Syncope is a symptom defined as a transient, self-limited loss of consciousness, usually leading to a fall. Thirty percent of the adult population will experience at least one episode of syncope. It accounts for approximately 3% of emergency department visits. A specific cause of syncope is identified in about 50% of cases during the initial evaluation.
Reflex (neurally mediated) syncopemay be due to excessive vagal tone or impaired reflex control of the peripheral circulation. The most frequent type is vasovagal syncope or the “common faint,” which is often initiated by a stressful, painful, or claustrophobic experience, especially in young women.
Enhanced vagal tone with resulting hypotension is the cause of syncope in carotid sinus hypersensitivity and postmicturition syncope; vagal-induced sinus bradycardia, sinus arrest, and AV block are common accompaniments and may themselves be the cause of syncope.
Orthostatic (postural) hypotension is another common cause of vasodepressor syncope, especially in elderly patients; in diabetic patients or others with autonomic neuropathy; in patients with blood loss or hypovolemia; and in patients taking vasodilators, diuretics, and adrenergic-blocking medications. In addition, a syndrome of chronic idiopathic orthostatic hypotension exists primarily in older men. In most of these conditions, the normal vasoconstrictive response to assuming upright posture, which compensates for the abrupt decrease in venous return, is impaired.
Cardiogenic syncope can occur on a mechanical or arrhythmic basis. There is usually no prodrome; thus, injury secondary to falling is common. Mechanical problems that can cause syncope include aortic stenosis (where syncope may occur from autonomic reflex abnormalities or ventricular tachycardia), pulmonary stenosis, hypertrophic cardiomyopathy, congenital lesions associated with pulmonary hypertension or right-to-left shunting, and LA myxoma obstructing the mitral valve. Episodes are commonly exertional or postexertional. More commonly, cardiac syncope is due to disorders of automaticity (sick sinus syndrome), conduction disorders (AV block), or tachyarrhythmias (especially ventricular tachycardia and SVT with rapid ventricular rate).
In patients with vasovagal syncope, treatment consists largely of education on the benign nature of the condition and counseling to avoid predisposing situations. Counterpressure maneuvers (squatting, leg-crossing, abdominal contraction) can be helpful in limiting or terminating episodes.
Medical therapy is reserved for patients with symptoms despite these measures. Midodrine is an alpha-agonist that can increase the peripheral sympathetic neural outflow and decrease venous pooling during vasovagal episodes.
Fludrocortisone and beta-blockers have also been used but generally provide minimal benefit. Selective serotonin reuptake inhibitors have shown some benefit in select patients. There is generally no indication for permanent pacemaker implantation in patients with vasovagal syncope unless prolonged, spontaneous episodes of syncope are recorded, especially in the absence of vasodepressor response on tilt-table testing.
If symptomatic bradyarrhythmias or supraventricular tachyarrhythmias are detected and felt to be the cause of syncope, therapy can usually be initiated without additional diagnostic studies. Permanent pacing is indicated in patients with syncope and documented severe pauses (greater than 3 seconds), bradycardia, or high-degree AV block (second-degree Mobitz type II or complete heart block) when symptoms are correlated to the arrhythmia.
An important consideration in patients who have experienced syncope, symptomatic ventricular tachycardia, or aborted sudden death is to provide recommendations concerning automobile driving restrictions
Other patients with symptomatic ventricular tachycardia or aborted sudden death, whether treated pharmacologically, with antitachycardia devices, or with ablation therapy, should not drive for at least 6 months. Longer restrictions are warranted in these patients if significant arrhythmias persist.