In His Own Words: One Clinician's Diagnostic Journey

Differential Diagnosis: The Process of Clinical Decision Making

Submitted by Vincente Mauri, PT, COM. from Santiago, Chile

"Treatment can be learned in a day, but to learn to diagnose takes a lifetime." — James Cyriax

"What you don't know, you don't see." — James Cyriax

The process of making decisions in the clinic comes from a balance between both what is scientifically known and proven and the specialist's intuition and experience. These two features are the foundations of clinical reasoning used daily and they reflect the process of making sense of a clinical encounter. But they alone are not enough. The IAOM advocates a very systematic evaluation process done the same way for each patient in order to maintain this balance, avoid common errors, and to improve efficiency.

In many cases, the examination process has to be achieved in a very short period of time due to expectations of productivity, which is mainly based on the number of patients treated during a specific period of time. Unfortunately a measurement of "quality" is not deemed as important as getting the examination done quickly. One possible solution to this situation is to develop and follow a well structured and organized differential diagnosis process that could help the clinician become as accurate as possible in as little time as possible.

Each IAOM course includes instruction in a basic clinical examination. The goal of this basic examination is to get the most information from the fewest tests. The purpose of the basic functional examination is to determine the tissue responsible for the patient's pain and any impairment that is interfering with healing. If this exam is performed in the same way every time, an examiner will become more efficient and more accurate. The other advantage of performing a consistent basic functional examination is that one can share findings easily with colleagues who use the same examination; a common professional language is created.

A Differential Diagnosis is choosing between several hypotheses that could explain the patient's symptoms. The process starts by gathering information through the patient's history, and the basic clinical examination. From the results the therapist will generate hypotheses that could explain the findings. Now the examination process must systematically eliminate hypothesis until only one is left. The examiner will chose between many possible special tests to achieve this goal. It is here where the therapist's expertise and clinical reasoning must play an important role, identifying which findings (from both history and clinical exam) are important and which special tests must be performed to identify the correct diagnosis. Too many special tests waste time and confuses the assessment;too few and the conclusion may be inaccurate.

There are 3 methods of differentiating between separate hypotheses that are used most commonly. One of these methods is called the hypothetic-deductive approach. Initial theories are generated very early during the initial presentation of the case, from existing knowledge, associations, and experience. Further questions or examination are oriented towards supporting or refuting these first ideas. If a hypothesis is discarded, usually because of special testing, an alternative one is considered; this requires further special tests. Both awareness of probabilities (prevalence) of disease and knowledge of causal pathways are important. This is the most common utilize by physical therapists.

The second common method is pattern recognition. A particular combination of symptoms, or even certain phrases used to describe a symptom, can be strongly suggestive of a particular diagnosis. People build up their own internal library of patterns on the basis of their experience and existing knowledge. With this type of method, the clinician biases the physical examination and relies primarily on what the patient is saying. This method could result in clinical mistakes, and inaccurate diagnoses. There is a risk of not excluding all possible explanations and "jumping to a conclusion." One will be better served by subjecting every patient to the same initial examination so as to limit evaluator bias.

The last common method of differential diagnoses relies on pathognomonic signs and symptoms; these signs and symptoms are those that almost guarantee a certain diagnosis. Unfortunately, most of these findings are rare and of little help in day to day practice and therefore the possible diagnoses would be limited.

Instead of adopting just one method, the physical therapist should be able to move freely from one to another. An examiner can be performing a hypothetic-deductive approach during the initial part of an examination, but at the same time, the clinician is open to identify any red flags (pattern recognition) that could be associated with other underlying pathologies or disorders that could influence the diagnostic process or the resulting treatment, or require referral to another medical specialist. The examiner should also be aware of the rare pathognomonic signs they may encounter.

Even though this procedure seems to be the right way to establish a diagnosis, some experts tend to make decisions in very different ways. Instead of constructing a differential diagnosis based on history and examination, they utilize a number of shortcuts (heuristics), based on knowledge and previous experience, which enable them to work much more quickly. There are many advantages to heuristics, such as very rapid processing and an ability to handle complex information without overload. There are, however, also a number of biases incorporated in the heuristics that may lead to poor decision making. These cognitive biases are common features of thinking that can occur during the clinical reasoning process, but they tend to lead to inaccurate conclusions.

1. Representative bias is the difficulty in estimating probabilities accurately, giving undue weight to small samples, or over-estimating the similarity between people or events.
   
   
2. Availability bias is the tendency to attribute too much value to information that is easily obtained, or to an event that is easily remembered because of particularly salient features.
   
   
3. Bias towards positive and confirming evidence at the expense of negative evidence.
   
   
4. Incorrect application or interpretation of tests.

For example, if a 17 year old girl with anterior knee pain is evaluated, representative bias could lead one to conclude that she has patellofemoral pain syndrome because that is a common problem in 17 year old girls. But a relatively high probability is not the same as a diagnosis. If the patient complains of a popping sensation during knee flexion one might decide that she has a meniscus lesion because the examiner recently saw a patient with a meniscus lesion who also had a popping sensation in the knee during knee flexion. This is an example of availability bias. If one chooses to do only 1 or 2 tests to prove their primary hypothesis correct, this is an incorrect application of special tests. When one is in a hurry, it is tempting to reach a conclusion as quickly as possible which could lead to a bias towards positive evidence.

All of the IAOM courses teach differential diagnosis of musculoskeletal conditions. Keeping in mind a few key points can assist with this process. Knowing the patient's age, gender, and usual activities can be quite useful as certain conditions are more prevalent in certain populations. Adhesive capsulitis is more common in women and diabetics for example. Teenagers are more likely to have an avulsion fracture than a tendon rupture at many different boney attachments. Keep in mind that negative findings and tests are just as important as positive ones. At the end of your examination process you should review all of your findings and make sure they all fit your hypothesis, including negative findings. If one suspects a patellar tendonitis but resisted knee extension was negative, re-consideration is required. Keep in mind the specificity and sensitivity of certain tests, when they are known.

References:

1. Interview with Valerie Phelps, PT, OCS, FAAOMPT, Founder and Current Education Director of the IAOM-US, instructor for IAOM courses in the USA and Europe and owner and practice director of Advanced Physical Therapy, AK USA.
   
   
2. The Clinical Reasoning "Guidelines". The College Faculty of the University of Washington School of Medicine.
   
   
3. Sizer P, Brismée JM and Cook C. Medical Screening for Red Flags in the Diagnosis and Management of Musculoskeletal Spine Pain. World Institute of Pain. Pain Practice, Volume 7, Issue 1, 2007 53-71.
   
   
4. Zimmermann P. Triage and Differential Diagnosis of Patients with Headaches, Dizziness, Low Back pain, and Rashes: A basic primer. Journal of Emergency Nursing. June 2002, 28:3.