3 Discordant Extracardiac Amyloid Proteins Highlighting Bystander Deposition in a Case of Light-Chain Amyloid Cardiomyopathy.

BACKGROUND: Accurate differentiation of immunoglobulin light-chain (AL) from transthyretin (ATTR) amyloid cardiomyopathy is critical, as misclassification may delay appropriate therapy and worsen outcomes. CASE SUMMARY: An 80-year-old woman with incidental gallbladder AL lambda and ApoA-IV amyloid deposits had elevated cardiac biomarkers and lambda monoclonal gammopathy. Bone marrow biopsy demonstrated AL and ATTR deposits. Echocardiography showed increased wall thickness, apical-sparing strain, and elevated filling pressures. Given these discordant findings, endomyocardial biopsy (EMB) was performed; proteomic analysis confirmed AL lambda deposition without ATTR, prompting anti-plasma cell therapy. DISCUSSION: This is a rare case of mixed extracardiac amyloid required EMB with proteomic analysis to resolve discordant findings and confirm AL cardiomyopathy. TAKE-HOME MESSAGES: Amyloid cardiomyopathy subtyping is essential because AL and ATTR amyloidosis treatment differ fundamentally. Underrecognized codeposited "bystander" proteins may confound typing; therefore, discordant extracardiac findings may not represent the myocardial subtype. When uncertainty persists, particularly with monoclonal gammopathy, EMB is essential to define cardiac amyloid subtype.