A 9-year-old boy has developed an itchy vesicular rash on his left leg that has spread to his other leg and to both arms and hands. What is the diagnosis?
The boy's mother reports that the rash appeared two days after he played in a park for several hours.
The rest of your examination is unremarkable.
Diagnosis: Poison ivy (Toxicodendron radicans) dermatitis
Poison ivy dermatitis is a major source of contact allergy in children, particularly during the summer. The causative plant and its relatives (including the commonly named poison oak and poison sumac) are classified under the botanical genus Toxicodendron but are sometimes included in the genus Rhus-hence the term "Rhus dermatitis" that is also used.
Poison ivy, the likely culprit in this case, is a three-leaved tall shrub or woody, rope-like vine that grows within grasses, on trees and fences, and in vacant areas. It flourishes in most regions of the continental United States, and is very common in some areas.
The dermatitis results from a delayed contact hypersensitivity type-IV reaction to the oleoresin (urushiol), of which the active sensitizing ingredient is pentadecylcatechol.
Characteristic lesions appear in susceptible children within one to three days after exposure, although they may develop as early as within 8 hours after exposure in a highly sensitized person. Seventy percent of people in the United States demonstrate a genetic susceptibility to Toxicodendron dermatitis when they are exposed to the sensitizing oleoresin in plants' leaves, stems, and roots.1
The eruption of poison ivy is characterized by pruritic red papules, vesicles, and bullae1 that are usually linear or angulated, either because of the manner of exposure to the plant in woods or the backyard or because scratching transfers the urushiol to adjacent areas (Koebner reaction). Black lacquer-like deposits that often appear within lesions when sap from any Toxicodendron species comes in contact with dry skin, and that are evident in this patient, are a reliable marker for this allergen.2 The nature of the silver-white crust on this patient's lesions is uncertain.
Linear lesions of poison ivy dermatitis can usually be distinguished from other dermatitic disorders, such as atopic dermatitis, which tend to be symmetric in distribution3 and that present in characteristic age-related patterns.3 Poison ivy can also be confused with insect bite, miliaria rubra, and contact dermatitis triggered by such allergens as sun block and fragrances.
When vesicles and bullae are prominent, poison ivy dermatitis must be distinguished from bullous impetigo, herpes simplex virus infection, and varicella infection.4
Exposed persons should wash thoroughly as soon as possible (within minutes of contact is best) to remove the oil from skin. Fresh allergen can be transmitted to other areas of the body by surface-to-surface contact and, within minutes, becomes fixed to skin, where the allergic reaction ensues.
Contaminated clothing and shoes should be washed with soap and water within a few minutes after exposure to remove the urushiol before it dries to prevent it from becoming antigenic indefinitely.5
A potent topical corticosteroid may help reduce the intensity of the reaction but does little to hasten involution of lesions.1 A systemic antihistamine or antipruritic may help ease the itching.