The Pattern Approach to Dermatologic Diagnosis

You might have wondered how dermatologists always seem to come up with some fantastic diagnosis or a comprehensive list of differential diagnoses after seeing a pet only briefly during a referral visit. These dermatologists then seem to know exactly which tests to run, and these tests are always ones that you didn't run before you referred the case. Many clinicians falsely believe that the diagnosis was reached because the dermatologist had seen cases just like theirs many times before. That may be true, but most of the time the reason is far more pragmatic; veterinary dermatologists quickly get into the habit of identifying cutaneous lesions properly and that provides critical information necessary to compile differential diagnoses and perform standard tests. There's nothing magical about it.

To do this effectively, you must know your primary and secondary lesions and be able to associate them with a predominant morphologic pattern. If you can't describe the morphologic pattern correctly, you can't diagnose the case correctly either in most cases.

To make a correct dermatologic diagnosis, we need to approach each case in three distinct steps. Of course, patient history and signalment are critical to narrowing down the diagnostic possibilities. However, this approach deals exclusively with the evidence you see during a clinical examination. If we follow these three steps, we'll arrive at a diagnosis in most cases.

Step 1. Determine Pattern

Take your knowledge of primary and secondary lesions and carefully evaluate the animal to determine the predominant pattern. This is done by asking yourself a series of questions in a specific order. Remember to select the predominant pattern. A dog with profound scaling and 3 pustules doesn't have a vesiculopustular disorder; it has an exfoliative dermatosis. A cat with a large hairless mass on its back has a papulonodular pattern, not alopecia.

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