Syncope

· blood pressure is regulated by vascular smooth muscle, venous valves, autonomic nervous system, and renin-aldosterone-angiotensin system, which all can affect the pathophysiology of syncope

· Normally, the pooling of blood in the lower parts of the body is prevented by: (1) pressor reflexes that induce constriction of peripheral arterioles and venules, (2) reflex acceleration of the heart by means of aortic and carotid reflexes, and (3) improvement of venous return to the heart by activity of the muscles of the limbs.

· With a decrease in circulating blood volume, cerebral blood flow can be maintained if cardiac output and systemic arterial vasoconstriction compensate, but when these adjustments fail, hypotension can lead to cerebral hypoperfusion and syncope

Definition: transient LOC with loss of postural tone, secondary to global hypoperfusion of cerebral cortex or focal hypoperfusion of reticular activating system

Differential:

• Neurally-mediated: vasovagal (neurocardiogenic), carotid sinus hypersensitivity, situational (e.g. micturition), orthostatic (drugs, autonomic dysfunction, volume)
• Cardiac: arrhythmia, conduction disease, valvular disease, HCM, PE, low-output HF
• Neurologic: CVA, TIA, seizure, vertebrobasilar insufficiency
• Neurally-mediated causes are benign, but cardiac and neurologic causes carry increased mortality
  • In 1/3 of cases, etiology indeterminate, but prognosis in these cases is not entirely benign as in some cases, there is probably underlying cardiac or neurologic disease that has not been elucidated

History is key in helping to differentiate between benign and dangerous causes:

• History of heart disease strongly predicts cardiac cause
• Sudden occurrence and palpitations suggest arrhythmia
• Syncope during effort suggests AS or HCM
• Position: syncope while supine suggests arrhythmia, upright suggests orthostasis
• Prodome: vasovagal syncope often preceded by abdominal discomfort, followed by nausea/vomiting during recovery
• Triggering events or situations (e.g., fear, pain, defecation, urination, cough)

Recommended initial evaluation:

• Orthostatic BPs
• Auscultation with provocative maneuvers (e.g. Valsalva)
• ECG
• Carotid sinus massage

(Christopher Woo MD, 1/28/11)

(Victoria Kelly MD, 5/5/11)