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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, 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 due to vertebrobasilar ischemia.


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

The first step in diagnosis is to determine if events before, during, and after the spell suggest a disease requiring prompt attention, such as hypoglycemia, meningitis, head trauma, cardiac arrhythmia, or acute pulmonary embolism. Next, the number of spells and their similarity or dissimilarity should be ascertained. If all spells are identical, a single pathophysiologic process is likely. Major differential features should then be considered, as discussed below.


Prodromal Symptoms (Aura)

Prodromal symptoms should be inquired about; for this a witness can be critical. The often brief, stereotyped premonitory symptoms (aura) at the onset of some seizures may help localize the responsible lesion. Progressive light-headedness, dimming of vision, and faintness suggest decreased cerebral blood flow leading to syncope.

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, 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 paresthesia, 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 seizures originating in the temporal lobe.


Tonic Stiffening & Clonic Movement

Generalized tonic–clonic (grand mal, or major motor) seizures are characterized by loss of consciousness, accompanied initially by tonic stiffening and subsequently ...

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