Complete Heart Block Unmasking Transthyretin Cardiac Amyloidosis: A Multimodality Imaging Case Report

Transthyretin cardiac amyloidosis (ATTR-CM) is an increasingly recognized infiltrative cardiomyopathy characterized by progressive myocardial amyloid deposition, leading to heart failure, arrhythmias, and conduction abnormalities. Recent studies suggest that ATTR-CM is not uncommon among older patients hospitalized with heart failure with preserved ejection fraction and left ventricular hypertrophy, highlighting that the disease remains substantially underdiagnosed. Conduction disturbances are common during the course of the disease, and advanced atrioventricular conduction disease may require permanent pacemaker implantation; however, complete heart block as the initial presentation remains uncommon. We report the case of a 70-year-old woman who presented with exertional syncope and was found to have complete atrioventricular block with a wide-QRS escape rhythm, requiring emergency temporary pacing followed by permanent dual-chamber pacemaker implantation. Transthoracic echocardiography demonstrated marked biventricular hypertrophy, biatrial enlargement, restrictive left ventricular filling pattern, and severely impaired global longitudinal strain with a characteristic apical sparing pattern, raising a strong suspicion of cardiac amyloidosis. Cardiac MRI revealed diffuse biventricular subendocardial late gadolinium enhancement and abnormal myocardial nulling, highly suggestive of amyloid infiltration. Hematological investigations excluded light-chain amyloidosis, and technetium-99m hydroxymethylene diphosphonate scintigraphy demonstrated Perugini grade 3 myocardial uptake, establishing the non-biopsy diagnosis of ATTR-CM. Tafamidis therapy was subsequently initiated. This case highlights complete heart block as an uncommon initial presentation of ATTR-CM. It underscores the pivotal role of multimodality imaging in achieving an early and accurate diagnosis, thereby enabling timely initiation of disease-modifying therapy.