Clinical Red Flags Raising Suspicion of Cardiac Amyloidosis

Demographic and Clinical Context

Cardiac amyloidosis should be suspected in patients older than 65 years presenting with heart failure and increased left ventricular (LV) wall thickness [3, 5]. Unexplained LV hypertrophy (LVH) or heart failure with preserved ejection fraction (HFpEF) at hospitalization warrants evaluation [5]. Heightened awareness is essential in patients with severe aortic stenosis, particularly the low-flow, low-gradient phenotype, or those undergoing aortic valve replacement [1, 5, 10]. A history of increasing frequency of hospitalizations and visits for heart failure without disease recognition in the 3 years preceding diagnosis is also a notable red flag [8]. Furthermore, intolerance to beta blockers, angiotensin-converting enzyme (ACE) inhibitors, angiotensin receptor blockers (ARBs), or angiotensin receptor neprilysin inhibitors (ARNIs), or a reduced requirement for antihypertensives, should prompt clinical suspicion [1].

Cardiac Signs and Symptoms

Patients often present with symptoms of predominant right heart failure, including lower extremity edema, ascites, and hepatic enlargement [8, 10]. Atrial arrhythmias, particularly atrial fibrillation and flutter, are common and may precipitate heart failure [8, 10]. Refractory atrial arrhythmias resistant to antiarrhythmic drugs or repeated ablation procedures should raise suspicion, especially in patients in their mid-60s or older [10]. Physical examination may reveal a normal or low pulse pressure, an elevated jugular venous pressure with a paradoxical rise on inspiration (Kussmaul sign), and a difficult-to-palpate apex beat [10]. Despite congestive heart failure, a third or fourth heart sound is rarely heard due to restrictive pathophysiology and impaired ventricular relaxation [10]. There is also an increased risk of intracardiac thrombus and stroke or systemic embolization, even in patients in sinus rhythm [8].

Differentiating ATTR and AL Cardiac Amyloidosis

Definition and Pathophysiology

Cardiac amyloidosis (CA) is a protein misfolding disorder characterized by the extracellular deposition of misfolded fibrillar proteins in the myocardium, leading to a diffuse infiltrative cardiomyopathy [8, 15]. The deposited amyloid exhibits pathognomonic apple-green birefringence under cross-polarized light after staining with Congo red [10, 15].

The two predominant forms affecting the heart are immunoglobulin light-chain (AL) amyloidosis and transthyretin (ATTR) amyloidosis [8, 15].

• AL Amyloidosis: This is a plasma cell dyscrasia in which abnormal plasma cells produce misfolded immunoglobulin light chains [8, 19]. Light chains are directly toxic to cardiomyocytes, inducing the elaboration of high levels of natriuretic peptides and cardiac troponin [19].

• ATTR Amyloidosis: This form arises from the misfolding of transthyretin, a protein produced primarily in the liver [10]. ATTR is subclassified based on the TTR gene sequence on chromosome 18 [19]:

Clinical Presentation and Course

Both AL and ATTR-CA typically present as heart failure with preserved ejection fraction (HFpEF), and a multitude of clinical symptoms cannot reliably distinguish AL from ATTR-CA [8]. However, their clinical trajectories and extracardiac manifestations differ significantly.

• AL Amyloidosis: Associated with more rapid progression of heart failure and a worse prognosis compared to ATTR [6]. Untreated, the median time from heart failure presentation to death in AL amyloidosis is approximately 6 months [9].

• ATTR Amyloidosis: Clinically better tolerated than AL amyloid cardiomyopathy [9]. Untreated survival after onset of ATTR disease is 4 to 15 years, depending on whether the disease primarily affects the heart or the nervous system [2]. The median time from heart failure presentation to death in untreated ATTR is 42 to 48 months [9]. ATTRwt is predominantly expressed in men beginning in the seventh decade [9].

