Apolipoprotein A-IV Cardiac Amyloidosis as a Diagnostic Pitfall: Beyond the Noninvasive Algorithm.

BACKGROUND: Cardiac amyloidosis is frequently underdiagnosed, and rarer forms (eg, nontransthyretin amyloidosis, light chain amyloidosis) can be challenging to uncover. CASE SUMMARY: A 71-year-old man with reduced kidney function presented with new-onset heart failure, concentric left ventricular hypertrophy, and an "apical sparing" strain pattern. Cardiac magnetic resonance confirmed an infiltrative phenotype with suggestive late gadolinium enhancement and parametric mapping pattern. However, bone scintigraphy and monoclonal protein screening were negative; therefore, an endomyocardial biopsy was performed. After negative immunohistochemistry, mass spectrometry-based proteomics finally identified apolipoprotein A-IV amyloidosis. DISCUSSION: Apolipoprotein A-IV cardiac amyloidosis represents a critical diagnostic blind spot and requires molecular proteomics for definitive typing. TAKE-HOME MESSAGES: A pathognomonic cardiac magnetic resonance phenotype for cardiac amyloidosis, despite negative bone scintigraphy and monoclonal protein screening, should prompt invasive investigation to identify rare amyloid types. Endomyocardial biopsy with mass spectrometry remains the definitive diagnostic technique for amyloid typing in such cases.