What is the treatment approach for an acute allergic reaction, including medication administration and supportive care?

Treatment of Acute Allergic Reactions

Epinephrine administered intramuscularly into the anterolateral thigh is the immediate first-line treatment for anaphylaxis and should never be delayed—there are no absolute contraindications to its use when treating anaphylaxis, as the risk of death from untreated anaphylaxis outweighs all other concerns. 1, 2, 3

Immediate Recognition and Assessment

Anaphylaxis involves two or more organ systems with rapid onset and includes:

• Cutaneous: Flushing, urticaria, angioedema, pruritus 3, 4
• Respiratory: Airway swelling, laryngospasm, bronchospasm, wheezing, difficulty breathing 3, 5
• Cardiovascular: Hypotension, tachycardia, syncope, thready pulse 3, 5
• Gastrointestinal: Vomiting, diarrhea, abdominal cramps 3

Critical pitfall: The most dangerous error is using antihistamines as primary treatment instead of epinephrine—this significantly increases risk of progression to life-threatening reactions. 1

First-Line Treatment: Epinephrine

Dosing (FDA-approved)

• Adults and children ≥30 kg: 0.3-0.5 mg (0.3-0.5 mL of 1:1000 solution) IM 3
• Children <30 kg: 0.01 mg/kg (0.01 mL/kg of 1:1000 solution) IM, maximum 0.3 mg per dose 3
• Route: Intramuscular into anterolateral thigh through clothing if necessary 3, 6
• Repeat: Every 5-10 minutes as needed based on clinical response 3

Administration technique

• Use needle at least 1/2 to 5/8 inch long to ensure intramuscular delivery 3
• Hold child's leg firmly to minimize injection-related injury 3
• Do not inject repeatedly at same site due to vasoconstriction causing tissue necrosis 3
• Monitor for severity of reaction and cardiac effects with each dose 3

Supportive Care

Positioning and Fluids

• Position patient recumbent with lower extremities elevated if tolerated 1
• Administer crystalloid fluids (normal saline) 20 mL/kg rapid bolus, repeated as needed for hypotension or incomplete response to epinephrine 1, 7, 2
• Up to 35% of intravascular volume can shift into extravascular space within minutes, requiring aggressive fluid resuscitation 2

Airway Management

• Supplemental oxygen for all patients 1
• Albuterol nebulizer for persistent wheezing: 1.5 mL (child) or 3 mL (adult) every 20 minutes or continuously 1
• Prepare for advanced airway management if laryngeal edema progresses 5

Special consideration: Patients with asthma are at particularly high risk for fatal anaphylaxis—wheezing in an asthmatic patient having an allergic reaction mandates immediate epinephrine. 1

Second-Line Adjunctive Medications

These should ONLY be administered AFTER epinephrine, never as substitutes or first-line agents. 1

H1 Antihistamines

• Diphenhydramine: 1-2 mg/kg IV or oral, maximum 50 mg 1, 8
• Oral liquid absorbs faster than tablets 1
• Continue every 6 hours for 2-3 days after discharge 1
• Never use alone—much slower onset than epinephrine 1

H2 Antihistamines

• Ranitidine (or famotidine): 1-2 mg/kg, maximum 75-150 mg IV or oral 1
• Combination of H1 and H2 antihistamines works better than either alone 1
• Continue twice daily for 2-3 days after discharge 1

Corticosteroids

• Methylprednisolone: 1 mg/kg IV, maximum 60-80 mg OR 1
• Prednisone: 1 mg/kg oral, maximum 60-80 mg 1
• Used primarily to prevent recurrent or protracted anaphylaxis, though evidence is limited 1
• Continue daily for 2-3 days after discharge 1

Special Populations and Situations

Patients on Beta-Blockers

• May develop refractory hypotension and bradycardia resistant to epinephrine 2
• Glucagon administration required: 20-30 μg/kg for children or 1-5 mg for adults IV 1, 2
• Epinephrine's effects are blocked at β-receptors in these patients 2

Pregnant Patients

• No contraindication to epinephrine use in pregnancy when treating anaphylaxis 7, 2
• Position with left uterine displacement to prevent aortocaval compression 7
• Administer crystalloid fluids 20 mL/kg as rapid bolus, repeated as needed 7
• Consider emergent cesarean section if persistent hypotension despite aggressive resuscitation 7
• Initiate perimortem cesarean delivery if persistent hypotension after 4 minutes of cardiac arrest 7

Observation Period

• Minimum 6 hours observation in monitored area from onset of reaction 9, 7
• Patients with Grade III-IV reactions require ICU admission 9, 7
• Monitor for biphasic reactions (recurrence without re-exposure): risk period 4-12 hours depending on severity 9, 5
• Patients with minimal symptoms (few hives that resolved) may discharge after 2 hours if physician assesses minimal risk 9

Discharge Planning

All patients require:

• Two epinephrine auto-injectors with proper training on use 1, 7, 10
• Written anaphylaxis emergency action plan 10
• Education on allergen avoidance 1, 10
• Referral to allergist-immunologist for trigger identification and consideration of immunotherapy 1, 7
• Continue diphenhydramine every 6 hours for 2-3 days 1
• Continue H2 antihistamine twice daily for 2-3 days 1
• Continue prednisone daily for 2-3 days 1

Critical Mortality Prevention Points

• Fatal anaphylaxis occurs in approximately 1 in 75,000 untreated or inadequately treated reactions 2
• Myocardial ischemia and dysrhythmias can occur even without pre-existing cardiovascular disease 2
• Hypoxia from airway swelling and bronchospasm leads to organ damage and death if untreated 2
• The risk of death from untreated anaphylaxis outweighs all concerns about treatment side effects 2