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The first way to approach a lesion or rash is to think in terms of the “reaction patterns”. These are five morphologic groups that help narrow your ddx. They are broad categories but will give direction to your thought process when presented with a lesion. They are:
| • | dermal (typically a biopsy diagnosis) |
the last two reaction patterns have sub-categories which are useful (in fact they are essential)
| • | bullous – intraepidermal vs. subepidermal |
| • | eczematous – acute vs. subacute vs. chronic |
This is the way you should be approaching every lesion during your first few months of training: simply put the lesion into a reaction pattern (and subcategory when appropriate). Then work on your ddx for within each reaction pattern. When you are comfortable with your reaction patterns (it is often eczematous which comes last, but your eye will develop and you will immediately recognize it after some time) and when you realize that many lesions either span two or three reaction patterns (which diminishes the usefulness of this approach) or cannot be classified in this way, and it is time to move on. But put morphology aside for a moment and learn differential diagnosis by distribution. This is perhaps the most useful way to approach dermatology, because it is the most accessible. You do not need to develop your “eye” for this, you simply have to learn what occurs where. Often this alone will generate a ddx that will contain the correct diagnosis. Then start to learn the other differential diagnoses: the "expanded" reaction patterns, arrangement, etc...) Notice that I have said nothing about therapy. You need to be able to come to a correct diagnosis, then you can always look up the therapy. Being a dermatologist is being a diagnostician.
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