Myocarditis I: Introduction

Myocarditis is inflammation of the myocardium, the heart's muscular layer. It results from various infectious agents like viruses and bacteria, as well as non-infectious causes including autoimmune disorders, radiation therapy, and chemotherapy drugs. Understanding myocarditis is essential for recognizing how different pathogens and toxins damage cardiac tissue.

Viral causes include Coxsackievirus A and B, influenza A, adenovirus, and parvovirus B19. Bacterial causes include Streptococcus pyogenes and Mycoplasma pneumoniae. Fungi like Aspergillus and parasites such as Trypanosoma cruzi also cause myocarditis. Additionally, rickettsial infections and spirochetal infections like Lyme disease can trigger myocardial inflammation.

Non-infectious causes include autoimmune disorders such as systemic lupus erythematosus and polymyositis, radiation therapy used in cancer treatment, and chemotherapy drugs like doxorubicin. Chemical exposures and certain toxins can also lead to myocardial inflammation. These factors directly damage myocardial cells or trigger inflammatory responses independent of infectious agents.

During the initial insult, a causative agent invades the myocardium, leading to direct cellular damage and necrosis of myocytes. This damage activates the immune system, triggering the release of cytokines and oxygen-free radicals. These inflammatory mediators attempt to eliminate the causative agent but simultaneously contribute to further myocardial injury and inflammation.

In some cases, the immune system mounts an autoimmune response where antibodies and immune cells mistakenly target healthy myocardial cells in addition to the causative agent. This autoimmune response exacerbates myocardial damage beyond the initial infection. The resulting inflammation can cause heart dilation, mural thrombi formation, and infiltration of blood cells around coronary vessels.

As inflammation progresses, extensive myocardial necrosis and destruction of interstitial collagen and elastin occur throughout the myocardium. This structural damage impairs the myocardium's integrity and function, leading to reduced cardiac output and impaired hemodynamic function. Severe cases result in chronic heart failure and significant structural heart changes requiring myocarditis clinical features and diagnostic tests.

In many cases of myocarditis, no specific infectious or non-infectious cause can be identified despite thorough investigation. These cases are classified as idiopathic myocarditis. The underlying etiology remains unknown, making diagnosis and treatment more challenging for clinicians managing patients with unexplained myocardial inflammation.