Rheumatology: recommends proceeding with muscle biopsy to differentiate polymyositis and inclusion-body myositis
Inclusion-body myositis is often misdiagnosed as polymyositis or dermatomyositis until identified by muscle biopsy findings (see How inflammatory myopathies develop), although suspicion is raised with a poor response to steroid therapy.
(13) The risk was highest in dermatomyositis (standardized incidence ratio [SIR] 6.2), next highest in inclusion-body myositis (SIR 2.4), and lowest in polymyositis (SIR 2.0).
(14) The most prevalent MSA, anti-Jo, is present in only about 20% of cases and correlates with interstitial lung disease, but has uncertain usefulness in differentiating between dermatomyositis, polymyositis, and inclusion-body myositis. (6,10,11,14,15)