Eye Conditions

Medical Information on Norrie's Disease

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

Authors

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
Holyrood Road
Edinburgh EH8 8AQ

Telephone Number : 0131 651 6078
Fax Number : 0131 651 6502
Textphone : 0131 651 6067

Email : viscotland@ed.ac.uk
Website : www.viscotland.org.uk

Medical Information on Norrie's Disease

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.

What is Norrie’s Disease?

Very rarely when a baby is growing in the womb not all the parts of the eye grow in the right way. In Norries’s disease the back part of the eye stops growing while the unborn child is still in the womb. This happens about half way through pregnancy. As a result when the child is born the light sensitive film at the back of the eye (retina) is not ready to work and the vision is poor.

What is the cause of Norrie’s Disease?

Growth factors help the unborn child to grow in the womb

When an unborn child is growing in the womb special chemicals called growth factors are needed. Certain parts of the body need certain growth factors to grow. In Norrie’s Disease the growth factor that helps the back part of the eye to grow does not work.

Genes hold the plan for growth factors

Genes are a chemical alphabet held in all the parts of the body. The body has a "built-in" plan to make sure all the growth factors work. This plan is written in our genes. Genes are stored as long threads folded up into thick ropes called chromosomes. Every cell of the body has exactly the same number and kind of chromosomes. There are 23 different chromosomes but each chromosome has a copy of itself so that there are 46 in total (23 pairs). One pair of chromosomes are called the sex chromosomes and hold the plan for making a person grow into a boy or a girl. There are two types of sex chromosomes called X or Y. The letters just describe the shape of the chromosome when seen under a microscope. If a child has an X and a Y sex chromosome then the child will develop into a boy (XY). If the sex chromosomes are both X then the child will develop in the womb into a girl (XX). Children inherit a sex chromosome from each parent. Mothers (XX) always pass on X chromosomes.

Only boys develop Norrie’s Disease

The gene that has the wrong plan for the important growth factor in Norrie’s Disease is on the X chromosome. If a girl (XX) has one X chromosome that does not work she will be protected from developing Norrie’s Disease because she has a protective spare. This is because her ‘spare’ working X chromosome makes enough growth factor to make the back part of the eye grow in the womb. Boys (XY) however have only one X chromosome, with no protective spare. If boys have an X chromosome that does not work then they do not make any growth factor to help the back part of the eye to grow. They develop Norrie’s disease. Most children with Norrie’s disease have inherited the affected X chromosome from their mother.

Might another child from the same mother develop Norrie’s Disease?

There is a 1 in 2 chance that another son would develop Norrie’s Disease and a 1 in 2 chance that he would not be affected. All daughters will not be affected by Norrie’s Disease but there is a 1 in 2 chance that a daughter would carry an affected X chromosome like her mother. This daughter would have a 1 in 2 chance of having a son with Norrie’s Disease.

Screening for Norrie’s disease can be performed on the unborn child if the Mother is a known carrier of an affected X chromosome.

Norrie’s disease is very rare and is not routinely screened for in the unborn child. If however a family already has a son with the condition then a special test can be performed on the Mother and an unborn son to see if the child will also develop the condition.

How does Norrie’s Disease affect a child’s vision?

Young children with Norrie’s Disease will feel their vision to be ‘normal’ as they have never known anything else but their own visual world. They will assume that everyone else has vision the same as their own and will not realise that other people see things differently.

Norrie’s Disease can affect vision in different ways depending on the degree to which the eyes are affected. If only mildly affected then the overall vision may be good enough for the child to see large print and recognise faces. However in Norrie’s Disease the back part of the eye is often affected to a severe level. The vision can be very poor. The child may have reduced sharpness of vision and may only see bright lights and large shapes.

After a few years the lens in the eye can become hazy. This can reduce the vision further. Sometimes the pressure in the eye can also go up. If the pressure cannot be reduced with drops then an operation may need to be performed.

Sometimes children can also develop fast ‘to and fro’ movements of the eyes (nystagmus) or a squint.

How does Norrie’s Disease affect a child?

About half of children with Norrie’s disease have learning difficulties. After a few years about one third will also develop poor hearing. Many children also develop epilepsy.

How is Norrie’s Disease diagnosed?

Most children can be diagnosed by an examination from an ophthalmologist. A special test of the genes can be performed which can help confirm the diagnosis.

How can doctors help?

Special low visual aids may improve vision. Ophthalmologists can treat any conditions of the eyes which sometimes develop after a few years.

The epilepsy can be treated with tablets.

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. See the ADVICE Section on the VI Scotland website.

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.

Most children with Leber’s Amaurosis have few problems getting around. The way they act can give the impression that their vision is normal. It is important however to be aware of their own special problems with vision.

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 most children will find schoolwork easier.

It is 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.

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.

Even if a child has very poor vision many useful and practical things can be done to help. See the ADVICE Section on the VI Scotland website.