Differential Diagnosis in ATTR-CM: 99mTc-PYP Scintigraphy and Diagnostic Workup

Clinical Summary

In ATTR-CM, the core diagnostic problem is distinguishing a potentially treatable infiltrative cardiomyopathy from more common causes of increased wall thickness, heart failure symptoms, conduction disease, or restrictive physiology before disease-specific treatment is interpreted in the right context.^2,5 The presenting phenotype is often nonspecific and can overlap with hypertensive heart disease, hypertrophic cardiomyopathy, aortic stenosis, HFpEF, ischemic disease, and other infiltrative or restrictive processes, so the practical question is when the overall pattern justifies moving beyond routine cardiomyopathy evaluation toward a structured amyloidosis workup.¹

Suspicion usually rises from a cluster rather than a single finding: ventricular wall thickening that appears out of proportion to ECG voltage, diastolic dysfunction or restrictive filling, atrial arrhythmias, conduction disease, heart failure symptoms that seem disproportionate to coronary or valvular findings, and extracardiac clues such as bilateral carpal tunnel syndrome, lumbar spinal stenosis, biceps tendon involvement, peripheral neuropathy, or autonomic features.^1-2 These findings make ATTR-CM more plausible, but none is individually diagnostic, and coexisting cardiac disease can blur the picture enough that even a suggestive clue does not end the differential.

Excluding AL amyloidosis remains central because it can closely mimic ATTR-CM clinically and on imaging. Serum free light chain testing together with serum and urine immunofixation helps identify or exclude a monoclonal protein signal that would redirect the workup toward a plasma cell dyscrasia.² A negative screen meaningfully narrows the differential, but it does not by itself establish ATTR-CM; conversely, a positive monoclonal result does not automatically exclude ATTR-CM, particularly in older patients in whom incidental monoclonal gammopathy may coexist and mixed disease remains possible.

Imaging then refines, rather than replaces, that probability assessment. Echocardiography can increase suspicion when wall thickening is accompanied by restrictive filling, atrial enlargement, valve or right-sided involvement, or strain patterns compatible with amyloid, yet overlap with hypertensive remodeling, hypertrophic cardiomyopathy, and aortic stenosis remains substantial.^2,5 Cardiac MRI can strengthen suspicion for infiltrative disease through diffuse myocardial involvement and characteristic tissue characterization findings, but it cannot determine amyloid type on its own.¹ Similarly, 99mTc-PYP scintigraphy can support ATTR-CM when the pattern is typical and the rest of the workup is concordant, but interpretation depends on the broader clinical context and on excluding AL amyloidosis and other mimics; false-positive, indeterminate, or mixed-disease scenarios still require caution.^1,5

Biomarkers such as NT-proBNP and troponin are best read as contextual markers of myocardial stress or injury rather than as discriminators between ATTR-CM and its mimics. Elevation can heighten concern when it appears disproportionate to the expected phenotype, but chronic kidney disease, atrial arrhythmias, aortic stenosis, ischemic cardiomyopathy, and noncardiac contributors can all produce similar abnormalities.² Genetic testing adds another layer once ATTR-CM is suspected: a pathogenic TTR variant supports hereditary disease, while a negative result does not exclude wild-type ATTR-CM.² The practical endpoint is a staged synthesis of phenotype, extracardiac context, monoclonal protein assessment, imaging, and, when needed, tissue confirmation, because ATTR-CM is most often misclassified when the evaluation stops at a single abnormal test or an incomplete explanation.