Myositis Mimics: Recognizing Red Flags and Diagnostic Pitfalls.

Purpose of Review: This review aims to provide clinicians with a practical framework for distinguishing idiopathic inflammatory myopathies (IIMs) from their numerous non–immune-mediated mimics, including hereditary, toxic, metabolic, and endocrine myopathies, by integrating clinical, serologic, imaging, electrophysiologic, and histopathologic clues. Recent Findings: IIMs represent a heterogeneous group of immune-mediated muscle diseases. Despite major advances in antibody discovery, imaging, and classification criteria, accurate diagnosis remains challenging because many non-immune-mediated myopathies can mimic IIMs. Such resemblance may lead to misdiagnosis, delay in genetic evaluation, prolonged exposure to offending agents in toxic myopathy, and unnecessary treatment with immunosuppressive therapy that carries significant adverse effects. The temporal course of weakness, pattern of muscle involvement, presence of extramuscular manifestations, and ancillary testing offer important diagnostic clues, but none are pathognomonic when interpreted in isolation. We introduce the mnemonic “MYOSITIS” to summarize key diagnostic red flags that should raise concern for mimicking disorders: Myopathic motor unit potentials without fibrillation potentials, Young age of symptom onset or positive family history, Onset atypical for IIMs, Seronegative or weakly positive myositis specific autoantibodies, Iatrogenic causes, Treatment refractoriness, Irregular weakness patterns, and Systemic features. Summary: Recognizing these pitfalls and adopting an integrated diagnostic approach that combines clinical pattern recognition with selective use of serologic, imaging, and genetic testing can help clinicians differentiate true IIMs from their mimics and ensure timely, accurate diagnosis and optimal patient management.