Myocarditis

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CHAPTER 66 Myocarditis

Myocarditis—acute inflammatory syndrome of myocardium—is a rare cause of acute chest pain and an important cause of dilated cardiomyopathy. The most common cause of myocarditis in the Western world is viral infection. The patient presents acutely with chest pain and dyspnea or, rarely, hemodynamic compromise of the fulminant type. Chronic myocarditis usually is manifested with symptoms and signs of dilated cardiomyopathy. Clinical presentations, biochemistry, and echocardiographic findings may overlap with those of myocardial infarction or ischemic heart disease. Imaging is critical in the diagnosis. Currently, cardiac magnetic resonance imaging (MRI) has proved to be the most robust and accurate noninvasive imaging tool, not only in the diagnosis of myocarditis but also in guiding targeted endomyocardial biopsy and in the subsequent follow-up. Treatment of myocarditis is usually supportive. Cardiac transplantation may be considered for patients with deteriorating cardiac function in dilated cardiomyopathy.

Definition

Myocarditis refers to focal or diffuse inflammation of the cardiac muscle associated with myocyte damage or necrosis and lymphocyte infiltration as a result of various infectious, immune, or nonimmune insults in the absence of significant acute or chronic ischemia.¹^–³

Prevalence and Epidemiology

Myocarditis is found in 1% to 10% of postmortem examinations; however, the true prevalence is unknown because of its insidious and variable clinical presentation.¹^,² It accounts for 20% of cardiac causes of sudden deaths in military recruits¹ and 40% of acute presentations of dilated cardiomyopathy.² Up to 50% of patients with acquired immunodeficiency syndrome (AIDS) have evidence of myocarditis on biopsy.²

Infections are a major cause of myocarditis. Viruses, such as coxsackieviruses A and B and other enteroviruses, adenovirus, influenza virus, and Epstein-Barr virus, are the most important causes of myocarditis in the United States.²^,⁴ A wide range of bacteria (Streptococcus, Chlamydia, Neisseria, Borrelia) and parasites (Trypanosoma, Toxoplasma, Trichinella) can also cause myocarditis. Various medications, such as doxorubicin (Adriamycin) and sulfonamides, and toxins, such as cocaine, have also been associated with myocarditis. Large-vessel vasculitis, such as Takayasu arteritis, and autoimmune diseases, such as systemic lupus erythematosus, sarcoidosis, and Wegener granulomatosis, are also important but rare causes of myocarditis.²^,⁴

Manifestations of Disease

Clinical Presentation

The clinical manifestation of myocarditis is variable, ranging from progressive dyspnea and weakness to left ventricular failure to sudden death. Patients may present with influenza-like symptoms, such as fever, fatigue, malaise, and arthralgia.¹^–³ Arrhythmia is common.⁴ In addition, chest pain with abnormal electrocardiographic (ECG) recordings and serum troponin elevation in patients with myocarditis can mimic acute myocardial infarction.⁵ Patients may also present at a late stage with dilated cardiomyopathy.⁴

Imaging Indications and Algorithm

Imaging plays a pivotal role in the diagnosis of myocarditis. In the past, endomyocardial biopsy was once considered the gold standard for diagnosis of myocarditis; however, it has been shown to have low sensitivity due to sampling error related to the patchy nature of the disease.⁶ Echocardiography and cardiac MRI should be part of the initial diagnostic evaluation in conjunction with ECG and serum troponin evaluation. More selected biopsy, if needed, may be performed with the MRI result as a guide.⁵ Coronary computed tomographic angiography (CTA) can be performed in the acute setting to exclude significant coronary arterial stenosis in patients with myocarditis who are presenting with chest pain, raised abnormal cardiac enzymes, and abnormal ECG changes.

