Sporadic inclusion body myositis

A definite diagnosis of sporadic inclusion body myositis can be made if the following features are present on muscle biopsy:

  • 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 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 six months
  • Age of onset greater than 30 years
  • Asymmetric muscle weakness affecting proximal and distal muscles of arms and legs, along with one of the following:
    • Weakness of finger flexors
    • Wrist weakness greater in flexor than extensor muscles
    • Quadriceps muscle weakness (equal to or less than the Medical Research Council grading system of 4)

Laboratory features

  • Serum creatine kinase less than 12 times normal
  • Muscle biopsy (as 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.