Unknown Dermatology Cases

Two posts ago, I mentioned that much of dermatology is pattern recognition. But what happens when something defies recognition? Several times in this rotation, I've come across diagnostic mysteries. But it reassures me that dermatologists go back to a systematic method of assessing the rash. Is it infectious, inflammatory, neoplastic, a primary skin disease, a drug rash, or a manifestation of another disease? In the end, a biopsy is always taken since that's the gold standard, but at least the thought process is fun.

We had one interesting case of an 18 year old with a presumed diagnosis of inflammatory bowel disease for 7 years. He had initially presented with severe anemia, weakness, and bloody diarrhea. Since then, he's had multiple colonoscopies with biopsies but none have been diagnostic. The gastroenterologists aren't even sure whether this is ulcerative colitis or Crohn's disease. When you learn about these diseases in medical school, they're completely distinct; every test asks about the differences between the two. Yet in real life, that diagnosis has so far evaded us. Several months ago, he started developing large erythematous ulcerating plaques on the backs of his lower legs. We expected this to be pyoderma gangrenosum or Sweet's syndrome, both associated with inflammatory bowel disease. Instead, the biopsy came back polyarteritis nodosa, a vasculitis. The attending even reviewed the slides himself. This really surprised us; the lesion didn't suggest a vascular component and there's no known association between inflammatory bowel disease and polyarteritis nodosa. However, then we began to wonder whether his presumed gastrointestinal symptoms could actually be due to visceral polyarteritis nodosa. That would explain why all the colonoscopic biopsies were nondiagnostic. Very interesting.