Definition and mechanism
Syncope is transient loss of consciousness due to global cerebral hypoperfusion, with rapid onset, short duration, and spontaneous complete recovery. The unifying final pathway is a drop in cerebral perfusion — whether from vasodilation/bradycardia (reflex), failure to maintain pressure on standing (orthostatic), or a sudden fall in cardiac output (arrhythmic or obstructive).
Classification
- Reflex (neurally mediated) — the most common and most benign. Vasovagal (emotional or orthostatic stress with a prodrome), situational (cough, micturition, defecation), and carotid sinus syndrome.
- Orthostatic — hypotension on standing from volume depletion, medications, or autonomic failure.
- Cardiac — the dangerous group. Arrhythmic (bradyarrhythmia or tachyarrhythmia) or structural/obstructive (severe AS, HCM, PE).
Red flags pointing to a cardiac cause
- Exertional or supine syncope
- No prodrome (“I just woke up on the floor”)
- Palpitations immediately preceding collapse
- Syncope causing significant injury or occurring while driving
- Structural heart disease, prior MI, or reduced EF
- Family history of sudden cardiac death at a young age
- An abnormal ECG (any of the features above)
ECG clues
The 12-lead is the highest-yield first test. Beyond overt arrhythmia, look for the substrate: conduction disease (AV block, bifascicular block), pre-excitation, channelopathy patterns (long/short QT, Brugada), and markers of structural disease or scar.
Workup
- History, exam, orthostatic vitals, and 12-lead in everyone — this risk-stratifies most patients
- Echocardiogram when structural disease is suspected
- Ambulatory monitoring matched to symptom frequency — Holter (daily), patch (weeks), or an implantable loop recorder for infrequent, unexplained, recurrent events
- Exercise testing for exertional syncope
- Tilt-table testing for suspected reflex syncope when the diagnosis is unclear
- EP study in selected patients with structural disease or conduction disease
Role of the EP study
Targeted, not a screening test. It can demonstrate sinus node dysfunction, infranodal conduction disease (prolonged HV), or inducible VT in the right substrate. A negative study does not rule out an arrhythmic cause, so it is used selectively and interpreted in context.
Management
- Reflex syncope — education, trigger avoidance, hydration and salt, counter-pressure maneuvers; pacing only for documented reflex asystole in older patients
- Bradyarrhythmic syncope — pacemaker for SND or AV block as the documented cause
- Tachyarrhythmic syncope — ablation or ICD per the mechanism and substrate
- Structural/obstructive — treat the lesion (e.g., aortic valve, HCM)