Causes
The cause of Myasthenia gravis and other autoimmune diseases such as rheumatoid
arthritis and lupus is not known. One theory is that the disease may be
triggered by a virus or other infection which has a similar structure to a part
of the acetylcholine receptor (which is found in the neuromuscular junction
and is needed for it to function normally). The antibodies that the immune system
produces to fight the virus then also mistakenly attack the receptors.
What we do know is that certain antibodies have been found in the blood of
people with myasthenia gravis that are built to attack acetylcholine receptors.
Sometimes women with myasthenia gravis who give birth transmit these particular
antibodies to their babies causing muscle weakness in the infant. This is called
neonatal myasthenia. It is different from congenital myasthenia because
the infant gets better in a few weeks as the antibody blood levels go down.
The other abnormality that often shows up in people with myasthenia gravis
is an overactive, overlarge, or otherwise malfunctioning thymus. The thymus
is a gland located where the neck joins the chest. It is important for the
normal development of the immune system. Normally, tht thymus is larger in childhood
than in adulthood, and becomes inactive around puberty. In people with myasthenia
gravis, however, it often keeps going into adulthood.
Some people with myasthenia gravis turn out to have a thymoma (a tumor
in the thymus). This is a type of cancer, and obviously it's frightening, but
in fact very few people die of thymomas (see "Treatment and Prevention").