Chronic Active Myocarditis and Inflammatory Cardiomyopathy　- Challenges in Diagnosis and Treatment

Myocarditis is a heterogeneous disease with diverse etiologies and clinical trajectories. Traditionally, its diagnosis has been guided by the Dallas criteria, which focus on histopathological features. Clinically, myocarditis is categorized as acute or chronic based on the duration since symptom onset. However, recent expert consensus, particularly in Western countries, has redefined myocarditis as either acute myocarditis or chronic inflammatory cardiomyopathy, including inflammatory dilated cardiomyopathy, reflecting advancements in viral genome analysis and histopathology. In 2023, the Japanese Circulation Society proposed the concept of chronic active myocarditis, a high-risk phenotype characterized by persistent inflammation and ongoing cardiomyocyte injury. The transition from acute myocarditis to its chronic phase involves complex immune mechanisms, with sustained myocardial inflammation driving ventricular remodeling and progression to heart failure. Cardiac magnetic resonance imaging and endomyocardial biopsy remain pivotal diagnostic modalities, though their diagnostic yield varies according to disease phase. Management strategies focus on heart failure treatment, arrhythmia control, and, in select cases, immunosuppressive therapy, particularly for virus-negative inflammatory cardiomyopathy. Although antiviral therapy has shown promise, its clinical efficacy remains uncertain. Given the evolving understanding of the chronic phase of myocarditis, further research is warranted to refine the diagnostic criteria and optimize personalized therapeutic strategies. This review gives a comprehensive overview of the pathophysiology, classification, and management of chronic myocarditis, with an emphasis on emerging disease concepts and their clinical implications.