Allergic Contact Dermatitis from Cashews
There is a society of dermatologists known as the Holmes Society which get together every year to challenge one another with pictures such as this one. They take their societal name from the hero of every diagnostician, Sherlock Holmes, and they thrive on extracting every possible clue from the clinical photos with which they are challenged. I am not a member, since I am primarily a derm surgeon, but the idea is very cool nonetheless. So, if we were to put on our deerslayer, grab our magnifying glass and try to extract all the clues, what exactly would we come up with? What would Sherlock say?
The first is the historical data. Primarily left hand, but to a degree it is bilateral. Second, it is episodic, otherwise I would not have described it as recurrent. Third, going to visual clues it is a bullous eruption. The bulla has small vesicles in the surface of it, which has significant implications. Fourth, it is pretty well localized to the central palm, with a little "bleeding off" of the rash onto the thenar eminence. The fingertips are spared!! So, given that spare bit of data, what are the possibilities?
We generally know that the differential for bullous eruptions run from the immunobullous diseases (BP, PV, DH and their ilk), epidermolysis bullosa, porphyria and pseudoporphyria, thermal trauma, chemical trauma, bullous fixed drug to spongiotic dermatitis. The fact that it is so localized pretty well rules out immunobullous disease. Thermal trauma? C'mon. Chemical? Same.
Recurrent bullous fixed drug would almost always have some measure of hyperpigmentation, and the fact it is sometimes on the other palm kills that diagnostic possibility. Wrong location for porphyria and pseudo, and it is only on palms, which makes EB very, very unlikely.So, if we run the ddx for spongiotic derm (contact, id, nummular, dyshidrotic and dermatophyte- the mnemonic is CINDD) then you can rule out id and nummular and dyshidrotic (it would be pretty symmetrical) and dermatophyte by history and by exam, so you are left with... contact.
Contact derm can be this bullous, especially after the third or fourth or umpteenth exposure, as was the case with this litttle person. In fact, the other clue this person had was an extremely itchy anal area, which could really nail down your diagnosis. So, what was the exposure?? In a word, Anacardiacea.
The Anacardiaceae are a group of plants that encompass a wide variety of fruits and nuts and shrubbery, but in this case, the Anacardiacea of choice for this patient was the humble cashew nut. That explains why the palmar distribution (how do you hold 'em before you pop 'em in your mouth?) and the relative sparing of the fingertips. Also explains the downstream symptomatology, too. The undigested allergenic oils are distributed on defecation to the perianal area, and patients can often get a significant spongiotic derm in the perianal area.
The Anacardiacea family encompasses poison ivy, poison oak, poison sumac, cashews, mangoes and the marula tree (for a hilarious video of animals getting drunk from the fermented fruit of the marula tree, click here: http://www.youtube.com/watch?v=D5E5TjkDvU0 ). Recent taxonomic reclassification had included pistachios in this group, although I have yet to see a patient who has gotten an allergic contact reaction from them. So, if they are allergic to rhus, they are possibly allergic to cashews. And mangoes.