Definition

Sudden, self-limited loss of consciousness and postural tone caused by transient global cerebral hypoperfusion, followed by spontaneous, complete, and prompt recovery.

Classification

Four major categories based on etiology¹

• Neurocardiogenic (most common): vasovagal, carotid sinus hypersensitivity, and situational.
• Orthostatic hypotension: hypovolemia, medication-induced (iatrogenic), and autonomic dysfunction.
• Cardiovascular
  • Arrhythmogenic: sinus node dysfunction, AV block, pacemaker malfunction, VT/VF, SVT (rare).
  • Mechanical: HCM, valvular stenosis, aortic dissection, myxomas, pulmonary embolism, pulmonary HTN, acute MI, subclavian steal, etc.
• Miscellaneous (not true syncope): seizures, stroke/TIA, hypoglycemia, hypoxia, psychogenic, etc.
  • Atherosclerotic cerebral artery disease is a rare cause of true syncope; the exception is severe obstructive four-vessel cerebrovascular disease (expect focal neurologic findings prior to syncope).

Epidemiology

• Common in general population: 6% of medical admissions and 3% of emergency room visits.²
• Incidence is similar among men and women; one of the largest epidemiologic studies revealed an 11% incidence during an average follow-up of 17 years, with sharp rise after age 70 years.²

Pathophysiology

• Two components of neurocardiogenic syncope are described as cardioinhibitory, in which bradycardia or asystole results from increased vagal outflow to the heart, and vasodepression, where peripheral vasodilation results from sympathetic withdrawal to peripheral arteries. Most patients have a combination of both components as mechanism.
• Specific stimuli (e.g., micturition, defecation, coughing, swallowing) may evoke a neurocardiogenic mechanism, leading to situational syncope.

Risk Factors

• Cardiovascular disease, history of stroke or TIA, and HTN have been shown to predispose patients to syncope.³
• Low body mass index (BMI), increased alcohol intake, and diabetes are also associated with syncope.³