Myocarditis

Definition

Myocarditis is inflammation of the heart muscle. It belongs to the spectrum of inflammatory myopericardial syndromes, which are distinguished by the predominant site of inflammation: pericarditis, myopericarditis (predominantly pericarditis with extension to the myocardium), perimyocarditis (predominantly myocarditis with extension to the pericardium) and myocarditis.

Classification

• By course: acute (typically a few days), subacute (weeks to months) and chronic (not resolved after a few months); there are no fixed time limits.
• Fulminant myocarditis: sudden, severe and life-threatening form with circulatory failure.
• Specific forms: giant cell myocarditis (rare, fulminant, with multinucleated giant cells on biopsy), eosinophilic myocarditis, drug-related hypersensitivity myocarditis, granulomatous myocarditis in sarcoidosis.

Aetiopathogenesis

• Viral (the most common infectious cause in North America and Western Europe): parvovirus B19 and human herpesvirus 6 (most often detected on biopsy), enteroviruses including coxsackie B virus, adenoviruses, Epstein-Barr virus, influenza viruses, coronaviruses including SARS-CoV-2, HIV.
• Other pathogens: bacteria (e.g. Borrelia burgdorferi, group A streptococci, staphylococci, tuberculosis), parasites (e.g. Chagas disease due to Trypanosoma cruzi, toxoplasmosis), fungi.
• Autoimmune and inflammatory: systemic lupus erythematosus, granulomatosis with polyangiitis, giant cell and Takayasu arteritis, idiopathic inflammatory myopathies, inflammatory bowel disease, sarcoidosis.
• Toxic causes: alcohol, cocaine; certain drugs (hypersensitivity myocarditis), including modern cancer drugs that activate the immune system, and anthracyclines; radiogenic.
• Genetic factors: variants in genes for inherited cardiomyopathies or arrhythmias may increase susceptibility.

Clinical features

• Frequently a preceding viral or respiratory infection, often 1–4 weeks before the onset of cardiac symptoms.
• Spectrum from minimal symptoms to fulminant heart failure and fatal arrhythmias; sudden cardiac death may be the first manifestation, possibly after palpitations or syncope.
• Signs of heart failure: fatigue, dyspnea, edema, crackles, raised jugular venous pressure, third or fourth heart sound, murmurs of mitral and tricuspid regurgitation; pulsus alternans in severe systolic dysfunction.
• With pericardial involvement pericarditic chest pain (aggravated by breathing and coughing, relieved by leaning forward) and a pericardial rub.
• Clues to the cause: fever and myalgia in infection, rash in hypersensitivity myocarditis, enlarged lymph nodes in sarcoidosis, fulminant course with arrhythmias in giant cell myocarditis.
• Progression to dilated cardiomyopathy is possible.

Histology

Endomyocardial biopsy is the gold standard: it shows an inflammatory infiltrate with necrosis of adjacent cardiomyocytes. Because of patchy involvement its sensitivity is low – a positive result confirms the diagnosis, a negative one does not exclude it. Characteristic findings are multinucleated giant cells in giant cell myocarditis and eosinophilic infiltrates in eosinophilic myocarditis.

Diagnosis

• Laboratory: cardiac troponin (high-sensitivity assays with a sensitivity of 64–100 % for acute myocarditis), full blood count; additional tests for the cause.
• ECG: normal or abnormal; frequently non-specific ST-T changes, sometimes infarct-like ST elevation; conduction delays, sinus tachycardia, ventricular tachycardia, ventricular fibrillation; AV block in Lyme carditis, giant cell myocarditis or sarcoidosis; low voltage with marked edema or dysfunction.
• Echocardiography: normal in early or mild cases; segmental wall motion abnormalities, dilatation and global systolic dysfunction, abnormal strain, frequently diastolic dysfunction.
• Cardiac MRI: often the decisive examination; typical late gadolinium enhancement subepicardial and mid-myocardial (in ischemia, by contrast, subendocardial), myocardial edema and hyperaemia. The diagnosis rests on the combined detection of edema and non-ischemic myocardial injury with T1- and T2-based techniques; signs of pericarditis or systolic dysfunction are supportive.
• Endomyocardial biopsy: a positive result confirms the diagnosis and can identify special forms such as giant cell or eosinophilic myocarditis; because of possible complications it is not a routine test.

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Further reading (open access)

1. MSD Manual Professional: Myocarditis
2. StatPearls: Acute Myocarditis
3. StatPearls: Viral Myocarditis

Cross-references