Rapid progression of monoclonal gammopathy of undetermined significance to cardiac amyloidosis with intracardiac thrombus: Unveiling via cardiac magnetic resonance.

We present the case of a 67-year-old male diagnosed with Monoclonal Gammopathy of Undetermined Significance (MGUS) who developed progressive peripheral edema and abdominal distension, raising concerns for right-sided heart failure. Initial investigations, including electrocardiography and transthoracic echocardiography (TTE), revealed findings suggestive of restrictive diastolic dysfunction, including low-voltage QRS complexes, concentric left ventricular hypertrophy, left atrial enlargement, and valvular thickening, despite a preserved ejection fraction (EF) of 62%. These findings raised suspicion for AL-type cardiac amyloidosis in the context of the patient's recent MGUS diagnosis. Although no thrombus was detected on TTE, cardiac magnetic resonance imaging (CMR) conducted 2 weeks later demonstrated findings supportive of cardiac amyloidosis, such as myocardial infiltration and increased extracellular volume, and identified a left atrial appendage thrombus which was not visualized on the initial TTE. This case highlights the diagnostic superiority of CMR over TTE in detecting intracardiac thrombi. While transesophageal echocardiography (TEE) remains the gold standard for thrombus detection, its semi-invasive nature limits its routine use in initial evaluations. The findings underscore the importance of early utilization of CMR in suspected cardiac amyloidosis cases, emphasizing its comprehensive noninvasive assessment capabilities for guiding timely diagnosis and management.