Restrictive Cardiomyopathy

• Myocardial fibrosis and increased stiffness: Fibrocytes and myofibroblasts accumulate within the myocardium, depositing excessive extracellular matrix collagen (particularly types I and III). This occurs through activation of transforming growth factor-beta (TGF-β) signaling cascades, which drives differentiation of cardiac fibroblasts to myofibroblasts. The excessive collagen increases myocardial passive stiffness (elevated chamber stiffness constant K), meaning minimal increases in ventricular volume require disproportionately large increases in filling pressure. At the molecular level, crosslinking of collagen by lysyl oxidase and advanced glycation end-products (AGEs) further increases stiffness. This fibrotic remodeling directly impairs diastolic relaxation and compliance, creating the pathognomonic "restrictive physiology" on hemodynamics (see below).

• Impaired diastolic relaxation and filling dynamics: The combination of myocardial stiffness and reduced myocardial relaxation (due to altered calcium handling and reduced early diastolic suction) forces the ventricles to operate on the steep portion of the diastolic pressure-volume curve. Early rapid filling ceases prematurely, and ventricular filling becomes increasingly dependent on atrial contraction (atrial "kick"), making these patients critically dependent on maintained atrioventricular synchrony. When atrial fibrillation develops (common in RCM), the loss of organized atrial contraction causes acute hemodynamic decompensation. At the cellular level, abnormalities in sarcoplasmic reticulum calcium ATPase (SERCA2a) and phospholamban dysfunction impair calcium reuptake and prolong relaxation time.

• Restrictive hemodynamic pattern and ventricular interdependence: The stiff ventricles operate in the "restrictive" pattern on cardiac catheterization: markedly elevated and equal (within 5 mmHg) diastolic pressures in both ventricles, steep diastolic pressure slopes (>7 mmHg/mL early diastolic filling), prominent "square root sign" (abrupt rise in early diastole followed by plateau), and increased ventricular interdependence. This interdependence means that volume redistribution between ventricles is exaggerated due to the shared septum and pericardial constraint, so that right ventricular filling pressures directly reflect left ventricular filling pressures. The elevated filling pressures (often 20-40 mmHg) lead to pulmonary and systemic venous congestion.

• Infiltration or replacement of myocardium: In secondary RCM (amyloidosis, sarcoidosis, hemochromatosis, Fabry disease), abnormal proteins or minerals directly replace myocardial tissue or trigger reactive fibrosis. Amyloid fibrils (transthyretin or light chain) deposit between myocytes, physically stiffening the tissue and triggering inflammatory responses that compound fibrosis. Iron deposition in hemochromatosis generates reactive oxygen species (ROS) and oxidative stress, promoting myocyte death and fibrous replacement. This pathophysiology explains why treating the underlying infiltrative disease (iron chelation, immunosuppression) may halt progression.

• Preserved systolic function and resultant hemodynamic paradox: Unlike dilated cardiomyopathy, ejection fraction remains normal or near-normal (>50%) because systolic shortening is preserved; however, the ventricles cannot fill adequately at normal pressures, creating a hemodynamic paradox wherein cardiac output becomes preload-dependent and highly sensitive to modest volume changes. This explains the clinical observation that RCM patients may appear euvolemic on exam yet have markedly elevated filling pressures, and that diuretics (while necessary to relieve symptoms) can cause acute decompensation if over-diuresed.

• Neurohormonal activation and progressive remodeling: Chronically elevated ventricular filling pressures activate the renin-angiotensin-aldosterone system (RAAS), sympathetic nervous system, and natriuretic peptide pathways. Initially, these compensatory mechanisms attempt to maintain cardiac output through increased contractility and heart rate; however, chronic RAAS and sympathetic activation promote further myocardial fibrosis via angiotensin II and catecholamine-mediated pathways, perpetuating a vicious cycle of progressive stiffening and functional decline.

• Idiopathic restrictive cardiomyopathy: Occurs in the absence of identified systemic disease; accounts for ~50% of RCM cases in Western nations. Increasingly, genetic mutations (sarcomeric and cytoskeletal genes including troponin I, troponin C, desmin, and cardiac myosin-binding protein) are being identified, suggesting that many "idiopathic" cases represent unrecognized genetic restrictive cardiomyopathy. Familial inheritance is documented in ~30% of idiopathic RCM cases. The pathophysiology involves progressive myocardial fibrosis with unknown etiology.

