Myasthenia gravis

Definition:

A disorder characterized by chronic weakness of voluntary muscles, which improves with rest and worsens with activity.

Causes, incidence, and risk factors:

Myasthenia gravis involves fluctuating levels of weakness of commonly used muscles. Weakness occurs when the nerve impulse does not adequately reach the muscle cells; this is caused by blockage of the action of transmitters (the chemicals that transmit signals from nerve cell to muscle cell). This is thought to be the result of an immune response of the body against itself (an autoimmune response). High levels of antibodies block the action of specific chemicals that are used to transmit messages from the nerve cell to the muscle cell (neurotransmitters).

The cause of autoimmune disorders such as myasthenia gravis is unknown. In some cases, it may be associated with tumors of the thymus (a tissue of the immune system). It also may be associated with thyrotoxicosis, rheumatoid arthritis, systemic lupus erythematosus, and other immune system disorders. In many cases, no other disorder is identified.

Myasthenia gravis affects about 3 out of 10,000 people. Except when the disorder is associated with thymus tumor (which is most common in elderly men), myasthenia gravis is most common in adult women. Symptoms may worsen with pregnancy or menstrual periods.

Pediatric myasthenia gravis can be distributed into 3 groups:

• neonatal myasthenia (develops in a newborn)
• congenital myasthenia (already present at birth)
• onset any time after birth, although before 1 year of age is rare