What is Myasthenia gravis?
Myasthenia gravis is a neurological disorder that causes weakness of muscles. Typically the weakness becomes worse with activity and improves with rest; this is known as fatigability. It is a disease of auto immunity in which abnormal antibodies interfere with the effective contraction of muscles.
Myasthenia gravis is caused by an abnormality at the neuromuscular junction. The neuromuscular junction is located at the nerve endings, where electrical impulses are transferred to the muscle fibres causing the muscle to contract. A chemical neurotransmitter, acetylcholine, is released at the nerve ending and acts on receptors located on the muscle fibre surface. In myasthenia gravis abnormal antibodies on the muscle fibre receptor prevents the effective action of acetylcholine. As a result the muscle fibres are not stimulated properly, they contract less effectively than normal and movement is impaired.
It is not fully understood why the body produces abnormal antibodies in this disease. However myasthenia gravis is associated with other autoimmune disease including rheumatoid arthritis, sarcoidosis, thyroid disorders and systemic lupus erythematosus (SLE). Myasthenia gravis is also linked with abnormalities of the thymus gland.
The thymus gland is situated in the upper part of the chest above the heart and behind the sternum. In the body the thymus gland plays an important role in the development of immunity and antibody production during life. It is normally enlarged in childhood but shrinks in size as the person matures. In 65% of myasthenia gravis cases the thymus gland is enlarged (hyperplasia), and in 10% a thymic tumour, called a thymoma, is present.
Prevalence and Incidence
Myasthenia gravis is a rare condition with an incidence of around 3 - 5 per 100,000 in the UK. It can occur at any age, but has two peaks of incidence -between 30 and 40 years, and between 60 and 70 years. Women are affected more often in the earlier decades and men more frequently in the later decades. Myasthenia gravis is also now being diagnosed more often in elderly people of both sexes.
Amended June 2008
