Inclusion body myositis

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.

Clinical features

  • 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)

Laboratory features

  • 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)

Considerations

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.

Associated Disorders

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

 
 
 

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