OUP accepted manuscript

To evaluate MRI changes to define muscle-lesion specific patterns in patients with antisynthetase syndrome (ASS), and compare them with those in other common idiopathic inflammatory myopathy (IIM) subtypes.Qualitative and semi-quantitative thigh MRI evaluations were conducted in patients with ASS, dermatomyositis (DM), and immune-mediated necrotising myopathy (IMNM).This study included 51 patients with ASS, 56 with DM and 61 with IMNM. Thigh MRI revealed muscle oedema (62.7%), myofascial oedema (90.2%), subcutaneous-tissue oedema (60.8%), and fatty infiltration of muscles (68.6%) in patients with ASS. Compared with IMNM, ASS and DM were associated with more frequent adductor-muscle relative sparing (40.6% vs 3.6%, p < 0.001, and 25.6% vs 3.6%, p < 0.001) and subcutaneous-tissue oedema (60.8% vs 23.0%, p < 0.001, and 57.1% vs 23.0%, p < 0.001). Although ASS and DM exhibited similar oedema patterns, there were certain subtle differences between them. The ASS group was less frequently symmetric (60.6% vs 88.4%, p = 0.005, and 60.6% vs 80.0%, p = 0.048), but more frequently showed myofascial oedema of the tensor fasciae latae (80.4% vs 48.2%, p < 0.001, and 80.4% vs 31.1%, p < 0.001) than either the DM or IMNM groups. The receiver operating characteristic curve analysis showed an optimal combination of thigh MRI findings had an area under the curve with 0.893 for diagnosing ASS.Thigh MRI in ASS exhibited frequent myofascial oedema. ASS oedema patterns resembled those of DM more than those of IMNM. Bilateral asymmetry, adductor-muscle relative sparing and remarkable myofascial oedema of tensor fasciae latae were the most characteristic ASS imaging findings.