Myasthenia gravis

Written by Dr Camilla Buckley, MRCP and Professor John Newsom-Davis, FRS, University Department of Clinical Neurology, Radcliffe Infirmary, Oxford

What is myasthenia gravis?

Myasthenia gravis (MG) is a chronic disease characterised by fluctuating levels of muscle weakness.

It is uncommon, affecting only 15 per 100,000 individuals in the UK, and although it can be a very serious condition, 90 per cent of patients live a near-normal life while continuing medical treatment.

What causes MG?

MG is the result of a selective overactivity of the body's immune system in which patients produce antibodies against a particular target in their own body.

Antibodies are the specialised types of protein made by the immune system in response, normally, to intrusion of a foreign body such as a virus.

In MG however the protein being attacked is a normal component of the body, which is therefore being attacked by its own immune system. This is what is termed an 'autoimmune disease'.

The target is a protein called the acetylcholine receptor (AChR), which is found at the junction between the nerve and the muscle (the neuromuscular junction) (See Figure 1).

In order for a nerve impulse to reach a muscle fibre and cause the muscle to contract, a minute packet of a signalling chemical, acetylcholine, must be released from the end of the nerve and cross the gap to the muscle membrane. Here the acetylcholine molecule fits in to the receptor of the muscle like a key fits in a lock, and the signal is passed: a process known as neuromuscluar transmission.

Under normal circumstances this process works very efficiently, but in MG, antibodies against the AChR reduce the efficiency of neuromuscular transmission, and as a result the person experiences muscle weakness particularly when they repeatedly try to use the same muscle.

Although we now understand how antibodies to the AChR cause muscle weakness, we do not know why patients with MG develop these autoantibodies.

Very rarely an individual can be born with a genetic abnormality that causes a disorder that resembles MG (congenital myasthenia).

Who is at risk of developing MG?

People who inherit a tendency to develop autoimmune disease are at increased risk of developing MG, so a patient with MG may have a relative with other autoimmune diseases such as diabetes or thyroid disease.

MG affects all races, and can develop at any age from childhood to extreme old age. In young patients MG is commoner in women, whereas in patients over 50 years old MG it is commoner in men.

How does the illness progress?

The disease varies tremendously between individuals, and in any one person the symptoms fluctuate with relapses and remissions.

MG can occasionally resolve spontaneously, but for most it is a condition that persists for life. With modern treatments, however, the symptoms can usually be well controlled.

What are the symptoms of MG?

The hallmark of MG is weakness of voluntary muscles without pain that gets worse with repeated or sustained use of the muscle (fatiguable muscle weakness).

Symptoms fluctuate and are usually worse at the end of the day, in hot weather, during or immediately after an infection, or just before a period in women.

In two thirds of patients with MG, the first muscles to be affected are those controlling eye movements (causing double vision) and those holding the eyelids up (causing drooping of the eyelids).

Almost all patients experience problems with these muscles at some stage.

In some patients, the MG only ever involves the eye muscles (ocular MG) while in others there are also problems with other muscles (generalised MG).

Ocular MG

Drooping of the eyelids (ptosis): intermittent; can affect one or both eyes.

Double vision (diplopia): may be intermittent; sometimes occurs only when looking in a particular direction.

Generalised MG

Patients usually have symptoms of ocular MG but in addition muscles in other parts of the body become affected.

Face and swallowing mechanism: affecting smiling, speech, chewing and swallowing.

Neck: causing difficulties in holding the head up.

Limb: causing difficulties in walking upstairs, and in holding the arms up (eg when brushing hair).

Lungs and diaphragm: causing shortness of breath when exercising or when lying flat.

How is MG diagnosed?

The history and examination of the patient can suggest the diagnosis but investigations are needed for confirmation.

Blood tests can detect the antibodies to the AChR that are found in 85 per cent of patients with generalised MG, and 50 per cent of patients with ocular MG.

Electromyography (EMG) is a very sensitive test, showing an abnormality in most patients with MG. An electrical stimulus is applied to a nerve and the response in the muscle is recorded.

Tensilon® test in which an injection of Edrophonium is sometimes given which results in a rapid, but short-lived, improvement in symptoms in many patients.

Chest CT scan is done to investigate the thymus as 10 per cent of patients with MG have a tumour of the thymus (a thymoma) that is usually benign. The thymus is part of the immune system and is a tissue situated in the middle of the chest, behind the breastbone. A thymoma shows up as a swelling usually visible on the chest scan.

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