Anaphylaxis (severe allergic reaction)

Anaphylaxis is a medical emergency. Though rare, this serious type of allergic reaction occurs after a person is exposed to an allergen (a substance they are allergic to), such as certain foods, medications, or insect stings.

The body's immune system quickly reacts, causing swelling of the face and throat, difficulty breathing, dangerously low blood pressure, and leaky blood vessels. These effects can lead to shock. If not treated quickly, anaphylaxis can be fatal.

Avoiding the allergen and knowing what to do if you are exposed are the keys to managing the problem.

Just about any substance can bring on an anaphylactic reaction. But anaphylaxis is most commonly caused by:

• allergy shots
• blood products
• foods such as peanuts, tree nuts, shellfish, eggs, fish, sesame, soy, milk, and wheat
• insect bites or stings
• latex (a type of natural rubber found in certain medical supplies)
• medications such as antibiotics, vaccines, anti-inflammatories and painkillers, and dyes used for CT or MRI scans

In up to 20% of cases no cause for the anaphylaxis is ever found. The medical term for this is idiopathic anaphylaxis.

When a person with an anaphylactic allergy is exposed to an allergen, their immune system goes into overdrive. The substances the body produces (e.g., histamine) are intended to protect the body from a foreign invader, but they overreact, causing the throat to swell up and the blood vessels to leak fluid. This leads to the symptoms of anaphylaxis and can be life-threatening.

People with asthma, seasonal allergies, or eczema are at a higher risk of anaphylaxis. Although anaphylaxis rarely occurs, it can happen at any age. Race and geographic location do not affect the risk of anaphylaxis.

The most common symptoms of anaphylaxis include:

• dangerously low blood pressure, dizziness, sweating, and fainting
• hives
• nausea, cramping, diarrhea, or vomiting
• rapid heart rate
• swelling of the face and throat
• wheezing or difficulty breathing

Sometimes, anaphylaxis can cause other symptoms as well. These include:

• chest pain
• flushing
• headache
• itching
• seizures

If you experience these symptoms, seek immediate medical attention. They will usually start within 15 minutes of exposure to the allergen. Anaphylaxis can be fatal if not treated. The usual causes of death are cardiovascular collapse, which means that the heart cannot supply the body with the oxygen it needs; or laryngeal edema, which means that the throat swells up and prevents the person from breathing in enough oxygen to survive.

Sometimes, the symptoms of an anaphylactic reaction can return anywhere from 1 to 72 hours after the first attack. This is called biphasic anaphylaxis and happens in about 20% of cases. The cause of biphasic anaphylaxis remains unknown, as does a method of predicting the return of symptoms.

Your doctor may wish to keep you for a period of observation after your first attack has been treated. If your anaphylaxis symptoms come back, seek immediate medical attention.

The diagnosis of anaphylaxis is based on symptoms. The doctor will evaluate your symptoms to rule out other conditions. In general, anaphylactic reactions are easier to recognize while they are happening and more difficult to diagnose after the attack. If anaphylaxis was not diagnosed during the attack, it can also be diagnosed after the fact using lab tests and a description of your symptoms.

The cause of an anaphylactic reaction is often clear – you may experience a reaction shortly after being stung by an insect, eating a certain food, or taking a certain medication. If the cause is not clear, additional tests may be done to determine the cause. These include keeping a diary, skin testing, and blood tests. Because anaphylaxis is a medical emergency, tests to find out the cause of the reaction will be delayed until after the reaction is treated.