University of Edinburgh
 

Medical information on Bardet-Biedl Syndrome

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 Bardet-Biedl Syndrome

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 Bardet-Biedl Syndrome?

Children with this condition develop visual impairment because their rod and cone photoreceptors stop working. This is often called retinitis pigmentosa. 'Retinitis' means disease or inflammation of the retina. 'Pigmentosa' refers to how the retina can look in this condition (the retina can often develop lots of small, dark specks of pigment). Children with Bardet-Biedl syndrome almost always also have other conditions. These may include:

  • Weight gain
  • Learning difficulties
  • High blood pressure and kidney disease
  • Extra fingers (polydactyly)
  • Delayed or absent puberty

What is Retinitis Pigmentosa?

Retinitis Pigmentosa 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 over a period of time. Some of these conditions do not only affect the eye but may also affect the rest of a child's body.

What is the cause of Bardet-Biedl Syndrome and Retinitis Pigmentosa?

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 part of the body may not work correctly. A child with Bardet-Biedl Syndrome has both copies of an important gene misprinted. This leads to photoreceptors breaking down and stopping working. It also leads to the other conditions seen in Bardet-Biedl Syndrome. . There are now 8 different genes know to cause Bardet-Biedl Syndrome. The different genes cause slightly different types of Bardet-Biedl Syndrome. In s ome types vision can be more affected than in others.

Most cases of Bardet-Biedl Syndrome occur by chance however occasionally the condition can occur as a result of the parents of a child being cousins or even a more distant relative. This is called 'recessive inheritance'. Some types of Bardet-Biedl Syndrome are inherited in more complex ways.

This is only a brief summary of the genetics of Bardet-Biedl Syndrome. There are exceptions to these general rules.

This is why it is important that families with a child affected by Bardet-Biedl Syndrome receive counselling from a specialist in genetics.

How Is Bardet-Biedl Syndrome Diagnosed?

Young children with Bardet-Biedl Syndromemay develop fast to-and-fro movements of the eyes called nystagmus. Less commonly children may have ‘Roving’ eye movements where the eyes appear to slowly wander around not fixing and staying still on any objects. Very rarely children may poke 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. They may notice that their child is not able to play normally in dim light. Sometimes they may notice that their child cannot see small toys but can see larger ones. Some parents notice that their child does see things well at the edges of their vision.  If parents 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 Retinitis Pigmentosa 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 Retinitis Pigmentosa these parts of the eye often 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 Rod-Cone Dystrophy is more likely. This test is called an Electroretinogram (ERG).

All these tests suggest that the child might have retinitis pigmentosa. If this eye condition is seen in a child with:

  • Weight gain
  • Learning difficulties
  • High blood pressure and kidney disease
  • Extra fingers (polydactyly)
  • Delayed or absent puberty

then a diagnosis of Bardet-Biedl Syndrome is very likely.

How Does Retinitis Pigmentosa affect the way a child sees?

Retinitis Pigmentosa can affect different children in different ways. Most young children who are born with rod and cone photoreceptors that do not work feel their vision to be 'normal'. At first they assume that everyone else has vision the same as their own as they have never known anything else but their own visual world. They do not realise that other people see things differently.

Some older children may notice that their vision is blurred around the edges and that their vision is especially poor in the dark. These children might mainly have a problem with their rod photoreceptors. Some other children will also notice that their central vision is blurred and that most colours are not bright. These children will have a problem with their cone photoreceptors as well. Some other children may only see bright lights or the movement of large objects. A few children with Retinitis Pigmentosa can have very poor sight and unfortunately are able to see very little.

Children with Bardet-Biedl syndrome tend, at first to have good vision. Often later in childhood blurred vision develops. Often it is the cone photoreceptors that are more affected than rod photoreceptors. When they reach adulthood the visual impairment does not usually become any poorer.

Is there any treatment for Retinitis Pigmentosa?

There is no good way to stop the sight loss. 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.

If spectacles, contact lenses or a Low Visual Aid (LVA) has been prescribed, it is important that encouragement is given to wear and use them. This will help your child see more clearly and ensure the vision parts of the brain grow and develop correctly.

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.

Many children with Retinitis Pigmentosa need early and specialist care from the local visual impairment support team. 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.

vis logo