Myocarditis occurs in approximately 5% of patients with Lyme disease, a systemic illness caused by the bacterial spirochete Borrelia burgdorferi (Chapter 9). Lyme myocarditis manifests primarily as self-limited conduction system disease, frequently requiring temporary pacemaker insertion.
Noninfectious causes of myocarditis include systemic diseases of immune origin, such as systemic lupus erythematosus and polymyositis. Drug hypersensitivity reactions affecting the heart (hypersensitivity myocarditis) may occur with exposure to a wide range of agents; such reactions typically are benign and only in rare circumstances lead to CHF or sudden death.
MORPHOLOGYInacutemyocarditis,theheartmayappearnormalordilated;inadvancedstages,themyocardiumtypicallyisflabbyandoftenmottledwithpaleandhemorrhagicareas.Muralthrombimaybepresent.
Microscopically,myocarditisischaracterizedbyedema,interstitialinflammatoryinfiltrates,andmyocyteinjury(
Fig.11.28 ).Adiffuselymphocyticinfiltrateismostcommon(see
Fig.11.28A ),althoughtheinflammatoryinvolvementisoftenpatchyandcanbe“missed”onendomyocardialbiopsy.Ifthepatientsurvivestheacutephaseofmyocarditis,lesionsmayresolvewithoutsignificantsequelaeorhealbyprogressivefibrosis.
Inhypersensitivity myocarditis, interstitialandperivascularinfiltratesarecomposedoflymphocytes,macrophages,andahighproportionofeosinophils(
Fig.11.28B ).Giant cell myocarditis isamorphologicallydistinctiveentitycharacterizedbywidespreadinflammatorycellinfiltratescontainingmultinucleategiantcells(formedbymacrophagefusion).Giantcellmyocarditisprobablyrepresentstheaggressiveendofthespectrumoflymphocyticmyocarditis,andthereisatleastfocal—andfrequentlyextensive—necrosis (Fig.11.28C ).Thisvariantcarriesapoorprognosis.
Chagas myocarditis ischaracterizedbytheparasitizationofscatteredmyofibersbytrypanosomesaccompaniedbyaninflammatoryinfiltrateofneutrophils,lymphocytes,macrophages,andoccasionaleosinophils(
Fig.11.28D
The clinical spectrum of myocarditis is broad; at one end, the disease is asymptomatic, and patients recover without sequelae. At the other extreme is the precipitous onset of heart failure or arrhythmias, occasionally with sudden death. Between these extremes are many levels of involvement associated with a variety of signs and symptoms, including fatigue, dyspnea, palpitations, pain, and fever. The clinical features of myocarditis can mimic those of acute MI. Clinical progression from myocarditis to DCM occasionally is seen.
Cardiac complications of cancer therapy are an important clinical problem. Cardiotoxicity can be associated with conventional chemotherapeutic agents, targeted drugs such as tyrosine kinase inhibitors, and certain forms of immunotherapy (e.g., immune checkpoint blockade for cancer). The anthracyclines doxorubicin and daunorubicin are the chemotherapeutic agents that are most frequently associated with toxic myocardial injury, which often takes the form of a dilated cardiomyopathy and heart failure. Anthracycline toxicity is dose-dependent (cardiotoxicity becomes progressively more frequent above a total dose of 500 mg/m2) and is attributed primarily to peroxidation of lipids in myocyte membranes.