University of Edinburgh
 

Medical Information on Rod-Cone Dystrophy

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 Rod-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 or twilight. 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 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 over a period of time. Dystrophy is a word for a condition 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.

Are there any other conditions associated with Rod-Cone Dystrophies?

Some children with Rod-Cone Dystrophies only have visual impairment. Many other children may have other conditions such as:

  • Poor Hearing
  • Learning Difficulties
  • Reduced Growth

The other associated conditions depend on what the cause of the Rod-Cone Dystrophy is.

Is there any treatment for Rod-Cone Dystrophy?

There is no good way to stop the sight loss in Rod-Cone Dystrophy. But many things can be done to help children with the condition.

What are the causes of Rod-Cone Dystrophies?

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

There are many reasons why children may get a Rod-Cone Dystrophy

Most Rod-Cone Dystrophies are caused by misprints in a 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 misprint 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 Rod-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. Please ask us for an information document on 'Ways children can inherit conditions'.

What are the names for some of the more common rod-cone dystrophies?

Leber's Amaurosis, Retinitis Pigmentosa (and Usher's Syndrome) and Batten's Disease are the names of some of the more common conditions where rod and cone photoreceptors do not work properly.

How is the diagnosis made?

Young children with Rod-Cone Dystrophy 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 Rod-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 Rod-Cone Dystrophy 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 Rod-Cone Dystrophy is more likely. This test is called an Electroretinogram (ERG).

Some Rod-Cone Dystrophies 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 Rod-Cone Dystrophies affect the way a child sees?

Rod-Cone Dystrophies can affect different children in different ways. At first most young children will feel their vision to be 'normal'. They will 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 may 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 twilight and 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. Many children with Rod-Cone photoreceptors can have very poor sight and unfortunately are able to see very little.

It is important to appreciate that other people will not realise, just by looking at your child that they may not see well. This is particularly important in twilight and the dark. Children with rod photoreceptors that do not work so well will have even poorer vision at these times of day. Crossing roads when going to and from school can be especially dangerous, as motorists will not appreciate that your child cannot see clearly.

What can be done to help?

There are no medicines or surgical treatments that will fix or improve photoreceptors. There are however lots of things that can be done to help children with Rod Cone Dystrophy make the most of their vision.

We use our vision to get around, learn new things and to meet other people and make friends. It is important to consider what your child's particular problems with vision might be now and in the future. If your child has been prescribed spectacles, contact lenses or a Low Visual Aid (LVA) it is important that they are encouraged to wear and use them. This will help your child see more clearly and ensure the vision parts of the brain grow and develop.

Bright light often causes discomfort and reduced vision (photophobia). A sunhat and sunglasses may help reduce symptoms of photophobia. Special lenses that get darker in brighter light (photochromic lenses) are also known to be useful.

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 and letters and words spaced more widely most children will find schoolwork easier. Sometimes placing reading books on a slope, which tilts the print towards the child, will improve reading speed as well. When reading it can be helpful to read one line at a time through a 'letter box' placed over the page. Placing a piece of blue tack below the line they are reading, at the beginning of the next sentence, can help some children find their way back to the start of the next line more quickly.

It is also 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. Placing one toy on a plain background will often help children see it more readily. Placing lots of toys of different size and colour close together on a patterned background can sometimes make them even more difficult to see.

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.

Infants and young children need to learn about the world around them. Home visiting teachers, physiotherapists and occupational and speech therapists may all add to the child's care and education. It is important to continue the programmes that they recommend. If the child is involved in family activities vision can improve and new skills can develop.

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.

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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