• Amyloidosis: The most common infiltrative cause of RCM worldwide. Transthyretin amyloidosis (ATTR) includes wild-type ATTR (ATTRwt, formerly "senile cardiac amyloidosis"), which predominantly affects elderly males and is increasingly recognized, and hereditary ATTR (ATTRv) due to transthyretin gene mutations. Light chain (AL) amyloidosis involves misfolded immunoglobulin light chains (κ or λ) typically from clonal plasma cells and often presents with simultaneous cardiac and renal involvement. Amyloid deposits in the myocardium create severe stiffness and trigger inflammatory responses; ATTRwt is being increasingly recognized as a cause of HFpEF in elderly patients. Diagnosis requires cardiac imaging (echocardiography showing characteristic "granular sparkling" appearance; cardiac MRI with late gadolinium enhancement; cardiac nuclear imaging with technetium-99m pyrophosphate tracer uptake), tissue biopsy with Congo red staining and mass spectrometry, and genetic testing for transthyretin mutations.

• Hemochromatosis: Both primary (hereditary) hemochromatosis (HFE mutations, most commonly HFE C282Y homozygosity) and secondary hemochromatosis (repeated transfusions, chronic liver disease) can cause RCM through iron-catalyzed oxidative damage to myocytes. Iron deposition in cardiac myocytes generates free radicals, impairing contractile function and triggering fibrotic replacement. Diagnosis is established by elevated ferritin and transferrin saturation, genetic testing, and cardiac MRI showing iron deposition in the myocardium (T2* relaxometry <20 ms indicates significant iron). Early iron chelation therapy can halt or partially reverse cardiac involvement.

• Sarcoidosis: Cardiac sarcoidosis (detected clinically in ~5% of sarcoidosis patients, but present on autopsy in ~25%) causes RCM through granulomatous infiltration of the myocardium. Can present with restrictive or dilated physiology; granulomas preferentially involve the basal septum and walls. Diagnosis requires clinical or histopathologic evidence of sarcoidosis plus imaging evidence of cardiac involvement (late gadolinium enhancement on cardiac MRI in a non-ischemic distribution, or FDG-PET tracer uptake). Early immunosuppressive therapy (corticosteroids ± immunosuppressive agents) may preserve cardiac function.

• Endomyocardial fibrosis (EMF): The leading cause of RCM in tropical Africa and the Middle East, affecting young patients (often <40 years). Characterized by fibrosis restricted to the endocardium and subendocardium, resulting in severe diastolic dysfunction and often involving the mitral and tricuspid valves. Etiology remains unclear but may involve hypereosinophilia, parasitic infections, or chronic inflammation. Presents with restrictive physiology and often requires surgical intervention (endomyocardial decortication). Prognosis is generally poor without intervention.

• Fabry disease: An X-linked lysosomal storage disorder caused by mutations in the α-galactosidase A gene, leading to accumulation of globotriaosylceramide in myocardial and other tissues. Presents with cardiac RCM, renal involvement, and neurologic manifestations. Males are more severely affected. Diagnosed by low α-galactosidase A activity in leukocytes and confirmed by genetic testing. Enzyme replacement therapy (imiglucerase) or substrate reduction therapy may slow progression if initiated early.

• Hypereosinophilic syndrome: Can cause RCM through eosinophilic infiltration of the myocardium. Characterized by sustained absolute eosinophil count >1,500/μL with documented tissue involvement. Cardiac involvement presents as restrictive physiology (early stage) progressing to dilated cardiomyopathy and thromboembolic complications (late stage). Diagnosis requires bone marrow examination, cytokine studies, and tissue involvement assessment. Treatment with corticosteroids and cytotoxic agents (imatinib, hydroxyurea) can prevent progression.

• Radiation-induced cardiomyopathy: Mediastinal radiation (for lymphoma, breast cancer, or other thoracic malignancies) can cause late restrictive cardiomyopathy decades after exposure, thought to result from chronic inflammation and fibrosis. Risk increases with doses >30 Gy to the heart.

• Drug-induced toxicity: Chemotherapy agents (anthracyclines, tyrosine kinase inhibitors) can cause restrictive cardiomyopathy, particularly when cumulative doses exceed thresholds.

• Genetic mutations in sarcomeric and cytoskeletal proteins: Familial restrictive cardiomyopathy can result from mutations in genes encoding cardiac troponins (TNNI3, TNNT2), tropomyosin (TPM1), cardiac myosin-binding protein C (MYBPC3), and thick filament proteins. These mutations alter protein-protein interactions and calcium sensitivity, directly increasing myocardial stiffness.

• Dyspnea and orthopnea: Exertional dyspnea is the cardinal symptom, resulting from elevation of left ventricular filling pressure (LVEDP) transmitted retrograde to the pulmonary circulation, causing pulmonary edema at low workloads. Patients often notice dyspnea with minimal exertion (NYHA functional class II or III at presentation). Orthopnea and paroxysmal nocturnal dyspnea (PND) occur as fluid redistributes to the lungs when supine. Unlike in systolic cardiomyopathy, dyspnea in RCM is often refractory to diuretics because patients operate on the steep portion of the diastolic pressure-volume curve; modest volume reduction may dramatically increase filling pressures relative to cardiac output, causing decompensation.

