Although food allergy is much more commonly suspected than proved and is usually not severe, most episodes that lead to anaphylaxis are due to peanut or tree-nut allergies. Sampson reviews the epidemiology, diagnosis, and management of peanut allergy.
For unexplained reasons, the incidence of peanut allergy has risen over the past few decades in developed countries, paralleling an increase in asthma and atopic disease. Among children in Great Britain, rates of peanut allergy manifested by clinical symptoms increased from 1.3 percent to 3.2 percent between 1989 and 1995. Between 1988 and 1994, up to 6 percent of Americans exhibited asymptomatic serologic evidence of sensitivity in the form of IgE antibodies to peanut proteins. Most cases of anaphylaxis due to peanuts occur in persons who also have a history of asthma or atopic dermatitis. About 25 to 35 percent of persons with peanut allergy also have sensitivity to tree nuts (e.g., walnuts, cashews, pistachios).
Symptoms can develop from minutes to hours after peanut ingestion. Oropharyngeal itching or tingling, nausea, and urticaria are common initial manifestations. More severe episodes may progress to respiratory difficulties, hypotension, or cardiac dysrhythmias.
The diagnosis of food allergy is not always straightforward. Careful history-taking can be aided by skin-prick testing for peanut allergy or radioallergosorbent testing (RAST) for antibodies to peanut proteins. Patients with IgE levels of at least 15 IU per mL (15 kU per L) have a 95 percent or greater incidence of peanut allergy, and confirmation by a controlled food challenge is unnecessary. The first episode of symptomatic peanut allergy usually occurs at toddler age, and reactions can become more severe over time.
Anaphylaxis caused by peanuts is often confused with a severe asthma attack because many affected persons also have asthma. The key clinical difference is the abrupt onset without a history of progressively worsening respiratory symptoms as would typically occur in a severe asthma flare.
Treatment of peanut allergy starts with prevention. Patients and parents need to be educated about checking food labels and avoiding high-risk situations (e.g., buffets, unlabeled candies and desserts, ice cream parlors). The author advocates providing a written action plan, including early administration of oral diphenhydramine and self-injection with epinephrine (see the accompanying table). Even if initial symptoms improve, further evaluation by a physician is prudent because some patients have a subsequent flare within the next several hours. Patients with anaphylactic reactions are typically treated with epinephrine, oxygen, nebulized albuterol, histamine H1-and H2-receptor antagonists, and systemic corticosteroids.