Wild-Type Transthyretin Amyloid Cardiomyopathy With Bone Marrow Involvement: Clinical Implications of Less Common Sites of Amyloid Deposition

Transthyretin amyloidosis (ATTR) is a progressive disease characterized by tissue deposition of transthyretin (TTR)-derived amyloid fibrils, commonly involving the heart, joints, and ligaments. However, deposition in less common sites, such as the bone marrow, has also been reported, but its clinical significance remains unclear. An 86-year-old man presented with mild heart failure. Amyloid was initially detected in synovial tissue during carpal tunnel release surgery, which prompted technetium-99m pyrophosphate (99mTc-PYP) scintigraphy, and an endomyocardial biopsy confirmed wild-type ATTR cardiomyopathy (CM). Due to an elevated serum-free light chain ratio, a bone marrow biopsy was performed to exclude immunoglobulin light chain (AL) amyloidosis or myeloma, revealing amyloid deposits positive for TTR and negative for both anti-κ and anti-λ antibodies, indicating ATTR-type deposition. A TTR stabilizer was initiated shortly after diagnosis. Although biomarker levels, such as B-type natriuretic peptide and troponin I, and echocardiographic findings were consistent with mild disease at presentation, both gradually worsened over the subsequent year, in parallel with increasing heart failure symptoms. This case highlights that TTR amyloid deposition in a less common site (e.g., the bone marrow), even when the initial clinical findings are mild, may reflect a more advanced stage of wild-type ATTR-CM.