ANAPHYLAXIS OVERVIEW

Anaphylaxis is a potentially deadly allergic reaction that is rapid in onset. It is most often triggered by foods, medications, and insect stings. There are many other possible triggers.

Anaphylaxis is an unpredictable condition. Many people who experience it have a known allergy and some have had one or more milder allergic reactions previously. Others, who are not even aware that they have an allergy, can suddenly experience severe anaphylaxis. Even the first episode of anaphylaxis can be fatal.

The severity of anaphylactic reactions can be minimized by recognizing the symptoms early, having proper medication available for self-treatment, and seeking emergency medical care promptly. This topic reviews the symptoms and diagnosis of anaphylaxis. Treatment and prevention of anaphylaxis are discussed separately. A separate topic discusses how to use an epinephrine autoinjector. (See "Patient information: Anaphylaxis treatment and prevention (Beyond the Basics)" and "Patient information: Use of an epinephrine autoinjector (Beyond the Basics)".)

ANAPHYLAXIS SYMPTOMS

Symptoms of anaphylaxis generally begin within minutes to an hour of exposure to a trigger. Less commonly, symptoms do not develop for several hours.

The most common symptoms of anaphylaxis are hives (urticaria) and swelling of the skin (angioedema), which occur in 80 to 90 percent of reactions. Respiratory symptoms occur in about 70 percent of reactions, and are especially common in people who also have asthma or another chronic respiratory disease. Extremely low blood pressure, causing lightheadedness, dizziness, blurred vision, or loss of consciousness (passing out) occurs in about 30 percent of reactions.

Anaphylaxis can cause symptoms throughout the body:

• Skin: Itching, flushing, hives (urticaria), or swelling (angioedema)
• Eyes: Itching, tearing, redness, or swelling of the skin around the eyes
• Nose and mouth: Sneezing, runny nose, nasal congestion, swelling of the tongue, or a metallic taste
• Lungs and throat: Difficulty getting air in or out, repeated coughing, chest tightness, wheezing or other sounds of labored breathing, increased mucus production, throat swelling or itching, hoarseness, change in voice, or a sensation of choking
• Heart and circulation: Dizziness, weakness, fainting, rapid, slow, or irregular heart rate, or low blood pressure
• Digestive system: Nausea, vomiting, abdominal cramps, or diarrhea
• Nervous system: Anxiety, confusion, or a sense of impending doom

A severe form of anaphylaxis causes sudden collapse without other obvious symptoms, such as hives or flushing. This form of anaphylaxis occurs most commonly after a person is given a medication into a vein or is stung by an insect.

Up to 20 percent of people with anaphylaxis have biphasic (two-phase) or protracted (prolonged) anaphylaxis. A person with biphasic anaphylaxis has a reaction that resolves and then recurs hours later without further exposure to the trigger. The late phase reaction usually occurs within eight hours, but may occur up to 72 hours after the initial symptoms. A person with protracted anaphylaxis has signs and symptoms that persist for hours or even days despite treatment, although this is rare.

ANAPHYLAXIS TRIGGERS

The trigger for a person's anaphylaxis may be obvious or it may be difficult to identify.

Common anaphylaxis triggers can include (figure 1):

• Foods: In children, hen's eggs, cow's milk, peanuts, tree nuts, fish, wheat, and soy are the most common food triggers.
  
  In teens and adults, peanuts, tree nuts, fish, and crustaceans (shellfish such as shrimp) are the most common triggers.
  
  Any food, including fruits and vegetables, and some spices and food additives, can cause anaphylaxis.
• Venom from insects, including bees, yellow jackets, wasps, hornets, and fire ants
• Venom from insects, including bees, hornets, wasps, and fire ants
• Latex from natural rubber, found in some latex gloves, balloons, condoms, sports equipment, and medical products
• Allergen immunotherapy ("allergy shots"), such as those given for the treatment of allergic rhinitis (hay fever)
• Exercise, either by itself, or after eating certain foods (eg, wheat, celery, seafood), medications (eg, aspirin), or exposure to cold air/water
• Less common triggers include exposure to airborne allergens (such as horse dander), human seminal fluid, and cold temperatures.

Sometimes a specific trigger cannot be identified, even after a thorough evaluation. This condition is called idiopathic anaphylaxis. (See "Patient information: Anaphylaxis treatment and prevention (Beyond the Basics)".)

IgE mediated anaphylaxis — In most people, anaphylaxis is caused by the presence of proteins called immunoglobulin E (IgE) antibodies. IgE antibodies are normally produced in the body for the purpose of fighting certain infections. In people with allergies, however, IgE is made in response to non-infectious substances, such as foods, medications, or insect venoms. This IgE then sticks to the outside of mast cells and basophils, a type of white blood cell.

If a person with IgE antibodies to a specific allergen is exposed to that allergen again, the cells may suddenly become activated. The activated cells release large amounts of inflammation-causing chemicals (including histamine) into the blood stream, causing anaphylaxis.

