University of Edinburgh
 

Medical Information on Leber's Congenital Amaurosis

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 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 Leber's Congenital Amaurosis

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. There are two types of photoreceptors named by their shape when looked at in fine detail. They are called 'rods' and 'cones'.

Rod and Cone Photoreceptors are good at seeing different things

Rods are good at 'seeing':

  • things that move
  • in the dark
  • but only in black and white
  • and in less detail.

Cones are good at 'seeing':

  • things that are still
  • in daylight
  • in colour
  • and in fine detail.

The covering of rod and cone photoreceptors at the back of the eye makes a thin film called the retina. The central bit of the retina is made up of cones. They help us see the central bit of vision that we use for reading, looking at photographs and recognising faces. The area of the retina around the central bit is made up of rods. The rods see the surrounding bits of vision and help us to walk around and not bump into things especially in the dark. 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 Leber's Amaurosis?

When a baby is growing in the womb sometimes the rod and cone photoreceptors in the retina do not grow properly. This is called a rod-cone dystrophy. Rod-Cone Dystrophy is the name given to a wide range of eye conditions. These eye conditions are all linked by a problem with the rod and cone photoreceptors. The photoreceptors either do not work from the day a child is born or else slowly stop working later on over a period of time. Dystrophy is a word for a condition that a child is born with. Some of these conditions do not only affect the eye but may also affect the rest of a child's body. Leber's Amaurosis is a particular type of rod-cone dystrophy in which both the rods and cones are both quite badly affected. Leber was the man who first described the condition. Amaurosis is another word for poor vision.

What is the cause of Leber's Amaurosis?

The body has a "built-in" plan to make sure all the parts of the body work correctly. This plan is written in our genes. Genes are a chemical alphabet stored in the body. Every person has two copies of each gene. If both copies of a gene have a misprint in the chemical alphabet then a small part of the body may not work correctly. A child with Leber's Amaurosis has inherited a copy of the same gene from each parent with a misprint and the wrong plan for making photoreceptors. This leads to photoreceptors breaking down and stopping working. This is called 'recessive inheritance'. Nearly all cases of Leber's Amaurosis are due to recessive inheritance.

Most cases of Leber's Amaurosis occur by chance (that is both parents having the same gene that is not working and the child getting two copies of each bad gene) however occasionally the condition can occur as a result of the parents of a child being cousins or even a more distant relative (where it is much more likely that the parents share the gene that is not working).

At the moment six different genes have been shown to cause Leber's Amaurosis. These six genes are known to cause almost half of all cases of children with Leber's Amaurosis. Testing for these genes is not widely available. In the future, when these tests become more common, they could help parents know if new children in their family are going to have the same eye condition. Knowing which gene is causing the sight loss may also help to say how severe the sight loss might end up being.

n the far distant future replacing the affected gene in a child's eye might help improve their vision.

How is the diagnosis made?

Young children with Leber's Amaurosis may develop:

  • Fast to-and-fro movements of the eyes called Nystagmus.
  • 'Roving' eye movements where the eyes appear to slowly wander around not fixing and staying still on any objects.
  • 'Eye Poking' where the child touches their eyes with their fingers.

Parents will often notice these signs. Sometimes parents also notice (by the way their child acts) that their child's vision is reduced. If they discuss their concerns with their Family Doctor an assessment can be arranged.

An eye doctor can check the way the eyes behave to bright lights. If the pupils of a child move slowly to a bright light then Leber's Amaurosis is more likely. Using a special instrument the eye doctor can look at the optic nerve and retina at the back of the eye. In children with Leber's Amaurosis sometimes these parts of the eye look different from normal.

There are also special tests that can be done to help the eye doctor decide what is wrong. These tests measure signals from the eyes when a child is shown a bright light. Sticky patches are placed around the eyes. The sticky patches are attached to wires that lead to a machine. The machine records the electrical signals made by the eyes. The record of the signals will help the doctors decide what the matter is. If the signals are weak or absent then Leber's Amaurosis is more likely. This test is called an Electroretinogram (ERG).

How does Leber's amaurosis affect the way a child sees?

Different children are affected in different ways by Leber's Amaurosis. Some children with Leber's Amaurosis can see the top letter on a eyesight chart from 3 meters (3/60) while others may only be able to see larger objects such as a person moving across a room. Many children with Leber's Amaurosis have very poor sight and unfortunately are able to see very little. In a small number of children vision can get a bit poorer as they get older.

Are there any other conditions associated with Leber's Amaurosis?

Some children with a condition like Leber's Amaurosis can also have:

  • Hearing loss
  • Learning difficulties
  • Kidney problems

It is not clear if these children really do have Leber's Amaurosis.

Is there any treatment for Leber's Amaurosis?

At the moment there is no way to stop the sight loss in Leber's Amaurosis. But many things can be done to help children with the condition.

What can be done to help?

We use our vision to get around, learn new things and to meet other people and make friends.

It is important however to be aware of your child's special problems with vision.

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. It will be very useful to contact your local special education department. Specialist teachers who are involved with children with sight loss can offer a lot of useful advice.

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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