While CA classically presents with clinical signs of restrictive cardiomyopathy, in daily practice, a restrictive pattern is present in fewer than half of the patients with CA. [
5]. Although both TTR and AL CA share similar cardiac functional and morphological characteristics when multimodal imaging techniques are appropriately combined with laboratory and genetic tests, identification of specifying types of CA improves. We recommend using multimodal cardiac imaging, including echocardiogram, cardiac MRI, and nuclear imaging, to differentiate CA, AL, and TTR. The accurate identification of the amyloid subtype is a cornerstone step because the light chain and transthyretin kinds have different prognoses and treatments. If untreated, survival ranges from less than six months for AL CA to three to five years for TTR [
6,
7].
While biopsy is a valuable method for obtaining tissue to confirm amyloid infiltration, Congo red staining, which identifies amyloid infiltration as typical apple-green birefringence using polarized light, does not differentiate between the two relevant types of CA. Although heart biopsy is more likely to reveal amyloid deposits in cases with cardiac involvement, tissue may be obtained from other sites, such as abdominal fat, bone marrow, and kidney [
8]. In this setting, immunohistochemistry and mass spectrometry can be used. Fat aspiration is a simpler procedure than endomyocardial biopsy. Although its performance is limited in TTR CA, it can be appropriate in AL, as 84% of patients have a positive result [
9].
Compelling evidence demonstrates that noninvasive imaging techniques can effectively diagnose TTR CA (Table
1). The echocardiogram is the first-line cardiac imaging method. Particularly in the early stage, it lacks specificity to precisely distinguish amyloid from nonamyloid infiltrative or hypertrophic heart diseases. The classical findings are biatrial enlargement, valvular and interatrial thickening, pleural and pericardial effusion, and biventricular hypertrophy with a bright and sparkling appearance with preserved left ventricular ejection fraction associated with a restrictive pattern with diastolic dysfunction [
10]. However, most of these findings are usually found in an advanced stage of disease and are also nonspecific to CA [
11]. The presence of a small A wave on mitral inflow Doppler, particularly in the absence of other features of restrictive LV filling, is a clue to identify atrial dysfunction. Cardiac magnetic resonance (CMR) is recognized for its ability to provide gold-standard morphological and functional assessment of the heart. CMR can also provide tissue characterization using multiple sequences, which typically include precontrast T2 imaging for edema and inflation, perfusion for microcirculation assessment, late gadolinium enhanced (LGE) for scar and fibrosis, and T1 mapping (pre- and postcontrast administration) for native T1 and extracellular volume (ECV). Characteristically, patients with CA demonstrate a nonischemic heterogeneous LGE pattern, ranging from transmural or subendocardial to patchy focal LGE, commonly in association with suboptimal myocardial nulling [
12]. A pattern of LGE including global subendocardial, transmural, and patchy LGE is very suggestive of CA, with high sensitivity (86%) and specificity (92%) [
13]. Although LGE is more common in patients with TTR CA, this finding should not be used to differentiate between subtypes [
14]. Recently, the presence of LGE has been shown to be a robust marker for mortality in both LC and TTR CA. Cardiac scintigraphy with bone tracers using a variety of agents (Tc-99m PYP/DPD/HMDP) has revolutionized the TTR CA diagnostic approach characterizing TTR amyloid deposits in the myocardium [
15] (Table
1). No plasma or urinary biomarker is available for the diagnosis of TTR CA [
11]. Nonetheless, a compound of unusually high plasma levels of N-terminal pro–B-type natriuretic peptide (NT-proBNP) and high troponin levels in a patient with a clinical phenotype of CA should indicate a diagnostic workup. One recent study showed that NT-proBNP is a biomarker that can be highly elevated in ATTRh amyloidosis, especially among asymptomatic carriers of a
TTR gene mutation or patients with neurological symptoms only [
16]. For ATTR-CA, cardiac biomarkers have also recently been used for staging and prognostic stratification. Different staging systems for ATTR-CA have been proposed: one that includes NT-proBNP (> 3000 pg/mL) and troponin T (> 0.05 ng/mL) [
17] and another that includes NT-ProBNP and estimated glomerular filtration rate (< 45 mL/min) [
18]. Cardiac biomarkers such as natriuretic peptides and cardiac troponins are well-established biomarkers to assess risk and to evaluate response to treatment in patients with AL amyloidosis [
19]. Nevertheless, data in AL amyloidosis does not apply to ATTR amyloidosis due to biological differences between the two diseases [
19].
