Itching to Know What Caused a Bulbous Eruption

My patient-an 84-year-old woman with hypertension-presented with itching and blistering of 3 days' duration on her right foot (Figure). Before the symptoms began, she had been working in her garden. She was hospitalized and given intravenous antibiotics and corticosteroids for presumed contact dermatitis and cellulitis. After 6 days, both the blistering and infection had completely resolved.
What was the likely cause of this patient's symptoms?
-Rebecca Galante, MD
Hammond, Ind
Your patient had a striking bullous eruption that involved the dorsum of 1 foot. Her history of working in the garden before the eruption occurred may or may not be pertinent. The diagnoses in the clinical differential fall into 3 broad categories: external contactant, infection or infestation, and autoimmune blistering disease. External contactant. Large bullae are sometimes seen following cutaneous exposure to potent antigens. In this setting, plants from the genus Rhus (poison ivy, poison oak, and poison sumac) and chemicals used in pesticides or insecticides are possible culprits. The acute onset of the eruption, outdoor activity, associated pruritus, and prompt resolution following corticosteroid therapy all favor this diagnosis. Unfortunately, it is impossible to identify with certainty the offending allergen. Infection or infestation. Severe blistering may also accompany some infections, including Vibrio septicemia and bullous erysipelas (a severe form of cellulitis); the latter can be caused by toxigenic strains of staphylococci or streptococci, as well as a variety of enteric pathogens. Although the clinical morphology would be compatible with that seen here, the absence of a history of penetrating trauma and the lack of fever or systemic signs of sepsis make such serious infections unlikely. Tinea pedis can produce bullous lesions, and this patient does appear to have some nail dystrophy, which suggests underlying onychomycosis. However, blistering associated with superficial fungal infection is rarely this severe and almost always appears on the plantar surface rather than the dorsum of the foot.
An atypical reaction to an insect bite might also cause this type of eruption. Flea bites and some spider bites can result in bullae of this magnitude in hypersensitive persons. Autoimmune disease. Cutaneous autoimmune disorders, such as bullous pemphigoid and pemphigus, can cause large blisters on any region of the skin. Pemphigoid, which is most common in older patients, is particularly likely to manifest on the lower extremities. However, autoimmune blistering diseases are typically bilateral and symmetric in distribution and would not be expected to respond to treatment as quickly as this patient's condition did. I favor a diagnosis of acute allergic contact. My second but somewhat distant diagnostic choice is bacterial bullous erysipelas.
-Ted Rosen, MD
Professor of Dermatology
Baylor College of Medicine
Houston