Medical Information on Third Nerve Palsyby Dr Andrew Blaikie for VI Scotland
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.
What this information is not for
This document is not a substitute for a consultation with a Health Professional. It should not 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
Medical Information on Third Nerve Palsy
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. The covering of photoreceptors at the back of the eye forms a thin film known as the retina. 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.
How are eye movements controlled?
Twelve pairs of nerves control all the different parts of the face and neck, including the eyes. The nerves are numbered and named from one to twelve. There are three nerves that control the movements of each eye. They happen to be the third, fourth and sixth nerves. These three nerves work together to tell the different muscles of the eye whether to pull or relax. The muscles then act together to move the eye in all directions of gaze to look at objects.
What is Third Nerve Palsy?
This describes a condition when the Third Nerve either does not work at all or is working poorly. This affects the movements of the eyes. Often the eye that is affected appears to be looking down and out. This is because the affected eye cannot look inwards or upward. The eyelid on the same side is also usually droopy. This is because the Third Nerve also works the eyelid on the same side.
What are the causes of Third Nerve Palsy?
There are many different causes of Third nerve palsy. Sometimes a child is born with this condition. This is known as Congenital Third nerve palsy. More often a child may develop the condition after birth. This is called Acquired Third nerve palsy.
There are three main causes of Acquired Third nerve palsy in childhood. These include:
Head injury is the most common cause of Acquired Third Nerve Palsy in childhood
Sometimes after a bad bang to the head children can develop a Third nerve palsy. This is most common after falling from a height or being involved in a road traffic accident. The thin nerve may become bruised or cut. Usually the nerve grows back over several months.
Meningitis sometimes causes Third Nerve Palsy
Sometimes the coverings of a child's brain can become infected. The coverings are called the meninges. If this becomes inflammed and infected the condition is called meningitis. This can cause damage to the Third Nerve and lead to a palsy. This usually improves over a few months after the infection is treated.
Brain Tumours may cause Third Nerve Palsy
A tumour is a condition where a part of the body grows more than it would normally. If a tumour of the brain grows it may press on the Third Nerve. This may cause the nerve to stop working.
After being damaged the Third Nerve may grow back
After being damaged the nerve may grow back. Often it grows back incompletely and does not fully recover. A mild Third nerve palsy may then persist. The nerve may also grow back incorrectly. This leads to unusual signs when the eye moves. This sometimes means that when the affected eye looks inwards the eyelid raises and the pupil becomes smaller.
How is the diagnosis made?
The diagnosis can be confirmed during an examination by an ophthalmologist or orthoptist. The typical direction an affected eye looks and the particular pattern of eye movement restriction helps to make a diagnosis.
What kind of visual difficulties is a child with Third Nerve Palsy likely to have and what can be done to help?
The eyes of a child with a third nerve palsy do not point in the same direction. This is called a squint. You might expect this to cause double vision. But it rarely does. The brain quickly learns to ignore the information coming from the eye that is squinting. This means however that the child is seeing with only one eye. One of the main benefits of seeing with two eyes is to be able to judge depth well. Children with a third nerve palsy can have difficulty judging depth. This may lead to problems with:
- Pouring juice into a cup
- Catching or hitting fast moving balls
- Judging how quickly a car is approaching while crossing roads
- Walking up or down stairs
At first a bit more care must be taken when doing these things. Most children however quickly learn new ways to judge depth and do not end up having too many difficulties.
What other problems is a child with Third Nerve Palsy likely to have?
An eye that is squinting may become lazy. This is also known as Amblyopia. It is not actually the eye that has become lazy; it is the special vision parts of the brain. The brain can only learn to see from an eye if it is pointing in the same direction as the other one. If the eyes of a child point in different directions it is very confusing for the brain. The brain cannot then match the images given to it. To make things simpler and understandable it quickly learns to ignore one image (usually from the eye squinting the most). This eye then becomes lazy or amblyopic.
There are several ways to avoid an eye becoming lazy. Hopefully by treating the cause of the third nerve palsy the eyes will become 'straight' and point in the same direction. This will avoid amblyopia developing. If after treating the cause of the palsy the eyes still squint an operation may help. The aim of an operation is to move some of the muscles of the eye around. This will strengthen some muscles and weaken others. This can help straighten the eyes. Covering the other eye that is squinting least with a patch will force the affected eye to 'see'. Combining these treatments will help avoid an eye becoming lazy or amblyopic.
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.