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Acute alterations in the level of consciousness always indicate
a serious medical problem, which must be comprehensively evaluated
and closely monitored. They may arise from both primary processes
within the central nervous system or may be caused by the secondary
effects of other systemic disorders. In either case, if the disruption
is severe enough, central control of respiratory or cardiovascular
function can rapidly deteriorate, leading to a life-threatening
emergency. The differential diagnosis for altered consciousness
or coma is broad, and imaging studies and laboratory tests alone may
not identify an etiology. A careful history of the events leading up
to the change in mental status and a full multisystem examination
are essential and will help guide the choice of diagnostic tests.
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Normal consciousness requires both maintenance of arousal and
the ability to focus attention and respond to the environment with
a full range of cognitive functions. Maintenance of arousal is mediated
by a complex network of interactions within the brain, prominently
involving the reticular activating system. This is a poorly delineated
brain-stem structure extending from the medulla to the rostral midbrain.
It receives input from the cerebral cortex and all major sensory
systems and projects to local structures within the brain stem and,
via ascending pathways, to the thalamus, the hypothalamus, and the
cerebral cortex. Stimulation of the reticular activating system
results in increased alertness, and destructive lesions produce
unresponsiveness.
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The cerebral hemispheres direct conscious response to environmental
stimuli. In contrast to the brain stem, where even small lesions
affecting the reticular activating system may produce coma, extensive
bilateral involvement of the cerebral cortex is necessary to cause
severe impairment of consciousness. Focal lesions rarely impair
consciousness unless they cause compression or edema of the contralateral
hemisphere or when multiple bilateral hemispheric lesions are present.
However, both focal and diffuse processes may provoke seizures,
which can alter consciousness both during an epileptic attack and
in the subsequent postictal state.
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Altered consciousness usually begins with mild confusion or lethargy. More
severe cases progress to obtundation, where the
patient is somnolent but arousable, then to stupor, where
a patient responds only to vigorous stimuli and immediately becomes
unresponsive when stimulation ceases. Coma refers
to true unresponsiveness to external stimuli, although there may
be reflexive, nonlocalizing motor responses to pain. Delirium describes
a state characterized by confusion, disorientation, and difficulty
maintaining attention.1
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Because terminology is used inconsistently among observers, and
because a patient’s exam may fluctuate, scoring systems
such as the Glasgow Coma Scale (Table 104-1)
are useful in documenting a patient’s level of consciousness
and changes over time. Although these scales provide prognostic
information when the etiology of altered consciousness is known,
they do not assist in establishing a diagnosis and are not a substitute
for a comprehensive history and examination.
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