What Causes MG?

MG is caused by a defect in the way nerves transmit signals to muscles. Normally, impulses travel down the nerve, and nerve endings release a substance called acetylcholine. Acetylcholine binds or attaches to receptors on the muscle and makes the muscle contract. The strength of the contraction depends on how much acetylcholine the muscles receive.

With MG, your immune system produces antibodies that block or destroy many of the receptor sites for acetylcholine in your muscles (the acetylcholine receptors). Because there are fewer receptor sites, the muscles get fewer nerve signals and become weak.

In another form of MG, your immune system produces antibodies to the MuSK (muscle specific tyrosine kinase) protein. This also interferes with the nerve-to-muscle communication and causes muscle weakness.

Although not fully understood, doctors believe the thymus gland likely plays a role in starting the abnormal immune response in some MG patients. This gland, which is large in infancy, is involved in the education and development of your immune system. Usually the gland shrinks and becomes dormant in adults, turning into fatty tissue in the upper chest behind the breastbone. With patients whose MG symptoms begin before age 40, the thymus gland is generally larger than normal, and may be responsible for triggering the immune attack on the neuromuscular junction. Some people with MG develop “thymomas” or tumors on the thymus gland. Generally thymomas are benign, but in rare cases they can become malignant. When a thymoma is discovered, surgery to remove it should be performed.

Physicians aren’t sure what triggers the onset of MG. They suspect there may be a combination of factors. For example, if you have an abnormal thymus gland or genetic predisposition, then an illness or other event that stimulates the immune system could trigger myasthenia.

This information was abstracted from the Myasthenia Gravis Fact Sheet published by the National Institute of Neurological Disorders (2007, December 13) and retrieved March, 27, 2008 from http://www.ninds.nih.gov/disorders/myasthenia_gravis.

Additional medical references were used as well. Reviewed by the MGF of Illinois Medical Advisory Board, September 2008.
Unless otherwise stated, the information provided here is of a general nature, composed by non-medical personnel. It is meant to be accurate and helpful advice for MG patients. It is not intended to be medical opinion, nor is it a substitute for personal professional medical care.