Syncope — Approach and Evaluation

• Vasovagal (Neurocardiogenic) Syncope — Central Mechanism: Begins with a triggering stimulus (emotional distress, pain, blood draw, prolonged standing) activating central autonomic pathways. This leads to inappropriate activation of parasympathetic tone via the vagus nerve combined with paradoxical withdrawal of sympathetic activity. The parasympathetic surge causes negative inotropic effects and bradycardia, while simultaneous sympathetic withdrawal eliminates the normal peripheral vasoconstriction that maintains blood pressure during standing. Blood pools in the dependent circulation; venous return decreases; cardiac output plummets; and cerebral perfusion falls below the critical threshold for consciousness maintenance. Importantly, the prodrome (lasting seconds to minutes) reflects initial sympathetic activation—patients report diaphoresis, nausea, pallor, and tachycardia—before the abrupt parasympathetic dominance causes bradycardia and hypotension. This biphasic autonomic response is pathognomonic for vasovagal syncope and explains why prodromal symptoms often precede loss of consciousness.

• Orthostatic Hypotension — Reflex Dysfunction: When a person assumes upright posture, gravitational redistribution causes ~500-800 mL blood to shift into dependent venous capacitance vessels. Normally, baroreceptor reflex arcs sense decreased central blood pressure and trigger sympathetic activation within 1-2 seconds, resulting in increased heart rate, myocardial contractility, and vasoconstriction to restore blood pressure. In orthostatic hypotension, this reflex arc is dysfunctional due to autonomic neuropathy (diabetes, Parkinson disease, amyloidosis), medications (vasodilators, diuretics, anticholinergics), or hypovolemia. Blood pressure fails to recover, and cerebral hypoperfusion develops over seconds to minutes. The defining diagnostic criterion is systolic BP drop ≥20 mmHg (or ≥10 mmHg if diastolic) within 3 minutes of standing.

• Cardiac (Arrhythmic) Syncope — Output Failure: Abnormal electrical activity or structural heart disease causes sudden reduction in cardiac output without the gradual prodrome typical of vasovagal events. Mechanisms include: (1) Tachyarrhythmias (ventricular tachycardia, supraventricular tachycardia with rapid ventricular response, atrial fibrillation) where excessively rapid rates prevent adequate ventricular filling during diastole, dramatically reducing stroke volume and cerebral perfusion; (2) Bradyarrhythmias (complete heart block, severe sinus bradycardia, sick sinus syndrome) where insufficient heart rate and contractility cannot maintain adequate cardiac output; (3) Structural heart disease (aortic stenosis, hypertrophic cardiomyopathy, pulmonary embolism) causing mechanical obstruction to forward flow or acute right ventricular pressure overload. Unlike vasovagal syncope, cardiac causes typically present without prodrome, with abrupt onset, and carry significant mortality risk if untreated. The pathophysiology is primarily mechanical or electrical failure, not reflex dysfunction.

• Cerebrovascular Insufficiency: Rarely, syncope results from transient reduction in vertebrobasilar blood flow due to subclavian steal, severe carotid stenosis, or vertebral artery dissection. These mechanisms are distinct from global hypoperfusion and typically occur in specific body positions or with head movement. The posterior circulation supplies the reticular activating system in the brainstem required for consciousness; critical reduction in posterior circulation flow can cause syncope even without marked systemic hypotension.

• Metabolic and Hypoxic Causes: Severe hypoglycemia (blood glucose <40 mg/dL) impairs neuronal function and can cause syncope through altered cerebral metabolism and reduced oxygen delivery to the brain. Similarly, severe hypoxemia (PaO₂ <40 mmHg) or hypercapnia causes loss of consciousness through impaired cerebral autoregulation and direct neural dysfunction.

• Vasovagal (Neurocardiogenic) Syncope: The most common cause overall (50-60% of syncope presentations), particularly in young, healthy individuals without structural heart disease. Typical triggers include emotional stress (frightening news, blood draw, medical procedures), pain, prolonged standing in warm environments, or crowded spaces. The prodrome is characteristic and mandatory for diagnosis. Patients often report clear memory of prodromal symptoms. Risk factors include young age, female sex, family history of syncope, and previous syncopal episodes. This is the "benign" syncope category—prognosis is excellent with appropriate counseling, and mortality is rare unless injury occurs during the event.

