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PATIENT 
Mr. B is a previously healthy 70-year-old man who underwent right upper lobectomy for localized squamous cell lung cancer 5 days ago. On morning rounds, he comments that he is in a military barracks and that he is ready to go home.
What is the differential diagnosis of delirium and dementia? How would you frame the differential?
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CONSTRUCTING A DIFFERENTIAL DIAGNOSIS
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Delirium and dementia are both syndromes of neurologic dysfunction that present as a “change in mental status.” Since the term “altered mental status” can be broad, categorizing the change as chronic or acute and then further classifying acute changes as fluctuating or nonfluctuating may be helpful (Figure 11-1). A patient whose mental status change is acute may have either fluctuating or nonfluctuating symptoms. A patient with acute, fluctuating mental status change is probably delirious. A patient with an acute but stable mental status change may have delirium but may also have a long list of other CNS insults (head trauma, CNS infection, hypoglycemia, CVA among others). Chronic mental change is caused by an irreversible change to the CNS, such as dementia, chronic psychiatric disease, or long-standing CNS injury.
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Delirium and dementia are thus very different syndromes. Whereas delirium is acute, usually reversible and nearly always has an underlying, non-neurologic etiology, dementia is chronic and seldom reversible. The definition of delirium from the DSM-V is:
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A disturbance in attention (ie, reduced ability to direct, focus, sustain, and shift attention) and awareness (reduced orientation to the environment).
The disturbance develops over a short period of time, represents a change from baseline attention and awareness, and tends to fluctuate in severity during the course of a day.
An additional disturbance in cognition (eg, memory deficit, disorientation, language, visuospatial ability, or perception).
The disturbances in Criteria A and C are not better explained by another preexisting, established, or evolving neurocognitive disorder and do not occur in the context of a severely reduced level of arousal, such as coma.
There is evidence from the history, physical exam, or laboratory findings that the disturbance is a direct physiologic consequence of another medical condition, substance intoxication or withdrawal, or exposure to a toxin, or is due to multiple etiologies.
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The DSM V now defines dementia as a major neurocognitive disorder and then defines the many underlying diseases. The definition of a major cognitive disorder is:
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Evidence of significant cognitive decline from a previous level of performance in 1 or more cognitive domains (complex attention, executive function, learning and memory, language, perceptual-motor, or social cognition) based on:
Concern of the individual, a knowledgeable informant, or the clinician that there has been a significant decline in cognitive function; and
A substantial impairment in cognitive performance, ...