Acute Myocarditis

Introduction and Etiology

• Acute myocarditis is defined by the World Health Organization (WHO) as an inflammatory heart disease associated with cardiac dysfunction.
• The inflammation affects the entire heart, encompassing the interstitium, coronary arteries, and the pericardium.
• The estimated incidence of myocarditis in the pediatric population ranges from 1 to 1.95 per 100,000 children annually.
• The etiology is highly diverse, ranging from direct infectious pathogens to immune-mediated and toxic triggers, though viral infections remain the most common cause in children and adolescents.

Pathophysiology

• The pathophysiological progression of acute myocarditis is classically divided into three distinct stages.
• Phase 1 (Invasion): The pathogenic trigger (e.g., cardiotropic virus) invades the myocardium, leading to direct myocyte degeneration, necrosis, and apoptosis.
• Phase 2 (Subacute/Immune Activation): The innate immune system is activated, recruiting natural killer cells, macrophages, and T-lymphocytes into the myocardium. Virus-infected cardiomyocytes are specifically targeted and degraded by cytotoxic T-cells, which contributes to further myocardial damage. Cytokines released during this intense inflammatory response, such as tumor necrosis factor-alpha and interleukin-1, actively inhibit the myocytes' ability to respond to adrenergic stimuli, leading to profoundly diminished cardiac contractility.
• Phase 3 (Healing and Remodeling): The ongoing immune reaction and virus elimination processes transition into a healing phase characterized by fibrosis of the damaged myocardium.
• In infants and toddlers, this inflammatory process frequently involves the entire heart as a diffuse pancarditis.
• In older children and adolescents, the inflammatory lesions tend to be more focal and localized.
• Failure of the myocardium to fully recover during the healing phase can lead to chronic loss of myocardial functional structure, transitioning into a dilated cardiomyopathy (DCM) phenotype.

Clinical Manifestations

• The clinical presentation of myocarditis in the pediatric population is exceptionally heterogeneous, ranging from asymptomatic isolated ECG changes to fulminant cardiogenic shock and sudden cardiac death.
• In neonates and children under 2 years of age, the disease frequently takes a fulminant course. These infants typically present with non-specific prodromal symptoms such as fever, apathy, feeding intolerance, and gastrointestinal symptoms, which rapidly progress to severe respiratory distress, profound tachycardia, hypotension, and shock.
• Older children and adolescents are more likely to present with distinct complaints of chest pain, palpitations, syncope, or near-syncope.
• A significant proportion of older pediatric patients initially present with normal or only mildly depressed left ventricular function compared to infants.
• In advanced cases with decompensated heart failure, patients manifest with diminished peripheral perfusion, hepatic congestion, peripheral edema, and pulmonary edema characterized by rales and wheezing.
• Arrhythmias, including new-onset supraventricular tachycardia, ventricular tachycardia, or varying degrees of atrioventricular block, are frequently the presenting sign.

Diagnostic Investigations

Auscultation

• The physical examination frequently reveals a tachycardia that is disproportionate to the degree of fever.
• Muffled heart sounds and an overactive precordial impulse may be appreciated.
• A pathologic third heart sound (S3) gallop rhythm is a classic finding indicative of ventricular dysfunction and failure.
• An apical systolic murmur of mitral insufficiency may be audible, resulting from left ventricular and mitral annular dilation.
• If the patient has concurrent pericardial involvement (myopericarditis), a rough, scratchy pericardial friction rub may be heard over the precordium.

Electrocardiogram (ECG)

• The ECG is abnormal in up to 93% of children with myocarditis, though the findings are largely nonspecific.
• Sinus tachycardia is the most frequent finding, occurring in 46% of patients without other identifiable causes like hypovolemia or pain.
• Low-voltage QRS complexes (defined as zenith-to-nadir <0.5 mV in limb leads or <1.0 mV in precordial leads) are highly suggestive of fulminant myocarditis, representing decreased active myocardium or the damping effect of myocardial edema.
• Diffuse, nonspecific ST-segment and T-wave abnormalities (inversion or flattening) are common and can closely mimic the pattern of acute myocardial infarction.
• Pathologic Q waves and ST-segment elevations may occur, mimicking an ischemic infarct pattern.
• High-grade conduction abnormalities, such as complete atrioventricular block or bundle branch blocks, indicate severe involvement of the conduction system.

Chest Radiograph (CXR)

• The chest radiograph is a valuable initial screening tool, demonstrating cardiomegaly in approximately 60% of cases due to left ventricular dilation.
• In fulminant or severe cases, the CXR will reveal pulmonary vascular prominence, interstitial infiltrates, and overt pulmonary edema.
• Pleural effusions may occasionally be present, reflecting biventricular heart failure.
• It is important to note that the cardiac silhouette may appear completely normal in the early stages of acute myocarditis before significant ventricular remodeling and dilation have occurred.

Echocardiography

• Echocardiography is the primary modality for evaluating the functional and structural impact of the disease.
• It frequently demonstrates diminished global or regional left ventricular systolic function and cardiac chamber enlargement.
• Isolated diastolic dysfunction with a preserved ejection fraction can also be a key indicator of myocardial inflammation.
• Functional mitral insufficiency is commonly identified secondary to left ventricular dilation.
• A concurrent pericardial effusion may be visualized, supporting the diagnosis of myopericarditis.

