In most cases, the cause of IBM is unclear. For some reason, the body’s immune system turns against its own muscles and damages muscle tissue in an autoimmune process.
Viruses might be a trigger for autoimmune myositis. People with the HIV virus, which causes AIDS, can develop a myositis, as can people with a virus called HTLV-1. Some myositis cases have followed infection with the Coxsackie B virus.
There are reports of myositis following exposure to certain drugs. Among the drugs that have been suspected of contributing to myositis are carticaine (a local anesthetic), penicillamine (a drug used to lower copper levels in the body), interferon-alpha (mostly used to treat cancer and hepatitis), cimetidine (used to treat ulcers), carbimazole (to treat thyroid disease), phenytoin (used to treat seizures) and growth hormone. The vaccine for hepatitis B also has been implicated in some cases.
Genetic inclusion-body myopathies can be inherited in either a dominant or a recessive pattern. Dominant genetic disorders require only one genetic flaw to show themselves. Recessive disorders require that both parents pass on a flaw in the same gene before their offspring can show signs of the disease. For more, see Facts about Genetics and Neuromuscular Disease. And, to learn more about getting a definitive genetic diagnosis, see The Genie's Out of the Bottle: Genetic testing in the 21st century.