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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.

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Seizures

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

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Syncope

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Syncope is loss of consciousness due to a reduced supply of blood to the cerebral hemispheres or 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.

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It is important to distinguish seizures from syncope because they have different causes, diagnostic approaches, and treatment.

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Approach to Diagnosis

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The initial step in evaluating a patient who has suffered a lapse of consciousness is to determine whether the setting in which the event occurred, or associated symptoms or signs, suggests that it was a direct result of a disease requiring prompt attention, such as hypoglycemia, meningitis, head trauma, cardiac arrhythmia, or acute pulmonary embolism. The number of spells and their similarity or dissimilarity should be established. If all spells are identical, then a single pathophysiologic process can be assumed. The following major differential features should be ascertained.

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Events at Onset of Spell

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Prodromal Symptoms (Aura)
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A detailed inquiry should always be made about prodromal and initial symptoms. A witness is 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 a given patient.

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Posture When Loss of Consciousness Occurs
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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.

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

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Focal Motor or Sensory Symptoms
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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.

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Affective or Cognitive Symptoms
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A sensation of fear, olfactory or gustatory hallucinations, or visceral or déjà vu sensations are commonly associated with seizures originating in the temporal lobe.

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Presyncopal Symptoms
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Progressive light-headedness, dimming ...

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