Visually Impairing Conditionsby Dr Andrew Blaikie for VI Scotland
Medical Information on Myopia
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 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 Myopia
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 Myopia?
Myopia is also known as 'short-sight'. This means that a child who is 'short-sighted' can see better at 'short' distance than 'long'. Often children with myopia can see clearly when reading a book but often find the television or the blackboard at school blurred.
Mild short-sight is a common and normal finding
Mild short-sight is a common and normal finding in about one in every twenty children. Every year for every one hundred children another one child will develop mild short-sight. By adulthood about one in four adults are short-sighted.
Why can objects at short distance be seen clearly but not at long distance?
A short-sighted (or myopic) eye cannot focus the light from an object at long distance sharply onto the retina at the back of the eye. Instead the light focuses to a sharp point in front of the retina. The vision is then blurred. If the object is brought nearer the eye, the point at which the light focuses sharply will move backwards onto the retina. An object at a short distance then becomes clear: the eye is 'short-sighted'.
Big eyes tend to be short-sighted
The bigger and longer and eye is the more likely light from a distant object will focus short of the retina. The focussing power of the cornea and lens are also important in causing myopia.
What is the cause of Myopia?
There are many reasons why a child might develop Myopia. Some of these include:
- Their parents are short-sighted (myopic) and they 'inherit' Myopia
- The way they use their eyes may lead to Myopia
- They are born premature and with low birth weight
- They have an eye condition that is seen along with Myopia
- They may have a condition of growth that causes their eyes to grow bigger than normal
If a child's parents are short-sighted the child is more likely to also be short-sighted
If a child has one parent who is short-sighted there is a one in three chance the child will also be short-sighted. If both parents are short-sighted then there is a one in two chance the child will be short-sighted. The child can be said to have 'inherited' the myopia.
Children who use their eyes lots for close work are more likely to become Myopic
There is some evidence that children who use their eyes more for looking at near objects, than other children, are more likely to become myopic. Children who are already myopic may increase their level of myopia. The reason why this occurs is not well understood.
Premature babies are more likely to become Myopic
Babies that are born earlier than usual and of lower birth weight have a one in two chance of becoming myopic. If they also develop a condition called Retinopathy of Prematurity then they are likely to become very short-sighted.
Some eye conditions can cause Myopia
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 may cause the eye to grow bigger and longer than usual. This leads to myopia. This is known as 'Form Deprivation Myopia'.
Some conditions of growth may cause Myopia
Some conditions of growth may cause Myopia. These conditions include:
- Marfan's Syndrome
- Stickler's Syndrome
- Ehlers-Danlos Syndrome
These conditions all tend to affect the way bones and joints grow. They also cause eyes to grow bigger than normal. This leads to myopia.
Myopia can often be seen along with other eye conditions
The bigger and longer an eye is the more likely it is to develop other eye conditions. These include:
- Retinal Detachment
- Macular Degeneration
What is Retinal Detachment?
Children who are short sighted have a higher chance of developing a retinal detachment. This is because eyes that are short-sighted are bigger. Although the outer covering of the eye is bigger the retina inside is not. The retina has to 'stretch' to cover the whole of the back of the eye. Sometimes a tear or a hole can appear in the 'stretched' retina. If this occurs water from within the eye can leak down behind the retina. As the water seeps in it causes the retina to come away (detach) from the back of the eye. A Retinal Detachment will then be present. See the VI Scotland Document on this condition.
What is Glaucoma?
Glaucoma is when the optic nerve is damaged by high pressure in the eye. This can cause visual impairment. See the VI Scotland Document on Glaucoma.
What is Macular Degeneration?
The macula is the central bit of the retina. This bit is for seeing in fine detail and in colour. It is used for things like reading, watching TV and recognising faces. In very big eyes, which are very myopic, the macula is prone to becoming thin and wearing out. This causes blurred vision, but only in the central part of vision. This does not commonly occur in childhood but may happen in adulthood. It is sometimes also known as Fuch's Spot or Myopic Degeneration.
What is a Squint?
A squint is when a child's eyes do not appear to be both looking in the same direction. Sometimes in Myopia one eye can point outwards (diverge). This called a divergent squint.
How is Myopia Diagnosed?
Sometimes parents and teachers notice, by the way a child acts, that their vision might be impaired. Children may have difficulty seeing the blackboard at school and 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 myopia during an examination. The level of Myopia can be assessed by shining a light, through different lenses, into the eye of the child. The level of myopia is measured in focussing power units called 'dioptres'. Minus units are used to describe myopia. Mild myopia is between zero and minus five dioptres. Moderate myopia is between minus five and ten dioptres. Severe or 'high' myopia is greater than minus ten 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, such as corneal dystrophy or retinopathy of prematurity.
Other eye conditions such as retinal detachment and squint can be treated by operations and wearing spectacles. Glaucoma can usually be treated with drops or a small operation.
Laser surgery has been used successfully to treat myopia 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 myopia can change during childhood and the early twenties 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 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.