• Fatigue and exercise intolerance: Due to limited cardiac output reserve. The stiff ventricles cannot increase stroke volume in response to exercise demands, and the heart rate response is often blunted, preventing adequate cardiac output augmentation. Fatigue is often disproportionate to the degree of ventricular dysfunction based on ejection fraction.

• Syncope or presyncope: Occurs in ~20% of RCM patients and indicates poor prognosis. Mechanisms include arrhythmia-induced hypotension (atrial fibrillation, ventricular arrhythmias), inadequate heart rate response to exercise, or sudden loss of atrial contraction. Syncope with exertion suggests severe diastolic dysfunction with inability to increase cardiac output.

• Palpitations: Often related to atrial fibrillation, which develops in 50-80% of RCM patients and causes acute clinical deterioration due to loss of the critical atrial "kick." Patients may also experience premature atrial or ventricular contractions.

• Peripheral edema and ascites: Reflecting right ventricular filling pressure elevation and elevated systemic venous pressure. Unlike in systolic cardiomyopathy, peripheral edema may occur despite relatively normal left ventricular function. Ascites can be prominent, leading to abdominal distension, early satiety, and weight gain.

• Hepatic congestion: Passive hepatic congestion from elevated right atrial pressure causes hepatomegaly (often firm, non-tender), hepatic synthetic dysfunction in advanced disease, and elevated transaminases and bilirubin. Severe congestion can lead to hepatic cirrhosis ("cardiac cirrhosis").

• Physical exam findings - Elevated jugular venous pressure (JVP): Markedly elevated JVP with prominent prominent S waves and reduced Y descent (rapid early diastolic descent followed by plateau reflecting restrictive physiology). The "square root sign" in venous pressure tracings mirrors the hemodynamic square root sign. JVP may be fixed and elevated despite diuresis.

• Physical exam findings - Heart sounds: S3 (ventricular gallop) is uncommon in RCM, in contrast to dilated cardiomyopathy; if present, suggests concurrent systolic dysfunction. S4 (atrial gallop) is common, reflecting forceful atrial contraction against the stiff ventricle. In advanced disease with atrial fibrillation, both S3 and S4 may disappear.

• Physical exam findings - Regurgitant murmurs: Functional mitral and tricuspid regurgitation secondary to annular dilation from chronic volume overload; murmurs are holosystolic and increase with inspiration (for tricuspid regurgitation).

• Pulmonary examination: Crackles (rales) at lung bases reflecting pulmonary edema; in severe cases, wheezing may occur ("cardiac asthma").

• Splenomegaly: Can occur from passive congestion, particularly in longstanding disease.

• Important clinical variants by etiology:
• Amyloidosis: May present with orthostatic hypotension (autonomic neuropathy in AL amyloidosis), proteinuria (renal involvement), peripheral neuropathy symptoms (ATTRv), or carpal tunnel syndrome (amyloid transthyretin variant). ATTRwt typically affects elderly males with subtle presentation initially mistaken for normal aging.
• Hemochromatosis: May have concurrent cirrhosis, diabetes (bronzed diabetes), hypogonadism, or arthropathy. Skin hyperpigmentation may be present.
• Sarcoidosis: May have pulmonary symptoms, hypercalcemia, uveitis, or constitutional symptoms of sarcoidosis.
• Endomyocardial fibrosis: Often presents in young patients from endemic areas with severe restrictive physiology, marked hepatomegaly/ascites, and embolic complications from endocardial thrombi.

• Clinical presentation and history: Progressive exertional dyspnea, orthopnea, PND, fatigue, syncope, and signs of right heart failure (elevated JVP, hepatomegaly, peripheral edema) in the absence of systemic hypertension or significant left ventricular hypertrophy history. Inquiry should include exposure history (mediastinal radiation, chemotherapy), family history (familial cardiomyopathy), and systemic symptoms (weight loss, joint pain, fever, travel to endemic areas suggesting sarcoidosis

Immediate stabilization

• Loop diuretics (furosemide): first-line for pulmonary and systemic congestion. Titrate cautiously — the stiff ventricle sits on the steep limb of the pressure–volume curve, so aggressive preload reduction drops cardiac output and precipitates hypotension and prerenal AKI.
• Restore sinus rhythm: loss of atrial kick causes abrupt decompensation. Per the 2023 ACC/AHA/ACCP/HRS Atrial Fibrillation Guideline, use rate control cautiously (bradycardia limits output) and pursue cardioversion/*amiodarone*; obtain TEE first because atrial thrombus is common. Anticoagulate — in cardiac amyloidosis, anticoagulation is generally advised for any AF given mechanical atrial failure.