The chemicals released from the cells cause the signs and symptoms typical of anaphylaxis. (See 'Anaphylaxis symptoms' above.) The allergic reaction can be so strong that it becomes life-threatening; for example, sudden severe swelling in the throat can lead to suffocation.

In some people with anaphylaxis, the reaction is caused by a process that does not involve allergens and IgE. However, the symptoms and treatment are the same.

ANAPHYLAXIS RISK FACTORS

Some people are more likely than others to experience anaphylaxis or to develop severe symptoms during anaphylaxis, for example, those who have one or more of the following:

• Previous sudden severe allergic reaction involving the whole body — People who have had allergic reactions to a particular substance in the past are at increased risk of anaphylaxis. However, the severity of past allergic reactions does not reliably predict the severity of future reactions; people with mild reactions in the past may experience severe anaphylactic reactions in the future.
• Asthma — People with asthma are more likely to have more severe respiratory problems during anaphylaxis. The combination of food allergy (especially to peanuts and tree nuts) and asthma seems to put people at risk for life-threatening episodes of anaphylaxis.
• Other diseases — People with chronic lung disease, especially older adults with chronic obstructive pulmonary disease (COPD) or emphysema, are at increased risk of complications during an anaphylactic reaction. People with coronary artery disease and other heart diseases are also at greater risk of developing complications during an anaphylactic reaction.

ANAPHYLAXIS DIAGNOSIS

The diagnosis of anaphylaxis is based upon symptoms that occur suddenly after being exposed to a potential trigger, such as a food, medication, or insect sting.

Is it anaphylaxis or another problem? — A number of other health problems can cause symptoms that are similar to those of anaphylaxis. These include a severe asthma attack, a heart attack, a panic attack, or even food poisoning. Evaluation by a specialist can help to clarify the diagnosis.

Tryptase is a protein that is released into the blood during an anaphylactic reaction. An increased amount of tryptase can sometimes be measured in a blood sample collected during the first three hours after anaphylaxis symptoms have begun, eg, after a medication has been taken or after an insect sting. However, tryptase levels are seldom elevated in food-induced anaphylaxis.

ANAPHYLAXIS TREATMENT

The treatment and prevention of anaphylaxis are discussed separately. (See "Patient information: Anaphylaxis treatment and prevention (Beyond the Basics)".)

WHERE TO GET MORE INFORMATION

Your healthcare provider is the best source of information for questions and concerns related to your medical problem.

This article will be updated as needed on our web site (www.uptodate.com/patients). Related topics for patients, as well as selected articles written for healthcare professionals, are also available. Some of the most relevant are listed below.

Patient level information — UpToDate offers two types of patient education materials.

The Basics — The Basics patient education pieces answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials.

Patient information: Anaphylaxis (The Basics)
Patient information: Food allergy (The Basics)
Patient information: Insect allergy (The Basics)
Patient information: Insect bites and stings (The Basics)
Patient information: Epinephrine auto-injectors (The Basics)
Patient information: Shortness of breath (dyspnea) (The Basics)
Patient information: Angioedema (The Basics)
Patient information: Peanut allergy (The Basics)
Patient information: Drug allergy (The Basics)

Beyond the Basics — Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are best for patients who want in-depth information and are comfortable with some medical jargon.

Patient information: Anaphylaxis treatment and prevention (Beyond the Basics)
Patient information: Use of an epinephrine autoinjector (Beyond the Basics)

Professional level information — Professional level articles are designed to keep doctors and other health professionals up-to-date on the latest medical findings. These articles are thorough, long, and complex, and they contain multiple references to the research on which they are based. Professional level articles are best for people who are comfortable with a lot of medical terminology and who want to read the same materials their doctors are reading.

Anaphylaxis: Rapid recognition and treatment
Bee, yellow jacket, wasp, and other Hymenoptera stings: Reaction types and acute management
Clinical manifestations of food allergy: An overview
Diagnosis of Hymenoptera venom allergy
Diagnostic evaluation of food allergy
Differential diagnosis of anaphylaxis in children and adults
Drug eruptions
Fatal anaphylaxis
Food-induced anaphylaxis
History and physical examination in the patient with possible food allergy
Immediate hypersensitivity reactions to radiocontrast media: Clinical manifestations, diagnosis, and treatment
Laboratory tests to support the clinical diagnosis of anaphylaxis
Pathophysiology of anaphylaxis

The following organizations also provide reliable health information.

• American Academy of Allergy, Asthma, and Immunology
  
  (www.aaaai.org/patients/gallery/anaphylaxis.asp)
• American College of Allergy, Asthma, and Immunology
  
  (www.acaai.org)
• Anaphylaxis Foundation and Anaphylaxis Network of Canada
  
  (www.anaphylaxis.org)
• The Anaphylaxis Campaign
  
  (www.anaphylaxis.org.uk)
• Food Allergy and Anaphylaxis Network
  
  (www.foodallergy.org/anaphylaxis/index.html)

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