Black spots on a toddler’s skin
You are called to the emergency room to evaluate a healthy 2-year-old boy with black spots on his legs that were noted yesterday evening. His younger brother developed similar black spots this morning. The boys are healthy, and the lesions are not symptomatic and appear to be superficial.
DIAGNOSIS: Black spot poison ivy
CLINICAL FINDINGS AND ETIOLOGY
The dark black-brown macules and patches range from 1 mm to 1 cm in size with thin surrounding rims of erythema on the patient’s arms and legs (Figure 1). Under dermatoscopic magnification the macules and papules follow normal skin markings in linear dark streaks, suggesting that they are superficial (Figure 2). A few dark areas can be reduced slightly in size using an alcohol pad, but most are persistent despite vigorous rubbing. Blood work and urinalysis are all within normal limits.
These skin findings are typical black spot poison ivy in a child who has no previous exposure to poison ivy or another member of the Toxicodendron genus. An estimated 85% of the North American population is sensitive to these plants, the most common of which are poison ivy (Toxicodendron radicans), poison sumac (Toxicodendron vernix), and poison oak (Toxicodendron diversilobum or Toxicodendron pubescens).1
The skin lesions resulting from the hypersensitivity reaction to this family of plants are usually erythematous, extremely pruritic, grouped or linear papules and/or vesicles appearing on exposed areas 24 to 48 hours after contact.
Black spot poison ivy is an uncommon presentation following exposure to urushiol or oleoresin, an irritant and allergen from the Toxicodendron genus.2 This plant resin oxidizes and turns black when exposed to air.3
Black spot poison ivy is rare because it requires exposure to concentrated sap. In one study, patients exposed to undiluted concentrations of urushiol developed black spots, while those exposed to a 1 to 50 dilution experienced papulovesicular dermatitis but not black spots.4
Given the superficial nature of these lesions, lack of symptoms, and the clinician’s suspicion of black spot poison ivy, a biopsy is deferred. Histopathology would have shown amorphous yellow material in the stratum corneum and epidermal areas of coagulation necrosis.5
The differential diagnosis of black macules and patches includes superficial purpura, marker or ink, tinea nigra, and black spot poison ivy. If the lesions appear necrotic, infectious and noninfectious vasculitis should be considered. It is important to distinguish the level of skin affected. The involvement of exposed areas and linear/geometric configuration of individual lesions suggest that the dark spots most likely originated from an outside source. A clinical history of exposure to poison ivy supports the diagnosis of black spot poison ivy. Interestingly, since these lesions usually occur with first exposure to poison ivy, a patient usually does not develop the typical itchy eczematous eruption characteristic of poison ivy.
The treatment of black spot poison ivy is similar to that of allergic contact dermatitis from poison ivy.6
Once the oleoresin is oxidized and bound to skin, the black spots cannot be removed with soap, water, or alcohol. The black spots gradually desquamate 1 to 2 weeks after formation without scarring. Patients should also clean or throw out clothing and evaluate for possible sources of poison ivy exposure.
In this case, the parents found poison ivy in the yard and developed blistering lesions of their own skin 2 days later.
1. Baer RL. Poison ivy dermatitis. Cutis. 1990;46(1):34-36.
2. Hurwitz RM, Rivera HP, Guin JD. Black-spot poison ivy dermatitis. An acute irritant contact dermatitis superimposed upon an allergic contact dermatitis. Am J Dermatopathol. 1984;6(4):319-322.
3. Guin JD. The black spot test for recognizing poison ivy and related species. J Am Acad Dermatol. 1980;2(4):332-333.
4. Mallory SB, Hurwitz RM. Black-spot poison-ivy dermatitis. Clin Dermatol. 1986;4(2):149-151.
5. Kurlan JG, Lucky AW. Black spot poison ivy: a report of 5 cases and a review of the literature. J Am Acad Dermatol. 2001;45(2):246-249.
6. Koo B, Lieb JA, Garzon MC, Morel KD. Five-year-old boy with a diffuse erythematous rash with black crusts. Diagnosis: black spot poison ivy (Rhus dermatitis). Pediatr Dermatol. 2010;27(4):395-396.
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