Medical Information on Cerebral Visual Impairment
by Dr Andrew Blaikie for VI Scotland
This document is written with the minimum use of medical terms and jargon. It is impossible to avoid all medical terms but where we have used them we have attempted to explain them as clearly as we can. Although the information is intended to describe most aspects of the condition each child is different and there will always be exceptions to the rule. As far as we can determine these pages are true and accurate and have been written in good faith.
What this information is not for
This document is not a substitute for a consultation with a Health Professional and should not to be used as a means of diagnosing a condition.
We hope the information will help you to:
- Have a better understanding of the condition
- Know what tests and treatments are normally available
- Know when to seek professional advice
- Be able to discuss the condition in a more informed way
- Make the most of consultations with carers, teachers and health professionals
- Be reassured and more able to cope
Due to staffing limitations we are not able to offer telephone or email advice to parents of children.
What we see is made in the brain from signals given to it by the eyes. What we see is in fact made in the brain. The brain makes sight from signals given to it by the eyes.
What is the normal structure of the eye?
The eye is made of three parts.
- A light focussing bit at the front (cornea and lens).
- A light sensitive film at the back of the eye (retina).
- A large collection of communication wires to the brain (optic nerve).
A curved window called the cornea first focuses the light. The light then passes through a hole called the pupil. A circle of muscle called the iris surrounds the pupil. The iris is the coloured part of the eye. The light is then focused onto the back of the eye by a lens. Tiny light sensitive patches (photoreceptors) cover the back of the eye. These photoreceptors collect information about the visual world. The covering of photoreceptors at the back of the eye forms a thin film known as the retina. Each photoreceptor sends its signals down very fine wires to the brain.
The wires joining each eye to the brain are called the optic nerves. The information then travels to many different special 'vision' parts of the brain. All parts of the brain and eye need to be present and working for us to see normally.
Cerebral Visual Impairment (CVI) is a condition where some of the special 'vision' parts of the brain and its connections are damaged. This causes visual impairment even though the eyes are normal. Often children with CVI actually have good visual acuity but can not 'make sense' of what they see. In most cases, once the damage has happened it does not get worse. As the child grows older the visual difficulties may slowly improve.
There are many different causes of damage to the 'vision' parts of the brain. The damage can also occur at different times in the child's life. Often it is not known what the cause of the damage to the brain is.
Most often damage occurs while the unborn child is still in the womb. The reasons for damage to the brain while in the womb are often not known or well understood. Some cases might be due to the lining of the womb becoming infected. This type of damage can also be associated with early (premature) birth.
Damage can also occur around the time of birth. If the baby becomes 'tired' during birth, damage can sometimes occur to the brain. This might be due to poor blood supply to the baby or difficulty with breathing. Premature babies are more prone to damage. This kind of brain injury is sometimes called Periventricular Leucomalacia (PVL).
Most children even if they do have problems in the womb or during birth do not have any problems with vision or the brain.
There are many other causes of damage to the brain that can affect vision after a child is born such as:
- Infection (meningitis, encephalitis and infected intracranial aneurysms)
- Conditions that run in families (hereditary or genetic conditions)
- Damage to the drainage of water from the brain (hydrocephalus)
- Serious head trauma
- Stroke (cerebrovascular accident)
If a child is suspected to have visual impairment an assessment can be organised. Sometimes it is the parents who notice (by the way their child acts) that their child's vision is impaired. If they discuss this with their Family Doctor an assessment can be arranged. In most children damage to the brain may already have been diagnosed. The doctors looking after the child may then also suspect poor vision.
CVI can be diagnosed in a child who has:
- Visual Difficulty
- Damage to the 'vision' parts of the brain
- But apparently normal eyes
A head scan will usually confirm the damage to the brain. This test is often done if it is thought that there is damage to the drainage of water from the brain (hydrocephalus). This condition can cause the shape of the brain to change, which can be seen on a scan.
There are other special tests that can be done. These tests measure signals from the 'vision' parts of the brain when a child is shown patterns on a screen. Sticky patches are placed on the back of the head. The sticky patches are attached to wires that lead to a machine. The machine records the electrical signals made by the brain. The record of the signals will help the doctors decide what the matter is. If the signals are reduced in size or slow then CVI is more likely. This test is called a Visual Evoked Potential (VEP).
Often the best way to find out if a child has Cerebral Visual Impairment is by asking questions. An eye doctor can find out from the parents and teachers of the child what kind of problems they seem to be having. The questions are based on the visual difficulties that commonly occur in children with CVI. If the child has difficulties that are typical of the condition then they are very likely to have CVI.
Most young children with Cerebral Visual Impairment feel their vision to be 'normal' as they have never known anything else but their own visual world. At first they assume that everyone else has vision the same as their own and do not realise that other people see things differently.
Cerebral Visual Impairment can affect vision in many ways. All or only one of the special 'vision' parts of the brain can be damaged to different degrees. If there has been lots of damage to many parts of the brain the vision can be very poor. However the child may still see movement and bright lights. Complete blindness due to brain damage is rare.
If there has been only a small amount of damage in only one part of the brain the visual problem can be a lot less impairing.
Children with CVI can have problems with:
- Getting Around
- Recognising Objects
- Focusing for near objects
- Fast eye movements
- Visual Field Loss
The 'dorsal stream' helps children get around safely and quickly
The many different 'vision' parts of the brain combine together to make two visual 'systems'.
One system helps the child to get around safely and quickly. It also helps the child pick objects up and avoid bumping into things and falling over. This visual system that tells the body how to get around is called the 'dorsal stream'. It is called a 'stream' because it is a flow of information about the visual world from one place to another like water flowing in a stream. 'Dorsal' describes the part of the brain where the system is.
