Inclusion body myositis (IBM) is the most common acquired muscle disease in people over the age of 45. More men have inclusion body myositis than women, and the disease is rarely seen in people younger than 45 years of age.
Inclusion body myositis is unlike all other forms of myositis in terms of symptoms, treatment, and who it affects. Symptoms of inclusion body myositis progress more slowly than the other types of myositis with weakness increasing gradually, often over years. For this reason it is not uncommon for individuals to realize that they had been experiencing symptoms for many years before they were diagnosed.
Unlike other forms of myositis, weakness in inclusion body myositis typically has an asymmetric distribution. While those with other subtypes of myositis typically exhibit proximal (closer to the body) muscle weakness in both the upper and lower limbs, those with inclusion body myositis show a characteristic pattern of distal (in the extremities) weakness in the upper limbs, particularly involving the muscles that flex the fingers, and proximal weakness in the lower limbs, most notably affecting the quadriceps.
Inclusion body myositis symptoms
The following are common symptoms of IBM:
- Frequent falls
- Difficulty walking
- Trouble climbing stairs or standing from a seated position
- A foot that seems to drop when walking, causing tripping
- Weakened hand grip and difficulty flexing the fingers
- Difficulty writing, manipulating keys, and other activities requiring finger control
- Weakness and noticeable shrinking of the quadriceps (main muscle of the thighs)
- Weakness in the forearm muscles
- Pain or discomfort as muscles weaken
- Difficulty swallowing
Some of the first signs of inclusion body myositis are falling, difficulty getting up from a chair, and weakened grip. Muscles most often affected early on in the course of the disease are those at the front of the thighs and fingers. Many people with inclusion body myositis notice shrinking (atrophy) in the forearms and thighs as the muscles become weaker. Trouble swallowing, or dysphagia, is also a common problem for those with inclusion body myositis, and in rare cases, may be the first symptom.
Inflammatory disease debate
There is currently some debate among myositis experts about whether or not inclusion body myositis is actually an inflammatory disease. Inflammatory cells are present in muscle tissue from those with inclusion body myositis, especially earlier in the disease process, but their role in causing muscle weakness is unclear. Anti-cN1A autoantibodies have also been found in some people with inclusion body myositis, supporting an immune-mediated mechanism of inclusion body myositis.
While the presence of inflammatory cells in muscle tissue is a common feature across many myositis subtypes, including inclusion body myositis, what sets inclusion body myositis apart is the presence of abnormal protein accumulations and tiny empty spaces called vacuoles within the muscle fibers. Both protein accumulations and vacuolar changes reflect a degenerative component of muscle damage and are considered potential alternative primary mechanisms in inclusion body myositis. This component may be the main reason that the disease does not respond to treatment with anti-inflammatory medications.
Most experts agree, however, that an eventual treatment and cure will likely require attention to both inflammation and muscle degeneration.
IBM and hereditary inclusion body myopathies
Inclusion body myositis should not be mistaken for hereditary forms of inclusion body myopathy (also referred to as VCP, MYH2, or GNE myopathy). This group of genetic diseases was previously known as hereditary inclusion body myopathy (HIBM). However, this term is no longer used in current classification. The preferred terminology is either rimmed vacuolar myopathy or the specific name indicating the defective gene. Although muscle biopsy findings in the hereditary myopathies share some of the same features seen in inclusion body myositis—rimmed vacuoles and inclusions in muscle cells—these two conditions are otherwise quite different.
The hereditary form of the disease is caused by a gene defect, not inflammation, and affects a younger group of individuals compared to inclusion body myositis. Muscle biopsy in these genetic conditions generally does not show inflammation. Muscle weakness is usually distal (in the extremities) and may include eye muscles and other areas of weakness depending on the specific underlying genetic defect.
As you go through the diagnosis process, be sure to discuss these distinctions with your doctor.
The IBM Functional Rating Scale is a tool doctors use to assess a person’s ability to perform daily activities, such as walking, dressing, and handling utensils. It consists of ten questions, each of which is answered using a number from 0–4. The numbers are added up to give the total score, with 40 being the highest possible.
Revised July 2025 by Teerin Liewluck, MD – neurology professor at Mayo Clinic, Rochester, MN, and TMA special volunteer.
Disclaimer: Information provided on this website is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider with any questions you may have regarding a medical condition.