Myocarditis is an inflammatory disease of the heart muscle, which mostly arises due to infections with cardiotropic viruses, especially infections with the coxsackie virus. The course of myocarditis, in contrast to endocarditis, is usually slower and less distinct.
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Histopathology of viral myocarditis

Picture: „Histopathology of viral myocarditis” by KGH. License: CC BY-SA 3.0


Definition of Myocarditis

Myocarditis is the inflammation of the myocardium, which contains the contractile muscle layer of the heart. Progression of this inflammatory disease may lead to heart failure.

Herzmuskelzellen

Picture: “Graphic of a Myocardiocyte (Heart Muscle Cell), including organelles and cell membrane functions” by OCAL. License: Public domain

Epidemiology of Myocarditis

Coxsackie Virus

Picture: “Coxsackie Virus” by Angusmclellan. License: Public Domain

The annual incidence of myocarditis is estimated to be about 17 cases per 100,000 individuals. Fatality occurs in less than 2% of all cases. In approximately 4% of cases of infection with coxsackie B virus, the heart will be affected as well. For other cardiotropic viruses, this number decreases by about 1%. When young adults die from sudden cardiac arrest, the autopsy will reveal myocarditis in approximately 10% of those patients.

Etiology of Myocarditis

Myocarditis may be caused by one or more of the following :

  1. Infectious organisms.
  2. Autoimmune disorders.
  3. Exogenous agents.
  4. Genetic predisposition.
  5. Environmental causes.

A distinction is made between infectious and noninfectious forms. The coxsackie B virus is often the trigger of the infectious form of myocarditis. Other entero-, adeno-, and influenza viruses, (e.g., Epstein-Barr virus, cytomegalovirus, hepatitis C virus, or human immunodeficiency virus [HIV]) can also lead to myocarditis. Bacteria are able to induce myocarditis as well. This includes primarily staphylococci, streptococci, enterococci, and the spirochete Borrelia burgdorferi. Fungi, protozoa, and parasites may also be the cause.

In rare cases, chronic systemic diseases such as rheumatoid arthritis, vasculitis, and collagenases can trigger noninfectious myocarditis. Medications may cause hypersensitivity myocarditis. An idiopathic Fiedler myocarditis or myocarditis following radiation therapy to the mediastinum is also possible.

Viral infections (most common) Bacterial infections (occasional)
  • Enteroviruses
  • Adenoviruses
  • Parvovirus B 19
  • Human herpesvirus 6
  • Dengue viruses
  • Cytomegalovirus
  • Coxsackievirus
  • Poliovirus
  • HIV
  • Bacteremia
  • Direct extension from contiguous focus
  • Bacterial toxin
Parasites
Trypanosoma cruzi
Miscellaneous causes
  • Toxins: chemotherapy (Doxorubicin), drugs, ethanol
  • Immunological causes: allergic reactions, post-transplant rejection, autoimmune

Classification of Myocarditis

Myocarditis can be classified according to its course or its pathogenesis. The disease may be acute or chronic myocarditis in nature. The pathogenesis can be differentiated based on whether it is infectious myocarditis, toxic myocarditis, idiopathic myocarditis, or autoimmune myocarditis.

Pathophysiology of Myocarditis

Histopathology of viral myocarditis

Picture: “Histopathology of viral myocarditis” by KGH. License: CC BY-SA 3.0

The virus damages myocardial cells, which triggers an immune reaction. This can cause the destruction of other myocardial cells or cause cross-reactions between viral and myocardial structures, which produce antibodies. This eventually leads to the inability of the heart to pump properly, resulting in heart failure (viral myocarditis).

Pathology of Myocarditis

Histologically, Myocarditis can be divided into the following categories:

  • Eosinophilic
  • Granulomatous
  • Lymphocytic
  • Contraction band necrosis, reperfusion

Initially, viral infections typically present with a serous exudate and hypereosinophilia. Later, the fibrotic replacement of cardiac muscle cells can occur. In bacterial infections, microabscesses that are rich in granulocytes and bacteria are present.

Symptoms and Clinical Presentation of Myocarditis

The onset and progression of myocarditis are slow and nonspecific, with symptoms such as fever, fatigue, or chest pain. During the course of the disease (days to weeks), signs of heart failure and arrhythmia, as well as tachycardia, can develop. Rarely, the course may be fulminant and similar to an infarction.

Unexplained heart failure Arrhythmia
Chest pain
  • Supraventricular tachycardia
  • Ventricular extrasystoles
May mimic acute MI Cardiac abnormalities developing during recognized systemic infection
E.g. fever, malaise, arthralgias, respiratory symptoms

Diagnostics of Myocarditis

Laboratory diagnostics of myocarditis

The patient’s medical history often reveals evidence of a previous infection. Clinical signs can be combined with laboratory values such as an elevated erythrocyte sedimentation rate (ESR) and increased C-reactive protein (CRP), troponin, and CK-MB levels, as well as with bacteriological and viral diagnostics. ECG may show nonspecific changes. Diagnosis is confirmed after detection of the pathogen in a myocardial biopsy.

Differential Diagnosis of Myocarditis

Similar diseases to myocarditis

A distinction must be made between postmyocardial cardiomyopathy, viral myocarditis as well as virus-positive myocarditis, and inflammatory dilated cardiomyopathy (DCM).

Treatment of Myocarditis

Therapeutic approaches for myocarditis

Initially, the treatment of myocarditis involves relieving the symptoms. Physical rest, abstinence from alcohol, thromboembolism prophylaxis, and the treatment of complications such as heart failure are indicated. Moreover, the therapeutic approach may be causal by treating the causative disease such as diphtheria or Chagas disease. As part of research studies, antiviral and immunosuppressive treatments are also used.

Supportive therapy – hemodynamically stable patients Chagas disease
Heart failure:

  • Angiotensin-converting enzyme inhibitors
  • ß-adrenergic blockers
Anti-trypanosomal therapy:

  • Benznidazole
  • Nifurtimox
Anti-viral therapy: limited applications Immunosuppression
Not for routine use

Complications of Myocarditis

Complications of myocarditis include:

Prognosis of Myocarditis

Partial or full clinical recovery can be expected, but relapse is also a possibility. Unresolved cases may lead to dilated cardiomyopathy. Indicators of advanced cardiomyopathy may warrant heart transplantation.

Review Questions

The answers can be found below the references.

1. Which is the most common trigger of infectious myocarditis?

  1. Staphylococci
  2. Streptococci
  3. Cardiotropic viruses
  4. Fungi
  5. Parasites

2. How can the diagnosis of myocarditis be confirmed?

  1. Measurement of troponin
  2. Increased ESR
  3. Myocardial biopsy
  4. Determining the CK-MB level
  5. Changes in the EKG

3. Which of the following is least likely to be part of the clinical manifestation of myocarditis?

  1. Serous exudate
  2. Hypereosinophilia
  3. Fibrotic replacements
  4. Hypertrophic myocytes
  5. Microabscesses rich in bacteria
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