What is inclusion body myositis?

Inclusion body myositis (IBM) is an inflammatory and degenerative muscle disease that causes painless weakening of muscle.

IBM gets worse slowly and is sometimes misdiagnosed as treatment-resistant polymyositis, another inflammatory muscle disease that causes muscle weakness. IBM also may be misdiagnosed as amyotrophic lateral sclerosis (ALS or Lou Gehrig's disease).

Symptoms of the disease usually begin after the age of 50, although the disease can occur earlier. IBM occurs more frequently in men than in women and is the most common muscle disease in people aged 50 and older.

What causes inclusion body myositis?

The cause of IBM is unknown. Because of the inflammation associated with IBM, some doctors think the disease is a form of autoimmune disorder. In this kind of disorder, the body’s immune system goes awry and attacks its own tissues – in this case, the muscles. Some experts have linked IBM to infection by a virus that has yet to be identified. Other researchers believe that the primary problem in IBM is an age-related inability of the muscle to deal with destructive chemicals.

What are the symptoms of inclusion body myositis?

In IBM, the onset of muscle weakness usually is gradual, occurring over months or years. Falling and tripping usually are the first noticeable symptoms. For some people, IBM begins with weakness in the hands. People with IBM may have:

  • Difficulty with gripping, pinching, and buttoning
  • Weakness of the wrist and finger muscles
  • Atrophy (shrinking or wasting) of the muscles of the forearms
  • Weakness and visible wasting of the quadriceps muscles (the large muscles on the front part of the thighs)
  • Weakness of the lower leg muscles, below the knees
  • Weakness of the esophageal muscles, which can cause dysphagia (difficulty swallowing) in about 30 to 40 percent of patients
  • Weakness of other muscle groups as the disease progresses