Extracardiac manifestations are largely defined by the specific amyloid precursor:

• ATTRv: Presents as familial amyloidotic polyneuropathy or familial amyloidotic cardiomyopathy [9]. Peripheral neuropathy begins as a length-dependent small-fiber sensorimotor neuropathy in the feet with ascending progression [9]. Autonomic neuropathy manifests as smooth muscle dysmotility (dysphagia, diarrhea, urinary retention), vascular dysregulation (orthostatic hypotension, erectile dysfunction), and anhidrosis [9]. Soft tissue disease, such as bilateral carpal tunnel syndrome, tendinopathy, and spinal stenosis, commonly precedes nerve or heart manifestations by one to two decades [9, 16].

• AL: May present with pathognomonic soft tissue findings of macroglossia or periorbital ecchymoses [14].

Diagnostic Evaluation

Accurate precursor protein identification is essential, as treatment is specific to the amyloid type and incorrect typing leads to inappropriate therapy [10, 14].

Biomarkers and Hematologic Testing

Persistent and unexplained elevations in cardiac troponins and natriuretic peptides are hallmark features of CA but are nonspecific [5].

• AL: Diagnosis requires evidence of a plasma cell dyscrasia. An abnormal increase in lambda (more common) or kappa free light chains with an abnormal ratio, and/or identification of a monoclonal band on immunofixation electrophoresis, is indicative [5]. Serum protein electrophoresis is insensitive and should not be obtained without subsequent immunofixation [5]. Tissue diagnosis remains a requisite for AL-CA [19].

• ATTR: Plasma cell testing abnormalities can be seen in up to 40% of patients with ATTR amyloidosis due to the increased incidence of monoclonal gammopathy of undetermined significance (MGUS) with age, highlighting the necessity of hematologic consultation in unclear scenarios [5]. Lower prealbumin (TTR) concentration may identify patients with ATTRv and inform prognosis in ATTRwt [5].

Imaging and Biopsy

Echocardiography and cardiac magnetic resonance (CMR) are important first tests that raise suspicion for CA [8]. Typical imaging features include increased left ventricular wall thickness greater than 12 mm, late gadolinium enhancement, or expanded extracellular volume greater than 0.40 [8, 17].

Radionuclide scintigraphy with bone-avid radiotracers (99mTc-PYP/DPD/HMDP) is the key diagnostic study for ATTR-CA [8]. A grade 2 or 3 positive scan can identify cardiac ATTR amyloidosis with nearly 100% specificity and 71% sensitivity, provided AL amyloidosis is excluded [8]. Exclusion of AL using serum-free light-chain assay and serum/urine immunofixation electrophoresis is critical to maintain this high specificity [8]. If AL is excluded and the bone scan is strongly positive, a biopsy is not required to diagnose ATTR-CA [16, 17].

Endomyocardial biopsy (EMB) remains the gold standard for TTR-CA diagnosis if hereditary ATTR-CA is suspected and the bone scintigram is negative [8, 17]. EMB is also necessary for AL-CA in the context of inconsistent testing or high clinical suspicion [19]. Amyloid-targeted PET tracers are the only clinically available tracers to image AL cardiac amyloidosis [8].

Genotyping

Once a diagnosis of ATTR-CA is confirmed, genotyping of TTR is a final critical step to distinguish ATTRv from ATTRwt, which has implications for treatment, prognosis, and raises the possibility of gene inheritance for first-degree relatives [3, 8]. A genotyping-only approach will fail to identify ATTRwt, the most common type of CA [3].

Key Distinctions: ATTR vs. AL Amyloidosis

Management

Therapeutic strategies are highly specific to the precursor protein.

• AL Amyloidosis: Therapy is based on treatment of the underlying hematological problem with chemotherapy or autologous stem-cell transplant [1]. New targeted plasma cell therapies have rapidly expanded treatment options and improved outcomes [19]. A 30% reduction in N-terminal prohormone brain natriuretic peptide after therapy serves as a marker of a favorable cardiac response and extended survival [19].

• ATTR Amyloidosis: Treatment is based on transthyretin stabilization and reduction of its production [1].

Diagnostic Role of Bone Scintigraphy and Free Light Chains in Cardiac Amyloidosis

The diagnostic evaluation of cardiac amyloidosis (CA) relies heavily on the integration of nuclear imaging and hematologic biomarkers to differentiate amyloid subtypes, specifically transthyretin amyloidosis (ATTR) from light-chain amyloidosis (AL).

