Eye Conditions

Medical Information on Cone Dystrophy

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.

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

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 Cone Dystrophy

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 Cone Dystrophy?

Cone Dystrophy is the name given to a wide range of eye conditions. These eye conditions are all linked by a problem with the cone photoreceptors. The photoreceptors either do not work from the day a child is born (stationary) or else slowly stop working during childhood (progressive). This usually leads to blurred vision, poor colour vision and a dislike for bright light (photophobia). Dystrophy is a word for a disease, which 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.

What are the causes of Cone Dystrophies?

There are many different causes of Cone Dystrophies. Sometimes nobody can say for sure why a child has a Cone Dystrophy. When no cause can be identified this is called Idiopathic.

There are many reasons why children may develop Cone Dystrophy

Most Cone Dystrophies are caused by a mistake in the child’s genes. Genes are a chemical alphabet stored in the body. Genes contain the body’s "built-in" plan to make sure all the parts of the body work correctly. If a gene has a mistake in the chemical alphabet then a part of the body may not work correctly. Sometimes many parts of the body do not work correctly. A child with a Cone Dystrophy has often been passed (inherited) a gene with a mistake in it from one or both parents. Sometimes by chance a new mistake occurs in the child’s genes and the parent’s genes are normal. There are many different ways a child can ‘inherit’ a condition. See the VI Scotland Information Document on ‘Ways Children Can Inherit Eye Conditions’.

There are two main types of Cone Dystrophy

In a child with Cone Dystrophy the photoreceptors either do not work from the day a child is born (stationary) or else slowly stop working during childhood (progressive). In Stationary Cone Dystrophies the condition tends to remain the same for all of a child’s life. In Progressive Cone Dystrophies the condition may become worse as the child gets older.

How is the diagnosis made?

Young children with Cone Dystrophy may develop:

• Fast ‘to and fro’ movements of the eyes called Nystagmus. See the VI Scotland Information Document on Nystagmus
  
• ‘Roving’ eye movements where the eyes appear to slowly wander around not fixing and staying still on any objects.
  
• Blurred vision, that is worse in bright light and better in dim light.
  
• Children may dislike bright light and try to avoid daylight and well-lit rooms. This is known as photophobia.

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 Cone Dystrophy 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 Cone Dystrophy sometimes these parts of the eye look different from normal. Often the central bit of the retina (the macula) can appear to have circular bands of different shades of pink and orange. Eye doctors often describe this appearance as ‘Bull’s Eye Maculopathy’.

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. A Cone Dystrophy is more likely if the signals are weak or absent. This test is called an Electroretinogram (ERG).

Sometimes it is not only vision that is affected

Some dystrophies of photoreceptors may present initially with symptoms that have nothing to do with vision. Conditions such as Batten’s Disease may present with increasing difficulty handling objects, increasing clumsiness moving around, gradual change in mood and personality, decreased attention span, slurred speech and poor memory. It may be only later that a problem with vision is noticed.

How do Cone Dystrophies affect the way a child sees?

Cone Dystrophies can affect different children in different ways. Most young children who are born with a stationary cone dystrophy 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.

Children with progressive cone dystrophies will however slowly notice problems with their vision and eyes. These include:

• Blurred vision, especially in the central bit of vision
• Dislike of bright light (photophobia)
• Poor colour vision
• Fast ‘to and fro’ movements of the eyes (nystagmus)

Most children with cone dystrophy can still see well in the outside bit of vision, away from the centre. They usually have no problem getting around although reading, recognising faces and seeing small toys can be difficult.

Is there any treatment for Cone Dystrophy?

There is no good way to stop the sight loss in Cone Dystrophy. 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 that children are encouraged to wear their spectacles. This will help the child see more clearly and reduce glare and photophobia. It will also help the vision parts of the brain to grow and develop correctly.

Most children with Cone Dystrophy 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.

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 Cone Dystrophy 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. See the ‘Advice’ section on the VI Scotland website.