Excerpt from Urticaria

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Background

Urticaria, commonly referred to as hives, is the most frequent dermatologic disorder seen in the ED. It appears as raised, well-circumscribed areas of erythema and edema involving the dermis and epidermis that are very pruritic. Urticaria may be acute (lasting less than 6 wk) or chronic (lasting more than 6 wk). A large variety of urticaria variants exist, including acute immunoglobulin E (IgE)–mediated urticaria, chemical-induced urticaria (non-IgE-mediated), urticarial vasculitis, autoimmune urticaria, cholinergic urticaria, cold urticaria, mastocytosis, Muckle-Wells syndrome, and many others.

Acute IgE-mediated urticaria is the most benign form of anaphylaxis. It usually occurs independently, but it may be accompanied by the more serious clinical manifestations of anaphylaxis, angioedema, and anaphylactic shock. The etiology of both acute and chronic urticaria are numerous (see Causes below). The etiologic agent is more likely to be identified in acute urticaria (40-60%) than in chronic urticaria (10-20%). The lesions of IgE-medicated urticaria usually last less than 24 hours and are often migratory, leaving no residual skin abnormalities. The lesions of urticarial vasculitis usually last longer than 24 hours, are both painful and pruritic, and often leave purpuric and hyperpigmented lesions.

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Pathophysiology

Urticaria results from the release of histamine, bradykinin, leukotriene C4, prostaglandin D2, and other vasoactive substances from mast cells and basophils in the dermis. These substances cause extravasation of fluid into the dermis, leading to the urticarial lesion. The intense pruritus of urticaria is a result of histamine released into the dermis. Histamine is the ligand for 2 membrane-bound receptors, the H1 and H2 receptors that are present on many cell types. The activation of the H1 histamine receptors on endothelial and smooth muscle cells leads to increased capillary permeability. The activation of the H2 histamine receptors leads to arteriolar and venule vasodilation.

This process is caused by several mechanisms. The type I allergic IgE response is initiated by antigen-mediated IgE immune complexes that bind and cross-link Fc receptors on the surface of mast cells and basophils, thus causing degranulation with histamine release. The type II allergic response is mediated by cytotoxic T cells, causing deposits of immunoglobulins, complement, and fibrin around blood vessels. This leads to urticarial vasculitis. The type III immune-complex disease is associated with systemic lupus erythematosus and other autoimmune diseases that cause urticaria.

Complement-mediated urticarias include viral and bacterial infections, serum sickness, and transfusion reactions. Urticarial transfusion reactions occur when allergenic substances in the plasma of the donated blood product react with preexisting IgE antibodies in the recipient. Certain drugs (opioids, vecuronium, succinylcholine, vancomycin, and others) as well as radiocontrast agents cause urticaria due to mast cell degranulation through a non-IgE mediated mechanism. The physical urticarias in which some physical stimulus causes urticaria include immediate pressure urticaria, delayed pressure urticaria, cold urticaria, and cholinergic urticaria. Finally, there are urticarias, especially chronic urticarias, for which no cause can be found, despite exhaustive efforts—the so-called idiopathic urticarias.

Frequency

United States

Urticaria affects 15-20% of the general population at some time during their lifetime.

International

The frequency of urticaria internationally is similar to that in the United States.

Mortality/Morbidity

Pruritus (itching) and rash are the primary manifestations of urticaria, and hyperpigmentation or hypopigmentation are rare. Acute urticaria is usually self-limited and commonly resolves within 24 hours but may last up to 6 weeks. Chronic urticaria lasts more than 6 weeks. Neither acute nor chronic urticaria results in long-term consequences other than anxiety and depression. The depression can be severe enough to lead to suicide in rare cases. Also, many of the diseases associated with chronic urticaria may cause very significant morbidity and mortality.

Race

No variation in race is noted.

Sex

Incidence rates for acute urticaria are similar for men and women; chronic urticaria occurs more frequently in women (60%).

Age

Urticaria can occur in any age group, although chronic urticaria is more common in the fourth and fifth decades.

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