Medical information on Astrocytoma of the Optic Chiasm
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:
- 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 optic nerves join about half way between the eye and brain, and then split up again. The join is called the optic chiasm. At the join, signals from the 'nose' side of each eye's visual world swap sides and continue travelling along the opposite side from where they started. The two optic nerves then join on to the brain. The brain is split into two halves, right and left. This means all the signals from the visual world on the child's right hand side are now travelling in the left side of the brain. It also means that all the signals from the visual world on the child's left hand side are now travelling in the right half of the brain.
The information then travels to the many different special 'vision' parts of the brain. The main bit of the brain that works vision is at the back of the head. It is called the occipital lobe. The joined up path that signals travel down from retina to optic nerve then optic chiasm then occipital lobe is called the visual pathway. There are two visual pathways, one on the right side of the brain and another on the left. All parts of both visual pathways need to be present and working for us to see normally.
An astrocytoma is a benign tumour of nerve tissue. A tumour is a condition where a part of the body grows more than it would normally. Benign means that bits of the tumour will not normally break off and spread to other parts of the body. It may, with time however grow and get bigger. The optic chiasm is made of nerve tissue. If such a tumour affects the optic chiasm then it is called an astrocytoma of the optic chiasm. More commonly it is also called an optic chiasm glioma. Glioma and astrocytoma are different names for the same thing.
An astrocytoma of optic chiasm may cause:
- Visual Field Loss
- Blurred Vision
Visual Field Loss
The visual field is the medical word for the full area of a child's visual world (what they see). Visual field loss may occur if damage occurs to a part of the visual pathway. The visual pathway that signals travel along includes the eye, optic nerve, optic chiasm and the special 'vision' parts of the brain. Damage to the visual pathway may cause blurring or even complete loss of bits of the visual world (visual field).
If damage occurs to the optic chiasm then a special type of visual field loss may occur. The child will lose the outside bits of the visual world on both sides. It is called a Bi-temporal Hemianopia. 'Bi' means both right and left sides. 'Temporal' means the outside bit of vision. 'Hemi' means half and 'anopia' means no vision.
It usually takes younger children longer to notice visual field loss. Older children may notice that something is wrong with their vision sooner. The visual field loss does not usually get better and may slowly get worse.
If the visual loss spreads into the central part of vision then the child may complain of blurred vision. Usually both eyes will become affected. Children may often only complain of blurred vision when the vision in their only remaining good eye becomes blurred. Often there can also be reduced colour vision.
Our eyes move quickly when we watch something that is moving quickly. Our eyes are still when we look at something that is still. This is so the eyes can focus sharply on an object. The eye can then give clear signals to the brain to make clear vision. In some children the eyes are always moving to-and-fro even though the object that they are looking at is still. This to-and-fro movement is called Nystagmus. Children do not know that they are doing it. Because the eye is shaking the vision is usually a bit blurred. This is like if a camera moves during a photograph. The photograph is also usually blurred. Surprisingly children do not complain of the visual world moving. The to-and-fro movements can be up and down, side to side or even in a circle. The movements of the eyes can be large or small and fast or slow.
What other conditions might an astrocytoma of the optic chiasm cause?
Astrocytomas of the optic chiasm are benign. This means they do not normally spread and grow to other parts of the body. They can however slowly grow and become bigger. The astrocytoma can then press and squeeze parts of the brain near the chiasm. This can lead to other conditions also developing. These may include:
- Problems With Growth And Development
- Head Bobbing
Hydrocephalus is a condition where the water pressure within the head rises. The water in the brain is called cerebrospinal fluid (CSF). The CSF flows in and around the brain through pipes and reservoirs called aqueducts and ventricles. If an aqueduct becomes blocked the CSF can no longer drain. When this happens the pressure in the head rises. A growing Astrocytoma may press and squeeze the aqueduct until it is so narrow that no more CSF can flow. The aqueduct is then blocked and the water pressure in the head may rise.
High pressure in the head can often cause headaches. A child might also feel sick and tired. Hydrocephalus may also cause blurred vision, squint and nystagmus. Please ask us if you wish further information on these conditions.
During a small operation a new drain can be placed that overcomes the blockage. This can help reduce the pressure and improve some of the symptoms.
Problems with growth and development
There is a small, but very important part of the brain that lies very close to the chiasm. This small part of the brain controls the growth and development of a child. It is called the pituitary gland. Sometimes an Astrocytoma of the optic chiasm can press on this gland and cause growth of the child to slow down or speed up. This may be treated with tablets.
Very rarely a child might develop a nodding movement of the head. The movement is often up and down like nodding but may be in any direction. This can sometimes be a sign of conditions of the chiasm or raised pressure in the head (Hydrocephalus).
Children with neurofibromatosis are more likely to develop an astrocytoma of the optic chiasm
There is a rare condition in which children are more likely to develop an Astrocytoma of the Optic Chiasm. The condition is called Neurofibromatosis. Many children with the condition have a parent or brother and sister with this condition as well. Quite often however a child may be the first in the family to be affected. As many as half of all children with an Astrocytoma of the Optic Chiasm will also have Neurofibromatosis. In this condition children usually develop small growths of nerve tissue and skin.
Children may notice that their vision is blurred or a bit is missing (visual field loss). Parents may notice that their child's eyes are moving to-and-fro or that they have a head bobble. Parents may also suspect, by the way their child acts, that their child's vision is impaired. If they discuss their concerns with their Family Doctor an assessment can be arranged.
The School nurse may notice the blurred vision during a vision screening examination. They can organise an examination at the eye clinic. An eye doctor at the clinic can also check for visual field loss. By also asking questions of the parents and teachers of the child the doctor can get a good idea about where and what kind of visual field loss the child may have.
A head scan may help confirm that an astrocytoma of optic chiasm is present.
In some older children and in many adults it is possible to measure the visual field with a machine. This is called perimetry. Most children cannot do this test and it is rarely recommended.
Many children with astrocytoma of optic chiasm do not need any treatment. This is because most astrocytomas grow so slowly that they do not cause any further problems. Regular checks at the clinic and scans can help to see if the astrocytoma is getting any bigger.
If the astrocytoma is getting bigger and causing new problems then treatment may help. There are three kinds of treatment. Any decision on treatment is discussed fully with the child and their parents. Most treatments can have side effects. The benefit to the child must always be weighed against any possible damaging side effects of the treatment.
Often X-ray treatment is recommended. This is called Radiotherapy. X-rays are normally used to take pictures of patient's lungs and broken bones. When concentrated and focused on to an astrocytoma they can help slow down its growth.
Sometimes drugs injected into a child's veins are also recommended. This is called chemotherapy. This can also help slow down growth of an astrocytoma.
Very rarely a surgeon might try to remove some of the astrocytoma by an operation. The chiasm is very difficult to operate on. Other parts of the brain might be damaged during an operation. Sometimes vision can be worse after the operation than before. An operation is not recommended very often.
What can parents, friends and teachers do to help?
There are however lots of things that can be done to help children with visual impairment make the most of their vision.
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.
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 visual impairment. Toys attached to a board covered in 'Velcro' are a good idea. The child can feel the toy and move it around making a loud 'cracking' noise. The toy will also not run away and fall off the board so easily. Toys that are in two parts, hinged by elastic, and then can come apart when pulled will keep the interesting of a child with poor vision for longer.
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.
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.