BackgroundIdiopathic inflammatory myositis (IIM) is a group of heterogeneous autoimmune diseases, characterized by myositis-specific (MSA) or myositis-associated autoantibodies (MAA). Muscle biopsy is the diagnostic gold standard, but correlations with elevated creatine level (CK) or MSA/MAA are unclear.ObjectivesTo evaluate the association between specific histological findings and clinical and/or serological features in IIM patients from a single tertiary Rheumatology referral center.MethodsWe retrospectively analyzed demographic and clinical data from medical charts, of 184 patients affected by IIM followed-up for >1 year. Muscle biopsies of 89 IIM patients were retrieved from deltoid, biceps or quadriceps muscle, snap-frozen and stored at -80°C and processed for routine histology and histochemistry.ResultsThe majority of our muscle biopsy cohort was represented by Dermatomyositis (DM) (39.3%), Polymyositis (PM) (29.2%) and Anti-synthetase syndrome (ASS) (13.5%), whereas the rest was composed of Overlap Syndrome (OS), Immune-Mediated Necrotizing Myopathy (IMNM) and Inclusion-Body Myositis (IMB) (13.5%, 2.2% and 2.2% respectively).DM presented perifascicular atrophy in 85% of cases and perivascular and perifascicular infiltration in only 40% of cases, similar to other types of IIMs. The overall histology finding was characterized by minimal inflammation and/or necrosis but major atrophy. In comparison, Polymyositis showed more endomysial infiltration and necrosis (Table 1). ASS was characterized by atrophy and perifascicular necrosis in 83% of cases, with perimysial and perivascular infiltration in 67% of cases. Those characteristics were observed both in anti-Jo-1+ (13.9%) as well as non-Jo-1+ ASS patients (13.2%). Non-Jo-1+ ASS patients presented endomysial infiltration, invading or enveloping the muscle fibers (not present in anti Jo-1+) (p: 0.036), with addition of rimmed vacuoles and aspecific clinical presentation. Moreover, in DM, anti-Mi2+ antibodies (9.3%) were significantly associated with perifascicular regeneration and endomysial infiltration, in comparison to both anti-TIF1γ+ (5.8%) and anti-NXP2+ patients (4.6%). No significant histopathological features were found in patients with anti-Ro52 autoantibodies (37.2%), that represented the most frequent MAA in our cohort.Table 1.BIOPSY CHARACTERISTICSASS 12 (%)PM 35 (%)DM 26 (%)p [OR] ASS vs PMp [OR] PM vs DMdiffuse atrophy3 (25)34 (97.1)8 (30.8)<0.0001 [0.010 (0.001-0.106)]<0.0001 [76.5 (8.8-660.6)]perifascicolar degeneration----29 (82.9)15 (57.7)----0.044 [3.544 (1.09-11.5)]prevalent necrosis----10 (28.6)1 (3.8)----0.017 [10 (1.19-84)]Jo1 12 (%)PL7/PL12 5 (%)p [OR]endomysial infiltration (invading fibers)03 (60)0.015 [inf]Mi2 8 (%)TIF1γ 5 (%)NXP2 4 (%)p [OR] Mi2 vs TIF1γp [OR] Mi2 vs NXP2perifascicolar regeneration8 (100)1 (20)00.007 [inf]0.002 [inf]endomysial infiltrate (surrounding fibers)6 (75)----0----0.061ConclusionNo significant associations emerged between CK levels, different types of IIM and MSA. Some histological features seem to define subtypes of DM. A precise definition of autoantibody profile in IIM could define not only a clinical phenotype but also the muscle damage distribution.Disclosure of InterestsNone declared