Inclusion body myositis
A definite diagnosis of inclusion body myositis can be made if the following features are present on muscle biopsy:
- Invasion of nonnecrotic fibers by mononuclear cells
- Muscle fibers harboring rimmed vacuoles
- Intracellular (within muscle fibers) accumulations of amyloid fibrils identified by fluorescent method, p62 or TDP-43 deposits detected by immunohistochemical studies, or 15-18 nm tubulofilaments observed by electron microscope.
If the muscle biopsy exhibits inflammation but other diagnostic features are not present, possible inclusion body myositis can be diagnosed with presence of the following clinical features and laboratory features:
Clinical features
- Duration of illness greater than 12 months
- Age of onset greater than 45 years
- Asymmetric muscle weakness, most severely affecting finger flexors and quadriceps
Laboratory features
- Serum creatine kinase less than 15 times upper limit of normal
- Muscle biopsy (as above)
- Electromyography consistent with features of an inflammatory myopathy (note that in addition to short-duration potentials, a typical finding of myopathy, long-duration potentials, resembling that observed in neurogenic disorders, are also commonly observed in inclusion body myositis)
A small proportion of people with inclusion body myositis may present with atypical features, such as an uncommon pattern of weakness including proximal weakness, foot drop, facial weakness, or isolated swallowing difficulty, as well as age of onset younger than 45 years, disease duration shorter than 12 months, or creatine kinase levels more than 15 times the upper limit of normal. In these cases, ancillary testing may help support the diagnosis, particularly in patients whose muscle biopsy shows only inflammation without rimmed vacuoles or intracellular protein accumulations. Useful studies include the presence of anti cN1A antibodies, mitochondrial abnormalities on muscle biopsy, and muscle imaging by MRI or ultrasound showing a pattern of common muscle involvement that is characteristic of inclusion body myositis.
Considerations
Family History. Rarely, inclusion body myositis may be observed in families. This condition is different from rimmed vacuolar myopathy (previously known as hereditary inclusion body myopathy) without inflammation. The diagnosis of familial inclusion body myositis requires specific documentation of the inflammatory component by muscle biopsy along with vacuolated muscle fibers, intracellular (within muscle cells) accumulations of amyloid fibrils, p62 or TDP-43, and 15-18 nm tubulofilaments, as well as negative genetic testing.
Myositis specific antibodies. The presence of antibodies specific to dermatomyositis, anti synthetase syndrome, or immune mediated necrotizing myopathy, especially at significant titers, should prompt consideration of another form of myositis rather than inclusion body myositis. This is particularly important when individuals lack the classic pattern of weakness seen in inclusion body myositis (finger flexor and quadriceps involvement) or when muscle biopsy does not show rimmed vacuoles and intracellular amyloid or protein accumulations. It is important to note that although anti cN1A antibodies are commonly found in those with inclusion body myositis, they are not considered specific to the disease. These antibodies can also be present in other forms of myositis, in individuals with autoimmune conditions such as lupus even without muscle involvement, and in those with motor neuron diseases, although at a lower frequency.
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.