Is it a Horse or a Zebra? The Importance of the Differential Diagnosis

As a physical therapist, we are often provided with a physician referral diagnosing the patient with some vague pathology ranging from “shoulder pain” to “lumbago.” There is a saying that the sound of approaching hooves is more likely to be the arrival of horses rather than zebras, so typically these diagnoses are more or less accurate. That being said, it is our responsibility to keep an eye out for the occasional zebra that comes walking into the clinic. The following case scenario is a perfect example of one of these striped horses that I evaluated several months ago…

“A 52 year old female presents to the clinic with a diagnosis of left sided sciatica. The patient reported having a gradual onset of posterior thigh and leg pain over the last seven months, with the majority of her symptoms currently isolated to her mid calf. Pain was rated as 2-5-9 (best, average, worst) with 0 indicating “no pain” and 10 indicating “worst pain.” The patient was unable to recall a specific mechanism of injury, but noted that the symptoms initially began after standing awkwardly from a seated position. Recently, the patient noticed having some intermittent tingling into her dorsum of her foot.”

What are your thoughts? Does it look like a horse at this point?

“The patient reported that her symptoms increased while walking and decreased at rest (but never completely went away). The patient stated that she was trying to stretch her calf consistently, but that it did not seem to positively or negatively change her symptoms.”

What about now? Any change to your initial diagnosis? What other information would you like to know?

“The patient’s past medical history revealed a recent heart attack (currently undergoing cardiac rehab and taking medication), hypertension, hypercholesterolemia and diabetes. The patient denied having any significant low back pain in the past.”

Seeing any stripes on that horse? What would be some of the main physical examination findings that you would want to know?

“Lumbar active range of motion was within normal limits (repeated motions were included in AROM testing without any reproduction of symptoms). Neurodynamic testing (Slump/Straight Leg Raise) were negative. Lumbar mobility testing with PA pressure proved to be normal and did not reproduce any symptoms.”

Just to summarize everything up to this point: insidious onset of leg pain that radiates from posterior thigh to lower leg, occasional tingling in the foot, symptoms range from a 2-9/10 and improve with recent (worst with walking), past medical history was significant for a recent heart attack (as well as HTN, DM, etc), lumbar screen was negative, and neurodynamic testing was within normal limits. Now what?

“The patient was asked to walk on a treadmill until symptoms were reproduced. Three minutes later, the patient reported having moderate discomfort throughout her leg. At this time, the evaluation was repeated, and once again, symptoms were not reproduced with testing.”

Here is your zebra and your “spidey sense” should be going off. The patient was instructed to continue with cardiac rehab, went to two aquatherapy sessions (which temporarily reduced her symptoms), but continued to report having constant calf pain. The referring physician was then contacted regarding a possible cardiovascular component to her symptoms, at which an ankle-brachial index test was performed. Normative values range between 1.0 to 1.4; our patient’s ABI was 0.70 (well below the abnormal range). Her symptoms were the result of peripheral artery disease, and she was referred for further vascular evaluation.

So what’s the lesson? If it walks like a duck, looks like a duck, but can’t quack, keep a short leash. Trust your skills, but also listen to your gut, because every once and a while a zebra might come trotting in to your clinic.


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