Chronic active myocarditis presenting as an isolated atrioventricular block: a case report.

Background: Although atrioventricular (AV) block is frequently observed in acute myocarditis, its occurrence as an isolated finding in chronic myocarditis is rare. Chronic active myocarditis represents an uncommon subset of myocarditis with a potentially poor prognosis. Herein, we report a rare case of chronic active myocarditis presenting solely as an isolated AV block. Case summary: A 51-year-old female with ulcerative colitis and bipolar disorder presented with complete AV block. Echocardiography revealed preserved cardiac function, whereas laboratory findings revealed persistently elevated troponin I levels. Despite extensive investigations, no definitive cause of conduction disturbance was identified, leading to pacemaker implantation. Ten months later, 18-fluoro-deoxyglucose positron emission tomography/computed tomography (18F-FDG-PET/CT) revealed diffuse cardiac uptake; endomyocardial biopsy demonstrated inflammatory infiltrates adjacent to the cardiomyocyte necrosis and fibrosis. These findings were consistent with those of chronic active myocarditis. As the patient's symptoms were mild and cardiac function was preserved, a conservative approach was adopted. Over the next 2 years, the patient's symptoms improved, cardiac function was preserved, and the FDG-PET/CT findings markedly diminished. However, high-uptake areas remained on FDG-PET/CT and re-elevation of troponin I was observed, indicating the persistence of subclinical inflammation within the myocardium. Discussion: AV block may be a clinical sequela of chronic active myocarditis and should not be considered idiopathic, even in the absence of inflammatory symptoms or systolic dysfunction. Such cases may carry a risk of poor long-term outcomes due to ongoing inflammation. Endomyocardial biopsy should therefore be considered to establish an accurate diagnosis, and long-term follow-up is essential.