Background Autoantibodies in the inflammatory myopathies identify unique subsets of patients with distinctive clinical features and provide useful diagnostic and prognostic information. However the association of individual autoantibodies with specific muscle biopsy features has rarely been studied.
Objectives To characterize muscle biopsy features associated with different autoantibodies in a cohort of patients with inflammatory myopathies from a single-center.
Methods Patients from the Johns Hopkins Myositis Center cohort diagnosed with probable or definite dermatomyositis (according to Bohan and Peter criteria) who had both a muscle biopsy read at Johns Hopkins and a known myositis autoantibody were included in the study. For each biopsy, the presence/absence of the following features were recorded: perifascicular atrophy, mitochondrial dysfunction (>5 COX negative fibers), perivascular inflammation, primary inflammation (invasion of non-necrotic fibers by mononuclear cells) and necrotizing myopathy (defined as biopsies with necrosis/degeneration without perifascicular atrophy or primary inflammation). As a complementary analysis, biopsy features of all anti-Jo1 positive patients with either dermatomyositis or polymyositis were compared through Fisher's exact test.
Results 37 patients with dermatomyositis were included in the study (79.5% definite). These included 11 with anti-Jo-1, 7 with anti-Mi-2, 11 with anti-TIF1-γ, 8 with anti-NXP2, and 5 with anti-PM-Scl. Biopsies from anti-Mi-2 positive patients showed frequent perifascicular atrophy (71.4%), perivascular inflammation (85.7%) and primary inflammation (57.1%); biopsies from anti-TIF1-γ positive patients showed frequent perifascicular atrophy (63.3%), mitochondrial dysfunction (55.6%) and perivascular inflammation (72.7%); biopsies from anti-NXP2 positive patients showed frequent perifascicular atrophy (50%) and biopsies from anti-PM-Scl positive patients showed frequent perivascular inflammation (80%) and primary inflammation (60%). Overall, 16.2% of DM patients had a necrotizing myopathy without perifascicular atrophy or primary inflammation. In the complementary analysis 7 Jo-1 polymyositis patients were identified. Biopsy features of anti-Jo1 patients with dermatomyositis and polymyositis were not statistically different (all p>0.05).
Conclusions The majority of anti-PM-Scl and anti-Mi-2 patients have primary inflammation, previously considered the hallmark of PM. Most biopsies from anti-TIF1-γ positive patients reveal mitochondrial dysfunction. 16.2% of DM biopsies have necrotizing myopathy. Biopsy features are similar between dermatomyositis and polymyositis in anti-Jo1 positive patients.
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Disclosure of Interest None declared
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