Imaging Technique and Findings

Radiography

Chest radiographs may be completely normal in patients with myocarditis, especially acutely (Fig. 66-1). Alternatively, cardiomegaly, interstitial or alveolar pulmonary edema, pleural effusions, and pericardial effusion may be detected as a result of left ventricular failure.² Less commonly, right ventricular failure may be manifested by right atrial enlargement, prominent azygos vein, and superior vena cava enlargement.

FIGURE 66-1 A 31-year-old patient who presented with acute chest pain, eventually diagnosed with acute myocarditis. A, An upright posteroanterior chest radiograph is essentially normal. The cardiac silhouette is at the upper limit of normal in size. There is no evidence of pulmonary edema or pleural effusions. B, Horizontal long-axis view of delayed postcontrast T1-weighted gradient-echo sequence shows dilation of the right ventricle (RV) relative to the left ventricle (LV) and linear subepicardial hyperenhancement of the anterior wall of the right ventricle (arrowhead). A nodular focus of non-subendocardial delayed hyperenhancement is also noted in the lateral myocardial wall of the left ventricle (arrow). C, Vertical long-axis view of delayed postcontrast T1-weighted gradient-echo sequence shows a well-defined oval focus of subepicardial delayed hyperenhancement (arrow) and subendocardial sparing (arrowhead). These features are consistent with acute myocarditis.

Ultrasonography

Echocardiographic findings of myocarditis are nonspecific, including segmental wall motion abnormalities and increased left ventricular volume.⁷ During the acute phase, transient left ventricular wall thickening can be observed, probably related to interstitial edema.⁸^,⁹ The average brightness of the myocardium is also higher in patients with myocarditis than in control patients.¹⁰

Computed Tomography

Recent literature shows that multidetector computed tomography (MDCT) may play a role in establishing the diagnosis of myocarditis in patients presenting in the acute setting as MDCT is more accessible and less time-consuming than MRI. In addition, ECG-gated MDCT also offers concurrent evaluation of the coronary arteries. However, the amount of iodinated contrast material administered must be considered in patients who may have already undergone conventional coronary arteriography.

On delayed enhanced images, nodular, patchy, bandlike enhancement of the mid and epicardial layer of the left ventricular wall can be seen in patients with myocarditis (Fig. 66-2).¹¹ In a study of 12 consecutive patients with acute chest pain consistent with myocardial ischemia and normal coronary angiogram, delayed enhanced MDCT performed 5 minutes after injection of contrast material demonstrated the same accuracy as MRI in differentiating between myocardial infarction and myocarditis in the acute phase.¹² Similarly, another study also showed good correlation in the extent and location of hyperenhancement at MDCT compared with cardiac MRI in the early phase of suspected acute myocarditis.¹³

FIGURE 66-2 A 40-year-old man with history of progressive exertional dyspnea for months. Axial view of contrast-enhanced CT scan in the delayed phase shows dilated left ventricle (LV) with patchy areas of subepicardial and transmural delayed hyperenhancement (arrows) in a nonvascular distribution involving the lateral wall and apex, consistent with myocarditis.

Magnetic Resonance

Cardiac MRI has been established as an important imaging tool in the diagnosis of myocarditis. By identifying inflammatory lesions in the myocardium, it helps guide endomyocardial biopsy for histopathologic confirmation of diagnosis. It is also an excellent modality for subsequent assessment of natural regression of inflammation or treatment response in patients already diagnosed with myocarditis.

ECG-triggered segmented cine steady-state free precession imaging may show segmental or global wall motion abnormality, such as hypokinesis or akinesis. Tissue characterization can be performed with ECG-triggered T2-weighted fat-saturated double-inversion recovery turbo spin-echo sequence (or T2 short tau inversion recovery [STIR]) and postcontrast delayed (10 minutes) T1-weighted inversion recovery gradient-echo sequence; the latter is the most important MR pulse sequence in the diagnosis of myocarditis (Fig. 66-3). T2-weighted images may show patchy areas of increased myocardial signal. Postcontrast T1-weighted gradient-echo images typically demonstrate ovoid or nodular, patchy, epicardial and mid-wall delayed hyperenhancement of myocardium in a nonsegmental vascular territory. Focal delayed hyperenhancement may become diffuse later in the disease process but with eventual regression (Fig. 66-4). Resolution of inflammation may be seen in 3 months. Persistent inflammation may lead to myocardial fibrosis and scar formation, resulting in myocardial thinning and dilated cardiomyopathy in some patients.

