University of Edinburgh
 

Medical Information on Glasses

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.

Medical Information on Glasses

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.

Why do some children need glasses?

Many children need to where glasses to help them see clearly. They may not see clearly because of a 'refractive error'. Short-sight (myopia), Long-sight (hypermetropia), and Astigmatism are all kinds of refractive errors.

What is refraction?

Refraction is the word used to describe the way light bends and changes direction when it passes through a lens. If the cornea and lens of the eye does not bend and change the direction of light, so as it focuses sharply onto the back of the eye (retina) then a 'refractive error' is said to be present.

Myopia

What Is Myopia?

Myopia is also known as Short-sight. A child with short-sight can see better at 'short' distance than 'long'. Often children with short-sight can see clearly when reading a book but often find the television or the blackboard at school blurred.

Mild myopia (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 myopia. By adulthood about one in four adults are myopic.

A 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'.

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 are the causes of Myopia?

There are many reasons why a child might develop Myopia. Some of these include:

  • Their parents are 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

Myopia can cause other eye conditions

The bigger and longer an eye is the more likely it is to develop other eye conditions. These include

Hypermetropia

What is Hypermetropia?

Hypermetropia is also known as 'long-sight'. This means that a child who is 'long-sighted' is more likely to see better at 'long' distance than 'short'. Often children with Hypermetropia can see clearly when watching the television or the blackboard but may find reading a book difficult. They may have to focus very hard to see near things clearly. This may cause eye strain and headache.

Mild hypermetropia is a common and normal finding in most young children. Many children 'grow out' of hypermetropia by adulthood.

For clear and comfortable vision light must be focussed sharply onto the retina at the back of the eye. Most hypermetropic eyes can focus the light from a far object. Many hypermetropic (or long-sighted) eyes have difficulty focussing the light from an object at short distance. Instead the light focuses to a sharp point in behind the retina. The vision is then blurred. If the object is taken further away from the eye, the point at which the light focuses sharply will move forwards onto the retina. An object at a longer distance then becomes clear: the eye is 'long-sighted'.

The smaller and shorter an eye is, the more likely light from a distant object will focus beyond the retina. The focussing power of the cornea and lens are also important in causing hypermetropia.

What are the causes of Hypermetropia?

Some children may have hypermetropia because their parents are Hypermetropic. The child then 'inherits' hypermetropia. Some children may have an eye condition that may cause Hypermetropia. 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 hypermetropia. Other conditions such as Retinitis Pigmentosa and Microphthalmia can also be associated with hypermetropia. See the VI Scotland information sheets on these conditions.

Most children are however long sighted by chance. They do not have a parent with long sight or any other condition of their eye.

Hypermetropia can cause other eye conditions

The smaller and shorter an eye is the more likely it is to develop other eye conditions. These include:

Astigmatism

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 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:

Refractive errors can all 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 a refractive error 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 are refractive errors 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. Parents may notice that their child's eyes also occasionally squint. If parents discuss their concerns with their Family Doctor an assessment can be arranged. The family doctor may initially suggest an appointment with an optometrist.

An optometrist can diagnose refractive errors during an examination. The level of refractive error can be assessed by shining a light, through different lenses, into the eye of the child. 'Dioptres' are the focussing power units used to measure refractive errors. Minus units are used to describe myopia. Plus units are used to describe hypermetropia. 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. Mild hypermetropia is between zero and plus three dioptres. Moderate hypermetropia is between plus three and ten dioptres. Severe or 'high' hypermetropia 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 some refractive errors 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 refractive error 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.

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.

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