by Nicole Evans M.D.

Created on: June 01, 2010

There are a huge number of causes of fainting or passing out.  The medical term for this loss of consciousness is “syncope”.  The causes of syncope may be psychological, drug or toxin related, neurogenic, cardiac, situational, vasovagal and more.

Fainting that occurs during defecation (bowel movements) or micturation (urination) is known as situational syncope.  This type of syncope can also occur from coughing and is more prone to occur in individuals who have “carotid hypersensitivity syndrome” or “carotid sinus syndrome”.

Vasovagal syncope is one of the most common causes of sudden loss of consciousness.  It occurs in individuals who have excessive tone of the vagus nerve or impaired reflex control of peripheral vasoconstriction.  These people may faint when excited by pleasant or unpleasant stimuli.

Passing out during a transient ischemic attack is known as neurogenic syncope.  A transient ischemic attack is essentially a stroke or cerebrovascular accident that lasts for less than 24 hours with out any residual deficits.

There are many cardiac causes of fainting.  These include arrhythmias such as atrial or ventricular fibrillation and supraventricular or ventricular tachycardias, inflow or outflow obstruction of the heart from acquired or congenital heart defects, cardiac ischemia (impaired perfusion of the heart) and/or decreased cardiac contractility resulting in decreased cardiac output.

Orthostatic hypotension is a fairly common cause of passing out.  This form of transient low blood pressure that occurs upon sitting up or standing may result from hypovolemia as occurs with dehydration or hemorrhage, from autonomic insufficiency as can result from diabetes, or from adrenal insufficiency.

There are numerous drugs and toxins that can cause a person to pass out.  Medications that may induce syncope include alpha-blockers, beta-blockers, nitrates, antiarrhythmic medications, antihypertensives, diuretics, barbituates, opiates, tricyclic antidepressants, digitalis, phenothiazines, adriamycin, and most illicit drugs.

Psychological issues can occasionally result in episodic loss of consciousness.  Fainting from psychological causes can occur in individuals with anxiety, hysterical attacks and those with disordered emotional impairment.

A few miscellaneous causes of syncope occur commonly enough to deserve mentioning.  These include anemia, hypoglycemia (decreased blood glucose), heat, hyperventilation and hypoxia (decreased oxygen levels in blood). 

There are many causes of syncope and often it is impossible to determine the exact cause after a single episode.  However, an individual who loses consciousness with associated symptoms before or after the event, for prolonged periods of time, or multiple times should have a thorough work up to determine the cause.

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