When the dorsal stream is damaged it is difficult to know precisely where things are in three dimensions. It can be difficult to:
- Use stairs without falling.
- Step onto pavements without tripping.
- Reach forward and grab a cup or handle.
Damage to the dorsal stream can also make it difficult to see a lot of different things at the same time. This means it can be difficult to find a toy on a patterned carpet or to see something that is pointed out in the distance amongst other things.
The 'ventral stream' helps a child recognise objects
The other system, called the 'ventral stream' helps us to recognise faces, objects and places. Damage to this system leads to problems:
- Recognising familiar faces.
- Knowing what common everyday objects are.
- Losing the way in places that should be well known to the child.
There are a number of other problems that can occur in children with CVI who still appear to have good vision. These include:
- Difficulty remembering things they have seen.
- Difficulty imagining 'seeing' things in their minds.
- Some children's vision can become 'tired' more quickly than others. This means that their ability to see can vary from one time to another.
- Difficulty reading. This can be due to lots of different reasons.
Children with CVI can have difficulty focusing when looking at near objects
The focusing power of the eye needs to increase when looking at a close object. In children with CVI the focusing power can be reduced. It can also become tired more easily. This is the usual situation for most adults when they become 40 or 50 years old. When this happens many adults need reading glasses. Some children with CVI may also benefit from reading glasses for the same reason.
Children with CVI may have difficulty making fast eye movements
'Fast' eye movements are called saccades. We use saccade eye movements to quickly change the direction that our eyes are looking. This helps us look at something that has suddenly changed position. This is so the eyes can follow and fix accurately on a fast moving object. The eyes can then give clear signals to the brain to make clear vision.
Fast eye movements are also important for reading. They help us to quickly move our eyes across the page of a reading book. Saccades are important in many other visual tasks.
Children with CVI may have difficulty making fast eye movements. These children may tend to make quick head turns when looking around a room or reading, rather than making fast eye movements.
By using a few tips described in the 'What can be done to help?' section children with this difficulty may find reading a bit easier.
What is Visual Field Loss?
Visual field is the medical word for the full area that we can see: our visual world. If an area of our visual world is blurred or missing with the rest clear then visual field loss is present. It is due to damage to some of the special vision parts of the brain. The relationship between brain damage and visual field loss is all opposite to what you might think.
The right side of the brain is responsible for seeing the left side of the visual world. The left side of the brain sees the right side of the visual world. If the right side of the brain is damaged the left side of the visual world may not be seen. In the same way the upper part of the back of the brain is responsible for seeing the lower part of the visual world. A child with damage in this area will not see the ground when looking straight ahead. The child may then tend to trip over things.
Are there other conditions associated with Cerebral Visual Impairment that can affect the Child?
Cerebral Visual Impairment is more common in children with Cerebral Palsy. Cerebral palsy is a condition of the brain that causes difficulty with movement and control of muscles.
There are no medicines or surgical treatments that will fix or improve brain damage. There are however lots of things that can be done to help children with CVI see better.
We use our vision to get around, learn new things and to meet other people and make friends. It is important to consider what your child's particular problems with vision might be now and in the future.
If your child has been prescribed spectacles, contact lenses or a Low Visual Aid (LVA) it is important that they are encouraged to wear and use them. This will help your child see more clearly and ensure the vision parts of the brain grow and develop.
Problems at school may be due to some of the reading books being hard to see. This often means it takes longer and more effort to do the work. If the size of print is increased and letters and words spaced more widely most children will find schoolwork easier. Good bright lighting and crisp black print on a clean white background will also make things easier. Sometimes placing reading books on a slope, which tilts the print towards the child, will improve reading speed as well. When reading it can be helpful to read one line at a time through a 'letter box' placed over the page. Placing a piece of blue tack below the line they are reading, at the beginning of the next sentence, can help some children find their way back to the start of the next line more quickly.
Some children may also benefit from using a computer program while reading. The program only shows one word of a sentence at a time. The word is in the middle of the computer screen. This reduces the need for fast eye movements. It can increase reading speed and reduce tiredness. One program is called Ace Reader. There are many others. A demonstration can be downloaded from www.acereader.com.
It is also worth watching carefully to find out what the smallest toys are that a child can see and play with. Then try to only play with toys that are the same size or bigger. Placing one toy on a plain background will often help children see it more readily. Placing lots of toys of different size and colour close together on a patterned background can make them 'invisible' to many children with CVI.
Recognising facial expressions can often be difficult. It is worth trying to find out at what distance facial expressions can be seen and responded to. Then always try to talk and smile from within this distance. This helps a child to learn what facial expressions mean and to copy them. There is a special part of the brain that helps children 'make sense' of faces. Sometimes this part is also damaged. These children may have difficulty responding to smiling even if their vision is clear enough. If the child has visual field loss try to place objects in the part of the child's vision that is working.
Cerebral Visual Impairment commonly occurs in children who have difficulty controlling both head and eye movements. Careful positioning of the head to prevent it falling to the side or falling forward can help a lot.
Infants and young children need to learn about the world around them. Home visiting teachers, physiotherapists and occupational and speech therapists may all add to the child's care and education. It is important to continue the programmes that they recommend. If the child is involved in family activities vision can improve and new skills can develop.
Even if a child has very poor vision many useful and practical things can be done to improve the ability of the child to get around, interact with other children and learn.
Who wrote these documents?
These pages are the consensus of opinion of many different people. They include parents of visually impaired children, visually impaired children themselves, Community Paediatricians, Ophthalmologists, Educationalists and Psychologists.
The main author and person responsible for their content is Dr Andrew Blaikie who was an Ophthalmology Research Fellow with Visual Impairment Scotland and is a member of the Royal College of Ophthalmologists.