– An allergic reaction that produces wheals and erythema in the dermis (urticaria) or the dermis and subcutaneous structures (angioedema).
Causes and Incidence
Urticaria and angioedema are caused by an allergen that provokes a histamine-mediated response. Common allergens include drugs, insect bites or stings, miscellaneous environmental factors, desensitization injections, and foods (eggs, shellfish, nuts, fruits). About 20% of the general population have an episode of urticaria or angioedema at some time. The peak incidence occurs in adults in their 30s.
A histamine response to the allergen induces vascular changes that result in vasodilation and itching. The histamine-mediated response also causes endothelial cells to contract, which allows vascular cells to leak between the cells via the vessel wall to form a wheal on the skin. A more diffuse swelling of subcutaneous tissue accompanies angioedema and is typically seen in the hands, feet, face, and upper airways.
Pruritus is followed by the appearance of wheals 1 to 5 cm in diameter. These may enlarge and develop a clear center and erythematous border. The wheals may occur singly or in crops. They may appear in one site, remain for several hours, disappear, and then appear in a new site. Angioedema involves edema of subcutaneous tissues, and the wheals are typically larger. Respiratory distress and stridor may be seen if the upper airways are affected.
Skin abrasion and secondary infection may occur as a result of scratching, and laryngeal edema may occur in angioedema.
The diagnosis is made by history and clinical examination. Other tests are unnecessary unless the urticaria is chronic and has no apparent allergic cause. In such cases underlying disease (e.g., lymphoma, polycythemia, systemic lupus erythematosus) should be ruled out.
Antihistamines to control histamine response; epinephrine subcutaneously for wheals and by mist for pharyngeal or laryngeal edema.
Identification and elimination of allergens; avoidance of scratching.