Wild-Type Transthyretin Amyloid Cardiomyopathy With Positive 18F-FDG/13N-Ammonia Cardiac Positron Emission Tomography

BACKGROUND: Wild-type transthyretin amyloid cardiomyopathy (wtATTR-CM) is increasingly recognized with advances in noninvasive imaging and disease-modifying therapies. However, atypical imaging findings may mimic other cardiomyopathies and delay diagnosis. CASE SUMMARY: N-ammonia positron emission tomography/computed tomography (PET/CT) showed matching perfusion defects with focal FDG uptake. Despite methotrexate therapy, he developed progressive heart failure. Serial echocardiography demonstrated increasing left ventricular wall thickness and worsening global longitudinal strain with apical sparing, raising suspicion for amyloidosis. Repeat PET/CT showed persistent perfusion defects but resolution of FDG uptake. Cardiac amyloid scintigraphy revealed grade 3 myocardial tracer uptake (heart-to-contralateral ratio 1.74). Negative monoclonal studies and transthyretin genetic testing confirmed wtATTR-CM. Methotrexate was discontinued, tafamidis was initiated, and symptoms improved. DISCUSSION: N-ammonia PET/CT may demonstrate myocardial FDG uptake, mimicking inflammatory cardiomyopathy such as cardiac sarcoidosis. Proposed mechanisms include localized inflammation from amyloid deposition, altered myocardial glucose metabolism, or coexistence of inflammatory processes. Serial multimodality imaging was essential for diagnostic clarification and timely targeted therapy. TAKE-HOME MESSAGES: wtATTR-CM may present with myocardial FDG uptake, mimicking inflammatory cardiomyopathy. Multimodality imaging is critical to avoid diagnostic delay and guide appropriate treatment.