Myasthenia Gravis

myasthenia gravis

Each muscle is supplied by a nerve which splits into smaller nerves that spread along the muscle fibres. There is a tiny gap between the ends of the nerves and the surface of the muscle. This gap is called the neuromuscular junction. The brain sends messages down the nerves to the muscles it wants to contract. The nerve endings release a chemical called a neurotransmitter into the neuromuscular junction. This neurotransmitter is called acetylcholine. The acetylcholine quickly attaches to receptors on the muscles. This in turn triggers the muscle to contract. There are many acetylcholine receptors on each muscle fiber.

What happens in myasthenia gravis and what causes it?

People with myasthenia gravis have a fault in the way nerve messages are passed from the nerves to the muscles. The muscles are not stimulated properly, so do not contract well and become easily tired and weak.

The fault is due to a problem with the immune system. Myasthenia gravis is an autoimmune disease. This means that the immune system (which normally protects the body from infections) mistakenly attacks itself. In most people with myasthenia gravis, antibodies are made which block, alter or destroy the acetylcholine receptors on muscles.

What causes the abnormal antibodies to be made?

The reason why the body's immune system starts to make abnormal antibodies against muscle receptors is not known.

Who gets myasthenia gravis?

Myasthenia gravis can develop at any age but most commonly affects women aged under 40 years and men aged over 60 years. It is not an inherited condition.

What part does the thymus gland have in myasthenia gravis?

The thymus gland is a small gland at the upper chest just behind the sternum (breastbone). It is part of the immune system. The thymus is abnormal in many people with myasthenia gravis, particularly in those who develop myasthenia gravis before the age of 40. The exact role of the cells in the thymus is not clear but they may have something to do with programming or making antibodies against acetylcholine receptors. For some people, removal of the thymus gland by an operation cures their myasthenia gravis.

What are the symptoms of myasthenia gravis?

The typical main symptom is weakness of muscles that gets worse with activity and improves with rest. In effect, affected muscles tire or become fatigued very easily.
The muscles around the eyes are most commonly affected first, as these are constantly used and can quickly tire. This causes drooping of the eyelid (ptosis), and double vision. If symptoms only affect the muscles around the eyes for longer than two years then the condition is unlikely to progress to other muscles. This is known as ocular myasthenia.
Muscles around the face and throat are also often affected. Difficulty in swallowing and slurred speech may be the first signs of myasthenia gravis. Weakness in the arms, hands, fingers, legs and neck may develop. Weakness in the chest muscles sometimes occurs.

Infection or stress can make symptoms worse.

How is myasthenia gravis diagnosed?

An examination by a doctor may find that you have muscle weakness which can suggest the diagnosis. A blood test can detect the abnormal antibody and confirm the diagnosis in most cases. Muscle and nerve tests may be needed in some cases where the diagnosis is not clear. A scan of the upper chest may be performed to calculate the size and shape of the thymus gland. Breathing tests are performed in those people who have weakness of the chest muscles.

What is the treatment for myasthenia gravis?

Anticholinesterase medicines

These medicines delay the breakdown of acetylcholine when it is released from the nerve endings. These medicines work best when the disease is mild and the level of antibody is low. The most commonly prescribed anticholinesterase medicine is called pyridostigmine.

Thymectomy (removal of the thymus)

This is an option in some cases. A thymectomy can improve symptoms in more than 7 in 10 people with myasthenia gravis and may even cure some.

Steroid medication

Steroid medication such as prednisolone tablets is often used in the treatment of myasthenia gravis. Steroids suppress the immune system and prevent the abnormal antibodies from being made. It may take several months to bring symptoms under control with steroids. Once improved, the dose is commonly reduced gradually to find the lowest dose needed to prevent symptoms.

Immunosuppressant medicines

An immunosuppressant medicine such as azothiaprine may be advised in addition to steroid medication. These medicines work by suppressing the immune system.

Combinations of medicines

A steroid plus an immunosuppressant tends to work better than either alone. Also, the dose of steroid needed is often less if an immunosuppressant is added which reduces the risk of side effects with steroids.

Myasthenic crisis

A myasthenic crisis is rare. This occurs when the muscles that control breathing weaken to the point that breathing becomes very difficult. Admission to hospital is usually needed and sometimes assisted breathing by a ventilator machine is performed for a short while.

Plasma exchange may be given for a myasthenic crisis. This is where your plasma is exchanged for blood donor plasma which is free of abnormal antibody.

What is the course of the disease and prognosis?

Without treatment, myasthenia gravis can become a serious, even life-threatening disease. However, treatment usually works well. For most people with myasthenia gravis, treatment significantly improves their muscle weakness. Most people with myasthenia gravis can lead normal lives and can be free of symptoms.

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