How do you differentiate between myocardial infarction (MI) and myocarditis in a patient presenting with symptoms suggestive of cardiac involvement?

Differentiating Myocardial Infarction from Myocarditis

The key to differentiating MI from myocarditis lies in coronary angiography findings (normal coronaries favor myocarditis), ECG patterns (diffuse ST elevation without reciprocal changes suggests myocarditis versus localized ST elevation with reciprocal depression in MI), wall motion abnormalities (noncoronary distribution in myocarditis versus coronary territory in MI), and cardiac MRI findings (myocardial inflammation and nonischemic patterns in myocarditis versus ischemic scar in coronary distribution for MI). 1

Initial Clinical Assessment

When a patient presents with chest pain, elevated troponin, and ECG changes, specific clinical features help distinguish between these diagnoses:

ECG Pattern Recognition

• Myocarditis typically shows: diffuse T-wave inversion, ST-segment elevation without reciprocal ST-segment depression, and prolongation of QRS complex duration 1, 2
• MI typically shows: localized ST-segment elevation with reciprocal ST-segment depression in a coronary artery distribution 3
• The absence of reciprocal changes is a critical distinguishing feature favoring myocarditis over MI 1

Patient Demographics and Context

• Consider myocarditis more strongly in: young patients, those with recent viral illness or fever, presence of skin manifestations (rash, red dots), and patients without traditional cardiovascular risk factors 4, 3
• Consider MI more strongly in: patients with cardiovascular risk factors, older age, and absence of recent infectious symptoms 5

Diagnostic Algorithm

Step 1: Initial Testing (All Patients)

Obtain ECG, high-sensitivity cardiac troponin, and transthoracic echocardiogram immediately 1, 2

Step 2: Echocardiographic Pattern Analysis

• Wall motion abnormalities in a noncoronary distribution strongly suggest myocarditis 1, 2
• Wall motion abnormalities corresponding to a specific coronary territory suggest MI 5
• Abnormal ventricular strain patterns can occur in both but are more diffuse in myocarditis 1

Step 3: Coronary Angiography Decision

Perform urgent coronary angiography when:

• Rising troponin with concerning ECG abnormalities and you cannot definitively exclude MI 1
• The patient has cardiovascular risk factors regardless of age 6
• Wall motion abnormalities could represent either diagnosis 5

Normal coronary arteries on angiography essentially confirm myocarditis in the appropriate clinical context, while significant coronary occlusion confirms MI 4, 6, 3

Step 4: Cardiac MRI for Definitive Diagnosis

In hemodynamically stable patients, cardiac MRI is the most sensitive method to differentiate these conditions 1, 2:

• Myocarditis shows: myocardial inflammation, nonischemic epicardial scar, pericardial effusion/enhancement, and edema patterns 1, 7
• MI shows: ischemic scar in coronary artery distribution, absence of inflammation markers 6
• CMR can detect both acute and previous infarctions that may have been clinically silent 6

Critical Pitfalls to Avoid

• Do not assume young age excludes MI: thrombophilic conditions (like factor V Leiden) can cause MI in young patients without traditional risk factors 6
• Do not rely solely on troponin elevation: both conditions cause troponin elevation, making this marker non-discriminatory 4, 3
• Do not delay angiography in uncertain cases: when clinical suspicion for MI exists, proceed to angiography rather than waiting for additional testing 1, 5
• Do not skip cardiac MRI in young patients with normal coronaries: MRI may reveal previous silent MIs or confirm myocarditis and guide management 6, 5

Management Implications Based on Diagnosis

If Myocarditis is Confirmed

• Hospitalize at an advanced heart failure center if definite myocarditis with any concerning features 2, 8
• Avoid NSAIDs in isolated myocarditis (increased inflammation and mortality risk) 8
• Restrict strenuous physical activity for 3-6 months 2, 8
• Plan follow-up testing (ECG, echocardiogram, ambulatory monitor, CMR) at 3-6 months 7, 8

If MI is Confirmed

• Proceed with standard acute coronary syndrome management and revascularization
• Investigate underlying thrombophilic conditions in young patients without traditional risk factors 6
• Initiate guideline-directed medical therapy for coronary artery disease

High-Risk Features Requiring Immediate Advanced Care

Regardless of the suspected diagnosis, transfer to an advanced heart failure center immediately if the patient exhibits: hypotension, cardiogenic shock, sustained ventricular arrhythmias, or advanced atrioventricular block 1, 2, 7