• Situational Syncope (Reflex Syncope Variant): Occurs during or immediately after specific activities. Micturition syncope is syncope during or just after urination, particularly common in young males at night; triggered by rapid bladder decompression and abdominal pressure changes. Defecation syncope occurs similarly with bowel evacuation. Cough syncope (or "tussal syncope") results from sudden intrathoracic pressure changes during paroxysmal coughing, seen in chronic obstructive pulmonary disease or chronic bronchitis. Swallow syncope follows esophageal stimulation. Exercise-induced syncope occurs during vigorous physical activity and warrants cardiac evaluation, as underlying structural heart disease or arrhythmogenic conditions must be excluded. Laughing syncope occurs with profound laughter. These reflex causes share the vasovagal mechanism but are distinguished by the specific trigger.

• Carotid Sinus Hypersensitivity: Exaggerated reflex response to carotid sinus stimulation causing abrupt bradycardia and/or vasodilation. Occurs predominantly in elderly men (>60 years). Triggered by head turning, tight collars, or minor neck pressure. Diagnosed by carotid sinus massage (CSM) during cardiac monitoring, showing asystole ≥3 seconds and/or systolic BP drop ≥50 mmHg with symptoms. Important cause of unexplained falls in elderly populations.

• Orthostatic Hypotension: Accounts for 10-15% of syncope cases, more common in elderly and those on vasoactive medications. Primary autonomic failure (pure autonomic failure, multiple system atrophy, Parkinson disease with autonomic dysfunction) involves progressive degeneration of autonomic nervous system. Secondary autonomic failure results from diabetes (most common cause of autonomic neuropathy), amyloidosis, Guillain-Barré syndrome, paraneoplastic syndromes, or prolonged bed rest. Medication-induced orthostasis is common with antihypertensives, diuretics, tricyclic antidepressants, antiparkinsonian agents, and vasodilators—particularly problematic when combined or in elderly patients. Hypovolemia from dehydration, hemorrhage, or excessive sweating impairs the baroreceptor reflex's ability to compensate for position change. Deconditioning and prolonged immobilization also predispose to orthostatic syncope.

• Arrhythmic (Cardiac) Syncope: Accounts for 10-20% of syncope cases but represents the highest-risk category with 1-year mortality up to 30% if untreated. Long QT syndrome (congenital or acquired) predisposes to torsades de pointes, a polymorphic ventricular tachycardia causing syncope. Congenital forms (Romano-Ward, Jervell-Lange-Nielsen) show genetic mutations affecting cardiac ion channels; acquired QT prolongation results from medications (antiarrhythmics, antipsychotics, macrolide antibiotics, fluoroquinolones) or electrolyte abnormalities (hypokalemia, hypomagnesemia, hypocalcemia). Brugada syndrome is an autosomal dominant channelopathy causing characteristic ECG changes (ST elevation in V1-V3) and predisposing to ventricular fibrillation. Catecholaminergic polymorphic ventricular tachycardia (CPVT) causes syncope during exercise or emotional stress through mutations in calcium-handling genes. Hypertrophic cardiomyopathy causes syncope through left ventricular outflow tract obstruction (particularly with Valsalva) or arrhythmias; presents with systolic murmur intensified by standing. Arrhythmogenic right ventricular cardiomyopathy predisposes to ventricular tachycardia, especially during exercise. Aortic stenosis causes syncope through fixed obstruction preventing adequate cardiac output increase with demand or through triggering arrhythmias. Acute pulmonary embolism causes syncope via acute right ventricular pressure overload and reduced left ventricular filling. Sick sinus syndrome and complete heart block cause bradycardic syncope; the latter is a medical emergency requiring pacemaker placement. Atrial fibrillation with rapid ventricular response in certain structural conditions can cause syncope. Brugada syndrome and Long QT are key genetic causes of sudden cardiac death in young individuals, making their recognition critical.

• Structural Heart Disease: Mitral stenosis reduces left atrial emptying and can cause syncope with rapid atrial fibrillation. Prosthetic valve dysfunction or endocarditis may cause syncope through mechanical obstruction or septic emboli. Dilated cardiomyopathy predisposes to arrhythmias or cardiogenic shock with syncope.

• Pulmonary Embolism: Causes syncope through acute increase in right ventricular afterload, reducing left ventricular preload and cardiac output. Classic presentation includes syncope with dyspnea, chest pain, and tachycardia; risk factors include recent surgery, immobilization, or malignancy.

• Cerebrovascular Causes: Subclavian steal causes vertebrobasilar insufficiency when arm exercise diverts flow from the vertebral artery; presents with syncope during or after upper extremity exertion. Severe bilateral carotid stenosis rarely causes syncope but can in specific circumstances. Vertebral artery dissection may cause syncope with head/neck pain and other neurologic symptoms.

