Abstract
<title>Abstract</title> <p>Background Current guidelines stratify typical atrial flutter (AFL) as carrying a lower thromboembolic risk than atrial fibrillation (AF), yet surface ECG differentiation between AFL and AF is frequently ambiguous[1]. We present a case in which isolated cavotricuspid isthmus (CTI)-dependent AFL—without inducible AF—led to a massive left atrial appendage (LAA) thrombus, highlighting the limitations of ECG-based risk stratification and the role of multimodality imaging. Case presentation A 66-year-old previously healthy man presented with exertional dyspnea and peripheral edema. Initial ECG was interpreted as AF with rapid ventricular response; contrast-enhanced CTA revealed a large lobulated LAA thrombus. After 3 months of anticoagulation with complete thrombus resolution on transesophageal echocardiography (TEE), electrophysiology study demonstrated isolated counterclockwise CTI-dependent typical AFL with no inducible AF. Althrough, combined CTI ablation, prophylactic pulmonary vein isolation and LAA closure with a 31-mm WATCHMAN FLX device were performed. At 3-month follow-up, CTA showed optimal device position and complete endothelialization, permitting discontinuation of anticoagulation. Conclusions Isolated typical AFL can independently drive massive LAA thrombogenesis, warranting anticoagulation and thromboembolic risk assessment analogous to AF. Surface ECG alone is unreliable for rhythm-stratified decision-making; multimodality imaging (CTA/TEE) is essential regardless of presumed rhythm.</p>