Cardiac Magnetic Resonance Imaging (CMR)

• CMR is currently considered the noninvasive reference standard for diagnosing acute myocarditis, offering detailed myocardial tissue characterization.
• The diagnosis relies heavily on the revised Lake Louise Criteria, which require the presence of both a T2-based criterion and a T1-based criterion to confirm acute myocardial inflammation.
• T2-Weighted Imaging: Demonstrates hyperintense signals indicative of global or regional myocardial edema.
• T1-Weighted Imaging and Late Gadolinium Enhancement (LGE): LGE signifies areas of myocardial necrosis and fibrosis.
• The LGE in myocarditis characteristically presents in a patchy, mid-myocardial, or subepicardial distribution, typically involving the inferolateral wall and basal septum, which sharply contrasts with the subendocardial distribution seen in ischemic coronary events.

Cardiac Catheterization and Endomyocardial Biopsy (EMB)

• While not performed universally due to the availability of CMR, EMB remains the gold standard for definitive histological diagnosis, especially in new-onset unexplained cardiomyopathy.
• The historical Dallas criteria (requiring inflammatory infiltrates with myocyte necrosis) have largely been replaced by the World Heart Federation (WHF) criteria due to poor interobserver reliability and sampling error.
• The WHF criteria define myocarditis by the presence of greater than or equal to 14 leucocytes/mm2 (including up to 4 monocytes/mm2) and the presence of CD3-positive T-lymphocytes greater than or equal to 7 cells/mm2.
• Quantitative immunohistochemistry and viral genome polymerase chain reaction (PCR) are now routinely performed on biopsy samples to identify specific viral triggers.
• Potential severe complications of the biopsy procedure include right ventricular perforation, cardiac tamponade, hemopericardium, and the induction of malignant arrhythmias.

Laboratory Studies

• Cardiac biomarkers, specifically Troponin I or T, are frequently elevated, reflecting active cardiomyocyte necrosis and injury.
• Brain natriuretic peptide (BNP) or N-terminal pro-BNP (NT-proBNP) levels are crucial for assessing the degree of ventricular wall stress and monitoring the trajectory of heart failure.
• General inflammatory markers such as C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), and procalcitonin are frequently elevated.

Management

Acute Medical Stabilization

• Initial management is primarily supportive and requires admission to an intensive care setting with strict bed rest and continuous ECG monitoring.
• In patients presenting with acute cardiogenic shock or fulminant heart failure, the judicious use of intravenous inotropes (such as milrinone, levosimendan, or epinephrine) is necessary to augment cardiac output.
• Inotropes must be used with extreme caution due to the highly arrhythmogenic environment of the inflamed myocardium.
• For patients with severe hemodynamic collapse unresponsive to pharmacotherapy, emergent mechanical circulatory support with Extracorporeal Membrane Oxygenation (ECMO) or a Ventricular Assist Device (VAD) is indicated as a bridge to recovery or cardiac transplantation.

Chronic Heart Failure and Arrhythmia Therapy

• In patients with compensated heart failure, standard anti-remodeling therapy is initiated, including Angiotensin-Converting Enzyme (ACE) inhibitors, beta-blockers, aldosterone antagonists, and diuretics.
• Clinically significant arrhythmias require aggressive management with specific antiarrhythmic agents, such as amiodarone, or the placement of temporary transvenous pacemakers for high-grade heart blocks.
• If life-threatening ventricular arrhythmias persist after a period of recovery, the implantation of an Implantable Cardioverter-Defibrillator (ICD) is strictly indicated.

Immunomodulatory and Antiviral Therapy

• The use of High-Dose Intravenous Immunoglobulin (IVIG) at 1 to 2 g/kg over 24 to 48 hours is frequently considered in the pediatric population, as it provides anti-inflammatory effects and may improve survival, though definitive randomized controlled data remains uncertain.
• Routine immunosuppressive therapy with systemic corticosteroids or azathioprine is generally contraindicated in active viral myocarditis, as it may enhance viral replication.
• Immunosuppression is reserved for specific, biopsy-proven non-viral etiologies, such as giant cell myocarditis, eosinophilic myocarditis, or virus-negative inflammatory cardiomyopathy.
• Specific antiviral therapy is rarely utilized but may be considered if a specific pathogen is isolated via EMB, such as the administration of ganciclovir for documented CMV myocarditis.

Long-Term Follow-Up and Lifestyle Modifications

• Patients require rigorous long-term follow-up with serial echocardiography, as a significant proportion will develop chronic dilated cardiomyopathy.
• Strict restriction from all competitive and vigorous sports is mandated during the active phase of the disease.
• Prior to resuming competitive athletic activities, the patient must undergo a comprehensive evaluation, including a resting echocardiogram, 24-hour Holter monitor, and an exercise stress test, no less than 3 to 6 months following the initial illness.
• Clearance for sports participation absolutely requires the normalization of ventricular systolic function, the absence of active inflammation, normalized cardiac biomarkers, and the absence of clinically relevant arrhythmias.