Definite diagnosis of inclusion-body myositis can be made if muscle biopsy features are diagnostic:
- Invasion of nonnecrotic fibers by mononuclear cells
- Vacuolated muscle fibers
- Either intracellular (within muscle fibers) amyloid deposits by fluorescent method of identification or 15-18 nm tubulofilaments by electron microscopy
If muscle biopsies exhibit inflammation and no other diagnostic features, possible inclusion-body myositis can be diagnosed with presence of clinical features and laboratory features listed below.
- Duration of illness greater than six months
- Age of onset greater than 30 years old
- Muscle weakness affecting proximal and distal muscles of arms and legs, along with one of the following-weakness of finger flexors, greater wrist flexor than wrist extensor weakness, or quadriceps muscle weakness (equal to or less than grade 4 MRC)
- Serum creatine kinase less than 12 times normal
- Muscle biopsy (see above)
- Electromyography consistent with features of an inflammatory myopathy (note that long-duration potentials are commonly observed and do not exclude diagnosis of sporadic inclusion-body myositis)
Family History. Rarely, inclusion-body myositis may be observed in families. This condition is different from hereditary inclusion-body myopathy without inflammation. The diagnosis of familial inclusion-body myositis requires specific documentation of the inflammatory component by muscle biopsy in addition to vacuolated muscle fibers, intracellular (within muscle fibers) amyloid, and 15-18 nm tubulofilaments.
Inclusion-body myositis occurs with a variety of other, especially immune-mediated conditions. An associated condition does not preclude a diagnosis of inclusion-body myositis if diagnostic criteria are fulfilled.
Credits: Modified with permission from Griggs RC, Askanas V, DiMauro S, Engel A, Karpati G, Mendell JR, Rowland LP. Inclusion body myositis and myopathies. Annals of Neurology. 1995;38:705-13.
Updated March 2012