What is the diagnostic approach for myocarditis?

Diagnostic Approach to Myocarditis

Endomyocardial biopsy remains the gold standard for definitive diagnosis of myocarditis, but a comprehensive diagnostic strategy using non-invasive methods including cardiac MRI should be employed first in most cases. 1

Clinical Presentation

Myocarditis can present with a wide spectrum of clinical manifestations:

• Acute chest pain: Often mimicking acute myocardial infarction or pericarditis with normal coronary arteries 1
• New onset or worsening dyspnea: At rest or with exercise, with or without signs of heart failure 1
• Arrhythmia symptoms: Palpitations, syncope, or aborted sudden cardiac death 1
• Unexplained cardiogenic shock: In fulminant cases 1

Diagnostic Algorithm

Step 1: Initial Evaluation

• ECG: Look for specific abnormalities including:

  • AV blocks (first to third degree) or bundle branch block
  • ST-segment/T-wave changes
  • Reduced R-wave height
  • Widened QRS complex (>120 ms predicts higher risk of death) 1
  • Low voltage (suggests myocardial edema) 1

• Laboratory tests:

  • Cardiac biomarkers: Troponin T/I (elevated in acute cases, but only present in ~34% of cases with subacute presentation) 1
  • Inflammatory markers: Erythrocyte sedimentation rate and C-reactive protein 1

Step 2: Imaging Studies

• Echocardiography: To identify:

  • Regional or global systolic/diastolic dysfunction
  • Ventricular dilatation
  • Wall motion abnormalities
  • Increased wall thickness (suggesting edema)
  • Pericardial effusion
  • Intracavitary thrombi 1

• Cardiac MRI: Currently the most important non-invasive diagnostic tool 1

  • Lake Louise Criteria (updated version requires): 1
    1. At least one T2-based criterion: Global/regional elevation of myocardial T2 or increased T2 signal
    2. At least one T1-based criterion: Late gadolinium enhancement (LGE) in a non-ischemic pattern (typically epicardial or midwall)
  • Look for:
    • Myocardial edema (T2-weighted imaging)
    • Hyperemia/capillary leak (early gadolinium enhancement)
    • Necrosis/fibrosis (late gadolinium enhancement)
    • Pattern of LGE distinguishes from ischemic injury (epicardial/midwall vs. endocardial) 1

Step 3: Definitive Diagnosis

• Endomyocardial biopsy (EMB): Gold standard for definitive diagnosis 1

  • Indications for EMB:

    • Life-threatening clinical presentation 1
    • Suspected giant cell myocarditis (higher sensitivity 80-85%) 1
    • Deterioration despite supportive treatment 2
    • Hemodynamic instability 3

  • Histological criteria:

    • Presence of inflammatory cells with necrotic myocytes 1
    • Immunohistochemical staining improves sensitivity (anti-CD3, anti-CD68, anti-HLA antigens) 1

Diagnostic Criteria for Clinically Suspected Myocarditis

According to the European Society of Cardiology, myocarditis is clinically suspected when: 1

1. ≥1 clinical presentation (chest pain, dyspnea, arrhythmia symptoms, etc.)
2. ≥1 diagnostic criteria from different categories:
   • ECG/Holter abnormalities
   • Elevated cardiac biomarkers
   • Functional/structural abnormalities on imaging
   • Tissue characterization by CMR

In the absence of:

• Significant coronary artery disease (stenosis ≥50%)
• Known pre-existing cardiovascular disease or other causes

Note: If the patient is asymptomatic, ≥2 diagnostic criteria should be met 1

Important Considerations and Pitfalls

• Troponin limitations: May be normal in subacute or chronic myocarditis; sensitivity only 34% in patients with symptoms for approximately 1 month 1

• CMR timing: Findings in acute myocarditis evolve from focal to diffuse patterns and can eventually resolve, so timing of imaging is important 1

• Viral etiology: Most common causes include enteroviruses (Coxsackie B), adenoviruses, parvovirus B19, and human herpes virus type 6 1

• Clinical course monitoring: Progressive wall motion abnormalities, persistent/fluctuating troponin levels, QRS widening, and frequent non-sustained ventricular arrhythmias may precede life-threatening arrhythmias 1

• Mortality risk: Acute fulminant myocarditis has high mortality risk, while uncomplicated cases have approximately 0% mortality 3

By following this systematic approach, clinicians can effectively diagnose myocarditis and stratify patients according to risk, allowing for appropriate management decisions to reduce morbidity and mortality.