Acute myocarditis is an active inflammatory condition of the myocardium characterized by an acute inflammatory cell infiltration, myocardial edema and cardiomyocyte injury with an onset that is usually less than one month 1-3.
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Epidemiology
The annual incidence was estimated to be 21-24 of 100000 4.
Males are significantly more commonly affected than women 5-7, even though the latter tend to be at higher risk for complications 5. Among children and adolescents, there is an increase of cases in the 2nd decade with a peak at about 15 years of age 7,8.
Diagnosis
Diagnosis of acute myocarditis can be established by a combination of clinical and imaging findings as well as pathological evidence of an acute inflammatory cell infiltrate within the myocardium 1-3.
Due to the steady decline of invasive biopsy rates, a combination of typical clinical findings and characteristic criteria on cardiac MRI to make the diagnosis has been found acceptable, which usually includes proof of myocardial necrosis e.g. by an increase of high-sensitivity troponins and cardiac MRI findings suggesting myocardial edema or a positive cardiac FDG-PET scan 1,2.
In addition, the time of <30 days after onset of symptoms is important for the diagnosis of acute myocarditis, otherwise the myocarditis is termed chronic 1-3.
Clinical presentation
Clinical presentation is variable in severity, ranging from asymptomatic to cardiogenic shock or hemodynamic instability including sudden cardiac death 9,10. Common symptoms include chest pain (including pericarditic or pseudo-ischemic), dyspnea, fatigue, syncope and fever 5,6. New-onset heart failure and arrhythmias are associated with a worse prognosis 4,6.
Prodromal respiratory or gastrointestinal symptoms, including a history of viral infection, are present in most cases 9.
Laboratory findings are usually characterized by elevated high-sensitivity troponin or other cardiac enzymes such as creatinine kinase-MB, and inflammatory markers such as ESR and CRP 1. Leukocytosis or eosinophilia might be present in some cases 1. A viral titer may be positive.
ECG abnormalities are common but often unspecific and include 6:
supraventricular tachycardia
ST-segment elevation or non-elevation, T wave inversion
AV block (first to third degree)
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bundle branch block
specifically a right bundle branch block in acute Chagas myocarditis
intraventricular conduction delay
new Q waves
low voltage
prolongation of the QRS complex duration (>120 ms) - poor prognostic factor 10
Complications
Complications of acute myocarditis include the following 2:
disease progression to chronic myocarditis or chronic inflammatory cardiomyopathy
severe arrhythmias
thromboembolism
sudden cardiac death (in 3-12% of cases) 5,7
Pathology
Acute myocarditis is characterized by the following 1-3,11-15:
mononuclear inflammatory cell infiltration of the myocardium, which can be focal or diffuse
histological evidence of myocardial injury including nonischemic myocardial necrosis and or cardiomyocyte degeneration
And this includes the following immunohistological criteria 12,13:
≥14 leukocytes/mm2 including up to 4 monocytes/mm2
presence of ≥7 CD3 positive T-lymphocytes/mm2
A setting with evidence of cardiomyocyte injury, necrosis or cell degeneration associated with an infiltration of inflammatory cells at the cardiomyocyte circumference in the myocardium indicates acute or chronic active myocarditis as per the Japanese 2023 JCS guidelines 2.
Subtypes
Pathological subtypes include the following 1,2,11-13:
lymphocytic
eosinophilic
granulomatous
Radiographic features
Echocardiography
Echocardiographic findings include the following 2,16:
myocardial thickening – suggesting myocardial edema
regional wall motion abnormalities
preserved or reduced ejection fraction
reduced longitudinal strain
CT
Coronary CTA might be performed to rule out coronary artery disease, the most important differential diagnosis.
Delayed CT images may show late iodine enhancement in a non-ischemic subepicardial or midwall distribution, similar to the pattern described for late gadolinium enhancement on cardiac MRI 16. Certain dual-energy applications have been proven useful for visualization 16.
As on cardiac MRI, myocardial extracellular volume (ECV) can be measured, which should be mildly increased.
MRI
Cardiac MRI serves as the noninvasive reference standard for myocarditis 15,16 and can be used to demonstrate main imaging features for acute myocardial inflammation according to the Lake Louise criteria 15-20:
hyperemia
myocardial necrosis and/or fibrosis
And the following supporting features 16-20:
regional or diffuse wall motion abnormalities including wall thickening
pericardial involvement
Late gadolinium enhancement (LGE) characteristically shows a nonischemic midwall or subepicardial pattern. Inferolateral, anteroseptal, other-LGE and no-LGE patterns have been described, of which no-LGE and inferolateral patterns are associated with a more beneficial outcome and the anteroseptal pattern with the worst prognosis 5.
Signal characteristics
-
cine SSFP:
regional and global wall motion abnormalities (hypokinesia, akinesia, dyskinesia)
pericardial effusion
-
T2/STIR black blood:
focal or diffuse hyperintensity
global T2 signal intensity ratio between myocardium and skeletal muscle >2 suggests myocardial edema
-
T1 mapping:
regional or diffuse increase in T1 relaxation time
mildly to moderately increased extracellular volume
T2 mapping: regional or diffuse increase in T2 relaxation time
IRGE: non-ischemic pattern late gadolinium enhancement often subepicardial or patchy midwall
Notably, the presence of myocardial edema characteristically evidenced on T2-weighted images or T2 mapping is classically a marker of acute disease and should be present in the setting of acute myocarditis 1,2, whereas its absence points to other differential diagnoses such as various cardiomyopathies or chronic myocarditis or inflammatory cardiomyopathy, where it can be absent 1,6,18.
Nuclear medicine
FDG-PET is an alternative noninvasive diagnostic tool 1,18, especially in patients, in whom an autoimmune disease is suspected and shows increased 18F-FDG uptake that can be focal or diffuse or focal on diffuse 18.
Radiology report
The radiology report should include the following:
morphology and functional analysis including left ventricular ejection fraction
wall motion abnormalities
presence of pericardial effusion/pericarditis
MRI
-
myocardial tissue properties, including:
presence, pattern and extent of late gadolinium enhancement
presence, location and extent of myocardial edema
abnormal T1 and T2 mapping values and extracellular volume
Treatment and prognosis
Management depends on presenting symptoms and in uncomplicated cases, this will most likely include diagnostic workup and no intense physical exercise for a period of 3-6 months 3, 22. In more complicated cases it will include supportive therapy, heart failure treatment, and cardiac rhythm monitoring 3,18 and in cases with a treatable underlying cause, it may also include specific therapies.
Endomyocardial biopsy should be considered in the setting of severe heart failure and cardiogenic shock, high-grade AV block, severe ventricular arrhythmias or where myocarditis has therapeutic implications e.g. in the setting of fulminant myocarditis or suspected immune checkpoint inhibitor-associated myocarditis 1,2,18.
The prognosis depends in part on the clinical symptoms and other factors, of which the following are associated with a potentially unfavorable outcome 4,23,24:
heart failure
autoimmune disorders
In addition the extent of and pattern of late gadolinium enhancement impact the prognosis, whereby no-LGE or an inferolateral pattern is associated with better outcomes and other non-specific and in particular anteroseptal patterns carry a worse prognosis 6.
Differential diagnosis
The most important differential diagnosis that has to be ruled out is acute coronary syndrome.
Other differential diagnoses that might mimic acute myocarditis on imaging include the following:
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