Neurocardiogenic: This is the most common cause of syncope in otherwise healthy persons, particularly younger persons. It is often precipitated by fear, anxiety, or other types of emotional distress. Its course is usually benign.

 Orthostatic hypotension: Orthostatic hypotension results from venous pooling and decreased cardiac output and fall in blood pressure. It may be due to volume depletion, anemia or acute bleeding, peripheral vasodilators (most notoriously the α-adrenoceptor blockers used to treat benign prostatic hypertrophy), or autonomic dysfunction (e.g., diabetic neuropathy, dysautonomia caused by central nervous system [CNS] disease).

 Carotid sinus hypersensitivity: This condition is suggested by syncope precipitated by neck movement, or by tight collars or ties. The diagnosis is made by carotid sinus massage (see Question 12).

 Cerebrovascular disease: Carotid artery stenosis usually leads to focal neurologic deficits rather that frank syncope (except perhaps in the very rare case of severe bilateral carotid artery disease, in which global cerebral hypoperfusion can occur). Vertebrobasilar disease is more likely to lead to syncope, although this is a rare cause of syncope in the general population.

 Tachyarrhythmias: Ventricular tachycardia (VT) and torsades de pointes are the most ominous cause of syncope. In patients with a history of prior myocardial infarction or those with significantly depressed left ventricular (LV) ejection fraction (less than 30% to 35%), the presumptive cause of syncope is VT until proven otherwise. Polymorphic VT and torsades de pointes is the presumptive cause of syncope in those with prolonged QT intervals because of drugs or congenital long QT syndrome, and in those with Brugada syndrome (see Question 17). Supraventricular tachycardia (SVT) may produce presyncope but does not usually produce overt syncope.

 Bradyarrhythmias: Syncope may be caused by intermittent complete heart block. Sick sinus syndrome is a general term covering multiple disorders of the conduction system. Tachy-Brady syndrome is the more appropriate term used to describe patients with intermittent atrial fibrillation who, when the atrial fibrillation terminates, then have a several or more second period of asystole before normal sinus rhythm and ventricular depolarization resume.

 Structural-functional: Aortic stenosis is the most common structural cause of syncope in older patients. The dynamic obstruction that occurs in hypertrophic cardiomyopathy (see Chapter 27) is the most common cause of structural-functional–mediated syncope in younger patients. Left atrial myxoma, causing functional mitral stenosis, is an extremely rare cause of syncope. Syncope can also occur with massive pulmonary embolism, which obstructs the pulmonary artery to such an extent that it compromises blood flow to the LV.