Peanut Allergy

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Epidemiology

In the United States, food allergies affect almost 2% of the  population. Over the past two decades, the prevalence and severity of food allergies has increased in this country. Among all food allergies, peanut allergy is the third most prevalent allergy occurring among young children and the most common food allergy in older children, adolescents and adults, with the median age of first allergic reaction being 14 months.

 

Among children who have peanut allergies, approximately 75% will experience a reaction on their first exposure with the potential outcome being severe or fatal.  Initial reactions are not predictive of later reactions.  In general, the peanut must be eaten before life-threatening reactions will occur.

 

There is an estimated 2 fold increase in reported peanut allergies among Westernized nations with the most likely culprits being peanuts used as a source of protein in health foods, the popularity of vegetarianism and the increased used of prepared foods. In 2002, the prevalence of peanut allergy among children under the age of 5 was 0.8% as compared to 1997 when the prevalence was 0.4%; however, the prevalence among adults has remained constant over this five-year period. The reasons why peanut allergies are increasing are unknown.

 

Approximately 20% of infants with eanut allergy will outgrow their allergy, especially if IgE levels are low.  Children should be re-evaluated  every 1-2 years as they get older.

 

Pathogenesis

Food allergies are mostly attributed to IgE mediated and or cell mediated mechanisms. When exposed to a particular allergen, food specific IgE antibodies are formed which bind to the Fce receptors on mast cells, basophils, macrophages and dendritic cells. Mediators are released as the allergens reach the IgE antibodies and the result is the production of vasodilatation, smooth muscle contraction, and mucus secretion.

 

Mast cells and macrophages release cytokines that attract and activate other cells which produces an inflammatory response that is more prolonged.

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Figure 1: Pathogenesis of Type I Hypersensitivity

 

Systems that are affected during an acute IgE mediated reaction include the skin, gastrointestinal tract, respiratory tract and cardiovascular system. Lymphocytes, more specifically food allergen-specific T cells, can also play a role by secreting cytokines that lead to a delayed inflammatory response affecting the skin, GI or respiratory tract. Infants and children are more susceptible to food allergies because of the immaturity of the immune system.

 

Clinical Manifestations

Patients with peanut allergy are at risk of developing anaphylaxis if exposed to peanuts. Symptoms of anaphylaxis include flushing, urticaria (Figure 2), pruritus, angioedema (Figure 3), cough, wheezing,  stridor, dyspnea, abdominal cramping, vomiting, diarrhea, dizziness, and syncope. 

 

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Figure 2: Urticaria
http://www.ebmedicine.net/topics.php?paction=showTopicSeg&topic_id=165&s...

 

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Figure 3: Angioedema
http://www.babyrashclinic.com/face-rash-on-baby-due-to-angioedema/

 

All symptoms may not be present in a given patient. Skin reactions are common, particularly urticarial and angioedema, occurring in 90% of cases of anaphylaxis. Respiratory symptoms occur in 70% of cases, and cardiovascular and gastrointestinal symptoms are each present in 45% of cases. Symptoms typically appear after 30 minutes and up to several hours after exposure. Symptoms can progress to anaphylactic shock. It is important to note the classic biphasic reaction of anaphylaxis in which patients have a second episode of anaphylaxis within 8 hours and up to 48-72 hours after exposure without additional exposure to allergen.

   

Diagnosis

The diagnosis of a peanut allergy is dependent on a clear and specific history confirmed by testing (Figure 4). It is important to note that many reported food allergies by parents or patients may not be actual allergies, so confirmation is required to make a diagnosis. Available tests include a food challenge test, an elimination diet test, skin and laboratory tests. 

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Figure 4: Diagnostic Algorithm for Food Allergies

 

The recommended method for a food challenge is to perform a double-blinded placebo-controlled food challenge in which the suspected allergen is administered to the patient in a clinical setting with appropriate resuscitative equipment and signs and symptoms are observed. However, a single-blinded open challenge that demonstrates symptoms may be diagnostic if there is a clear history and confirmatory laboratory tests.

 

The absence of symptoms after elimination of peanuts and peanut products from the diet combined with a confirmed history of anaphylaxis after ingestion of peanuts is also diagnostic of a peanut allergy.

 

Skin prick tests and laboratory tests that measure IgE or allergen-specific IgE can assist in the diagnosis of peanut allergy, but are not diagnostic alone. These tests can have many false positives, but can be helpful when used in conjunction with history and the above tests.

 

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Skin prick testing on the back of a child.  www.psychiatrictimes.com

 

Treatment

Elimination of foods containing peanuts and peanut products are the most appropriate treatment for peanut allergy. Injectable epinephrine, such as an EpiPen or Auvi-Q, and a written emergency plan in case of accidental ingestion should be given to children with peanut allergy, along with appropriate education on how to use the injectable epinephrine device.

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CLICK HERE for a video on how to use the injectible EPI PEN.

 

Patients should be aware of the biphasic nature of the reaction and they should CALL 911 OR GO TO THE EMERGENCY DEPARTMENT AFTER USE.

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Families should also be counseled to keep their EpiPen on their person (or at school/work), as leaving in the car can expose it to extremes of heat and cold thus making the medication ineffective.

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Furthermore, since it is an autoinjector, the EpiPen once expired should be brought to the clinic and placed in a sharps box to avoid accidental injection into the digits of trash / waste handlers.

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Current recommendations do not require women to avoid allergenic foods such as peanuts during pregnancy or breast feeding. Infants should be exclusively breast fed until 4-6 months of age, at which time infants should be introduced to solid foods. It is no longer recommended to avoid potentially allergenic foods at this time such as nuts. Infants may be given any food that does not present a choking hazard. Only one new food should be tried at one time with several days in between introduction of new foods, and honey, water and cows milk should be avoided.

 

References