Inclusion body myositis (IBM) is a progressive inflammatory disease of the muscles. It is the most common among such conditions, but is distinct from hereditary inclusion body myopathies (hIBM). As such it is often referred to as sporadic inclusion body myositis (sIBM).
Myositis refers to the inflammation of the muscles, whereas myopathy refers to disease of the muscle fibers leading to impaired function.
Muscle pain/ache (myalgia) is a symptom of IBM. Image Credit: Diy13 / Shutterstock
What are the clinical characteristics/symptoms of IBM?
IBM usually develops after the age of 50 and is characterised primarily by progressive weakness of the limbs (legs, arms, hands) in addition to weakness of the facial muscles, caused by muscle inflammation and atrophy (wasting).
Symptoms may vary in severity between individuals, but the condition is typically more debilitating and progresses faster in older people.
The main symptoms of IBM are:
- Weakness of the leg muscles (especially the quadriceps) and the outermost muscles of the upper and lower limbs
- Muscle degeneration (muscle atrophy) of the affected muscles
- Reduced tendon reflexes (hyporeflexia)
- Muscle pain/ache (myalgia) – though this is not seen in all cases
- Elevated serum/blood creatinine phosphokinase levels, which act as a diagnostic marker
The symptoms of IBM are progressive and set in slowly, sometimes over months or years to develop. IBM is also more common in men than in women. The prevalence of IBM is 10-70:1 000 000 and is higher in people over 50.
Weakness of the limb muscles as well as those of the head and neck can lead to impairment in performing the activities of daily living. These may include:
- A greater tendency to trip or fall, leading to repeated injuries
- Impaired grip which may show in difficulty in turning a key or buttoning a shirt, or using a pen
- Trouble walking up steps/stairs or rising from a sitting position
- Difficulty in swallowing or chewing – this may lead to repeated choking
While IBM does not typically reduce lifespan, it can lead to the inability to perform activities of daily living and therefore may require assistive devices such as wheelchairs or walkers (Zimmer-frames) within a decade of the onset.
What causes IBM?
The exact cause of IBM is still unknown. It is thought that various immunological, environmental and genetic factors contribute to the development of IBM. Unlike hIBM (hereditary myopathy), sIBM/IBM (sporadic myositis) is not genetically inherited.
There are two distinct processes that operate in IBM, autoimmunity and degeneration. The muscles of affected individuals contain inflammatory cells as well as a specific autoantigen called NT5C1A. The body’s immune system begins to attack healthy muscle tissue, leading to increased populations of white blood cells within muscle tissue.
IBM is also a degenerative disease in that the muscle itself begins to waste away (atrophy) because of the formation of deep holes called vacuoles, inclusion bodies composed of abnormally clumped proteins such as TDP-43 or amyloid-beta begin to deposit within these vacuoles. The inclusion bodies are the classical hallmark of IBM, giving rise to its name.
The two processes may happen in parallel with each other, promoting each other, or they may occur independent of one another. What triggers either process is still unknown, although some research has suggested a role for abnormal immune responses against specific infections caused by certain viruses (e.g. HTLV-1).
How is IBM treated?
There is no cure for IBM, or any standard course of treatment to slow or halt disease progression. Furthermore, classic anti-inflammatories and immunomodulatory medications do not seem to work for IBM.
Only symptomatic management is available, especially specialized exercise therapies tailored to the patient’s needs by specialist physiotherapists or occupational therapists. Repetitive simple exercises can help maintain muscle function. Aerobic exercise and resistance training (appropriately suited to the individual) may even improve the quality of life – although not all exercises are appropriate as some may enhance inflammation.
In summary, IBM is a sporadic inflammatory disease of the muscles leading to progressive weakness and wasting, particularly of the limb muscles. The resulting progressive weakness begins to affect activities of daily living leading to the inevitable need for assistive devices. The prognosis for IBM is variable, depending on the severity of the disease. However, it is progressive and degenerative, and therefore will only get worse with time. With no effective disease-modifying therapies, only specialist exercise regimes may, in some cases, improve the quality of life.