Role of Bone Scintigraphy

Bone scintigraphy utilizing bone-seeking radiotracers is a cornerstone of the non-invasive diagnostic algorithm for CA. The commonly employed radiotracers include 99mTc-pyrophosphate (PYP), 99mTc-3,3-diphosphono-1,2-propanodicarboxylic acid (DPD), and 99mTc-hydroxymethylene diphosphonate (HMDP) [18].

Mechanism and Diagnostic Criteria

These radiotracers bind to amyloid fibrils, allowing for the visualization of myocardial infiltration. Myocardial uptake is typically graded using the semiquantitative Perugini score, ranging from grade 0 (no uptake) to grade 3 (cardiac uptake exceeding rib uptake) [18].

Non-invasive diagnostic criteria for ATTR-CA require the following [15][18]:

• Typical echocardiographic or cardiac magnetic resonance (CMR) features of amyloidosis.

• Grade 2 or 3 myocardial radiotracer uptake on planar and single-photon emission computed tomography (SPECT) imaging.

• Absence of a clonal plasma cell dyscrasia, confirmed by serum and urine testing.

When grade 2 or 3 uptake is observed in the absence of a monoclonal protein, bone scintigraphy demonstrates a specificity of 100% for ATTR-CA [18]. This high specificity has enabled the widespread adoption of a non-biopsy diagnostic approach for ATTR-CA [18].

Technical Considerations and Limitations

Planar imaging alone is insufficient for diagnosis. SPECT imaging is mandatory to confirm true myocardial retention and to exclude false positives caused by blood pool uptake [18]. SPECT with CT attenuation mapping is preferred to accurately colocalize tracer uptake with cardiac anatomy [18].

Despite its high specificity, the sensitivity of nuclear scintigraphy is approximately 70% [18]. False-negative scans may rarely occur in certain ATTRv genotypes [15]. False-positive results can be attributed to AL cardiac amyloidosis, recent myocardial infarction, diffuse myocardial scarring, overlying rib fractures, hydroxychloroquine toxicity, and unusual amyloidosis variants such as ApoA1 [18].

Role of Free Light Chains and Monoclonal Protein Assessment

The evaluation for a monoclonal protein is a prerequisite step in the CA diagnostic algorithm. It is essential for interpreting bone scintigraphy results and for identifying AL-CA, which is a plasma cell dyscrasia [14][18].

Required Laboratory Testing

To adequately exclude a clonal dyscrasia, the following serum and urine tests must be performed [15][18]:

• Serum free light chain (FLC) assay: Quantifies kappa and lambda light chains and calculates their ratio. The ratio is considered abnormal if it is less than 0.26 or greater than 1.65 [18].

• Serum and urine immunofixation electrophoresis (IFE): Identifies monoclonal proteins.

Serum protein electrophoresis (SPEP) alone is insufficiently sensitive for identifying AL amyloidosis and should not be used as a standalone test [1][18].

Interpretation in the Diagnostic Algorithm

• Exclusion of AL-CA: If monoclonal proteins are absent and bone scintigraphy demonstrates grade 2 or 3 uptake, a diagnosis of ATTR-CA is confirmed [18].

• Suspicion for AL-CA: An abnormal increase in lambda (more common) or kappa FLCs with an abnormal ratio, or the identification of a monoclonal band on IFE, is indicative of a plasma cell dyscrasia [1]. In the proper clinical context, these findings increase suspicion for AL-CA but are not independently diagnostic [1].

• Need for Biopsy: If monoclonal gammopathy is demonstrated, endomyocardial biopsy (EMB) is required, as bone scintigraphy cannot reliably distinguish ATTR from AL in this setting [18]. Furthermore, if scintigraphy is negative or equivocal but clinical suspicion for CA remains high, biopsy is also indicated [18].

