A 59-year-old woman with Fitzpatrick Skin Type I presented to the health center with a 1-day history of a linearly distributed rash on the volar aspect of her right wrist (Figure 1). The rash developed rapidly within hours of doing yard work, which included the removal of weeds from her garden. She noted intense pruritus and burning at the site of the rash. She denied the use of any new skin products, harsh chemicals, or irritants before the development of the rash. She also denied using any topical therapeutics to treat the rash. She reported taking diphenhydramine 25 mg every 8 hours to reduce pruritus.
The patient's past medical history was significant for asthma and gastroesophageal reflux disease. She also reported a history of varicella as a child. In addition, she noted medication allergies to penicillin, cephalosporin, and sulfa drugs. Her immunizations were up-to-date, with a recent inoculation of the Shingrix (zoster) vaccine. She denied taking any new medications in the past 2 weeks. Her current medications included ranitidine for gastroesophageal reflux disease, olopatadine eye drops for allergic conjunctivitis, and betamethasone inhaler to control her asthma symptoms.
What is your diagnosis?
- Herpes zoster
- Bullous pemphigoid
- Lichen planus
- Contact urticaria
- Insect bite reaction
This patient was diagnosed with a toxin-mediated (nonimmunologic) contact urticaria. The diagnosis was made after it was determined she was exposed to stinging nettles in the area above her leather gloves while gardening. Stinging nettles (Urtica dioica) have deep green elm-shaped leaves with an irregular edge that contain sharp hairs (Baumgardner, 2016). The leaves of this plant extend singularly from a central stalk and bloom with yellow flowers (Baumgardner, 2016; Figure 2). Members of the family Urticaceae cause most of toxin-mediated contact urticaria, with Urtica dioica as the most common cause of this condition in the United States (Baumgardner, 2016).
The precise pathogenesis of toxin-mediated contact urticaria involves sharp hairs, or trichomes (nettles), on the leaves of inciting plants that, when rubbed against, dislodge to reveal a needle-like hollow containing a multitude of irritating chemical cocktails including histamine, acetylcholine, and serotonin (Bolognia et al., 2012). Fluid then leaks into the area of contact, causing a wheal and flare reaction (Mahar & Papier, 2019). These wheals are intensely pruritic and will subside once exposure to the causative agent has ceased (Mahar & Papier, 2019). A persistent tingling sensation in the area of injury may last for 12 hours or more after the urticaria has resolved (Bolognia et al., 2012).
Treatment of contact urticaria typically requires only the avoidance of any inciting factors, as the condition is benign and self-limited (Baumgardner, 2016). However, antihistamines, oral analgesics, and topical steroids may be used to control associated pruritus and burning (Baumgardner, 2016). Although contact urticaria secondary to plants is fairly common in the United States, it can be mistaken for other skin conditions, including herpes zoster, bullous pemphigoid, lichen planus (LP), and insect bite reactions.
Herpes zoster is because of the reactivation of the varicella zoster virus, which manifests as a painful eruption of grouped vesicles on an erythematous base (Bolognia et al., 2012). The lesions of herpes zoster typically involve only one dermatome with common locations on the trunk and face (Carter, 2020). Although the burning discomfort the patient experienced is typical of herpes zoster, the distribution of skin lesions did not correlate with this diagnosis (Carter, 2020).
Bullous pemphigoid is a chronic autoimmune subepidermal blistering disease with a tendency to occur in older populations (Bolognia et al., 2012). Pruritic erythematous patches and urticarial plaques without bullae may be present early in the disease, presenting similarly to contact urticaria (James et al., 2011). The patient's age as well as history of contact with the nettle plant led to the correct diagnosis in this particular case.
Features of LP include flat-topped violaceous papules and plaques in a linear distribution that favor the wrist, forearm, genitalia, distal lower extremities, sacrum, and oral mucosa (Wolff et al., 2017). This condition is often very pruritic and may develop in areas of skin injury (Koebner effect; Wolff et al., 2017). Although the location and quality of this patient's rash could have been associated with LP, the appearance of the lesions was not typical.
Insect Bite Reaction
An arthropod bite or sting can cause a localized inflammatory reaction manifesting as erythematous papules with associated swelling, redness, pain, burning, and pruritus—all symptoms this patient experienced (Craft et al., 2019). Her history of time spent outdoors while gardening necessitated the consideration of insect bites in the differential diagnosis. However, the presentation and distribution of the lesions, along with her history of contact with the inciting plant, made contact urticaria the more likely diagnosis.
In summary, the correct diagnosis of nonimmunologic contact urticaria is largely tied to the appropriate history taking on the part of the dermatology nurse. Therefore, it is critical for these providers to take the time to ask patients about all possible contact exposures, including plant-based toxins, when evaluating patients with a suspicious rash for contact urticaria.
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