The Case:You are called to the emergency room to evaluate a toddler with a diffuse, itchy skin rash that erupted a week ago, the day after he received his mumps/measles/rubella vaccination.
You are called to the emergency room to evaluate a toddler with a diffuse, itchy skin rash that erupted a week ago, the day after he received his mumps/measles/rubella vaccination. He appears well while running around the room, laughing and playing with the nurses. His parents report that his skin improves for a few hours after receiving Benadryl.
Urticaria, also known as hives or wheals, is a common, usually self-limited reactive skin pattern.
Triggers include foods, additives, dyes, infections, inhalants, medications, rheumatologic disorders, and malignancies.
The most common causes in children are foods, drugs, and physical and contact urticaria. Despite careful detective work, the etiology often is not identified before the urticaria resolves. Lesions present as itchy, edematous discrete and/or confluent red macules, papules, or plaques that may clear centrally as they expand
They are often surrounded by pallor. They vary in shape and size, from a few millimeters to many centimeters. Anatomically, they usually involve the dermis, but occasionally lesions extend into the deeper soft tissue resulting in angioedema. Urticaria are dynamic and transient, regressing and reappearing at various sites within minutes to hours. Individual lesions last for less than 24 hours, but acute urticaria may recur for up to 6 weeks and chronic urticaria for months to years.
Histamine released from mast cell granules is the primary trigger of urticaria.
Degranulation of mast cells is most often mediated by IgE Type 1 hypersensitivity reactions. Urticaria can also be complement-mediated in Type 3 hypersensitivity reactions or through a nonimmunologic reaction in which inciting agents such as aspirin or NSAIDs trigger release directly. Some patients develop physical urticaria in which environmental factors such as extremes of temperature, physical trauma, and emotional stress trigger release of histamine. Binding of histamine to H1 and H2 receptors in the skin results in pruritis and vasodilation of blood vessels, respectively. Transudate leaking into the superficial dermis manifests as wheals.
The differential diagnosis includes morbiliform drug rash, viral exanthem, erythema multiforme, erythema marginatum, urticarial vasculitis, and hereditary angioedema. Some morbiliform drug eruptions can be pruritic and appear similar to wheals, but they are usually fixed rather than transient and migratory. Fixed prupuric papules should suggest vasculitis and are readily differentiated from urticarial. Most viral exanthems tend to be more persistent than urticaria, while clinical and histologic findings can exclude the other disorders.
Diagnosis of specific triggers may require some detective work, and patients should avoid these triggers. Allergen testing can be performed to determine triggers, if not evident from the history.
First-line agents include H1 antagonists, and H2 antagonists can be added if refractory. Patients should avoid aggravating factors such as heat, alcohol, aspirin, and tight or itchy clothing. Our patient’s hives waxed and waned for 3 weeks, and resolved uneventfully.
Chronic urticaria has been arbitrarily distinguished from acute urticaria in that its symptoms persist for greater than 6 weeks.
In many cases, the etiology remains elusive after a thorough investigation, and the term chronic spontaneous urticaria is used. Forty percent of these patients have associated autoimmune processes. Chronic urticaria is often refractory to first-line treatment, and severely symptomatic children may require systemic anti-inflammatory and/or immunosuppressive therapy.
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