I.I.7 - Contribution of myopathological analysis to myositis classification in a multidisciplinary approach

Abstract

Diagnosis of inflammatory myopathies is based on evaluation of a certain clinical syndrome, laboratory results, electromyography and the myopathological assessment of a muscle biopsy. Recently, several additional parameters have been taken into consideration; among these, serological auto-antibody testing is the most relevant. The current classification of inflammatory myopathies (IIMs) comprises polymyositis (PM), dermatomyositis (DM), immune-mediated necrotizing myopathy (IMNM), sporadic inclusion body myositis (sIBM), and non-specific myositis (NSM). This classification has been a subject of intense critique at various levels. First, the entity of polymyositis has been questioned, since many of these patients developed either sIBM later on, or should better be grouped among the IMNM group or the NSM/overlap myositis group. Second, dermatomyositis encompasses a multitude of subgroups with different clinical, therapeutical and prognostic implications. It has been shown recently that certain subgroups can be defined by different myositis specific auto-antibodies, such as TIF1γ, Mi-2, NXP2 or MDA-5. Anti-TIF1γ+ and NXP2+ adult patients show an increased risk of systemic malignancy, while anti-Mi-2+ patients harbour a classical dermatomyositis with good response to treatment, and NXP2+ juvenile DM patients have a strongly increased risk of developing calcinosis. The morphology and in situ immune mechanisms of the skeletal muscle of these subentities are presented. Third, necrotizing myopathy was recently added to the IIMs; however, IMNM still comprises a heterogeneous group of diseases. Recently interesting advances have been achieved, illuminating the immunopathogenesis from anti-SRP+, anti-HMGCR+ patients and anti-synthetase syndrome-associated myositis. These entities will be discussed in the light of clinical, serological, morphological and immunological characteristics.