What are the diagnostic criteria for viral myocarditis?

Diagnostic Criteria for Viral Myocarditis

Viral myocarditis is suspected when a patient presents with at least one clinical symptom (chest pain, dyspnea, fatigue, palpitations, or syncope) combined with at least one diagnostic criterion from ECG, cardiac biomarkers, or cardiac imaging findings. 1

Clinical Presentation Requirements

The diagnosis begins with identifying one or more of the following symptoms: 1

• Acute chest pain (pericarditic or pseudoischemic pattern)
• New-onset dyspnea (days up to 3 months) at rest or with exertion, with or without signs of heart failure
• Fatigue or malaise
• Palpitations or unexplained arrhythmia symptoms
• Syncope or aborted sudden cardiac death
• Unexplained cardiogenic shock

The acute viral phase lasts only 1-3 days and is characterized by myocyte necrosis from direct viral replication. 1 After this acute phase, the immune response may persist for weeks to months, potentially leading to chronic post-infectious autoimmune myocarditis independent of viral genome detection. 1

Mandatory Diagnostic Criteria (At Least One Required)

ECG/Holter Abnormalities

Any of the following new ECG findings support the diagnosis: 1

• Conduction abnormalities: First- to third-degree AV block, bundle branch block, sinus arrest, or intraventricular conduction delay
• ST-segment and T-wave changes: ST-segment elevation or T-wave inversion
• Arrhythmias: Ventricular tachycardia/fibrillation, atrial fibrillation, supraventricular tachycardia, or frequent extrasystoles
• Other changes: New Q waves, reduced R-wave height, or low voltage

However, ECG findings alone are neither specific nor sensitive enough for definitive diagnosis or to rule out inflammatory heart disease. 1

Cardiac Biomarkers

• Elevated troponin T or troponin I 1
• Elevated creatine kinase 1

Serum markers of inflammation are not very sensitive, and routine viral serology testing is not specific enough to be clinically useful. 1

Cardiac Imaging Abnormalities

Echocardiography may demonstrate: 1

• Regional or global systolic or diastolic dysfunction
• Ventricular dilatation (with or without)
• Increased wall thickness secondary to edema (in acute phase)
• Pericardial effusion
• Intracavitary thrombi

Cardiac MRI is the preferred advanced imaging modality and should show: 1

• Updated Lake Louise Criteria (2018): At least one T2-based criterion (elevated myocardial T2 or increased T2 signal indicating edema) PLUS at least one T1-based criterion (elevated T1, elevated extracellular volume, or late gadolinium enhancement) 1
• LGE pattern: Mid-myocardial or subepicardial distribution (not subendocardial, which suggests ischemia) 1
• Native T1-mapping: Detects subtle focal disease with 90% sensitivity, 91% specificity, and 91% accuracy—superior to T2-weighted imaging and LGE alone 1

Having both T2 and T1 criteria provides high specificity for acute myocarditis; having only one criterion still supports the diagnosis but with lower specificity. 1

Essential Exclusions Before Diagnosis

Acute coronary syndrome and stress-induced cardiomyopathy must be excluded, especially in patients presenting with chest pain, heart failure, or new arrhythmia. 1 This is critical because myocardial injury in viral infections can have multiple causes including type 1 and type 2 myocardial infarction, takotsubo cardiomyopathy, cytokine storm, and pulmonary emboli. 1

Gold Standard Confirmation (When Indicated)

Endomyocardial biopsy remains the gold standard for definitive diagnosis, requiring histologic evidence of inflammatory infiltrates with myocyte necrosis that is nonischemic in origin. 1, 2

Biopsy is indicated in: 1

• Rapidly deteriorating cardiac function despite supportive treatment
• Suspected giant cell myocarditis (requires immediate immunosuppression)
• Fulminant presentation with hemodynamic compromise
• Evaluation for cardiac transplantation candidacy

When strict histologic criteria are applied, the yield is only 5-10% among patients with recent-onset systolic dysfunction and heart failure. 1 The definite diagnosis requires comprehensive histological, cytological, immunohistological, and molecular investigations (PCR for viral genome detection) in pericardial fluid and peri/epicardial biopsies. 1, 2

Common Viral Pathogens

The most frequently identified viruses in endomyocardial biopsies in Western Europe are: 1

• Parvovirus B19 (most common DNA virus)
• Human herpesvirus 6 (HHV-6)
• Coxsackie B and other enteroviruses (important in acute/fulminant cases)
• Adenoviruses
• Epstein-Barr virus (EBV)

Coinfection with two or more viruses occurs in a substantial minority of cases. 1

Critical Diagnostic Pitfalls

• Do not rely on viral serology alone: Increasing serum antibody titers provide only circumstantial evidence and are of little clinical help. 1 There is no correlation between antiviral antibodies in serum or virus isolation from throat/rectal swabs with positive PCR analyses in pericardial tissue. 1

• Normal cardiac MRI indicates good prognosis: A normal MRI in suspected myocarditis indicates good long-term prognosis independent of clinical and other findings. 1

• Myocardial edema without fibrosis suggests recovery potential: Persistent LGE at 4 weeks after onset indicates adverse prognosis, particularly when combined with early gadolinium enhancement. 1