Allergic reactions range from localized urticaria to life-threatening anaphylaxis. Anaphylaxis refers to the most severe form of immediate hypersensitivity reaction and encompasses both IgE-mediated reactions and anaphylactoid reactions, which do not require a previous sensitizing exposure.
Anaphylaxis may occur within seconds or be delayed over an hour after an exposure; rapid reactions are associated with higher mortality. Common exposures are foods, medications, insect stings, and allergen immunotherapy. Many cases are idiopathic. Criteria for anaphylaxis describe an acute progression of organ system involvement that may lead to cardiovascular collapse. Organ system involvement can include dermatologic (pruritus, flushing, urticaria, erythema multiforme, angioedema), respiratory tract (dyspnea, hypoxia, wheezing, cough, stridor), cardiovascular (dysrhythmias, collapse, arrest), gastrointestinal (cramping, vomiting, diarrhea), genitourinary (urgency, cramping), and eye (pruritus, tearing, redness). A biphasic response may occur causing recurrence of symptoms up to 11 hours after the initial exposure, but clinically important events are uncommon. Patients on β-blockers are susceptible to an exaggerated allergic response and may be refractory to first-line treatment.
Angioedema is caused by edema in the dermis, often of the face, neck, or extremities. Angioedema may accompany allergic reactions, or it may be triggered by angiotensin-converting enzyme inhibitors (ACEIs) or due to hereditary angioedema. Urticaria is a pruritic cutaneous reaction that may be associated with allergic reactions but may also be nonallergic. Many urticarial reactions are caused by viruses.
DIAGNOSIS AND DIFFERENTIAL
Anaphylaxis is a clinical diagnosis. A history of exposure to an allergen, such as a new drug, food, or sting may make the diagnosis obvious. There is no specific test to verify the diagnosis in real time. Anaphylaxis should be considered in any rapidly progressing multi-system illness. Workup is directed at ruling out other diagnoses while stabilizing the patient. The differential depends on the organ systems involved and may include myocardial ischemia, gastroenteritis, asthma, carcinoid, epiglottitis, hereditary angioedema, and vasovagal reactions.
EMERGENCY DEPARTMENT CARE AND DISPOSITION
Resuscitation must begin with airway, breathing, and circulation. Patients should be placed on a cardiac monitor with pulse oximetry and intravenous access obtained.
Administer oxygen as indicated by oximetry. Angioedema or respiratory distress should prompt early consideration for intubation.
Limit further exposure. This may be as simple as stopping an intravenous drug or removing a stinger. First-aid measures, ice, and elevation may be helpful for local symptoms. Gastric decontamination is not recommended for ingested allergens.
First-line therapy for anaphylaxis is epinephrine. In patients without cardiovascular collapse, administer 0.3 to 0.5 mg (0.3 to 0.5 mL of 1:1000; pediatric dose, 0.01 mg/kg) intramuscularly in the thigh. The dose may be repeated every 5 minutes as needed. Patients who are refractory to IM dosing or with cardiovascular compromise should receive intravenous epinephrine. A bolus of 100 μg (place 0.1 mL of 1:1000 in 10 mL normal saline) can be infused over 5 ...