Differential Diagnosis- Understanding How a Doctor Thinks

One thing that I think will help patients get more out of their doctor's visit is understanding the concept of a "Differential Diagnosis" and how this affects how doctor's think.

A "Differential Diagnosis" is a list of all the possible explanations for what may be causing a patient's symptoms.  When a patient comes to see me, the first thing I want to know is what their main symptom is, what we sometimes refer to as a "Chief Complaint".  Since I am a Sports & Spine specialist, most commonly this may be an injured body part.

Once I know the area of concern, I can start formulating a list of the things that may affect the patient.  For example, let's say the patient's chief complaint is "pain near the shoulder blade" (.... doctor-speak, I would call that peri-scapular pain, but I actually prefer the lay term of pain near the shoulder blade).  Given that chief complaint, I would start formulating a list of things that cause cause the symptom (this list may be intimidating, so for those not familiar with the terminology, the point is that I am thinking of a big list):

Chief Complaint: Right shoulder blade pain

Differential Diagnosis:
- Middle Trapezius/ Rhomboid/ Levator Scapula/ Serratus muscle strain
- Right lower cervical facet arthropathy
- Notaliga paresthetica caused by lower cervical radiculopathy
- Cervical myelopathy
- Costo-transverse joint dysfunction
- Thoracic facet syndrome
- "T4" syndrome
- Insertional Enthesopathy from the common tendon insertion of the Rhomboids and Serratus Anterior
- Thoracic herniated disk
- Thoracic radiculopathy
- Referred pain from gall bladder
- Thoracic compression fracture
- Aortic aneurysm
- Postural fatigue from the thoracic paraspinals
- Somatic manifestation of anxiety
- Contusion from blunt trauma
- Metastatic lesion from cancer
- Primary pulmonary tumor
- Tuberculosis

This list could go on for a while.  Because of the nature of my practice, there are certain things that are more likely or less likely, depending on referral source, patient demographics, etc

At this point, I like to go through a structured interview to help ruling in and ruling out different causes.  Some of these causes are rare (e.g., tubercolosis), but because the consequence of missing this diagnosis is so high, I ask anyway.  For example, I may ask about tripping and falling, which may seem irrelevant to someone with pain in the shoulder blade.  However, if the person has a spinal cord compression (cervical myelopathy), often the first sign is a loss of balance.  This is why the patient is paying for my expertise and judgment- I've learned over time which details matter and which details don't.

The patients who allow me to work my way through the structured interview usually find the process rewarding and time efficient.  To me, the process is very similar to how I solicit expertise from my attorney or accountant.  Rather than asking my accountant questions about the tax code I read about on the internet, I let him guide me through those issues that I should be most concerned with, and at the end I'll let him know if there are things I still was wondering about.  There usually isn't - he knows what he is doing.

Usually, based just on chief complaint, age, and gender, the top 3 things on my list would account for over 80% of what is going on.  I still go through this process, though, because the 20% of the time that something else is involved is pretty high.

After performing my structured interview, I like to perform my physical examination.  This again helps me alter the probability for each of the items within my differential diagnosis.  Some examination maneuvers are, again, not obvious.  For example, for a patient like the one we are describing, I will usually perform a Babinski test, when I scrape the bottom of their bare feet.  Many patients wonder why I ask them to take off their shoes for shoulder pain, but for the less than 1% of patients who have positive Babinski test, it may be a critically important indication that they have a more serious neurologic condition.

Only after I have performed my structured interview and physical examination will I look at tests, like MRI imaging or electrodiagnostic studies.  Many patients will ask (.... or not really ask ... they state) "I don't understand why you are asking these questions- I already had an MRI."  The reason is that most tests have significant limitations.  For example, the false positive rate in spine MRIs can be EXTREMELY high- in some patient populations - well over 70%.

After this process, I then formulate what I think the most likely cause of the patient's symptoms are, formulate a treatment plan, and do my best to educate the patient on what I am thinking and why I think that way.   I like to think it works well.

Now, here is the key detail for patients- I've spent well over Malcolm Gladwell's recommended 10,000 hours refining the process, and I am very good at it.  When patients allow me to practice in the way that I've practiced, my success rates are high, and more importantly, the PATIENT's success rates are high.

Sometimes, however, the patient prefers that I modify the way I think about things to fit another approach.  While I am capable of doing this, it takes me out of my primary skill set.  Just like Michael Jordan is not as good a baseball player as he is a basketball player, I am better at being a physician than I am at anything else, and when I am asked to work in a way that is different than how I have practiced (.... and this is why we call it a "Physician Practice"), I am not as good.

This reminds me of a story Dr. Drew Pinsky told about his experiences with prostate cancer.  The main points that he learned from interacting with his own urologist, who saved him from prostate cancer:
1. Don't ask for special treatment.  Ask for treatment most consistent with the way the doctor normally does things
2. You are paying for the judgment of a highly educated and experienced professional.  Allow them to use their judgment

I would add a few suggestions as well:
1. Show up on time.  Nothing throws a physician off more than feeling time pressure. It's also a courtesy to every other patient seen that day
2. Make the doctor's job easier.  Filling out paperwork, bringing imaging and records, allows the doctor freedom to think, rather than looking stuff up
3. Have a clear chief complaint.  The differential thinking process starts with a chief complaint.
4. Don't worry about what factors led you to the doctor's office.  Many patients want to give a narrative story of what events led to them visiting the office.  It is much more important to have a chief complaint- that's when I can start problem solving.
5. If something seems irrelevant, go with it anyway.  The doctor is probably going through their differential thinking process, and trying to rule in and rule out different causes
6. If at the end, you are still confused, let the doctor know
7. Clearly state your goals for the visit

Hope this is helpful