SN7 SHORT NOTE 5M

Describe myocarditis with its causes, morphology and clinical features.

Pathology · 2nd Professional — Para-Clinical

Definition

Myocarditis is inflammation of the myocardium with injury to the myocytes not caused by ischaemia.

Causes

Category	Agents
Viral — the commonest	Coxsackie A and B (the classical cause), echovirus, adenovirus, influenza, HIV, cytomegalovirus, SARS-CoV-2
Bacterial	Diphtheria (exotoxin), meningococcus, borrelia (Lyme disease), Streptococcus
Parasitic	Trypanosoma cruzi — CHAGAS disease, a major cause in South America; toxoplasma; trichinella
Immune mediated	Rheumatic fever, SLE, drug hypersensitivity (methyldopa, sulphonamides), transplant rejection
Toxic and other	Doxorubicin, cocaine, catecholamines, radiation; sarcoidosis; giant cell myocarditis

Morphology

Gross — the heart may appear normal, or be dilated and flabby with a mottled myocardium; mural thrombi are common
Microscopy — the diagnostic combination is an interstitial inflammatory infiltrate with myocyte necrosis
Lymphocytic infiltrate in viral myocarditis — by far the commonest pattern
Eosinophilic infiltrate in hypersensitivity myocarditis
Giant cells in giant cell myocarditis and in sarcoidosis, the former carrying a very poor prognosis
Chagas disease — parasitic pseudocysts within myocytes, with a mixed inflammatory infiltrate

CLINICAL PEARL

The infiltrate alone is not enough — there must be myocyte necrosis. Inflammatory cells are found in the myocardium in many conditions, and the Dallas criteria require damage to adjacent myocytes before myocarditis can be diagnosed. Without that requirement the diagnosis would be made far too often.

Clinical Features

Highly variable — from entirely asymptomatic to fulminant cardiac failure and sudden death
Typically a young patient with a preceding viral illness, presenting with fatigue, dyspnoea, palpitations and chest pain
Arrhythmias and conduction block, which may be the only manifestation; diphtheria characteristically causes heart block
Raised troponin with normal coronary arteries — myocarditis is an important mimic of myocardial infarction in the young
ECG — non-specific ST and T changes, arrhythmias, sometimes pericarditic changes when the pericardium is also involved
Cardiac MRI shows characteristic late gadolinium enhancement and has largely replaced biopsy for diagnosis

Applied Aspects

An important cause of sudden death IN the young, and of dilated cardiomyopathy in survivors
Most viral myocarditis resolves completely, but a minority progress to dilated cardiomyopathy
Endomyocardial biopsy remains the gold standard but is limited by sampling error, since the disease is patchy
Treatment is supportive — management of failure and arrhythmia, and strict avoidance of exercise during the acute phase, which otherwise increases mortality
Immunosuppression helps only in specific forms — giant cell myocarditis, sarcoidosis and autoimmune disease; it is not used for viral myocarditis
Diphtheritic myocarditis remains a preventable cause in India where immunisation coverage is incomplete, and is treated with antitoxin given early

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