• Transient loss of consciousness characterized by unresponsiveness and loss of postural tone with spontaneous recovery; usually brief and caused by cerebral hypoperfusion
  • System(s) affected: cardiovascular, nervous



  • Approximately 20–35% of adults report ≥1 episode during their lifetime; 15% of children <18 years of age
  • The prevalence in institutionalized elderly (>75 years of age) is 23%.

Etiology and Pathophysiology

Systemic hypotension secondary to decreased cardiac output and/or systemic vasodilation leads to a drop in cerebral perfusion and resultant loss of consciousness.

  • Cardiac obstructions to outflow (e.g., pulmonary embolus [PE], hypertrophic cardiomyopathy, aortic stenosis)
  • Cardiac arrhythmias
  • Noncardiac
    • Reflex-mediated vasovagal (neurally mediated syncope [NMS]): inappropriate vasodilation leading to neurally mediated systemic hypotension and decreased cerebral blood flow; situational (micturition, defecation, cough, pain, emotions, hair combing)—most common cause in adult cases
    • Orthostatic hypotension (OHT): volume depletion, pregnancy, anemia, medications
    • Drug/alcohol induced
    • Primary autonomic failure: pure autonomic failure, Parkinson
    • Secondary autonomic failure: diabetes, amyloidosis
    • Carotid sinus hypersensitivity
  • The vast majority of pediatric cases represent benign alterations in vasomotor tone.

Risk Factors

  • Heart disease (acquired or structural)
  • Dehydration
  • Medications (e.g., antihypertensives, antiarrhythmics, diuretics)
  • Presence of a primary autonomic degenerative disorder

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