Anaphylaxis

Anaphylaxis is an acute allergic reaction following antigen exposure in a previously sensitized person; it requires immediate and specific therapy. It is usually mediated by immunoglobulin E antibodies and involves release of chemical mediators from mast cells and basophils. Anaphylactoid reactions, which are more common and less severe, are triggered by nonantigenic agents and are the result of direct release of these chemical mediators. Anaphylaxis or anaphylactoid reactions may occur after exposure to pollen, drugs, foreign serum, insect stings, diagnostic agents such as iodinated contrast materials or fluorescein, vaccines, local anesthetics, and food products. The most important parameter for predicting such an attack is a history of a previous allergic reaction to any other drug or possible antigen. Unfortunately, a history of known sensitivity may not always be elicited. Studies indicate that the prevalence of anaphylaxis has been increasing steadily since 2008.

Anaphylaxis is particularly important to the ophthalmologist, in view of the increasing number of surgical procedures and fluorescein angiograms performed in the office setting. It is estimated that allergic reactions to fluorescein (including urticaria) occur in up to 1% of all angiograms. In 1 survey, the overall risk of a severe reaction was 1 in 1900 patients, including a risk of respiratory compromise in 1 in 3800 subjects. If diaphoresis, apprehension, pallor, a rapid and weak pulse, or any combination thereof develops in a patient after administration of a drug, the patient should be considered to have an allergic reaction until proven otherwise. The diagnosis is confirmed if the patient experiences associated generalized itching, urticaria, angioedema of the skin, dyspnea, wheezing, or arrhythmia. Anaphylaxis may rapidly lead to loss of consciousness, shock, cardiac arrest, coma, or death.

Once an acute allergic reaction is suspected, prompt treatment is indicated:

• Oxygen should be administered to patients in respiratory distress.

• Epinephrine (0.3 mL of a 1:1000 solution) injected intramuscularly in a limb opposite to the antigenic agent exposure site is usually effective for the maintenance of circulation and blood pressure.

• IV volume expansion may be necessary to restore and maintain tissue perfusion. Methylprednisolone should be administered for serious or prolonged reactions. When given early, corticosteroids help control possible long-term sequelae.

• Antihistamines are helpful in slowing or halting the ongoing allergic response but are of limited value in acute anaphylaxis.

• Tracheotomy or cricothyrotomy is indicated when laryngeal edema is unresponsive to the previous methods or when oral intubation cannot be performed.

• All patients with anaphylaxis or anaphylactoid reactions should be kept under observation for at least 6 hours.

In cases of mild allergic reactions, the physician can administer 25–50 mg of diphenhydramine hydrochloride orally or intramuscularly and observe the patient closely to determine whether further treatment is necessary. Pretreating high-risk patients with an antihistamine, corticosteroids, or both prior to fluorescein angiography may reduce the risk of an allergic reaction. In all cases of anaphylaxis, supportive treatment should be maintained until the emergency medical team arrives.

For patients with a known history of anaphylaxis, personal emergency kits containing epinephrine are available and can be used until medical help arrives. The kits are designed to allow self-treatment by the patient or administration by a family member or an informed bystander. One commercially available allergy kit contains a syringe and needle preloaded with 0.6 mL of 1:1000 epinephrine. The physician who prescribes this kit must give detailed instructions concerning the use of the device. Epinephrine auto-injectors are also available. Each contains a spring-loaded automatic injector, which does not permit graduated doses to be given but automatically injects 0.3 mg of epinephrine (0.15 mg in the pediatric version) when the device is triggered by pressure on the thigh. The epinephrine ampules contained in these self-treatment kits have a limited shelf life and should be replaced when the expiration date is reached or if the solution becomes discolored. Any person given epinephrine requires 4–6 hours of observation to ensure that there is no rebound effect. In recent years, there has been renewed interest in treatment of allergic reactions with sublingual immunotherapy, in which the patient is exposed to the offending antigen via the gastrointestinal system to improve tolerance.

Excerpted from BCSC 2020-2021 series: Section 1 - Update on General Medicine. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.