Myocarditis and Cardiomyopathies

1. Myocarditis — definition

Inflammation of the myocardium in which the inflammatory process is itself the cause of myocyte injury (rather than a reaction to a pre-existing injury). It also involves the conduction system, reducing contractility and provoking arrhythmias.

2. Morphology

• Heart may look normal or dilated; the ventricular myocardium is flabby, often mottled by patchy/diffuse foci of pallor and/or hemorrhage.
• Microscopically: interstitial inflammatory infiltrate with focal myocyte necrosis immediately adjacent to the inflammatory cells.

3. Etiology & pathogenesis

• Viral (most common): Coxsackievirus A & B, poliovirus, influenza A/B. Mechanisms = direct cytolytic injury OR immune cross-reaction with myosin heavy chain / T-cell–mediated reaction.
• Non-viral infectious: Trypanosoma cruzi, Toxoplasma gondii, trichinosis.
• Non-infectious: systemic immune disease (SLE), drugs, hypersensitivity reactions.

4. Histological types

Type	Key features
Lymphocytic (most common)	Survivors of the acute phase either resolve with no residua or heal by progressive fibrosis (scarring)
Hypersensitivity	Interstitial + perivascular infiltrate of lymphocytes, macrophages, eosinophils; eosinophilic pattern, linked to hypereosinophilic syndrome
Giant cell	Widespread infiltrate with multinucleated giant cells (macrophage fusion) + lymphocytes, eosinophils, plasma cells; aggressive
Chagas	Infiltrate contains trypanosomes alongside inflammatory cells

5. Clinical features

Broad spectrum from asymptomatic → sudden cardiac death; in between: fatigue, dyspnea, arrhythmias, fever, chest pain. May progress to dilated cardiomyopathy (DCM).

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6. Cardiomyopathies (CM) — overview

A diverse group of diseases of intrinsic myocardial dysfunction that impair the heart’s ability to pump (progressive heart failure). CM is a diagnosis of exclusion — coronary, valvular and hypertensive disease must be ruled out first.

• Primary / intrinsic (cardiac origin): DCM, arrhythmogenic right ventricular CM (ARVC), hypertrophic CM (HCM), restrictive CM (RCM).
• Secondary / systemic (multiorgan origin): metabolic/storage (amyloidosis, hemochromatosis), inflammatory (viral myocarditis, Chagas), toxic (chemotherapy, alcohol).

7. Functional comparison of the three patterns

Pattern	LV EF	Mechanism of failure	Representative causes
Dilated	< 40%	Systolic dysfunction (contractility ↓)	Genetic, alcohol, peripartum, myocarditis, hemochromatosis, doxorubicin (Adriamycin), idiopathic
Hypertrophic	50–80%	Diastolic dysfunction (compliance ↓)	Genetic (sarcomere), Friedreich ataxia, storage disease, infant of diabetic mother
Restrictive	45–90%	Diastolic dysfunction (compliance ↓)	Amyloidosis, radiation fibrosis, idiopathic

8. Dilated cardiomyopathy (DCM)

• Hallmark: progressive cardiac dilation + systolic (contractile) dysfunction with hypertrophy.
• Morphology: heart enlarged 2–3× (up to ~800 g), all four chambers dilated and flabby → pumping failure → EF < 25% → blood stasis → mural thrombi → thromboembolism. Mitral insufficiency from valve-ring stretch; endocardial fibrosis. Micro: non-specific myocyte hypertrophy with enlarged nuclei + scattered interstitial fibrosis.
• Pathogenesis (heterogeneous): genetic (25–30% autosomal dominant; dystrophin/desmin mutations), viral (Coxsackie/enterovirus remnants), alcohol (acetaldehyde toxicity ± vitamin B1 deficiency), peripartum (late gestation/postpartum; ~50% resolve).
• Clinical: ages 20–50, slowly progressive CHF, major arrhythmia risk, embolism from mural thrombi; poor prognosis (~50% die within 2 yr); transplant is definitive.

9. Arrhythmogenic right ventricular cardiomyopathy (ARVC)

Autosomal dominant desmosomal protein mutation → RV wall thinned by fatty infiltration + fibrosis → loss of contractile tissue → right-sided HF, ventricular arrhythmias and sudden cardiac death.

10. Hypertrophic cardiomyopathy (HCM)

• Hallmark: myocardial hypertrophy without dilation, impaired diastolic filling (stiff ventricle), outflow obstruction in ~⅓.
• Morphology: asymmetric septal hypertrophy → subaortic stenosis / outflow obstruction; banana-shaped LV cavity. Micro: severe myocyte hypertrophy, myocyte disarray (chaotic end-to-side arrangement), interstitial fibrosis.
• Pathogenesis: point mutations in sarcomeric proteins, most often β-myosin heavy chain → ↑ myofilament activation → hypercontractility, ↑ energy use.
• Clinical: reduced stroke volume, exertional dyspnea + systolic ejection murmur, myocardial ischemia, AF → mural thrombus.

11. Restrictive cardiomyopathy (RCM)

• Primary ↓ ventricular compliance → impaired diastolic filling; the heart squeezes well but cannot relax. Ventricles normal/slightly enlarged, no cavity dilation; interstitial fibrosis.
• Endomyocardial fibrosis: children/young adults in Africa; dense subendocardial fibrosis → restriction.
• Loeffler endomyocarditis: endocardial fibrosis + large mural thrombi; peripheral hypereosinophilia → eosinophil granule release → endocardial necrosis/scarring; reversible with tyrosine kinase inhibitors.

💡 High-yield: Myocarditis = inflammation causing myocyte necrosis (most often Coxsackievirus), classically lymphocytic; can progress to DCM. The three functional CM patterns: DCM = systolic (dilated, EF < 25%, mural thrombi, alcohol/peripartum/genetic), HCM = diastolic (asymmetric septal hypertrophy, β-myosin mutation, myocyte disarray, outflow obstruction, exertional SCD in young athletes), RCM = diastolic (stiff, amyloid/Loeffler/endomyocardial fibrosis). ARVC = desmosome mutation, fibrofatty RV, SCD.