Table 1Noninvasive imaging techniques and features in cardiac amyloidosis
Echocardiogram | • Classical findings: biatrial enlargement, valvular and interatrial thickening, pleural and pericardial effusion, biventricular hypertrophy with a bright and sparkling appearance with preserved left ventricular ejection fraction, and a restrictive pattern with diastolic dysfunction. • A regional pattern of strain with severe impairment of strains at the middle and basal segments and relative apical sparing of longitudinal strain [ 20]. • Myocardial deformation analysis, identifying changes in its measurement on 2-dimensional speckle tracking imaging with a high prevalent rate (93 to 100%) [ 21]. It has been observed that this strain pattern, known as apical sparing or “cherry on top,” is not specific to CA, since it is also present in other conditions such as aortic stenosis, cardiotoxicity, and dilated cardiomyopathy. • The ejection fraction strain ratio (a ratio of LV ejection fraction/global longitudinal strain > 4.1) improves its accuracy [ 22]. |
Cardiac Magnetic Resonance (CMR) | • Provides tissue characterization using multiple sequences. • In patients with atrial fibrillation and in patients with some metallic devices, its application may be limited or restricted. • Typical findings are a nonischemic heterogeneous LGE pattern, ranging from transmural or subendocardial to patchy focal LGE, commonly in association with suboptimal myocardial nulling [ 12]. • An abnormality in the gadolinium kinetics, also occurring because of systemic amyloid infiltration, drops the blood pool signal to null before the myocardial signal [ 23]. • A global subendocardial, transmural, and patchy LGE pattern is very suggestive of CA [ 13]. • More recently, different groups worldwide have investigated the utility of novel CMR metrics based on T1 mapping techniques, with very promising results [ 23‐ 27]. |
Nuclear Medicine | • Among the available bone tracers, the most studied has been 99mTc-DPD, which seems to be much more specific to TTR CA compared with AL CA [ 28, 29]. • 99mTc-DPD is unable to differentiate between inherent and wild-type TTR CA [ 30]. • Typically, patients with TTR CA have a visual grade ≥ 2, and LA CA patients commonly have no uptake. • It was demonstrated that more than 1 in 5 patients with AL CA have significant uptake of Tc-99m PYP/DPD/HMDP (grades 2 and 3). • The semiquantitative approach using Tc-99m PYP relies on the heart-to-contralateral-lung uptake (H/CL) ratio of > 1.5 at 1 h after tracer administration. This approach was able to precisely differentiate TTR CA from AL CA with high sensitivity (97%) and specificity (100%) [ 31]. • The semiquantitative approach is also gaining attention because unlike the visual score, it may also provide prognostic information [ 31]. |
Several diagnostic algorithms have been proposed that incorporate a multimodal imaging approach. The majority of these proposed algorithms start with an investigation to identify classical clinical (TTR gene-positive, aging, low-flow low-gradient aortic stenosis, neuropathy, carpal tunnel syndrome, biceps tendon rupture, lumbar spinal stenosis) and imaging red flags (Table
1). The first step is to rule out AL CA, and depending on the results of the serum-free light chain level and serum and urine immunofixation studies, cardiac scintigraphy using a bone tracer is recommended. Currently, an endomyocardial biopsy is reserved for equivocal imaging findings or in patients with discordant clinical and imaging findings.