Even since then, it has evolved into a pet peeve of mine.  The
brain is a rather important organ, and when it shows acute signs of
dysfunction, you’d think doctors would notice and pay attention.
 All too often, they do not.  

Why is this so important?

A recent paper in the BMJ indicates:

The paper is available freely on the Epocrates site, href="http://www.epocrates.com/dacc/0407/deleriumElderbmj0407.pdf">here
(PDF). (Delirium in Older People, BMJ
2007;334:842-6, doi: 10.1136/ bmj.39169.706574.AD)

The authors seem to agree with my take on the problem:

Based upon the description, it would seem that it would be hard for an
attentive physician to miss the diagnosis.  But it is missed
about half the time.  Why?  

For one, physicians tend to see patients for brief periods of time.
 As a result, the most important symptoms (fluctuating level
of consciousness) is not observed.  Two, physicians often
think that the patient either has a psychiatric condition, or dementia.
 Three, it often is frustrating to interview delirious
patients.  Frustration dulls one’s diagnostic acumen.
 

Just to make life interesting for the physician, delirium can be
superimposed upon a psychiatric condition, or dementia.  In
fact, delirium often co-exists with dementia.  

In order to recognize delirium when you see it, just keep in mind three
things.  One, it is acute.  So when you see someone
with impaired mental status, the single most important question is: How
long has he/she been like this?  If you don’t know, you need
to find out.  Simple as that.

When I was an intern, I was called once to see a patient who “had just
developed schizophrenia.”  When?  ”Just a few hours
ago.”  In fact, it was an elderly person with lung cancer who
had become hypoxic.  She was hallucinating, was paranoid, and
I suppose those symptoms might have looked like those of schizophrenia.
 But it was not that at all, of course.

Two, the severity fluctuates over hours or days.  In a
hospitalized patient, the nursing staff will notice this long before
the doctor does.  The family may know, too.  Picking
up on this means the doctor has to talk to the nurses, maybe even read
what they write in the chart.  A heavy burden, I know.
 Talking to the family can be even more helpful.  

Three, when you find yourself frustrated while interviewing a patient,
the fact that you are frustrated is a diagnostic sign.  I
know, we are accustomed to seeing signs in the patient, not in
ourselves, but sometimes your own emotional state can be a valid source
of information.  

Try to figure out exactly what it is that is causing the frustration.
 Is the patient looking at the TV instead of at you?
 It may not be rudeness; it may be the attentional problem
seen in delirium.  Can the patient not provide a clear
chronology of the course of illness?  It could be an acute
worsening of cognitive ability.  When you ask a question, does
the patient go on and on about the topic of the question, but not
really give you an answer?  Again, cognitive impairment.
 Or does the patient start to answer, then trail off?
 It’s the fluctuating level of consciousness.