Wednesday, February 23, 2005

Miscellaneous Essay on Medical Topic:
Differential Diagnosis

Recently I was asked about some of the challenges involved in psychiatric diagnosis of adolescents.  This turns out to be a difficult topic.  Firstly, psychiatric diagnosis is an arcane practice, shrouded in obscurity.  Secondly, adolescents can be rather obscure themselves. 

In order to understand the topic fully, it is necessary to start at the beginning.  That means ancient Greece.  The term diagnosis comes from the Greek roots diagignoskein "discern, distinguish;" and gignoskein "to learn."  Thus, a diagnosis is established when the doctor learns the distinction between the disease that the patient has, and other, similar diseases that might present the same clinical picture.  In order to have a full understanding of the term, diagnosis, one also must understand the term, differential diagnosis

Lets take an example.  A patient comes in to the emergency department and tells the doctor: "I have chest pain and shortness of breath."  Immediately, the doctor thinks 'OK, the differential diagnosis is pneumonia, heart attack, pulmonary embolism.'  (The actual list of possibilities is much longer than that, but let's keep it simple for now.)  A differential diagnosis, then, is a list of possible diagnoses.   Medical people often use the term differential, in this context, to mean differential diagnosis; for example, "What's the differential in this case?"  Once the doctor learns the initial list of symptoms, he or she starts thinking of a differential diagnosis.  The doctor then asks questions and does tests to pare down the list.  A good diagnostician does this is a systematic way.

Back to the emergency department example, the first thing the doctor is going to want to know is: "What is the patient's temperature?"  This question divides the list of possibilities into two groups: febrile illnesses, and nonfebrile illnesses. The next question is: "When did this start?"  This divides the possibilities into acute, subacute, and chronic categories.  If you think of the differential diagnosis as a big circle that contains all the possibilities, then each question draws a line through that circle, cutting out some of the possibilities.  Ideally, it is possible to get to the point that only one possibility is left.  At that point, the doctor has made the diagnosis. 

The alert reader will have noticed that I put the word "ideally" in italics.  The really alert reader already will have guessed why I did that.  I did that to alert the reader to the fact that the ideal is not always achieved.  That is, sometimes the differential cannot be narrowed to a single entity. 

Those of us who tend to see life like a flow chart will start to wonder what happens in such a case, if the differential cannot be narrowed beyond a certain point.  At first, it would seem that you get to a decision point on the chart, then can't decide which branch to take.  A computer program would just stop there, and send an error message to the standard output. 
Operation Not Defined
Fortunately, doctors are not computers (at least not yet).  In such a situation, the doctor looks at the relative probabilities of each item in the differential, weighs the relative risks and benefits of each possible course of action, and decides upon a working diagnosis.  A working diagnosis is a diagnosis that is known to be uncertain, but which is used to define a course of action.  Note that the working diagnosis is not necessarily the most likely item in the differential.  This is a very important point. 

Back to the example.  Let's say that the doctor has interviewed the patient and has done a physical exam.  She or he now thinks that the most likely diagnosis is pneumonia.  However, it still is possible that the patient has a pulmonary embolus .  A PE can be very serious.  It can kill a person within minutes, in fact.  In that case, the patient may be sent down to radiology for a lung scan.  The doctor will write an order.  In the spot for "diagnosis," he or she might put: "Rule out PE."  Thus, for the purposes of the test, the working diagnosis is R/O PE, even though the most likely diagnosis is pneumonia.  When the lung scan is done, the differential diagnosis may be changed.  If the scan gives an equivocal result, in may be designated as "low probability."  Then the working diagnosis may be "pneumonia, R/O PE."  The PE is still on the list, but has been demoted from the top of the list. 

What this illustrates is that the diagnosis is not a static entity.  It also is not something that can be defined in one dimension.  It can change over time, as more information becomes available.  It also can change over time, as the disease process evolves.  Sometimes, a disease starts out with only a few vague, nonspecific symptoms.  As the disease progresses, more symptoms may emerge.  Tests that were negative in the early stage may be positive if repeated later on.  Sometimes patients tell the doctors different things on different days.  Sometimes they notice symptoms that were present before, but which they had not noticed or mentioned at the time of the first evaluation.  Sometimes family members step up and speak out about symptoms that the patient did not mention. 

A couple of years ago, I had a patient that I would have sworn had depression.  He was not getting better.  The reason was a mystery.  One day his sister called and asked, "did he tell you that he drinks a pint of vodka every night?" 

"Ma'am, I can't tell you anything about him, but thank you for letting me know."

Mystery solved. 

Adolescence is a time when everything is evolving.  Thus, the diagnosis is a moving target.  Psychiatric conditions often produce symptoms that are difficult to put into words.  Adolescents tend have varying capacities to describe subtleties, to think in abstract terms, and to disclose private details about their lives.  Usually, a teenager's capacity for self disclosure ranges from "very limited" to "none at all."  Parents, of course, will describe symptoms or behaviors that they are observed, but they tend to not be objective, may have their own agenda, and may have been misled, deliberately.  Sometimes one parent says one thing, and the other says the exact opposite.  For these reasons, and more, the psychiatric diagnosis of an adolescent often remains at the stage of the differential diagnosis for a long time.  Sometimes, kids come it with all kinds of symptoms, the symptoms later go away, and you never find out what was going on. 

For some examples, see the section in the Medscape Resource Center on Schizophrenia.  They have an article called Looking for Childhood Schizophrenia: Case Series of False Positives
Case A was a 12-year-old girl with an 8-month history of auditory/visual hallucinations and paranoid ideations that had been treatment-resistant to antipsychotics. Her premorbid history was fairly functional and family history was noncontributory. On the unit, she functioned well and never exhibited any formal thought disorder. Staff noted clinical improvement off medication and subsequent impairment during times of family conflict. She was discharged off medication and her symptoms subsided when the social stressors in her life were eliminated. Her final diagnosis was psychotic disorder not otherwise specified (NOS).
Note that, by citing this particular article, I do not mean to imply that false positives are common.  Rather, I point out the possibility of false positives in order to make a point.  Although the differential diagnosis of adolescents is fraught with difficulty, the stakes can be high.  As mentioned in this review article in the NEJM:
Immediate treatment of a patient after a first psychotic episode improves his or her long-term outcome and does not obscure the later differential diagnosis.88
As a general rule in medicine, anytime there is an opportunity to improve the long-term course of an illness, that opportunity should be taken.  This underscores the importance of the diagnostic process.  Having a full understanding of the diagnostic process is essential in order to avoid missed opportunities.  Yes, there are risks involved in treatment, and one does not want to be exposed to those risks if the diagnosis is a false positive, but the consequences of missing an opportunity can be serious. 

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