Pathoanatomical and Pathophysiological Basics

Myocarditis is an inflammatory heart disease that affects to variable degrees the myocytes, the interstitial and perivascular connective tissue, the coronary arterioles and the capillaries, and, in rare cases, the large epicardial coronary vessels. The course of the disease is in a continuum of three distinguishable, progressive stages: viral infection, autoimmune reaction, and dilated cardiomyopathy. The disease can also result in complete recovery.

The most frequent infectious agents in Western Europe are probably parvovirus B19 and enteroviruses. For example, there is cardiac involvement in ~0.5 to 1 % of infections with Coxsackie B viruses. Recent studies with better methodologies of virus detection (in situ hybridization) PCR suggest that the following viruses can cause a myocarditis:

Parvovirus B19

Coxsackie viruses B3 and B4

Some Coxsackie A virus serotypes

Some echovirus serotypes

Adenoviruses

Hepatitis C virus

Moreover, for numerous other viruses there is evidence for an etiological association between viral infection and myocarditis. Indeed, viral myocarditis is the most frequent inflammatory myocardial disease.

Subsequently, the inflammatory reaction is accelerated by activation of T cells, cytokines, and autoantibodies. For example, coxsackieviruses themselves, but also activation of matrix metalloproteinases and other enzyme systems, in part enhanced by cytokines, lead to changes in the cytoskeleton and in the final stage to dilated cardiomyopathy.

The natural history of the disease is either chronic or acute and usually benign with only transient symptoms. There are only rarely fulminant courses, in which left ventricular failure, total AV block or ventricular tachyarrhythmias occur within hours or days.

Indication

In most cases myocarditis is a suspected diagnosis with transient symptoms and has a generally good prognosis. Therefore, in these patients a noninvasive diagnostic work-up is sufficient and cardiac catheterization not required. The gold standard for noninvasive diagnosis of myocarditis is currently cardiac MRI. The essential findings are

Direct detection of edema in STIR (short TI inversion recovery) images

Relatively early enhancement after contrast medium administration (compared with skeletal muscle)

Increased delayed enhancement after contrast medium administration, which is predominantly located subepicardially