Diagnostic Utility of Auto-Antibodies in Inflammatory Muscle Diseases

To date, there are four main groups of idiopathic inflammatory myopathies (IIM): polymyositis (PM), dermatomyositis (DM), immune-mediated necrotizing myopathy (IMNM) and sporadic inclusion body myositis; based on clinical presentation and muscle pathology. Nevertheless, important phenotypical differences (either muscular and/or extra-muscular manifestations) within a group persist. In recent years, the titration of different myositis-specific (or associated) auto-antibodies as a diagnostic tool has increased. This is an important step forward since it may facilitate, at a viable cost, the differential diagnosis between IIM and other myopathies. We have now routine access to assays for the detection of different antibodies. For example, IMNM are related to the presence of anti-SRP or anti-HMGCR. PM is associated with anti-synthetase antibodies (anti-Jo-1, PL-7, PL-12, OJ, and EJ) and DM with anti-Mi-2, anti-SAE, anti-TIF-1-γ and anti-NXP2 (both associated with cancer) or anti-MDA5 antibodies (associated with interstitial lung disease). Today, over 30 myositis specific and associated antibodies have been characterised, and all groups of myositis may present one of those auto-antibodies. Most of them allow identification of homogenous patient groups, more precisely than the classical international classifications of myositis. This implies that classification criteria could be modified accordingly, since these auto-antibodies delineate groups of patients suffering from myositis with consistent clinical phenotype (muscular and extra-muscular manifestations), common prognostic (cancer association, presence of interstitial lung disease, mortality and risk of relapse) and treatment responses. Nevertheless, since numerous auto-antibodies have been recently characterised, the exact prevalence of myositis specific antibodies remains to be documented, and research of new auto-antibodies in the remaining seronegative group is still needed.