Patient with a solitary papular lesion with central punctum (likely insect bite), no local swelling, who then develops lip edema, generalized urticaria, abdominal pain, headache and flushing—what is the appropriate management and should pressure immobilisation for a suspected snake bite be applied?

Management of Systemic Allergic Reaction to Insect Bite

This patient is experiencing anaphylaxis and requires immediate intramuscular epinephrine (0.3–0.5 mg in the anterolateral thigh), followed by urgent transport to the emergency department; this is NOT a snake bite and pressure immobilization is contraindicated. 1, 2

Clinical Recognition: Anaphylaxis, Not Snake Envenomation

The progression from a localized papular lesion to lip edema, generalized urticaria, abdominal pain, headache, and flushing defines systemic anaphylaxis from an insect sting. 3 This constellation of symptoms—urticaria plus angioedema (lip swelling) plus gastrointestinal symptoms (abdominal pain)—meets criteria for anaphylaxis requiring epinephrine. 1, 4

Why This Is Not a Snake Bite

• Snake bites in North America typically cause local tissue injury with red, warm, tender, and swollen wounds, often progressing to hemorrhagic bullae, not a solitary papular lesion with central punctum. 3
• The central punctum with rapid onset of systemic urticaria and angioedema is pathognomonic for insect sting hypersensitivity, not snake envenomation. 3, 1
• Snake venom from pit vipers (>95% of North American venomous bites) causes cytotoxic local tissue damage, hypotension, bleeding, and muscle fasciculations—not the immediate-onset generalized urticaria and lip angioedema seen here. 3

Why Pressure Immobilization Is Contraindicated

Pressure immobilization bandaging is potentially harmful for this patient and should NOT be applied. 3 The 2024 American Heart Association guidelines explicitly state that pressure immobilization is not recommended for North American envenomations because >95% involve cytotoxic venom, and this technique may worsen local tissue injury. 3 More critically, this patient has an IgE-mediated allergic reaction to insect venom, not envenomation, making pressure immobilization both irrelevant and potentially dangerous by delaying definitive treatment. 1, 2

Immediate Management Algorithm

Step 1: Administer Epinephrine Without Delay

Give epinephrine 0.3–0.5 mg intramuscularly into the anterolateral thigh (vastus lateralis) immediately. 1, 2, 4 Fatal sting reactions are directly associated with delayed epinephrine administration, and antihistamines or corticosteroids are NOT substitutes. 3, 1

• Use the anterolateral thigh because of optimal muscle size, location, and blood flow for rapid absorption. 4
• Do NOT inject into the deltoid, buttock, digits, hands, or feet due to risk of inadequate absorption or tissue necrosis. 4
• Repeat the dose every 5–15 minutes if symptoms persist or recur during transport. 1, 2

Step 2: Position and Transport

Keep the patient supine with legs elevated to prevent "empty-ventricle syndrome" from blood pooling during anaphylactic shock. 3 Transport by emergency medical services to the emergency department while maintaining this position. 3, 1

Step 3: Adjunctive Medications (Secondary to Epinephrine)

After epinephrine administration, consider:

• H1 antihistamine: Give cetirizine 10 mg orally (or IV diphenhydramine 25–50 mg if oral route unavailable) to attenuate ongoing histamine-mediated symptoms. 1
• H2 antihistamine: Add ranitidine 50 mg IV or famotidine 20 mg IV to block H2 receptors, which may help with cardiovascular and gastrointestinal symptoms. 3
• Corticosteroids: Administer methylprednisolone 125 mg IV or prednisone 50 mg orally to reduce risk of biphasic reactions, though this does NOT replace epinephrine. 1, 2

Critical Pitfalls to Avoid

• Do NOT delay epinephrine while administering antihistamines or corticosteroids—these are adjuncts only and have been linked to fatal outcomes when used as primary therapy. 3, 1
• Do NOT apply pressure immobilization bandaging, tourniquets, ice, suction, or electric shock therapy—all are ineffective or harmful. 3
• Do NOT assume this is a snake bite based on a single papular lesion; the rapid systemic allergic response is diagnostic of insect sting hypersensitivity. 1, 2
• Do NOT prescribe antibiotics for the initial swelling, as this is allergic inflammation, not infection. 1, 2, 5

Post-Acute Management

Emergency Department Observation

• Observe for at least 4–6 hours for biphasic reactions, which occur in up to 20% of anaphylaxis cases. 1
• Monitor vital signs, respiratory status, and cardiovascular stability. 4

Discharge Planning

Prescribe two epinephrine auto-injectors (0.3 mg for adults, 0.15 mg for children 15–30 kg) with explicit instructions to carry them at all times. 1, 2 Some patients require more than one dose during a single anaphylactic episode. 2

Mandatory Allergist Referral

Refer to an allergist-immunologist for venom-specific IgE testing and consideration of venom immunotherapy (VIT). 3, 1 VIT reduces the risk of future systemic reactions from 25–70% to <5% and is highly effective. 3, 2 Testing should be performed at least 6 weeks after the reaction if initial tests are negative, as venom-specific IgE may be temporarily undetectable immediately post-reaction. 3

Patient Education

• Teach avoidance strategies for stinging insects (yellow jackets nest in the ground; hornets build large nests in trees). 3
• Instruct on proper epinephrine auto-injector technique and indications for use (any systemic symptoms after a sting). 3, 1
• Provide medical identification bracelet or necklace indicating insect sting allergy. 3
• Explain that the risk of a severe reaction with future stings is 25–70% without VIT, but <5% with VIT. 3, 2

Special Considerations

Coexisting Conditions

• Patients with asthma, cardiovascular disease, or those taking β-blockers or ACE inhibitors are at higher risk for severe reactions and should be counseled accordingly. 3, 4
• There is no contraindication to epinephrine use in life-threatening anaphylaxis, even in patients with heart disease, though they require closer monitoring. 2, 4

Mastocytosis Screening

Consider baseline serum tryptase measurement, as 3–5% of patients with sting anaphylaxis have underlying mastocytosis, which predisposes to very severe reactions. 3