A life-threatening systemic hypersensitivity reaction to contact with an allergen; it may appear within minutes of exposure to the offending substance. Manifestations include respiratory distress, pruritus, urticaria, mucous membrane swelling, gastrointestinal disturbances (including nausea, vomiting, abdominal pain, and diarrhea), and vascular collapse. Virtually any allergen may trigger an anaphylactic reaction, but among the more common agents are proteins such as antisera, hormones, pollen extracts, Hymenoptera venom, and foods; drugs (especially antibiotics); and diagnostic agents such as IV contrast material. Atopy does not seem to predispose to anaphylaxis from penicillin or venom exposures. Anaphylactic transfusion reactions are covered in Chap. 9.
Time to onset is variable, but symptoms usually occur within seconds to minutes of exposure to the offending antigen:
Respiratory: mucous membrane swelling, hoarseness, stridor, wheezing
Cardiovascular: tachycardia, hypotension
Cutaneous: pruritus, urticaria, angioedema
Made by obtaining history of exposure to offending substance with subsequent development of characteristic complex of symptoms.
Mild symptoms such as pruritus and urticaria can be controlled by administration of 0.3–0.5 mL of 1:1000 (1.0 mg/mL) epinephrine SC or IM, with repeated doses as required at 5- to 20-min intervals for a severe reaction.
An IV infusion should be initiated for administration of 2.5 mL of 1:10,000 epinephrine solution at 5- to 10-min intervals, and volume expanders such as normal saline, and vasopressor agents, e.g., dopamine, if intractable hypotension occurs.
Epinephrine provides both α- and β-adrenergic effects, resulting in vasoconstriction and bronchial smooth-muscle relaxation. Beta blockers are relatively contraindicated in persons at risk for anaphylactic reactions.
The following should also be used as necessary:
Antihistamines such as diphenhydramine 50–100 mg IM or IV
Nebulized albuterol or aminophylline 0.25–0.5 g IV for bronchospasm
Glucocorticoids (medrol 0.5–1.0 mg/kg IV); not useful for acute manifestations but may help alleviate later recurrence of hypotension, bronchospasm, or urticaria.
For antigenic material injected into an extremity consider: use of a tourniquet proximal to the site, 0.2 mL of 1:1000 epinephrine into the site, removal of an insect stinger if present.
Avoidance of offending antigen, where possible; skin testing and desensitization to materials such as penicillin and Hymenoptera venom, if necessary. Individuals should wear an informational bracelet and have immediate access to an unexpired epinephrine kit.
For a more detailed discussion, see Austen KF: Allergies, Anaphylaxis, and Systemic Mastocytosis, Chap. 317, p. 2707, in HPIM-18.