Disease-directed therapy (the actual high-yield step)

• ATTR cardiac amyloidosis: transthyretin stabilizer — tafamidis — is recommended by the 2022 AHA/ACC/HFSA guideline to reduce mortality and hospitalization; newer stabilizers and TTR-silencing agents are entering practice.
• AL amyloidosis: urgent hematology referral for plasma-cell–directed therapy (daratumumab-based regimens ± autologous stem cell transplant).
• Hemochromatosis: phlebotomy, or iron chelation (deferasirox) when transfusion-dependent.
• Cardiac sarcoidosis: corticosteroids ± steroid-sparing immunosuppression; the HRS expert consensus statement guides ICD and pacing decisions for high-grade AV block or ventricular arrhythmia.
• Endomyocardial fibrosis: surgical endocardectomy with valve repair/replacement.

Disease-related

• Atrial fibrillation with hemodynamic collapse (emergency): filling is atrial-kick dependent, so loss of organized atrial contraction abruptly drops stroke volume. Signal: new irregularly irregular rhythm with hypotension, rising lactate, or flash pulmonary edema — proceed to synchronized cardioversion.
• Intracardiac thrombus and systemic embolism (emergency): atrial mechanical failure (amyloid) and endocardial fibrosis with mural thrombus (EMF) create stasis even in sinus rhythm. Signal: acute stroke, limb ischemia, or thrombus on TEE.
• High-grade AV block and sinus node dysfunction (emergency): amyloid and sarcoid granulomas infiltrate the conduction system. Signal: syncope with bradycardia or complete heart block — pacing.
• Ventricular arrhythmias and sudden cardiac death (emergency): scar-based reentry, especially in cardiac sarcoidosis. The shockable rhythms are ventricular fibrillation / pulseless VT; defibrillate immediately.
• Refractory right heart failure: relentless venous congestion producing ascites, cardiac cirrhosis with rising bilirubin and INR, protein-losing enteropathy, and cardiac cachexia.
• Cardiorenal syndrome: elevated renal venous pressure plus low forward flow. Signal: creatinine rising despite congestion — a marker of end-stage disease.
• Pulmonary hypertension from chronic post-capillary pressure transmission, worsening tricuspid regurgitation.

Treatment-related

• Over-diuresis: preload dependence means modest volume loss can cause syncope, prerenal azotemia, and low-output shock. Signal: orthostatic hypotension with a rising BUN/creatinine ratio.
• Digoxin toxicity: fibril binding concentrates the drug in myocardium — arrhythmia and GI symptoms at "therapeutic" levels.
• Vasodilator/beta blocker intolerance: profound hypotension, compounded by amyloid autonomic neuropathy.
• Chelation toxicity: deferasirox causes renal and hepatic injury and cytopenias.
• Immunosuppression/chemotherapy: corticosteroid metabolic effects in sarcoidosis; bortezomib-related neuropathy and infection risk in AL amyloidosis.
• Anticoagulation: bleeding, particularly with amyloid-associated acquired factor X deficiency and vascular fragility.

• Biatrial enlargement with normal-sized, non-dilated ventricles and preserved EF is the echo signature. Massive atria plus small ventricles on a stem = restrictive physiology.
• Low-voltage (or pseudo-infarct) ECG despite thick walls on echo is the classic voltage–mass discordance of cardiac amyloidosis. Concentric "hypertrophy" without hypertension and without high voltage is amyloid, not HCM — HCM has high voltage.
• Apical sparing on longitudinal strain ("cherry on top" / bull's-eye) and granular sparkling myocardium are the amyloid buzzwords.
• The single best next step in a suspected amyloid stem: send serum/urine immunofixation and serum free light chains first to exclude AL disease, then obtain technetium-99m pyrophosphate (PYP) scintigraphy. Grade 2–3 PYP uptake with negative light-chain screening establishes ATTR non-invasively — no biopsy needed. Ordering PYP before excluding AL is the classic trap, since AL can also take up tracer.
• The association examiners love: elderly man with bilateral carpal tunnel syndrome, lumbar spinal stenosis, biceps tendon rupture, and HFpEF → wild-type ATTR (ATTRwt).
• RCM vs constrictive pericarditis — the perennial distractor. Both give Kussmaul sign, elevated JVP, and a dip-and-plateau/square-root tracing. Constriction shows respirophasic ventricular interdependence (discordant RV/LV systolic pressures), pericardial thickening/calcification, and **preserved or increased mitral annular e' (annulus reversus); RCM shows a reduced e'**, markedly elevated BNP, and no pericardial abnormality. Constriction is surgically curable by pericardiectomy — do not miss it.
• Pulsus paradoxus points to tamponade, not RCM. Do not use it to separate RCM from constriction.
• Endomyocardial biopsy remains the gold standard when imaging is inconclusive; Congo red staining with apple-green birefringence under polarized light plus mass spectrometry typing confirms and subtypes amyloid.
• Drugs to avoid: digoxin and verapamil in amyloid; over-diuresis in any RCM.