Making the Diagnosis of Myositis: Definition and Classification of Myositis

Abstract

Myositis is a heterogeneous group of autoimmune diseases encompassing a large spectrum of clinical manifestations including some patients presenting with no muscle involvement. Thus, the clinician must be aware that arthritis, certain cutaneous features, interstitial lung disease (ILD), or dysphagia may dominate the clinical picture. Further, autoantibody detection, muscle immunohistochemistry, and advances in muscle imaging such as MRI have modified the approach to diagnosis and treatment in myositis. In the last decades, several diagnostic and classification criteria were developed for myositis, with varying strengths and limitations. In this chapter, we will review criteria developed for polymyositis (PM), dermatomyositis (DM), immune-mediated necrotizing myopathy (IMNM), overlap myositis (OM), cancer-associated myositis, and sporadic inclusion body myositis (sIBM). We will also discuss the 2017 EULAR/ACR classification criteria for juvenile and adult myositis led by international myositis experts using a robust data-driven process.