties due to cerebral visual impairment along with approaches that can be used to help.

Problem
Approaches
Reduced clarity of vision.
Enlarge text
Double space text
Present text in small sections.
Reduce distractions
Limit tiredness
Colour vision and contrast sensitivity impairment
Bright and clear educational material and toys.
Distinct colour boundaries
Good contrast
Lack of vision on one side


Tracing of text with a finger or ruler
Turning text vertically or obliquely
Appropriate seat position in classroom
Turning of head to check the hemianopic side
Careful guidance around new environments
Training in crossing roads
Turn plate to eat food
Lack of vision down below

As with hemianopia
Regularly looking down to check the ground ahead
Tactile guide to ground height
Impaired ability to move the eyes
Movement of the head
Enlarging text
Double spacing text
Tracing of text with a finger or ruler
Impaired ability to see movement
Television programs with limited movement
Educational material with limited movement
Careful training or guidance in crossing roads.
Difficulty finding a toy in a toy box or an item of clothing in a pile or wardrobe
Separate storage of favourite items
Organised storage systems
Always store in same location
Avoid clutter
Colour coding and labels
Difficulty finding an object on a patterned background.
Use plain carpets, bedspreads and decoration.
Difficulty finding food on a plate
Avoid patterned plates
Avoid sauces/ gravy
Separate food portions
Problems seeing a distant object
Use zoom on video/ digital camera to view

Problems reading

Enlarge text
Double space text
Masking surrounding text
Computer programs to present information
Difficulty finding someone in a group.

Wear obvious identifier
Always stand in same location
Waving
Speak
Tendency to get lost
Training in seeking and identifying landmarks
Visit new locations at quiet times
Problems with floor boundaries, steps, kerbs and uneven surfaces
Avoid patterned floor surfaces
Bannister
Mark edge of stairs
Good lighting
Tactile guides to gage the height of the ground
Approach obstacles with
“Look- Slow- Check- Go”
Activities to improve coordination
Inaccurate visually guided reach
Reaching beyond an object to gather it
Activities to improve coordination
Occupational therapy
Difficulty ‘seeing’ when talking at the same time 
Limit conversation when walking
Identify obstacles by tactile stimulation
Frustration at being distracted

Limit distraction
Minimise background clutter
Minimise background activity.
Quiet table at school
Difficulty recognising people and photographs

Introductions
Training in identifying voices
Consistent identifiers worn
Training to recognise identifiers
Difficulty recognising shapes and objects
Training to identify and recognise identifiers
Training in tactile recognition
Difficulty reading facial expression
Training in recognising facial expressions
Expression of mood by tone of voice
Explanation of mood in words
Getting lost in known places
Training in orientation.
Encouraging leading
Incorporating landmarks in Mnemonics /Poems
Difficulty in new environments
Training in orientation.
Encourage exploration
–Visit at quiet times
–Hide and Seek
–Treasure Hunts
Visual fatigue
Prolonged visual processing
Minimise clutter
Reduce distractions
Reduce detail and complexity
Well earned breaks
Social problems
Good understanding and support at school
Identify problems and solutions
Encourage child to overcome them
Well known informed peer group
Find activities child enjoys and can excel in


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One bitterly cold Friday in December well over a year ago, in Glasgow, Scotland, I was privileged to observe Gordon Dutton doing his characteristically thorough assessments of children at the Vision Clinic in the Royal Hospital for Sick Children, just weeks before he retired from that post. It was an occasion I am not likely to forget as he patiently quizzed parents about the child's history and tested the child's vision with tools he has developed over the years. Gordon has been leading the field of cerebral visual impairment for some years now. He is a prolific writer on the subject and we look forward with great interest to his new book for teachers on CVI. He has been an enormous inspiration to many in recent years, myself included. Many thanks, Gordon, for submitting this article on such a vital and under-resourced topic.     - Maurice


Gordon N Dutton 


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