A 33-year-old female presented with a progressively worsening rash for one week. The patient just finished hiking the John Muir Trail, a backpacking trip that encompassed three weeks and over 240 miles. On the last days of the trip, the patient started to develop a severely itchy, red rash on both feet. She tried using a topical anti-fungal, which seemed to make the rash worse. She now has swelling and difficulty walking. The rash does not involve the hands or other parts of the body. She denies fever, open wounds, nausea, vomiting, or systemic symptoms, and has never had a similar rash before.
Skin: Diffuse edema and erythematous maculopapular rash to both feet, with vesicles and bullae overlying the dorsal and plantar surfaces of toes and feet. No rash proximal to the ankles. No petechiae or purpura noted. Normal hands and palms.
The rash has both vesicles and bullae which narrow the differential to contact dermatitis and dyshidrotic eczema. Without petechiae or purpura, it is less likely vasculitis (such as exercise-induced vasculitis). There is no fever, spreading redness, or systemic signs, and it is bilateral, making cellulitis less likely. There were no known exposures to poison oak and the patient never walked without shoes or socks. There were no known tick bites, the hike was in California, and the rash did not involve the palms, making an infectious cause such as Rocky Mountain Spotted Fever unlikely. The rash became worse with topical anti-fungal cream, making fungal infection less likely.
The most concentrated areas of the rash are on the plantar surface of the foot and toes. Upon further inspection, it appears in a pattern that may be consistent with sports tape being used during hiking for blisters and plantar fasciitis pain. The patient later received patch testing by dermatology and was diagnosed with a colophony allergy. In this case, colophony was found in the sports tape causing severe allergic contact dermatitis on the feet. This is a T-cell-mediated reaction caused by repeated exposure to an allergen on the skin. Colophony is a mixture of many different compounds that are all derived from pine trees and is a common ingredient in medical and sports tapes. It is also sometimes used in making shoes.