Excerpt from Anaphylaxis

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Background

Anaphylaxis is a severe allergic reaction that is rapid in onset and may cause death. There are usually prominent dermal and systemic signs and symptoms. The full-blown syndrome includes urticaria (hives) and/or angioedema with hypotension and bronchospasm. The classic form, described in 1902, involves prior sensitization to an allergen with later re-exposure, producing symptoms via an immunologic mechanism. An anaphylactoid reaction produces a very similar clinical syndrome but is not immune-mediated. Treatment for both conditions is similar, and this article uses the term anaphylaxis to refer to both conditions unless otherwise specified. A recent consensus report reviewed the definition and management of anaphylaxis.¹

For a CME/CE activity, see Anaphylaxis. Also see Medscape's Allergy & Immunology Resource Center.

Pathophysiology

Rapid onset of increased secretion from mucous membranes, increased bronchial smooth muscle tone, decreased vascular smooth muscle tone, and increased capillary permeability occur after exposure to an inciting substance. These effects are produced by the release of mediators, which include histamine, leukotriene C4, prostaglandin D2, and tryptase.

In the classic form, mediator release occurs when the antigen (allergen) binds to antigen-specific immunoglobulin E (IgE) attached to previously sensitized basophils and mast cells. The mediators are released almost immediately when the antigen binds. In an anaphylactoid reaction, exposure to an inciting substance causes direct release of mediators, a process that is not mediated by IgE. Increased mucous secretion and increased bronchial smooth muscle tone, as well as airway edema, contribute to the respiratory symptoms observed in anaphylaxis. Cardiovascular effects result from decreased vascular tone and capillary leakage. Histamine release in skin causes urticarial skin lesions.

The most common inciting agents in anaphylaxis are parenteral antibiotics (especially penicillins), IV contrast materials, Hymenoptera stings, and certain foods (most notably, peanuts). Oral medications and many other types of exposures also have been implicated. Anaphylaxis also may be idiopathic.

Frequency

United States

The true incidence of anaphylaxis is unknown, partly because of the lack of a precise definition of the syndrome. Some clinicians reserve the term for the full-blown syndrome, while others use it to describe milder cases. Fatal anaphylaxis is relatively rare; milder forms occur much more frequently. The frequency of anaphylaxis is increasing, and this has been attributed to the increased number of potential allergens to which people are exposed. Up to 500-1,000 fatal cases of anaphylaxis per year are estimated to occur in the United States. A recent review concluded that the lifetime prevalence of anaphylaxis is 1-2% of the population as a whole.²

International

Reactions to insects and other venomous plants and animals are more prevalent in tropical areas because of the greater biodiversity in these areas.

Mortality/Morbidity

Approximately 1 in 5000 exposures to a parenteral dose of a penicillin or cephalosporin antibiotic causes anaphylaxis. More than 100 deaths per year are reported in the United States. Fewer than 100 fatal reactions to Hymenoptera stings are reported each year in the United States but this is considered to be an underestimate. One to 2% of people receiving IV radiocontrast experience some sort of reaction. The majority of these reactions are minor, and fatalities are rare. Low molecular weight contrast causes fewer and less severe reactions.

Race

Well-described racial differences in the incidence or severity of anaphylaxis do not exist. Cultural and socioeconomic differences may influence exposure rates.

Sex

Differences have been reported in the incidence and prevalence of anaphylactic reactions between men and women. Overall, women have a higher incidence of anaphylaxis than men, but, in some series of children, males predominate.²

Age

Anaphylaxis occurs in all age groups. Food allergies are more common in the young, whereas more drug reactions occur in adults, possibly due to greater exposure. Although prior exposure is essential for the development of true anaphylaxis, reactions occur even when no documented prior exposure exists. Thus, patients may react to a first exposure to an antibiotic or insect sting. Elderly persons have the greatest risk of mortality from anaphylaxis due to the presence of preexisting disease.

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