Caveats in Interpretation

Interpretation of FLC assays can be complicated by concurrent clinical conditions. In chronic kidney disease, it is common to observe a kappa predominance with an abnormal FLC ratio but normal immunofixation, which can confuse the diagnostic picture [1]. Additionally, plasma cell testing abnormalities can be seen in up to 40% of patients with ATTR amyloidosis due to the increased incidence of monoclonal gammopathy of undetermined significance (MGUS) with age [1]. Hematologic consultation is recommended in unclear scenarios to aid in the interpretation of complex laboratory results [1].

Based on the provided context, characteristic electrocardiographic and echocardiographic findings vary significantly depending on the underlying cardiovascular pathology. The following sections synthesize the key diagnostic features for distinct clinical scenarios.

Electrocardiographic Findings

Acute Coronary Syndromes and Ischemia

In patients presenting with suspected acute coronary syndrome (ACS), an ECG should be obtained within 10 minutes of first medical contact [11]. Characteristic findings include:

• ST-segment elevation: Identifies patients with ongoing ischemia who are candidates for immediate reperfusion therapy [2].

• ST-segment depression and symmetric T-wave inversions: When at least 0.2 mV in depth, these are indicative of myocardial ischemia in the absence of ST-elevation myocardial infarction (STEMI) and correlate with a higher risk of death or recurrent ischemic events [2].

• Normal or nondiagnostic ECGs: An initial ECG may be normal in more than one-third of patients with non-ST-elevation ACS (NSTE-ACS). Serial ECGs at 15- to 30-minute intervals are recommended for patients with diagnostic uncertainty or recurrent symptoms [11]. Right-sided lead placement should be considered if ischemia is clinically suspected despite a standard 12-lead ECG being nondiagnostic [2].

Heart Failure

In the evaluation of heart failure (HF), the ECG is rarely normal but may display nonspecific findings; its positive predictive value in this setting far exceeds its negative predictive value [9]. Characteristic findings include:

• Sinus tachycardia: Reflects sympathetic nervous system activation in advanced HF or acute decompensation [9].

• Atrial arrhythmias: Atrial fibrillation, present in 20% to 30% of decompensated HF patients, may provide etiologic clues and therapeutic targets [8, 9].

• Ventricular ectopy: Identifies patients at increased risk for sudden death, particularly when the ejection fraction is very low (e.g., < 30%) [9].

• QRS voltage changes: Increased voltage suggests left ventricular hypertrophy, whereas low voltage may indicate infiltrative disease or pericardial effusion [9].

• Q waves: Suggest that HF may be secondary to ischemic heart disease [9].

• Interval prolongations: A prolonged PR interval may indicate intrinsic conduction disease or infiltrative cardiomyopathy (e.g., amyloidosis). A prolonged QT interval may reflect electrolyte abnormalities or drug effects, identifying risk for torsades de pointes [9].

Pulmonary Hypertension

ECG abnormalities can raise suspicion of pulmonary hypertension (PH) and provide prognostic information [4].

• Right axis deviation: Has a high predictive value for PH in adults with unexplained dyspnea on exertion [4].

• RV hypertrophy patterns: In the setting of PH, the ECG typically shows evidence of RV hypertrophy [16].

• Normal ECG limitations: A normal ECG does not exclude PH. However, a normal ECG combined with normal biomarkers (BNP/NT-proBNP) is associated with a low likelihood of PH in patients referred for suspected PH or those at risk following an acute pulmonary embolism [4].

Bradycardia and Conduction Disturbances

For patients with suspected bradycardia (resting heart rate < 60 beats/min, or sinus bradycardia < 50 beats/min), a 12-lead ECG is the initial diagnostic test of choice (Class I, Level of Evidence B) [10].

• AV block characterization: Evaluation of the PR interval, QRS duration, and the presence of escape rhythms helps determine the level of atrioventricular block or sinus node dysfunction [10].

• Infiltrative/conduction disease clues: A prolonged PR interval may suggest infiltrative cardiomyopathy such as amyloidosis [9].

Arrhythmias and Inherited Syndromes

The resting ECG can reveal signatures of electrophysiologic disturbances that predispose patients to arrhythmias [6].

• Preexcitation: Delta waves seen in Wolff-Parkinson-White syndrome [6].

• Channelopathies: Prolongation or shortening of the QT interval, or right precordial ST-segment abnormalities characteristic of Brugada syndrome [6].

