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, GI 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 drug reactions 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. Eighty to ninety percent are uniphasic; however, 10−20% are biphasic where anaphylactic symptoms return about an hour after resolution of the initial symptoms:
Respiratory: mucous membrane swelling, hoarseness, stridor, wheezing
Cardiovascular: tachycardia, hypotension
Cutaneous: pruritus, urticaria, angioedema
Diagnosis is made by obtaining history of exposure to offending substance with subsequent development of characteristic complex of symptoms.
Treatment of first choice is 0.3–0.5 mL of 1:1000 (1.0 mg/mL) epinephrine IM, with repeated doses as required at 5- to 20-min intervals for a severe reaction. The pt should be placed in the supine position to support venous return and prevent “empty heart syndrome.”
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:
Normal saline, and vasopressor agents, if intractable hypotension occurs.
Antihistamines such as diphenhydramine 50–100 mg IM or IV.
Nebulized bronchodilators for bronchospasm.
Oxygen; endotracheal intubation or tracheostomy may be necessary for progressive hypoxemia.
Glucocorticoids (methylprednisolone 0.5–1.0 mg/kg IV); not useful for acute manifestations but may help alleviate later recurrence of hypotension, bronchospasm, or urticaria.
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.