C. Intrinsic and extrinsic psychic stimuli 1. Fear, anxiety (presyncope is more common) 2. Sight of blood 3. Hysterical
II. Failure of sympathetic nervous system innervation (postural–orthostatic hypotension)
A. Peripheral nervous system autonomic failure (peripheral neuropathy, autonomic neuropathy 1. Diabetes 2. Pandysautonomia 3. Guillain-Barré syndrome 4. Amyloid neuropathy 5. Surgical sympathectomy 6. Antihypertensive medications and other blockers of vascular sympathetic innervation and presynaptic α-agonsits 7. Pheochromocytoma
B. Central nervous system (CNS) autonomic failure 1. Primary autonomic failure (idiopathic orthostatic hypotension) 2. Multiple system atrophy (parkinsonism, ataxia, orthostatic hypotension) 3. Lewy-body and Parkinson diseases 4. Spinal cord trauma, infarction, and necrosis 5. Centrally acting antihypertensive and other medications
III. Reduced cardiac output or inadequate intravascular volume (hypovolemia)
A. Reduced cardiac output 1. Cardiac arrhythmias a. Bradyarrhythmias i. Atrioventricular (AV) block (second and third degree) with Stokes-Adams attacks ii. Ventricular asystole iii. Sinus bradycardia, sinoatrial block, sinus arrest, sick sinus syndrome b. Tachyarrhythmias i. Episodic ventricular tachycardia ii. Supraventricular tachycardia (infrequently causes syncope) 2. Myocardial: angina, infarction, or severe congestive heart failure with reduced cardiac output 3. Obstruction to left ventricular or aortic outflow: aortic stenosis, hypertrophic subaortic stenosis, Takayasu arteritis 4. Obstruction to pulmonary flow: pulmonic stenosis, tetralogy of Fallot, primary pulmonary hypertension, pulmonary embolism 5. Pericardial tamponade
B. Inadequate intravascular volume (hemorrhage); dehydration
IV. Other causes of episodic faintness and syncope
A. Hypoxia
B. Severe anemia
C. Cerebral vasoconstriction caused by diminished CO2 as a result of hyperventilation 1. Without Valsalva (faintness common, syncope rare) 2. With Valsalva: purposeful (“mess trick”) or induced by crying in children
D. Hypoglycemia (faintness frequent, syncope rare)
E. Anxiety (panic) attacks
F. Environmental overheating
This list of conditions causing faintness and syncope is deceptively long and involved, but the usual types are reducible to a few well-established mechanisms. So as not to obscure these mechanisms by too many details, only the varieties of fainting commonly encountered in clinical practice and those of particular neurologic interest are discussed in the following text.
The Common Faint (Vasodepressor Syncope)