• Structural disease: Epsilon waves in arrhythmogenic right ventricular cardiomyopathy/dysplasia (ARVC/D) [6].

Muscular Dystrophies

• Duchenne muscular dystrophy (DMD): The ECG is abnormal in the majority of patients. Classic findings include tall R waves and increased R/S amplitude in V1, deep narrow Q waves in the left precordial leads, a short PR interval, and right ventricular hypertrophy [5].

• Becker muscular dystrophy (BMD): ECG abnormalities are present in up to 75% of patients, including tall R waves and increased R/S amplitude in V1. In those with dilated cardiomyopathy, a left bundle branch block is common [5].

Pericarditis and Pulmonary Embolism

• Pericarditis: Diffuse ST-segment elevation not corresponding to a specific coronary anatomic distribution, along with PR-segment depression, helps distinguish pericarditis from acute MI [2].

• Pulmonary embolism: Most commonly associated with sinus tachycardia, but may also cause a rightward shift of the axis, manifesting as an S-wave in lead I with a Q-wave and T-wave in lead III [2].

Echocardiographic Findings

Heart Failure

Echocardiography is generally the single most useful test in evaluating the cause of decompensation in a patient with HF [8].

• Global and regional function: Can assess global systolic and diastolic function, regional wall motion abnormalities, valvular function, hemodynamics (including filling pressures and cardiac output), and pericardial disease [8].

• Tissue Doppler ratios: The ratio of peak early diastolic transmitral blood flow velocity (E) to peak early diastolic mitral annular tissue velocity (Ea) is additive to natriuretic peptides in diagnosing decompensated HF. An E:Ea ratio > 15 predicts a pulmonary capillary wedge pressure (PCWP) > 15 mm Hg [8].

Cor Triatriatum

Transthoracic echocardiography is the mainstay of diagnosis for this condition [1].

• Membrane visualization: Can clearly demonstrate the obstructive membrane, often from parasternal long-axis, short-axis, or apical four-chamber views [1].

• Doppler assessment: Color Doppler shows where blood flow passes through the membrane and assesses for obstruction, with gradients measured by spectral Doppler [1].

• Alternative modalities: If acoustic windows are insufficient, transesophageal echocardiography (TEE) is an excellent choice to show the posteriorly located membrane within the left atrium. Rarely, cardiac magnetic resonance (CMR) or CT may be required to demonstrate the membrane and pulmonary vein anatomy [1].

Pulmonary Regurgitation

• RV volume overload patterns: In the absence of pulmonary hypertension, pulmonary regurgitation on an ECG presents as RV diastolic overload, specifically an rSr (or rsR) configuration in the right precordial leads [16].

Kawasaki Disease

• Coronary involvement: Echocardiography can detect coronary involvement from the second week of illness onward and is used to monitor disease progress [7].

Disease-Specific Pharmacotherapy for Transthyretin Amyloidosis (ATTR)

Therapeutic strategies for transthyretin amyloidosis (ATTR) target the amyloidogenic protein at various stages of its pathogenesis. Current disease-specific pharmacologic approaches include TTR silencing (or knockdown), TTR tetramer stabilization, and emerging TTR amyloid fibril disruption [1]. Additionally, orthotopic liver transplantation (OLT) was historically utilized to replace the production source of mutated TTR, though it is now rarely employed due to the availability of modern pharmacotherapies [2, 8].

TTR Stabilizers

TTR stabilizers are orally available small molecules that bind to the thyroxine-binding sites of the TTR tetramer, stabilizing it kinetically and preventing its dissociation into amyloidogenic monomers [2, 3].

• Tafamidis: A benzoxazole derivative lacking nonsteroidal antiinflammatory drug (NSAID) activity, tafamidis is approved for ATTR cardiomyopathy and polyneuropathy. In the phase III ATTR-ACT trial, it demonstrated a reduction in all-cause mortality and cardiovascular hospitalizations, with the most significant clinical benefit observed in patients at NYHA functional class I and II [3, 4]. It also attenuated the decline in 6-minute walk test distance and Kansas City Cardiomyopathy Questionnaire Overall Score (KCCQ-OS) [3].

