Definition:
Myocarditis = infectious or immune-mediated inflammation of the myocardium (the muscular layer of the heart wall). It disrupts the heart's electrical conduction and pumping function and can lead to serious complications such as heart failure or arrhythmias.
Etiology:
Infective Causes:
- Most commonly viral → Coxsackie virus, EBV (Epstein-Barr virus), HCV.
- Bacterial causes: Corynebacterium diphtheriae (produces toxins damaging to the heart), Brucella, Borrelia (Lyme disease), Haemophilus influenzae.
- Parasitic: Trypanosoma cruzi (causes Chagas disease, endemic in Latin America).
Non-infective Causes:
- Often associated with rheumatic fever or autoimmune diseases like systemic lupus erythematosus (SLE).
In autoimmune causes, the body attacks its own myocardium as part of systemic inflammation.
Other Causes:
- Toxins: alcohol, certain chemotherapeutic or antipsychotic drugs (e.g., doxorubicin, clozapine).
- Post-transplant rejection: due to immune attack on the donor heart.
- Electric shock, heavy metal poisoning (e.g., iron, mercury).
Pathogenesis:
- Infiltration of myocardium by pro-inflammatory immune cells (lymphocytes, macrophages).
- Inflammation damages cardiac myocytes, leading to:
- Swelling of the tissue
- ↓ Contractile ability → ↓ cardiac output (CO)
- To compensate, tachycardia occurs to maintain CO.
- In severe or chronic cases, fibrosis replaces damaged muscle → dilated cardiomyopathy → long-term systolic dysfunction.
The reduction in ejection fraction is due to both myocyte injury and fibrosis-induced stiffness of the ventricles.