Cardiac infections are classified by the affected site: endocardium, myocardium, or pericardium. As the pathophysiology, clinical presentation, differential diagnosis, and treatment of myocarditis and pericarditis overlap significantly, these will be discussed together.
EPIDEMIOLOGY AND PATHOPHYSIOLOGY
Myocarditis is an inflammation of the myocardium; the term myopericarditis describes the frequent additional involvement of the pericardium. Pericarditis involves only the pericardium. Isolated myocarditis is often relatively asymptomatic and therefore frequently misdiagnosed. Thus, the true incidence is unknown, although autopsy studies have demonstrated occult myocarditis in up to 1% of the general population. For unclear reasons, young men more frequently develop myocarditis as well as pericarditis.
The pericardium provides a protective barrier and is composed of two layers: visceral and parietal. The visceral layer is firmly attached to the epicardium, whereas the parietal layer moves freely within the mediastinum. Approximately 20 mL of fluid is normally present within the pericardial sac. Fluid accumulation within the pericardial sac may result in cardiac tamponade if the pericardium does not have sufficient time to stretch, as compliance increases slowly over time. Thus, the rate rather than the absolute amount of fluid accumulation in the pericardial sac is the most important determinant of tamponade physiology.
Cardiac infections may spread directly from one intracardiac region to another (from endocardium toward pericardium or vice versa). Alternatively, pleural or mediastinal infections can extend into the pericardium and cause cardiac infections.