• Acoramidis: Designed to mimic the naturally occurring T119M "rescue mutation," acoramidis is approved for cardiomyopathy. The ATTRibute-CM trial showed it reduced all-cause mortality by an absolute 6.4% (25% relative risk reduction) and cardiovascular hospitalizations by a 50% relative risk reduction over 30 months. It also attenuated the rise in NT-proBNP and slowed the decline in 6-minute walk distance and KCCQ-OS [3].

• Diflunisal: An NSAID repurposed for ATTR, diflunisal stabilizes the TTR tetramer similarly to tafamidis. Used off-label, it improved symptoms of amyloid polyneuropathy and demonstrated improved echocardiographic markers and survival in retrospective studies. It is considered a cost-effective alternative in selected patients, particularly those with an estimated glomerular filtration rate (eGFR) greater than 45 mL/min, requiring careful monitoring of volume status and renal function [1, 3].

TTR Silencers

TTR silencers suppress hepatic TTR production, significantly reducing circulating TTR levels and halting or reversing neurologic disease progression [2, 9].

• Patisiran: An intravenously administered small interfering RNA (siRNA) targeting hepatic TTR mRNA, given every 3 weeks. It reduces circulating TTR by 80% to 85% and requires premedication with corticosteroids and histamine receptor blockers to manage infusion reactions [9]. It is approved for ATTRv polyneuropathy, with or without cardiomyopathy. While the APOLLO-B trial showed statistically significant improvements in functional capacity and cardiac function in cardiomyopathy, the FDA declined expanding approval for ATTR-CA alone due to unclear clinical meaningfulness [9].

• Vutrisiran: A subcutaneously administered RNA interference therapeutic given every 3 months. It is approved for ATTRv disease with neuropathy and, based on the HELIOS-B trial, recently approved for ATTR cardiomyopathy. The trial demonstrated a lower risk of death and recurrent cardiovascular events compared to placebo (hazard ratio 0.72 in the overall group) and a lower risk of death through 42 months [9].

• Inotersen: A subcutaneously administered antisense oligonucleotide given weekly. It demonstrated efficacy in ameliorating polyneuropathy in the NEURO-TTR study and was approved for ATTRv polyneuropathy with or without cardiomyopathy. Due to toxicities of thrombocytopenia and glomerulonephritis (both occurring at a 3% rate), it requires a Risk Evaluation and Mitigation Strategy with weekly platelet count monitoring and biweekly renal function monitoring. It is no longer marketed in the United States [9].

• Eplontersen: A ligand-conjugated antisense oligonucleotide administered subcutaneously. It has demonstrated efficacy in ATTRv polyneuropathy and is currently being evaluated in a prospective randomized trial for ATTR-CA [9].

Emerging Therapies

• CRISPR-Cas9 Gene Editing: One-time gene editing therapies, such as nexiguran ziclumeran (NTLA-2001), are in phase III clinical trials and offer the potential for permanent silencing of TTR expression [2, 9].

• Antifibrillar Agents: Humanized monoclonal antibodies are in late-phase clinical development to target preexisting amyloid deposits. By recognizing misfolded TTR epitopes, these agents mobilize macrophages and monocytes to disrupt existing amyloid deposits. Agents targeting TTR deposits include NNC6019 (Coramitug) and ALXN2220 (NI006) [2, 17].

Advanced Surgical Therapies

Orthotopic liver transplantation (OLT) replaces the factory of mutated TTR with wild-type TTR. It is now rarely utilized and limited to patients with ATTRv amyloid, early peripheral neuropathy (V30M ATTR), and minimal systemic amyloid burden. Patients with extensive amyloid burden often experience posttransplant disease progression due to wild-type TTR complexing on preexisting amyloid deposits, a process known as seeding [2, 8]. Orthotopic heart transplantation (OHT) may be considered for advanced heart failure in carefully selected patients with wild-type or cardiac-restricted variant disease, with outcomes comparable to nonamyloid indications. Combined OHT and OLT is largely obsolete due to contemporary pharmacotherapies [8].

Summary of Approved Therapies