A 16-year-old girl presents for evaluation of a rash on her left forearm, which was 2 enlarging red areas that became increasingly pruritic and edematous. What's the diagnosis?
A 16-year-old girl presents for evaluation of a rash on her left forearm (Figure). Two days earlier, she was volunteering in the community, cleaning up a park by clearing plants and placing them in recycling bags, when she experienced sudden onset of a burning sensation on her left forearm associated with 2 enlarging red areas that became increasingly pruritic and edematous. The following day, she developed black macules on her forearm that she was unable to wash off.
Diagnosis: Urushiol dermatitis/”Black-spot” poison ivy
Allergic contact dermatitis as a result of poison ivy, poison oak, and poison sumac is an exceedingly common cause of Type IV hypersensitivity reaction. It results from allergens contained in urushiol, the resinous sap found in the stems, roots, and leaves of plants belonging to the genus Toxicodendron when the plants are damaged.1 This reaction is characterized by a pruritic, erythematous rash, with papules and vesicles often in a linear distribution.
“Black-spot” poison ivy is an uncommon presentation of this common reaction, and as such may cause diagnostic confusion and alarm. Black enamel-like deposits are often seen on the surface of the plants themselves in areas of trauma, and this can be used in the field to identify plants from the genus Toxicodendron. Leaves crushed between sheets of white paper cause the paper to turn dark brown within minutes and black by 24 hours as a result of oxidation of the urushiol.1 It is thought that this reaction occurs due to oxidation of concentrated urushiol under certain conditions, and it is reproducible on application of this substance to the skin, initially with black discoloration and after several days developing a lacquer-like appearance. These spots are adherent and cannot be removed.1,2
Interestingly, the resin has the potential to cause dermatitis indefinitely3,4 and cannot be removed from clothing by washing.5 Histologically, biopsied specimens demonstrate the resin on the surface of the stratum corneum with a neutrophilic infiltrate and areas of epidermal necrosis beneath it (in contrast to the typical presentation of poison-ivy dermatitis without black discoloration, in which biopsy reveals superficial perivascular infiltration with no evidence of resin and no associated necrosis). This suggests both an irritant and allergic contact dermatitis.
There have been several case series of such “black-spot” poison ivy dermatitis,5,6 with patients often initially presenting concerned about more serious pathology, including melanoma. Treatment mirrors that as for classic contact dermatitis and includes topical steroids and oral antihistamines with the addition of oral steroids only as needed in extensive cases or those that involve the face or groin.
The patient was advised of the diagnosis and treated with triamcinolone 0.1% topical ointment, with topical mupirocin for any open areas. The following day, after a swim meet, she re-presented to the emergency department with new red raised eruptions around her eyes and neck, likely from contact with her arm or a delayed eruption from the original contact. She was treated with oral prednisone and did not require further follow-up.