Isolated atrial myocarditis presenting with atrial arrhythmias and diffuse thickening of the atrial wall diagnosed via atrial biopsy: a case report.

Background: Isolated atrial myocarditis is rarely diagnosed in non-open-heart surgical cases. We report a case of isolated lymphocytic atrial myocarditis presenting with atrial arrhythmias, syncope, and atrial wall thickening, confirmed histologically by transcatheter atrial biopsy and multimodality imaging. Case summary: A 69-year-old woman presented to the emergency department with general fatigue and a syncopal episode. Electrocardiogram revealed sick sinus syndrome (SSS) with a heart rate of 40 beats/min. Laboratory tests indicated a mild inflammatory response without cardiac enzyme level elevation. Transthoracic echocardiography demonstrated left atrial wall thickening and mild pericardial effusion. Contrast-enhanced computed tomography revealed a thrombus in the left atrial appendage, 18F-fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT) showed diffuse uptake in both atria without ventricular involvement. Three weeks later, the patient presented with atrial fibrillation. An endomyocardial atrial biopsy was performed to histologically investigate the cause of atrial wall thickening, which revealed lymphocytic myocarditis, thereby ruling out malignant lymphoma and amyloidosis. No causative virus was identified, and immunosuppressive therapy was not initiated because the patient showed spontaneous improvement in atrial arrhythmia and atria wall thickening. Anticoagulation therapy resulted in complete resolution of the thrombus within 3 months. Follow-up FDG-PET/CT at 5 months confirmed the resolution of atrial uptake, and the patient remained symptom-free without SSS or AF recurrence at the 1-year follow-up. Discussion: In cases of isolated atrial myocarditis presenting with atrial arrhythmias and atrial wall thickening, a combination of imaging modalities and histological evaluation by atrial biopsy may be helpful in establishing a definitive diagnosis.