• References [3]
Causes of Syncope
Causes % of cases
Neurocardiogenic 21%
Cardiac 9%
Orthostatic hypotension 9%
Unknown 37%

  • References [1,3]
Neurocardiogenic features
  • Long history of syncopal episodes, particularly if they started before the age of 40 years
  • Syncope related to pain, fear, or an unpleasant sight, sound, or smell
  • Syncope occurring during prolonged standing or during a meal
  • Syncope related to a crowd and/or hot place
  • Presyncopal symptoms are present (e.g. sweating, pallor, hot/cold sensation, nausea, vomiting)
  • Syncope related to urination, defecation, swallowing, coughing, laughing, or sneezing
  • Syncope after exercise
  • Syncope related to carotid sinus pressure (e.g. head turning, tight collars, shaving)
  • Younger patients and absence of heart disease
Orthostatic hypotension features
  • Syncope that occurs upon rising to a standing position or after prolonged standing
  • Syncope that occurs upon standing after exertion
  • Treatment with antihypertensives, particularly if they were recently started or doses were recently increased
  • Patients with neurologic conditions (e.g. Parkinson's, diabetic neuropathy)
  • Syncope that occurs after eating
  • Patients who are dehydrated
Cardiac features
  • Older age (> 60 years) and male sex
  • Syncope during exercise or when lying down
  • New-onset syncope at an older age (> 40 years)
  • Sudden onset of palpitations immediately before syncope
  • Absence of prodromal symptoms (e.g. sweating, pallor, nausea, vomiting)
  • Unrelated to fear or other trigger
  • Family history of unexplained sudden death at a young age (< 50 years)
  • History of structural heart disease
  • History of coronary artery disease and/or CHF
  • History of previous arrhythmia

  • References [1,8]
Orthostatic hypotension (OH) testing criteria
Immediate (initial) OH criteria
  • Any one of the following within 15 seconds of standing:
    • Fall in SBP ≥ 20 mmHg (some guidelines use > 40 mmHg)
    • Fall in DBP ≥ 10 mmHg (some guidelines use > 20 mmHg)
    • Decrease in SBP to < 90 mmHg
    • The EFAS/AAS/EAN guidelines use the higher cutoff
Classic OH criteria
  • Any one of the following within 3 minutes of standing:
    • Fall in SBP ≥ 20 mmHg (if patient has supine hypertension, fall of ≥ 30 mmHg is required)
    • Fall in DBP ≥ 10 mmHg
    • Decrease in SBP to < 90 mmHg
  • NOTE: Fall in BP must be sustained, meaning it lasts for minutes
Delayed OH criteria
  • Any one of the following that develops after 3 minutes of standing:
    • Fall in SBP ≥ 20 mmHg
    • Fall in DBP ≥ 10 mmHg
    • Decrease in SBP to < 90 mmHg
  • Other
    • In delayed OH, blood pressure usually decreases gradually
    • Fall in BP must be sustained, meaning it lasts for minutes
Postural orthostatic tachycardia syndrome (POTS) criteria
  • Both of the following within 10 minutes of standing:
    • Increase in heart rate of ≥ 30 bpm (≥ 40 bpm in patients 12 - 19 years old) or heart rate > 120 bpm
    • Does not meet blood pressure criteria for OH
  • Other
    • POTS is defined as an inappropriate heart rate increase with standing that does not meet the criteria of OH
    • POTS rarely causes syncope, but presyncopal symptoms (e.g. lightheadedness, palpitations, weakness, blurred vision) are often present
    • The mechanism behind POTS is not completely understood but may include severe deconditioning, immune-mediated processes, excessive venous pooling, and hyperadrenergic state. See POTS treatment for more.

  • Reference [1]
Low-risk syncope features that support discharge from the ER
Syncopal event features
  • Prodromal symptoms consistent with reflex syncope (e.g. lightheadedness, sweating, nausea, cold/hot sensation)
  • Syncope related to pain, fear, or an unpleasant sight, sound, or smell
  • Syncope with prolonged standing or in crowded, hot places
  • Syncope during a meal or postprandial
  • Syncope triggered by cough, defecation, or urination
  • Syncope related to head rotation or pressure on the carotid sinus (e.g. tight collars, shaving)
  • Syncope related to standing
Past medical history
  • Long history (years) of recurrent syncope with low-risk features including the current episode
  • Absence of structural heart disease
Physical exam
  • Normal exam
  • Normal ECG
  • In young patients with unexplained syncope and no history of cardiac disease, no family history of sudden death, no supine syncope or syncope during sleep or exercise, no unusual triggers, and a normal ECG, the chance of cardiac syncope is very low.

  • References [1,4]
Other conditions to consider when evaluating LOC
Differentiating syncope from a seizure can sometimes be challenging. Some features that may be helpful include the following:
  • Event trigger: Syncope - common | Seizure - uncommon
  • Prodrome: Syncope - palpitations, lightheadedness, pallor, sweating | Seizure - specific auras (e.g. sounds, tastes, smells, tingling)
  • Shaking (myoclonus): Syncope - brief, asymmetrical | Seizure - longer, symmetrical, chewing or lip smacking
  • Tongue biting: Syncope - rare | Seizure - more common
  • Duration of LOC: Syncope - 10 - 30 seconds | Seizure - may last minutes; most are < 5 minutes
  • Post-event: Syncope - full alertness returns within a minute | Seizure - confusion and fatigue that lasts minutes
  • Incontinence: not a reliable indicator; can happen with both
Transient ischemic attack (TIA)
  • Does not typically cause total LOC
  • Focal neurologic deficits are present
Cerebral bleed
  • Progressive impairment of consciousness
  • Severe headache
  • Other neurological signs
Subclavian steal syndrome
  • Subclavian steal syndrome is a rare condition where blood is rerouted to the arm from the vertebral artery because the subclavian artery is stenosed or occluded. If the arm is used excessively, diminished cerebral perfusion from the vertebral artery may occur, and consciousness is impaired.
  • Symptoms are related to arm exercise, typically the left arm
Metabolic disorder / Intoxication
  • Impairment of consciousness is progressive and long as opposed to sudden and brief
  • Cataplexy is a sudden onset of muscle weakness triggered by emotions, usually laughter
  • Cataplexy does not cause LOC, and it is primarily seen in patients with narcolepsy
  • LOC lasts minutes to hours and is frequent
  • Eyes closed during event