Diagnostic value of additional histopathological fascia examination in idiopathic inflammatory myopathies

Background and purpose: Correct diagnosis of idiopathic inflammatory myopathies (IIM) may prevent harm from both lack of treatment in IIM patients and unnecessary treatment in non-IIM patients. However, it is unknown whether additional histopathological fascia examination may contribute to diagnosing IIM. Methods: Thirty-two magnetic resonance imaging guided en bloc biopsies from patients diagnosed with IIM (except inclusion body myositis) from 2010 to 2017 were reviewed: dermatomyositis (DM) (n = 6), non-specific/overlap myositis (NM/OM) (n = 11), immune-mediated necrotizing myopathy (n = 12) and anti-synthetase syndrome (n = 3). Muscle biopsy specimens were examined according to the 2004 European Neuromuscular Centre (ENMC) criteria. Fascia was subsequently examined for the presence of lymphocytic infiltrates. Isolated fascia involvement was defined as the presence of lymphocytic infiltrates in the fascia/epimysium on histopathology in the absence of any ENMC muscle histopathology/immunohistochemistry criteria. Results: One patient with DM (17%) and one patient with NM/OM (9%) had isolated fascia involvement. One patient with immune-mediated necrotizing myopathy (8%) and one patient with anti-synthetase syndrome (33%) had fascia involvement, albeit in combination with muscle involvement. Conclusion: Histopathological fascia examination may contribute to early diagnosis of DM and NM/OM in a small proportion of patients.