Emergency Medicine · Anaphylaxis and Envenomation
The facts most likely to be tested
Epinephrine is the first-line treatment for anaphylaxis and should be administered via intramuscular injection into the mid-outer thigh.
Large local reactions are characterized by erythema, edema, and pruritus extending beyond 10 cm, typically peaking at 48 hours, and do not require systemic treatment.
Systemic anaphylactic reactions are mediated by IgE-dependent mast cell and basophil degranulation, leading to rapid onset of hypotension, bronchospasm, and angioedema.
Honeybees leave a barbed stinger in the skin, which should be removed by flicking or scraping to prevent further venom injection.
Yellow jackets and wasps do not leave a stinger and are capable of multiple stings due to their smooth stingers.
Venom immunotherapy (VIT) is the definitive long-term management for patients with a history of systemic allergic reactions to Hymenoptera stings.
Toxic reactions occur after multiple stings and manifest as systemic symptoms like vomiting, diarrhea, rhabdomyolysis, or hemolysis due to the direct effect of the venom load.
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A 34-year-old male presents to the emergency department after being stung by multiple insects while gardening. He reports immediate generalized urticaria, wheezing, and a feeling of throat tightness. On physical exam, his blood pressure is 88/50 mmHg and his heart rate is 124 bpm. He has no history of prior allergic reactions. There are no stingers visible on his skin.
What is the most appropriate next step in management?
Intramuscular epinephrine
The patient is experiencing anaphylaxis, a life-threatening systemic reaction. Intramuscular epinephrine is the gold standard treatment and must be administered immediately to reverse hypotension and bronchospasm.
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Etiology / Epidemiology
Includes bees, wasps, and ants. History of prior systemic reaction is the primary risk factor for future anaphylaxis.
Clinical Manifestations
Ranges from local reaction to anaphylaxis. Look for urticaria, angioedema, and wheezing.
Diagnosis
Clinical diagnosis. Serum specific IgE testing is indicated only after a systemic reaction to confirm sensitization.
Treatment
Intramuscular epinephrine is the first-line treatment for anaphylaxis. Do not delay for antihistamines.
Prognosis
Most local reactions resolve in 24-48 hours. Venom immunotherapy is curative for high-risk patients.
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Epidemiology & Etiology
Hymenoptera stings are common, with most reactions being localized. Patients with a prior systemic reaction have a 30-60% risk of recurrence upon re-sting. Occupational exposure (beekeepers, landscapers) increases risk.
Pertinent Anatomy
Bees possess barbed stingers that remain in the skin, leading to continued venom injection. Wasps and hornets have smooth stingers, allowing for multiple stings from a single insect.
Pathophysiology
Venom contains enzymes (phospholipase, hyaluronidase) causing local tissue destruction. Systemic reactions are IgE-mediated type I hypersensitivity responses. Massive envenomation can lead to rhabdomyolysis and acute renal failure.
Clinical Manifestations
Local reactions present with erythema, edema, and pruritus. Systemic reactions manifest as anaphylaxis with hypotension, laryngeal edema, and bronchospasm. Monitor for the globus sensation as a sign of impending airway compromise.
Diagnosis
Diagnosis is clinical. Serum specific IgE (RAST) is the gold standard to confirm sensitization, but should be delayed 4-6 weeks post-reaction to avoid false negatives. Skin prick testing is highly sensitive but carries a risk of systemic reaction.
Treatment
Intramuscular epinephrine (1:1000) in the mid-outer thigh is the mandatory first-line treatment for anaphylaxis. Avoid subcutaneous administration due to poor absorption. Adjuncts include H1/H2 blockers and corticosteroids, but these are never substitutes for epinephrine.
Prognosis
Local reactions are self-limiting. Venom immunotherapy is the gold standard for preventing future systemic reactions in sensitized individuals. Patients with history of anaphylaxis must carry an epinephrine autoinjector at all times.
Differential Diagnosis
Cellulitis: delayed onset (24-48h) with fever and spreading erythema
Spider bite: often presents with necrotic center or target lesion
Scombroid poisoning: mimics anaphylaxis but associated with fish ingestion
Vasovagal syncope: bradycardia and hypotension without urticaria
Hereditary angioedema: recurrent swelling without pruritus or urticaria