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Consciousness is lost when the function of both cerebral hemispheres or the brainstem reticular activating system is compromised. Episodic dysfunction of these anatomic regions produces transient, and often recurrent, loss of consciousness. There are two major causes of episodic loss of consciousness: seizures and syncope.


Seizures are disorders characterized by temporary neurologic signs or symptoms resulting from abnormal, paroxysmal, and hypersynchronous electrical neuronal activity in the brain.


Syncope is loss of consciousness due to reduced blood flow to both cerebral hemispheres or the brainstem. It can result from pancerebral hypoperfusion caused by vasovagal reflexes, orthostatic hypotension, or decreased cardiac output, or from selective hypoperfusion of the brainstem resulting from vertebrobasilar ischemia.


Seizures and syncope have different causes, diagnostic approaches, and treatment.

First determine whether the setting in which the event occurred, or associated symptoms or signs, suggests a direct result of a disease requiring prompt attention, such as hypoglycemia, meningitis, head trauma, cardiac arrhythmia, or acute pulmonary embolism. Assess the number of spells and their similarity or dissimilarity. If all spells are identical, then a single pathophysiologic process can be assumed. Major differential features should be ascertained as discussed below.


Prodromal Symptoms (Aura)

Inquire about prodromal and initial symptoms. A witness may be critical. The often brief, stereotyped premonitory symptoms (aura) at the onset of some seizures may localize the central nervous system (CNS) abnormality responsible for seizures. Note that more than one type of aura may occur in an individual patient. A sensation of fear, olfactory or gustatory hallucinations, or visceral or déjà vu sensations are commonly associated with seizures originating in the temporal lobe. Progressive light-headedness, dimming of vision, and faintness suggest decreased cerebral blood flow (eg, simple faints, cardiac arrhythmias, orthostatic hypotension).

Posture When Loss of Consciousness Occurs

Orthostatic hypotension and simple faints occur in the upright or sitting position. Episodes that also or only occur in the recumbent position suggest seizure or cardiac arrhythmia as a likely cause, although syncope induced by strong emotional stimuli (eg, phlebotomy) can also occur in recumbency.

Relationship to Physical Exertion

Syncope induced by exertion is usually due to cardiac arrhythmias or outflow obstruction (eg, aortic stenosis, obstructive hypertrophic cardiomyopathy, or atrial myxoma).

Focal Symptoms

Focal motor or sensory phenomena (eg, involuntary jerking of one hand, hemifacial paresthesias, or forced head turning) suggest a seizure originating in the contralateral frontoparietal cortex. A sensation of fear, olfactory or gustatory hallucinations, or visceral or déjà vu sensations are commonly associated with ...

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