What is high-risk syncope?

High-Risk Syncope: Definition and Identification

High-risk syncope is defined as syncope associated with cardiac causes, structural heart disease, or conditions that increase the risk of sudden cardiac death, carrying significantly higher morbidity and mortality compared to other forms of syncope. 1

Cardiac Causes of High-Risk Syncope

• Cardiac arrhythmias - including sinus node dysfunction, atrioventricular conduction disorders, paroxysmal supraventricular and ventricular tachycardias, and inherited arrhythmia syndromes 1
• Structural cardiac disease - including valvular disease (especially aortic stenosis), acute myocardial infarction/ischemia, hypertrophic cardiomyopathy, atrial myxoma, acute aortic dissection, pericardial tamponade, and pulmonary embolism 1
• Inherited channelopathies - such as Long QT Syndrome, Brugada Syndrome, and catecholaminergic polymorphic ventricular tachycardia 1

Risk Stratification Factors

Short-Term High-Risk Markers (≤30 days)

• Abnormal ECG findings - including conduction abnormalities, ventricular hypertrophy, and arrhythmias 1
• History of structural heart disease or heart failure 1
• Exertional syncope - particularly concerning for hypertrophic cardiomyopathy or arrhythmic causes 1
• Syncope without prodromal symptoms - absence of nausea/vomiting preceding the event 1
• Older age (>65 years) 1
• Persistent abnormal vital signs - especially hypotension (systolic BP <90 mmHg) 1
• Palpitations preceding syncope - suggesting arrhythmic cause 1

Long-Term High-Risk Markers (>30 days)

• Male sex 1
• Advanced age 1
• Presence of ventricular arrhythmias 1
• Structural heart disease - especially with reduced ejection fraction 1
• Heart failure 1
• Cerebrovascular disease 1
• Diabetes mellitus 1
• High CHADS-2 score 1

Specific High-Risk Conditions

Long QT Syndrome

• QTc interval ≥450 ms 1
• Lifetime risk of syncope or sudden death increases with QT prolongation: 5% with QTc ≥440 ms, 20% with QTc 460-500 ms, and 50% with QTc ≥500 ms 1
• Syncope in these patients is an ominous finding, typically representing self-terminating episodes of torsades de pointes 1

Brugada Syndrome

• Distinctive ECG pattern with ST elevation in anterior precordial leads (V1-V2) 1
• Patients presenting with syncope have approximately 30% risk of sudden cardiac death within 2 years 1

Cardiomyopathies

• In patients with severe heart failure, syncope predicts higher mortality compared to similar patients without syncope 1
• In hypertrophic cardiomyopathy, syncope (especially during exertion) is a significant predictor of sudden death 1

Prognostic Implications

• Cardiac syncope has a one-year mortality rate of approximately 18-33% compared to 6% for non-cardiac causes 1
• One-year incidence of sudden death is about 24% in patients with cardiac causes versus 3-4% in other groups 1
• Poor outcomes are primarily related to the severity of the underlying disease rather than syncope itself 1
• Structural heart disease is the major risk factor for sudden cardiac death and overall mortality in syncope patients 1

Clinical Approach to Identifying High-Risk Syncope

1. Initial assessment should include detailed history, physical examination, and 12-lead ECG, which can diagnose up to 50% of cases 2

2. High-risk features in history:

   • Syncope during exertion or while supine 1
   • Palpitations preceding loss of consciousness 1
   • Family history of sudden cardiac death 1
   • Absence of prodromal symptoms 1

3. High-risk physical findings:

   • Heart murmurs suggesting valvular disease 1
   • Signs of heart failure 1
   • Persistent hypotension 1

4. High-risk ECG findings:

   • Bifascicular block or QRS duration >120 ms 3
   • Mobitz type II or complete heart block 3
   • Ventricular tachycardia or rapid paroxysmal supraventricular tachycardia 3
   • Long or short QT intervals 3
   • Brugada pattern 3
   • Evidence of arrhythmogenic right ventricular cardiomyopathy 3
   • Q waves suggesting myocardial infarction 3

Important Caveats

• Young patients (<40 years) with normal hearts and no ECG abnormalities generally have excellent prognosis even after syncope 1
• The mere presence of heart disease doesn't necessarily mean syncope is cardiac in origin - reflex mechanisms can still occur in these patients 1
• Multiple causes of syncope may coexist, especially in elderly patients 1
• Recurrence of syncope doesn't necessarily indicate increased mortality risk but is associated with quality of life impairment and injury risk 1

By systematically evaluating these risk factors, clinicians can identify patients with high-risk syncope who require urgent evaluation and treatment to prevent adverse outcomes including sudden cardiac death.