FIGURE 66-3 A 35-year-old woman with clinically suspected acute myocarditis. CT angiography of the chest was negative for pulmonary embolism and acute aortic syndrome. Horizontal long-axis view of T2-weighted dark blood double-inversion recovery sequence shows foci of hyperintensity within the mid-myocardium, more prominent within the lateral myocardium (arrow), consistent with focal inflammation related to myocarditis. T1-weighted delayed postcontrast images showed corresponding hyperenhancement of left ventricular myocardium and also pericardial enhancement consistent with myopericarditis.

FIGURE 66-4 An 18-year-old man with clinically suspected acute myocarditis. A, Vertical long-axis view of delayed postcontrast T1-weighted gradient-echo sequence shows a nodular focus of delayed hyperenhancement (arrow) of the basal inferior left ventricular (LV) wall. B, Short-axis view of delayed postcontrast T1-weighted gradient-echo sequence shows subepicardial and mid-wall myocardial delayed hyperenhancement (arrows) involving the lateral wall of the left ventricle (LV). C, Short-axis view of delayed postcontrast T1-weighted gradient-echo sequence performed after 2 months shows interval decrease in the area of delayed hyperenhancement (arrow), consistent with resolving myocarditis.

A large study showed that areas of delayed hyperenhancement are associated with active inflammation defined by histopathologic examination.¹⁴ In a study that included 44 patients with myocarditis, inferolateral wall delayed hyperenhancement was a common feature, although other segments were also involved. About two thirds of all lesions were subepicardial in location.¹⁵ Another study suggested that the delayed hyperenhancement pattern may be different between myocarditis caused by parvovirus B19 (PVB19) and myocarditis caused by human herpesvirus 6 (HHV6). Patients with PVB19 infection tended to have presentations similar to that of myocardial infarction and had subepicardial delayed hyperenhancement in the lateral wall. Patients with HHV6 or combined PVB19/HHV6 infection presented with new-onset heart failure and had septal delayed hyperenhancement; this group of patients more frequently progressed toward chronic heart failure.¹⁶

Delayed hyperenhancement detects irreversible myocardial injury but is unable to differentiate acute from chronic myocarditis.¹⁷ The sensitivity may be limited because myocarditis does not always result in irreversible myocardial injury.¹⁸

Perfusion cardiac MR imaging can also play a role in differentiating acute myocardial infarction from myocarditis in patients with acute chest pain. A prospective study showed that all the patients with acute myocardial infarction had a segmental early subendocardial defect on first-pass perfusion and corresponding segmental subendocardial or transmural delayed hyperenhancement in a predominantly anteroseptal or inferior vascular distribution. All but one of 24 patients with myocarditis had no early perfusion defect and focal or diffuse nonsegmental non-subendocardial delayed hyperenhancement predominantly in the inferolateral location.¹⁹

T2 imaging has also been used to identify acute myocarditis or edema of the myocardium as part of the acute inflammatory process. Because myocarditis usually results in global, instead of focal, high T2 signal abnormality, the signal intensity of the myocardium is compared with that of skeletal muscle.¹⁷ Areas of high T2 signal intensity have been shown to correlate with serum markers of acute myocardial injury.²⁰

In the setting of suspected chronic myocarditis, however, a study of 83 patients showed that increased calculated myocardial global relative enhancement on T1-weighted images and edema ratio on T2-weighted images indicating inflammation were common findings. Delayed hyperenhancement, however, had low sensitivity and accuracy.²¹ It was suggested that if MRI is normal in this setting, endomyocardial biopsy may not be necessary.