• Metabolic Causes: Hypoglycemia (usually <40 mg/dL) impairs cerebral function; risk factors include diabetes on insulin/sulfonylureas, prolonged fasting, or alcoholism. Hypoxemia from respiratory failure, carbon monoxide poisoning, or severe anemia. Severe electrolyte derangements (profound hyponatremia, hypernatremia, hypokalemia) alter neuronal excitability.

• Psychiatric and Factitious: Panic disorder may present with symptoms mimicking syncope but true loss of consciousness is absent; patients report anxiety and hyperventilation with subsequent lightheadedness. Psychogenic non-epileptic seizures (PNES) present similarly; diagnosis requires video-EEG. Factitious syncope is rare but must be considered in medical system-involved individuals.

• Prodromal Symptoms (Vasovagal Syncope): A characteristic prodrome lasting seconds to minutes before loss of consciousness is the hallmark of reflex syncope and virtually excludes cardiac arrhythmias. Patients report diaphoresis (profuse sweating despite cool environment), nausea, lightheadedness or dizziness, visual changes (blurred or tunnel vision, "graying out"), auditory changes (tinnitus, muffled hearing), and facial flushing. These reflect sympathetic activation—elevated catecholamine levels, increased heart rate (initially), and skin blood flow redistribution. The prodrome provides a window of opportunity for intervention (lying down, leg crossing, hand clenching) to abort the syncopal episode.

• Abrupt Onset Without Prodrome (Cardiac Syncope): The absence of warning symptoms is red flag for cardiac causes, particularly arrhythmias. Eyewitnesses report sudden collapse "out of the blue." This presentation indicates the need for urgent cardiac evaluation including ECG, telemetry, and echocardiography.

• Loss of Consciousness: True syncope involves complete, sudden loss of consciousness with unresponsiveness. Duration is typically 10-20 seconds; longer durations (>5 minutes) suggest seizure disorder rather than syncope. Eyewitnesses rarely report convulsive movements in simple syncope, though brief myoclonic jerks (usually 1-2 seconds) can occur due to cerebral hypoperfusion and do not indicate primary seizure disorder.

• Pallor and Cyanosis: Pallor reflects profound vasoconstriction and reduced skin perfusion; cyanosis develops if loss of consciousness is prolonged and respiratory effort diminishes. Both indicate severe hypoperfusion.

• Incontinence: Urinary or fecal incontinence during syncope suggests either prolonged unconsciousness or seizure-like activity from cerebral hypoperfusion; incontinence alone does not distinguish syncope from seizure.

• Rapid Recovery: Return to consciousness is typically immediate or within minutes of lying down (which restores cerebral perfusion). Post-ictal confusion is absent or minimal; patients regain orientation rapidly. Prolonged confusion suggests seizure disorder rather than syncope.

• Post-Syncopal Aching or Soreness: Patients often report muscle soreness from the fall or brief convulsive movements during the hypoperfusion state but lack the post-ictal fatigue seen after seizures.

• Situational Context: Syncope during blood draw or medical procedure is highly suggestive of vasovagal syncope. Syncope with prolonged standing in warm, crowded spaces suggests vasovagal or volume depletion. Syncope during exercise raises concern for structural heart disease or inherited arrhythmia syndromes and requires urgent evaluation. Syncope while driving is particularly dangerous; patients should be counseled against driving until etiology is determined and treated.

• Syncope in Supine Position: Should immediately raise suspicion for arrhythmia, as gravity-related volume redistribution should not occur lying down; true syncope in this position is rare unless cardiac output acutely fails.

• Physical Exam Findings: Orthostatic vital signs (systolic BP drop ≥20 mmHg or diastolic drop ≥10 mmHg from supine to standing, or reproducible symptoms with position change) indicate orthostatic hypotension. Cardiac murmurs (systolic ejection murmur of aortic stenosis, holosystolic murmur of mitral regurgitation, harsh systolic murmur of hypertrophic cardiomyopathy) suggest structural disease. Irregular pulse or bradycardia suggest arrhythmia. Focal neurologic deficits argue against simple syncope and suggest stroke or other neurologic process. Stigmata of autonomic neuropathy (decreased vibration sense, diminished reflexes, peripheral neuropathy) support orthostatic hypotension diagnosis.

