Negative bone scintigraphy in wild-type transthyretin cardiac amyloidosis

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Abstract
Background: Amyloidosis is a rare systemic disease due to the extracellular tissue deposition of a fibrillar-shaped misfolded protein, called amyloid. Only two types of proteins commonly affect the heart leading to an infiltrative cardiomyopathy: immunoglobulin light chain and transthyretin (TTR) cardiac amyloidosis (CA). Despite the promising role of emerging imaging modalities, such as strain echocardiography, cardiac magnetic resonance and bone scintigraphy, its diagnosis is still often missed or delayed due to their inherent limitations and to a nonspecific clinical scenario with frequent concomitance of cardiac comorbidities. The gold standard for a definite diagnosis still remains endomyocardial biopsy, but in rare cases Congo Red staining could provide false negative results, as in our case, requiring immunoelectron microscopy. Case presentation: A middle-aged male adult presented to the emergency department for relapse of heart failure. Echocardiography and cardiac magnetic resonance, along with the history of bilateral carpal tunnel syndrome, were suspicious for TTR-CA. The diagnosis, however, was hampered by concomitant cardiac comorbidities and conflicting results of imaging modalities. In fact bone scintigraphy was negative, as well as Congo Red Staining on myocardial tissue samples obtained by endomyocardial biopsy. Given the high clinical suspicion, immunoelectron microscopy was performed, showing TTR amyloid fibrils deposits, that confirmed the diagnosis. A genetic analysis excluded and hereditary form. The patient was then referred to a specialist center for specific treatment. Conclusions: This is a rare case of a TTR-CA with a negative Bone Scintigraphy and Congo red staining, which demonstrated that CA is frequently misdiagnosed because of the low specific clinical manifestations and the results of imaging modalities that sometimes could be misleading, with subsequent delayed diagnosis and correct treatment.

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