Myasthenia Gravis (MG) is a neuromuscular autoimmune disease that affects the use of muscles – normal communication between the nerve and the muscle is interrupted, leaving the muscle weak and fatigued.
An autoimmune disease is one where the body’s immune system appears to attack healthy tissue and produces so many antibodies (immune blood proteins that recognise and fight infections and other foreign things in the body) that the healthy tissue becomes damaged.
With MG, it is the voluntary (striated) muscles that are weakened. Voluntary muscles are the muscles that we can control, where the message is transmitted via our nervous system to contract the muscle. They are the muscles in our legs, our arms and our neck; those that move the eyeball and keep the eyelids open; some of those involved with facial expression, and those involving chewing, swallowing and breathing. MG does not affect bowel and bladder function or the myasthenic’s mental capacity. Nor is it restricted to humans!
For a muscle to contract, the chemical acetylcholine normally transmits nerve impulses to muscle fibres at the place where the nerve and muscle connect (the neuromuscular junction). The physical weakness of the myasthenic’s muscle is caused by a defect at this neuromuscular junction. However, whilst the he myasthenic has a problem with the transmission of nerve impulses to the muscle, the nerves and muscles themselves may remain normal.
Research has shown that most myasthenics form abnormal antibodies against the acetylcholine receptor (sites at which the chemical can be received on the surface of the muscle). The number of acetylcholine receptors in a myasthenic is reduced due to an attack on the receptors by the body’s immune system – if receptors are missing, the response of the muscle to the nerve impulses is poor, and so weakness occurs. Scientists are investigating what triggers the body to develop an autoimmune response. In many myathenics, the thymus gland appears to be involved.
The onset of MG may be sudden, with severe and generalised muscle weakness, but more often the symptoms in the early stages are subtle and variable, making it difficult to diagnose correctly. Some have only ocular myasthenia involving the eye muscles and eyelids; others have mainly swallowing difficulties or slurred speech; others have generalised MG affecting many muscle groups.
Muscle weakness may develop over a few days or weeks, or remain at the same level for long periods of time. The severity varies at different times of the day, and from person to person. The maximum extent of involvement in an individual patient usually manifests itself within the first 5 to 7 years, and thereafter it tends not to be progressive, even though muscle involvement and severity of weakness may still fluctuate from hour-to-hour and day-to-day.
MG is normally not a degenerative disease – that is, with continued use, the muscle itself should not deteriorate. A distinctive feature of MG, however, is fluctuating weakness of muscles, made worse by use of those muscles and improved by rest of the same muscles. Consequently, a person with MG experiences phases of muscular weakness that alternate with periods of normal health.
MG is not considered a hereditary disease as there is no well documented case of MG occurring in a child of a mother with MG. However, around 12% of babies born to myasthenic mothers suffer from neonatal myasthenia where the baby develops a feeble cry, poor sucking, and respiratory distress. These symptoms disappear over the first few months of life. It is rare that infants born to non-myasthenic mothers have myasthenic symptoms – these infants are said to have congenital myasthenia.
These days, MG is not considered fatal. It can be life threatening if muscle weakness interferes with breathing. It is treatable, but at this stage, not curable. Treatments provide patients with a marked relief of symptoms and often allow them to lead full, normal lives.