• High-Risk Clinical Features: Syncope in elderly patients, those with known heart disease, syncope during exertion, abrupt onset without prodrome, family history of sudden death, and

Initial evaluation (Class I in all patients)

• History, eyewitness account, and physical exam: the highest-yield "test" — establishes trigger, prodrome, position, and recovery pattern.
• Orthostatic vital signs: supine, then at 1 and 3 minutes standing. A fall in systolic BP ≥20 mmHg or diastolic BP ≥10 mmHg within 3 minutes defines orthostatic hypotension.
• 12-lead ECG: the single test indicated for every syncope patient. Look for conduction disease (bifascicular block, PR/QRS prolongation), QTc prolongation (a QTc above roughly 500 ms is the range associated with torsades risk), coved ST elevation in V1–V2 (Brugada type 1), epsilon waves and anterior T-wave inversion (ARVC), short PR with delta wave (WPW), or LVH with deep septal Q waves (HCM).
• Targeted labs only (hemoglobin if bleeding suspected, glucose, troponin/D-dimer if the history suggests ACS or PE). Broad reflexive lab panels are explicitly discouraged.

Directed and confirmatory testing

• Echocardiography: when structural disease is suspected (murmur, exertional syncope, abnormal ECG) — quantifies aortic stenosis gradient, LVOT obstruction, RV strain.
• Ambulatory rhythm monitoring matched to symptom frequency: Holter for daily events, external loop/patch or mobile telemetry for weekly, and an implantable loop recorder — the effective gold standard for infrequent, unexplained, suspected-arrhythmic syncope.
• Exercise stress testing for exertional syncope; electrophysiology study when structural disease plus conduction abnormality persists unexplained.
• Tilt-table testing: reasonable (Class IIa) for suspected vasovagal syncope when the diagnosis is uncertain, and for delayed orthostatic hypotension or POTS. It is not needed for classic vasovagal syncope and should not be used to gauge therapy response.
• Carotid sinus massage in patients over 40 with unexplained syncope: asystole ≥3 seconds and/or systolic BP fall ≥50 mmHg with symptom reproduction is diagnostic. CSM requires continuous ECG and BP monitoring with resuscitation capability at hand, and is contraindicated in patients with a carotid bruit, known significant carotid stenosis, or stroke/TIA within the preceding ~3 months because of embolic stroke risk — precisely the elderly vasculopathic group in whom it is otherwise tempting.
• Routine head CT/MRI, carotid ultrasound, and EEG are Class III (no benefit) absent trauma, focal deficits, or true seizure features.

Immediate stabilization

• ABCs, continuous telemetry, IV access, point-of-care glucose: treat the reversible cause first — volume resuscitation for hemorrhage/dehydration, dextrose for hypoglycemia.
• Unstable bradyarrhythmia: atropine, then chronotropic infusion (dopamine or epinephrine) and transcutaneous pacing as a bridge to a transvenous or permanent pacemaker (ACLS/ACC-AHA-HRS bradycardia pathway).
• Unstable tachyarrhythmia: synchronized cardioversion; pulseless VT or ventricular fibrillation is defibrillated per ACLS.
• Disposition: high-risk features (structural heart disease, abnormal ECG, heart failure, exertional or supine syncope, family history of sudden death) warrant admission or an observation unit with monitoring.

Reflex/vasovagal syncope — first line is non-pharmacologic

• Patient education, trigger avoidance, and liberalized salt and fluid intake in patients without hypertension or heart failure.
• Physical counterpressure maneuvers (leg crossing, handgrip, arm tensing) at prodrome onset — Class IIa; raises venous return and aborts the reflex.
• Medication withdrawal: deprescribe the offending vasodilator, diuretic, alpha blocker, or tricyclic. This is often the definitive intervention in the elderly.

Pharmacologic escalation for recurrent, disabling reflex syncope

• Alpha-1 agonist — midodrine: Class IIa when there is no hypertension, heart failure, or urinary retention; increases peripheral vascular tone.
• Mineralocorticoid — fludrocortisone: expands plasma volume; a reasonable second-line option.
• SSRIs may be considered in refractory cases.
• Beta blockers are not routinely useful and may be considered only in patients over roughly age 40 — a favorite distractor.

Definitive/device and surgical therapy

• Permanent pacemaker for symptomatic sinus node dysfunction, high-grade or complete AV block, and cardioinhibitory carotid sinus syndrome.
• ICD for syncope with structural heart disease or inherited channelopathy at high sudden-death risk, per the ACC/AHA/HRS ventricular arrhythmia guideline.
• Catheter ablation for SVT/WPW or monomorphic VT; aortic valve replacement for symptomatic severe aortic stenosis (ACC/AHA valvular guideline — syncope is a Class I indication); septal reduction for obstructive HCM.

