University of Edinburgh
 

Medical Information on Rod Monochromatism

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:

Due to staffing limitations we are not able to offer telephone or email advice to parents of children.

Medical Information on Rod Monochromatism

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 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':

Cones are good at 'seeing':

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 Rod Monochromatism?

Some children are born with cone photoreceptors that do not work correctly. The only photoreceptors that work are the rods. This usually leads to blurred vision, no colour vision and a dislike for bright light (photophobia). The medical word for colour is 'chromat'. Mono means one. The name tries to describe how a child with Rod Monochromatism might see and what with. It means seeing in only one colour using rod photoreceptors. Another name for this condition is Achromatopsia. If 'A' is placed at the beginning of a word it changes the word to mean 'without' or 'no'. Achromatopsia means to see 'without colour' or 'no colour'. This name also tries to describe how a child with Rod Monochromatism might see.

The condition does not get any worse as the child grows up. Because of this it is often described as 'stationary'. A medical word for a condition which a child is born with is 'dystrophy'. Another common name for Rod Monochromatism is 'Stationary Cone Dystrophy'. This should not be confused with 'Progressive' Cone Dystrophies where the condition may become worse as the child gets older. Progressive Cone Dystrophy is a different eye condition from Rod Monochromatism, Achromatopsia and Stationary Cone Dystrophy. But Rod Monochromatism, Achromatopsia and Stationary Cone Dystrophy all refer to the same eye condition. They mean the same thing in different words.

What is the cause of Rod Monochromatism?

We all have a "built-in" plan to make sure all the different 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 Rod Monochromatism has inherited a copy of the same gene from each parent with a misprint and the wrong plan for making cone photoreceptors. This means that when a child is born the cone photoreceptors do not work.

Most cases of Rod Monochromatism 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'.

This is only a brief summary of the genetics of Achromatopsia. There are exceptions to these general rules. This is why it is important that families with a child affected by Achromatopsia receive counselling from a specialist in genetics.

How is the diagnosis made?

Young children with Rod Monochromatism may develop:

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 Monochromatism 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 Monochromatism 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. Rod Monochromatism is more likely if the signals are weak or absent. This test is called an Electroretinogram (ERG).

How does Rod Monochromatism affect the way a child sees?

Rod Monochromatism can affect different children in different ways. Some children have an 'incomplete' or 'partial' kind of Rod Monochromatism. These children tend to have slightly better vision when they grow up.

All young children with any kind of Rod Monochromatism will however 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 might see things differently.

Cone Photoreceptors are found in the centre part of the retina. This bit 'works' the central part of the visual world. Cones are good at 'seeing':

Because children with Rod Monochromatism have Cone photoreceptors that do not work they will have problems with:

Most children with Rod Monochromatism 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. Children with reduced vision often develop fast to-and-fro movements of the eyes. This is called Nystagmus and usually gets better as a child grows up. If you wish more information on Nystagmus please ask us.

Long-sight (hypermetropia) is common in children with Rod Monochromatism. This means that their eyes find it easier to focus on things in the distance than for near. Spectacles may improve vision for near and far in children with Rod Monochromatism and Hypermetropia. If you would like the VI Scotland document on Hypermetropia, please let us know.

What can be done to help?

There is no good way to fix the Cone photoreceptors that are not working. But many things can be done to help children maximize their vision and fulfill their potential.

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

It is important, if children have been prescribed spectacles, that they are encouraged to wear them. Often a dark tint to the spectacle lens 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. Some children find ‘wrap-around’ glasses very helpful. When a child is older tinted contact lenses can be worn to help in the same way. Sun hats or baseball caps with a large sun visor can also help to reduce photophobia.

Most children with Rod Monochromatism have few problems getting around. The way they act can often give the impression that their vision is normal. It is important however to be aware of some of their own personal difficulties 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 Rod Monochromatism 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 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 a member of the Royal College of Ophthalmologists.

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