Myositis Mimics—a Clinical Approach to a Diagnostic Challenge

Abstract

Idiopathic inflammatory myopathies (IIMs) encompass a heterogeneous group of disorders generally referred to as myositis. Myositis mimics are common and their overlapping features with IIMs can pose a diagnostic challenge. This review highlights the current knowledge of various myositis mimics, analyzes key distinguishing features from IIMs, and summarizes practical tips in assessing a patient with possible myositis. A new classification criteria for IIMs has been proposed by the European League against Rheumatism/American College of Rheumatology in 2017. Advances in medical genetics have expanded the spectrum of hereditary myopathies, and effective treatment has become available for some. Magnetic resonance imaging (MRI), although not without limitations, is more widely used to differentiate myositis mimics from IIMs. In the end, one must consider the clinical gestalt in formulating a diagnosis. A good clinical history, examination, and judicious use and interpretation of ancillary tests are key in approaching a patient with possible IIMs. Monitoring treatment response is also paramount, not only as a clue to reconsidering the diagnosis but also to prevent unwanted side effects and symptom exacerbation from steroids.