Complications of the syncopal event itself

• Traumatic injury: the most common complication overall. Absent prodrome means no protective bracing, so facial fractures, dental injury, hip fracture, and head strike predominate. Recurrent unexplained syncope with facial injury is a clue to arrhythmic rather than reflex etiology.
• Intracranial hemorrhage: emergency. Suspect in anticoagulated or elderly patients with head strike; signaled by persistent headache, vomiting, focal deficit, or GCS that fails to normalize. Non-contrast head CT is indicated here — but only here, not routinely.
• Sudden cardiac death: emergency. The feared outcome of unrecognized arrhythmic syncope; the sentinel finding is syncope during exertion or while supine, or a family history of premature sudden death.
• Motor vehicle or occupational accidents: syncope at the wheel or at height converts a benign reflex event into a lethal one.
• Rhabdomyolysis and pressure injury: from prolonged "long-lie" after collapse in an elderly patient; signaled by markedly elevated creatine kinase and acute kidney injury.
• Missed life-threatening mimic: emergency. Pulmonary embolism, aortic dissection, ruptured ectopic pregnancy, and GI hemorrhage can all present as syncope; tachycardia, hypotension, anemia, or hypoxemia after the event should redirect the workup.

Complications of therapy

• Midodrine: supine hypertension (the dose-limiting effect — check BP lying down), piloerection, scalp pruritus, and urinary retention from alpha-mediated bladder neck contraction.
• Fludrocortisone: sodium retention causing edema, hypertension, and heart failure decompensation, plus hypokalemia and metabolic alkalosis from mineralocorticoid-driven distal potassium wasting.
• Beta blockers: symptomatic bradycardia and worsened hypotension — one reason they are not first line.
• Excess salt/fluid loading: pulmonary edema in patients with HFrEF or CKD.
• Pacemaker/ICD implantation: pneumothorax, pocket hematoma, lead dislodgement, cardiac perforation with tamponade (emergency — hypotension with distended neck veins and muffled heart sounds), and device infection with lead endocarditis.
• Inappropriate ICD shocks: usually from atrial fibrillation with rapid ventricular response or lead fracture; presents as recurrent painful shocks and can itself be arrhythmogenic; an electrical storm is an emergency.

• The 12-lead ECG is the single best next step in essentially every syncope stem after history, exam, and orthostatic vitals. If the question offers head CT, carotid ultrasound, or EEG for an uncomplicated syncope patient without trauma or focal deficits, that is the distractor — the 2017 ACC/AHA/HRS guideline calls routine neuroimaging Class III.
• Prodrome is probabilistic, not absolute. A long autonomic prodrome — diaphoresis, nausea, tunnel vision, warmth — favors reflex syncope; no warning at all, or a prodrome consisting only of palpitations, chest pain, or dyspnea, favors a cardiac cause. Elderly patients with reflex syncope are frequently amnestic for any prodrome, and a minority of arrhythmic syncope does have brief warning, so never use prodrome alone to exclude arrhythmia. Classic vasovagal syncope at a blood draw in a young person needs no tilt table — reassurance, salt/fluid, and counterpressure maneuvers suffice.
• Exertional syncope in an adolescent or young athlete means hypertrophic cardiomyopathy until proven otherwise — murmur that increases with Valsalva and standing (decreased preload); get an echocardiogram. In an older adult with a crescendo–decrescendo murmur radiating to the carotids and delayed pulses, it is aortic stenosis, and syncope is a Class I indication for valve replacement (ACC/AHA valvular guideline).
• Syncope while supine or during sleep cannot be gravity-mediated — think arrhythmia (complete heart block, VT, long QT, Brugada).
• Brief myoclonic jerks and incontinence do not make it a seizure. The discriminators are a lateral tongue bite, prolonged postictal confusion, and an aura; syncope recovers within seconds to minutes with immediate reorientation.
• Orthostatics: ≥20 mmHg systolic or ≥10 mmHg diastolic within 3 minutes of standing. Carotid sinus hypersensitivity: asystole ≥3 seconds or systolic drop ≥50 mmHg on carotid sinus massage in an older man whose collar or head-turning triggers collapse — but CSM is done only under continuous ECG/BP monitoring and is contraindicated with a carotid bruit, known carotid stenosis, or stroke/TIA in the past ~3 months (embolic stroke risk).
• Recurrent unexplained syncope with a nondiagnostic monitor → implantable loop recorder, not more echocardiograms.
• The one association examiners love: syncope plus dyspnea, pleuritic chest pain, and hypoxemia after immobilization or surgery = pulmonary embolism, from acute RV pressure overload reducing LV preload — not vasovagal, even if the patient was standing.