- Sudden, unexpected, and transient loss of consciousness and postural tone.
- Spontaneous and full recovery.
Syncope can be defined as a sudden, transient loss of consciousness and postural tone that fully resolves spontaneously without specific
intervention (eg, cardiopulmonary resuscitation [CPR], electrical
or chemical cardioversion). The common pathophysiologic mechanism responsible
for most syncopal spells is a transient reduction in cerebral blood
flow and cerebral hypoperfusion. Reduced cerebral blood flow from
cardiovascular and neurocardiogenic causes accounts for most cases
in which a diagnosis can be made. Even when cerebral blood flow is
normal, a reduced delivery of such essential cerebral nutrients
as oxygen and sugar can occasionally cause altered consciousness.
Syncope is a common condition experienced by up to 50% of adults in a lifetime. It is responsible for about 3% of hospital
admissions and 4% of emergency department visits. Physicians
are frequently consulted to evaluate this symptom and—more
commonly—presyncope, dizziness, or lightheadedness, which
may have a similar pathogenesis.
Syncope has many causes (Table 24–1),
most of which have a benign prognosis. Because cardiac causes are associated with greater morbidity and mortality, early recognition of structural heart disease or other cardiogenic causes is important in order to prevent sudden death or injury.
Table 24–1. Major Causes of Syncope. |Favorite Table|Download (.pdf)
Table 24–1. Major Causes of Syncope.
|Obstruction to blood flow|
|Prosthetic valve dysfunction|
|Congenital heart disease|
|Pump failure (myocardial infarction or ischemia)|
|Arrhythmias (decreased cardiac output)|
|Sick sinus syndrome|
|Atrioventricular block (Adams-Stokes attacks)|
|Torsades de pointes|
|Carotid sinus hypersensitivity|
|Situational (tussis, micturition, defecation, deglutition)|
|Cerebral vascular insufficiency|
|Extracranial vascular disease|
Obstruction to Blood Flow
Any obstructive structural lesion of the left or right side of
the heart can critically reduce the cerebral blood flow. Exertional
symptoms are common with obstructive lesions because cardiac output
does not rise normally with exercise and cerebral perfusion is not
maintained. Obstruction to left ventricular outflow occurs with aortic
valve stenosis, mitral stenosis, left atrial myxoma, prosthetic
aortic or mitral valve dysfunction, and hypertrophic cardiomyopathy.
The ventricular arrhythmias that can occur with valvular heart disease
may be responsible for both exertional and nonexertional syncope
as well as sudden death.
Lesions that obstruct flow through the right side of the heart
include right atrial myxoma, pulmonary stenosis, tricuspid stenosis,
pulmonary hypertension, and pulmonary emboli. Limitations to right ventricular
outflow diminish the cardiac output and the ability to increase
the output with exertion. Exertional syncope is common with severe
pulmonary hypertension and severe pulmonic stenosis.
Some congenital heart diseases, such as tetralogy of Fallot,