Inclusion Body Myositis (IBM) is a rare, acquired, progressive muscle disorder that leads to weakness and atrophy, primarily affecting adults over 50. It is classified as an inflammatory myopathy, but unlike polymyositis or dermatomyositis, IBM has a unique pathology involving both immune-mediated and degenerative components. This distinct combination makes IBM resistant to standard immunosuppressive treatments typically used for related muscle conditions.
How Inclusion Body Myositis Affects the Body
The most characteristic feature of IBM is slow, progressive muscle weakness and wasting, often developing insidiously over several years. The weakness is typically asymmetrical, meaning it may be more pronounced on one side of the body. The disease selectively targets specific muscle groups in a unique distribution.
The lower body is often affected first, specifically the quadriceps muscles. Weakness here can cause the knees to buckle unexpectedly, leading to frequent falls and difficulty standing up or climbing stairs. In the upper body, the forearm and finger flexors are prominently weakened. This compromises dexterity, making simple tasks like turning a key, buttoning a shirt, or maintaining a strong grip increasingly challenging.
A serious and common complication of IBM is dysphagia, or difficulty swallowing, which affects 30% to 50% of patients. This occurs because the muscles of the pharynx and esophagus become weak, and it can result in coughing or choking while eating or drinking. If not managed, dysphagia carries the risk of aspiration, where food or liquid enters the lungs, potentially leading to pneumonia.
The Cellular Basis of the Disease
The disease name comes from microscopic findings, reflecting a pathology that is both inflammatory and degenerative. Muscle fibers show invasion by inflammatory cells (primarily cytotoxic CD8+ T lymphocytes), suggesting an autoimmune attack. However, unlike other myositides, the muscle fibers also display distinct signs of degeneration.
These degenerative signs include “inclusion bodies,” which are clumps of abnormal proteins accumulating within the muscle cells. Pathologists also observe “rimmed vacuoles,” which are tiny, bubble-like spaces inside the muscle fibers. These vacuoles are often rimmed with membranous material and contain protein aggregates, including amyloid and other misfolded proteins.
The presence of both immune-mediated damage and degenerative protein accumulations complicates the understanding of the disease’s origin. It remains unclear whether the initial trigger is an autoimmune response that leads to degeneration, or if protein misfolding is the primary problem that subsequently attracts the immune system. Cellular abnormalities, such as mitochondrial dysfunction, also contribute to the muscle fiber damage observed in IBM.
Confirming the Diagnosis
Diagnosing IBM can be challenging because its symptoms may mimic other muscle disorders, often taking several years from symptom onset. The workup begins with a thorough clinical assessment, noting the characteristic weakness pattern in the quadriceps and forearm flexors. Initial blood tests measure creatine kinase (CK), an enzyme released when muscle is damaged. In IBM, CK levels are often only mildly elevated or normal, contrasting with the significantly higher levels seen in other inflammatory myopathies.
Electromyography (EMG) and nerve conduction studies (NCS) evaluate the electrical activity of the muscles and nerves. The EMG typically shows signs consistent with a myopathy, but may also show features resembling nerve damage. The definitive test required to confirm the diagnosis is a muscle biopsy, where a small tissue sample is surgically removed from an affected muscle.
The pathologist examines the biopsy to visualize features unique to IBM. This includes observing inflammatory cells invading non-necrotic muscle fibers, along with “rimmed vacuoles” and the abnormal protein clumps known as “inclusion bodies.” The presence of these specific microscopic findings is necessary to establish a definite diagnosis.
Managing Life with Inclusion Body Myositis
Currently, there is no available treatment that can slow or stop the progression of Inclusion Body Myositis. The disease is largely unresponsive to corticosteroids and other immunosuppressive drugs, which are the mainstays of therapy for related inflammatory myopathies. Therefore, the management strategy focuses entirely on supportive care aimed at maintaining function and maximizing the individual’s quality of life.
Physical therapy (PT) and occupational therapy (OT) are cornerstones of managing the condition. Supervised exercise programs, including strengthening and stretching activities, are recommended to help maintain muscle strength and mobility. Occupational therapy helps individuals adapt to progressive weakness by teaching new techniques and recommending modifications for daily tasks.
As the disease progresses, many individuals will require various assistive devices to maintain independence. These devices can include braces, canes, walkers, and eventually a wheelchair, particularly as quadriceps weakness makes walking difficult and unsafe.
Specialized interventions are necessary to address the risk of dysphagia. Speech therapists can assess swallowing function and recommend dietary texture modifications or specific swallowing techniques to minimize the risk of aspiration. While IBM is a slowly progressive disorder that leads to increasing disability, it does not typically affect life expectancy.