Inclusion body myositis (IBM) is one of the inflammatory myopathies, a group of muscle diseases that involves inflammation of the muscles or associated tissues, such as the blood vessels that supply the muscles. The disease is characterized by a slowly progressive weakness affecting the muscles of the wrists and fingers, the muscles of the front of the thigh, and the muscles that lift the front of the foot.
Invasion of muscle tissue by inflammatory cells is one characteristic of IBM, but the disease is distinct from other inflammatory myopathies in that muscle degeneration also occurs. IBM is named for the clumps of discarded cellular material — the "bodies" — that collect in the muscle tissues.
The vast majority of cases of IBM are sporadic, with no known cause. A relatively small number of cases are inherited, and several genes have been identified. In genetic forms of IBM, inflammation isn’t as major a part of the picture. For this reason, these forms are often called inclusion-body myopathy.
Sporadic IBM is believed to involve an autoimmune response, in which the immune system attacks the body’s own tissues. Reported triggers for this response, and the subsequent development of IBM, have included certain viruses and an array of medicinal drugs.
Despite the involvement of inflammation in sporadic IBM, anti-inflammatory therapies have not been successful in treating the disease. Learning more about the genetic causes of the rare forms of inherited IBM may lead to a better understanding of sporadic disease as well. MDA-funded researchers are developing models of one form of inherited IBM, due to mutation in the gene for myosin, the molecular motor that drives muscle contraction. Other MDA scientists are exploring mutations in valosin-containing protein (VCP), responsible for multi-system proteinopathy, in which IBM occurs along with a bone disease and motor neuron degeneration.