Visually Impairing Conditions
by Dr Andrew Blaikie for VI Scotland
Medical information on Astigmatism
For whom is this information intended?
The information contained in this document is intended for use primarily by parents, other members of the family and older children with visual impairment. The information will also be of use to interested health professionals, carers and teachers.
The purpose of each information document
The purpose of the information is to explain:
- The way the eyes and brain normally work to make 'vision'
- The reason why vision may become impaired by a specific condition
- The cause of the condition
- The effects of the condition on the child's vision
- How the condition is diagnosed
- What can be done to help
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. Inevitably there will be some mistakes. We apologise for this.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
After reading the information we hope you will:
- 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
These pages represent the consensus of opinion of many different people who 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 is an Ophthalmology Research Fellow with Visual Impairment Scotland and a member of the Royal College of Ophthalmologists. We are very keen that parents and children feedback to VI Scotland about these information documents. If you have any questions or comments regarding the information please contact:
Visual Impairment Scotland
Scottish Sensory Centre
The University of Edinburgh
Edinburgh EH8 8AQ
Telephone Number : 0131 651 6078
Fax Number : 0131 651 6502
Textphone : 0131 651 6067
Medical information on Astigmatism
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 the parts of the brain and eye need to be present and working for us to see normally.
What is Astigmatism?
The cornea at the front of the eye is shaped almost as smooth and round and as evenly as a football. This allows light from an object to focus sharply as a single point on to the retina. This makes clear vision. Astigmatism is when the curve of the cornea at the front of the eye is more like a football with someone sitting on it. This means that in one direction the curve is steeper than the other. This means that when light shines into an eye with astigmatism it does not focus to a single sharp point. Instead the light is smeared causing blurred vision. The greater the difference in steepness of the curves of the cornea, the greater the astigmatism and the more blurred vision will be.
Mild Astigmatism is a common and normal finding
Mild astigmatism is a common and normal finding in many young children. As they grow up the cornea changes shape and most children lose the astigmatism. A few children will still however have astigmatism into adulthood.
What Is The Cause Of Astigmatism?
There are a few reasons why a child might develop Astigmatism. Some of these include:
- Their parents have astigmatism and they 'inherit' it
- They have an eye condition that is seen along with astigmatism
- An eye operation may lead to astigmatism
- An injury to the cornea may lead to astigmatism
Most children have astigmatism by chance. They do not have a parent with it or any other condition of their eye that may be causing astigmatism.
Some Eye Conditions Are Seen Along With Astigmatism
For normal growth of the eye light needs to enter the eye without being blocked. If a child has a hazy cornea (corneal dystrophy) or lens (cataract) not all the light can enter the eye and the vision will be blurred. This disturbs the normal growth of the eye. This may lead to astigmatism.
Astigmatism is more commonly seen in eyes that also have optic nerve hypoplasia, retinitis pigmentosa, albinism or nystagmus. These conditions themselves, also often cause blurred vision.
There is an eye condition where the cornea changes shape during childhood. This may cause high levels of astigmatism and blurred vision. This condition is called keratoconus. See the VI Scotland document on this condition.
An upper eyelid that is positioned slightly lower than normal (ptosis) can sometimes cause astigmatism. The weight of the lid can press on the eye and change the shape of the cornea leading to blurred vision.
See the VI Scotland documents on these conditions.
Astigmatism Can Lead To Amblyopia
If light from an object cannot be focussed sharply on to the back of the eye then the child may develop a 'lazy' eye. This is also known as Amblyopia. It is not actually the eye that has become lazy; it is the special vision parts of the brain. The brain can only learn to see as clearly as the picture given to it by the eyes. If the brain has not been given a sharp, clear picture by the eye because of astigmatism then it cannot learn to see clearly. If spectacles are worn to help focus the light then amblyopia may be prevented. Sometimes however even with correct spectacles vision may still be blurred. This is because the brain has not developed the power to see clearly. This is called Amblyopia. Amblyopia is more common in a child who also has a squint.
How is Astigmatism Diagnosed?
Sometimes parents and teachers may notice, by the way a child acts, that their vision is impaired. Children may have difficulty seeing the blackboard at school or hold books very close to read. They may narrow their eyelids and half close their eyes when looking at things in the distance. If parents discuss this with their Family Doctor an assessment can be arranged.
An eye doctor or optometrist can diagnose astigmatism during an examination. The level of astigmatism can be assessed by shining a light, through different lenses, into the eye of the child. The level of astigmatism is measured in focussing power units called 'dioptres'.
What can be done to help?
Spectacles or contact lenses can usually sharpen vision. Sometimes the vision will not be perfect. This can be because of other problems with the eye.
Laser surgery has been used successfully to treat myopia (short sight) and astigmatism in adults. It involves altering the shape of the cornea to focus light from objects on to the back of the eye. Because the level of astigmatism can change during childhood and often gets better it is never performed on children.
How can parents, family, friends and teachers make a difference?
We use our vision to get around, learn new things and to meet other people and make friends.
It is important that children are encouraged to wear their spectacles, contact lenses or Low Visual Aids (LVAs). This will help the child see more clearly. It will also help the vision parts of the brain to grow and develop correctly.
Even if a child has very poor vision many useful and practical things can be done to help.