Myocarditis

Myocarditis (rare plural: myocarditides) is a general term referring to inflammation of the myocardium. 

Clinical presentation

Clinical presentation is variable in severity, ranging from asymptomatic to cardiogenic shock, but it typically is associated with other viral symptoms, including fever and malaise. It typically occurs 7-10 days after the onset of the systemic illness.

Chest pain may occur, in a variety of typical and atypical presentations.

Lab values are typically non-specific, with increased ESR and leukocytosis. Creatine kinase, CK-MB, and troponins may be elevated. A viral titer may be positive.

ECG

Commonly sought due to the clinical presentation, but demonstrates poor sensitivity. Findings include:

• sinus tachycardia

  • non-specific, most common overall electrocardiographic manifestation

• concave, diffuse ST-segment elevation

• interventricular conduction delays

  • right bundle branch block common in acute Chagas myocarditis

• prolongation of the QRS complex duration (>0.12 seconds)

  • poor prognostic factor ¹³

Pathology

Myocarditis has several etiologies. Inflammation from viral etiologies is thought to be caused both by direct cellular damage by the infectious agent and also by involvement by the host's immune system.

Etiology

Infectious etiologies

• viral (most common etiology): e.g. Coxsackievirus, Echovirus, arbovirus

• bacterial, e.g. Corynebacterium diphtheriae, Streptococcus pyogenes, Staphylococcus aureus, Borrelia burgdorferi

• fungal, e.g. Candida spp.

• parasites, e.g. Trypanosoma cruzi

Non-infectious etiologies

• drugs

  • antibiotics

  • antituberculous agents

  • anticonvulsants

  • anti-inflammatories

  • diuretics

  • non-prescription/recreational drugs

  • Covid-19 vaccines¹⁵

• radiation

• transplant rejection

• giant cell myocarditis

• systemic disease

  • systemic lupus erythematosus (SLE)

  • scleroderma

Markers

Although non-specific, cardiac creatine kinase (CK) and troponins I, T and C (TnI, TnT and TnC) are elevated.

Classification

Myocarditis is classified into four categories based on the clinical and pathologic presentation: fulminant, acute, chronic active, and chronic persistent.

Radiographic features

CT

Not always useful but some case reports describe certain features that can be helpful such as delayed myocardial enhancement (i.e. similar to MRI) with iodinated contrast ^10,11.

MRI

• cine SSFP

  • regional or global wall motion abnormalities are common but nonspecific (biventricular wall motion abnormality, however, is the main predictor of death or transplantation)

  • pericardial effusion is reported in ~45% (range 32-57%) of patients with myocarditis

• T2 black blood

  • T2 myocardial hyperintensity is compatible with edema

  • T2 hyperintensity may be global and difficult to detect

• early gadolinium enhancement

  • regional vasodilatation and increased blood volume due to the inflammation in myocarditis causes early postcontrast enhancement

• late gadolinium enhancement

  • late enhancement in myocarditis is an indication of irreversible myocardial necrosis and fibrosis.

  • distribution of enhancement is variable but classically involves the subepicardial myocardium (mid-interventricular and focal transmural patterns are also possible)

Lake Louise consensus criteria 2018

The original Lake Louise criteria ⁵ was updated in 2018. The following two main criteria (T1 and T2 criteria) now have to be fulfilled for the diagnosis of acute myocarditis ¹⁴:

• T2-weighted: any of the following

  • standard T2 sequences: regional high signal

  • standard T2 sequences: global signal intensity ratio (myocardium/skeletal muscle) ≥2

  • T2 mapping: increased T2 relaxation times

• T1-weighted: any of the following

  • late enhancement imaging: non-ischemic (subepicardial or mid-myocardial) late enhancement

  • native T1 mapping: increased T1 relaxation times or extracellular volume

• supportive criteria:

  • signs of pericarditis: effusion or pericardial late enhancement

  • regional or global wall motion abnormalities

Echocardiography

Primarily useful to exclude alternative diagnoses which present similarly, including ischemic heart disease, and evaluate for the presence or absence of a pericardial effusion (and potential tamponade physiology). Findings common in myocarditis include ¹²:

• global left ventricular systolic dysfunction

• diastolic dysfunction

• regional wall motion abnormalities

Endomyocardial biopsy

Endomyocardial biopsy is considered the gold standard of diagnosis, although it is subject to sampling error and there is a risk of perforation or tamponade. Endomyocardial biopsy is graded according to the Dallas criteria, with gradations of myocarditis, borderline myocarditis, and no myocarditis.

A typical appearance of myocarditis on MRI in the correct clinical setting may obviate biopsy.