12th March 2024

Myocarditis

Background 

Myocarditis is defined as evidence of inflammation within the myocardium that is not of ischaemic origin. As a result of the clinical manifestations of myocarditis being so variable, the true incidence is impossible to determine. However, studies have reported that myocarditis is present in up to 9% of routine postmortem examinations. Prognosis is variable and dependent on the underlying cause of myocarditis. The majority of patients with viral myocarditis are asymptomatic and improve spontaneously, whilst other patients may present in acute decompensated cardiac failure requiring intensive care support. 

Aetiology 

Causes of myocarditis include:

• Viral: coxsackie A/B most common, adenovirus, influenza, EBV, SARS-CoV-2, polio virus, CMV, HHV-6, 
• Infections: bacterial (Brucella, Mycoplasma, Haemophilus influenzae, Mycobacteria) and fungal (Aspergillus, Candida)
• Protozoan: Trypanosoma cruzi (causing Chagas disease), Toxoplasmosis 
• Drugs:/Toxins: cyclophosphamide, anthracyclines, ethanol, cocaine
• Autoimmune conditions: inflammatory bowel disease, connective tissue diseases, sarcoidosis
• Vaccinations: smallpox vaccine, SARS-COV-2 vaccine

Pathology

Viruses induce myocardial damage in a variety of ways. Enteroviral replication causes cytopathic damage, triggering both cellular and humoral immune responses. The resultant myocardial necrosis releases self-antigens, which are detected by the immune system. These self-antigens have the capability to induce auto-immune reactions within the myocardial tissue, which further exacerbates myocarditis.

Clinical manifestations

The presentation of myocarditis can vary, ranging from asymptomatic to signs and symptoms of heart failure. Signs and symptoms include:

• Chest pain, which is often sharp in nature
• Shortness of breath
• Palpitations secondary to electrical disturbance causing arrhythmias 
• Fever
• Heart failure: peripheral oedema, orthopnoea, PND 

Given myocarditis is often due to a viral illness, patients may report a preceding viral or respiratory illness and present with viral complaints, including rash, joint pains, anorexia, vomiting and diarrhoea. 

Diagnosis

Blood tests typically show raised inflammatory markers (ESR and CRP). Cardiac enzymes troponin and CK-MB are also raised, reflecting myocardial injury. Of note, normal troponin values do not exclude myocarditis. 

Common ECG findings in myocarditis include ST segment elevation and T wave inversion. Conduction abnormalities such as AV blocks will be evident on the ECG. Prolongation of the QRS is an independent risk factor for mortality. 

Where myocarditis is suspected, patients should have an echocardiogram with subsequent serial echocardiograms as follow-up. Typical findings include regional wall motion abnormalities, depressed right or left ventricular ejection fraction, pericardial effusion or global ventricular dysfunction. 

Coronary angiogram is recommended in all patients to exclude coronary artery disease as a cause of the clinical picture. Cardiovascular magnetic resonance (CMR) imaging is useful in providing a non-invasive study which can aid in the diagnosis of myocarditis. Typical features diagnostic of myocarditis include increased oedema and late gadolinium enhancement.  

The gold standard for definitive diagnosis of myocarditis is endomyocardial biopsy (EMB). This has the added value that it can help identify the aetiology of myocarditis. Samples from EMB are analysed by RT-PCR amplification of the viral genomes and immunohistochemistry. 

Management 

Treatment is symptomatic with hemodynamically stable patients with myocarditis-induced heart failure managed with beta-blockers, angiotensin- converting enzyme inhibitors and diuretics. Specific antiviral agents are available based on the EMB viral PCR results, but these should be given after seeking specialist advice from the microbiology team. 

Patients with signs of heart failure with haemodynamic instability are managed in a high-dependence critical care setting where cardiovascular support can be provided and workup initiated for potential cardiac transplantation. 

Patients should avoid excessive physical activity for six months following myocarditis. This advice is the same for athletes and non-athletes. 

Clinical outcomes in myocarditis remain highly variable. Up to half of patients with acute myocarditis show a resolution of their symptoms within the first few weeks. However, up to 25% of patients will deteriorate further and either die as a result or progress to severe dilated cardiomyopathy. 

References

• Caforio et al. 2013. Current State of Knowledge on Aetiology, Diagnosis, Management, and Therapy of Myocarditis: a Position Statement of the European Society of Cardiology Working Group on Myocardial and Pericardial Diseases. European Heart Journal. Available at https://pubmed.ncbi.nlm.nih.gov/23824828/
• Myocarditis. BMJ Best Practice 2022. Available at https://bestpractice.bmj.com/topics/en-gb/244
• Pericarditis. BMJ Best Practice 2022. Available at https://bestpractice.bmj.com/topics/en-gb/3000214
• Manda YR, Baradhi KM. Myopericarditis. 2021 Jul 26. Available at https://pubmed.ncbi.nlm.nih.gov/30521197/