You are called to the emergency room to calm some panicked parents who brought their healthy 2-year old boy in for evaluation of black spots on his legs which were noted when he awoke this morning (fig. 1). It seems that his younger brother developed similar black spots later this morning. The boys are healthy and the lesions are not symptomatic. What’s the diagnosis?
Diagnosis and Clinical Presentation
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 (Fig. 1). Under dermatoscopic magnification the macules and papules follow normal skin markings in linear dark streaks suggesting that they are superficial and an “outside job”(fig. 2). A few dark areas could be reduced slightly in size using an alcohol pad, but most were persistent despite vigorous rubbing. Blood work and urinalysis were 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 other 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.
Epidemiology and pathogenesis
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 1:50 dilution experienced papulovesicular dermatitis but not black spots(4). A black spot test is sometimes used to identify poison ivy. In this test, sap from the suspected plant is squeezed onto white paper(3). If the paper darkens, the plant belongs to the toxicodendron genus.
Given the superficial nature of these lesions, lack of symptoms, and the clinicians suspicion of black spot poison ivy, a biopsy was 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 non infectious 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, the patients usually do 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 1-2 weeks after formation without scarring. Patients should also clean or throw out clothing and evaluate for possible sources of poison ivy exposure. In our patient, the parents found poison ivy in the yard, and developed blistering lesions of their own skin two days later. It is important for physicians to promptly recognize this condition and reassure patients and parents to avoid unnecessary laboratory studies. .
Black spot poison is an unusual presentation of poison ivy contact dermatitis but important to recognize to prevent unnecessary laboratory studies and parental anxiety.
- 1. Baer RL. Poison ivy dermatitis. Cutis 1990;46:34-6. 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;6:319-22. 3. Guin JD. The black spot test for recognizing poison ivy and related species. J Am Acad Dermatol 1980;2:332-3. 4. Mallory SB , Hurwitz RM. Black-spot poison-ivy dermatitis. Clin Dermatol 1986;4:149-51. 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;45:246-9. 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;27:395-6.