The linear pattern of the vesicles and their distribution on the patient’s arms prompted the family physician (FP) to suspect that this was a case of allergic contact dermatitis (ACD) caused by exposure to a plant. While drug eruptions can cause all kinds of rashes—including vesicular eruptions—it would be rare for them to cause a perfect linear pattern. Upon further questioning, the FP learned that the patient had been gardening in her backyard a few days before the eruption started. This additional information supported a diagnosis of Rhus dermatitis from poison ivy. (Depending on the plants growing in the region, it could also have been poison oak.)
Toxicodendron (Rhus) dermatitis (poison ivy, poison oak, and poison sumac) is caused by urushiol, which is found in the sap of this plant family. Clinically, a line of vesicles can occur from brushing against one of the plants. The linear pattern can also occur from scratching and dragging the urushiol across pruritic skin with the fingernails. If ACD involves extensive skin areas (>20%), systemic steroid therapy is often required and offers relief within 12 to 24 hours. Severe poison ivy/oak is treated with oral prednisone for 2 to 3 weeks. Methylprednisolone should be avoided because the dose and duration are insufficient and can lead to a rebound contact dermatitis at the end of the short course.
In this case, the patient didn’t need an oral steroid. The patient was happy knowing the diagnosis and that the eruption would go away spontaneously. The FP suggested over-the-counter calamine lotion to sooth the itching and also asked the patient if she wanted a prescription for a topical steroid. The patient said that she would like one as a backup in case the over-the-counter lotion didn’t work, so the FP gave her a prescription for 0.1% triamcinolone cream to be applied once to twice daily.
While the evidence for topical steroids in Rhus dermatitis isn’t strong, the risk of adverse effects from topical steroids is much less than the risks associated with weeks of an oral steroid. During a future visit for her hypertension, the patient indicated that the poison ivy had gone away uneventfully.
Photos and text for Photo Rounds Friday courtesy of Richard P. Usatine, MD. This case was adapted from: Usatine R. Contact dermatitis. In: Usatine R, Smith M, Mayeaux EJ, et al, eds. Color Atlas of Family Medicine. 2nd ed. New York, NY: McGraw-Hill; 2013:591-596.
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