What is the appropriate treatment for an allergic reaction, specifically anaphylaxis?

Treatment of Anaphylaxis

Intramuscular epinephrine administered immediately into the anterolateral thigh is the only first-line treatment for anaphylaxis and should never be delayed for any other intervention. 1, 2, 3

Immediate First-Line Treatment: Epinephrine

Epinephrine is the drug of choice and must be administered promptly at the onset of apparent anaphylaxis. 1 If there is any doubt about the diagnosis, it is better to give epinephrine than to withhold it. 1

Dosing and Administration

• Adults and children ≥30 kg: 0.3-0.5 mg (0.3-0.5 mL of 1:1000 solution) intramuscularly 1, 3
• Children <30 kg: 0.01 mg/kg (up to maximum 0.3 mg) intramuscularly 1, 3
• Route: Intramuscular injection into the anterolateral thigh (vastus lateralis muscle) is superior to subcutaneous or deltoid administration because it achieves faster and higher plasma levels 4, 3
• Repeat dosing: Every 5-15 minutes as needed if symptoms persist 4, 3

Critical Pitfall to Avoid

Never inject epinephrine into the buttocks, digits, hands, or feet. 3 The anterolateral thigh is the only appropriate site for emergency intramuscular administration.

Supportive Measures (Concurrent with Epinephrine)

These interventions should occur simultaneously with epinephrine administration, not before or instead of it:

• Position patient supine with legs elevated (unless respiratory difficulty present) 4
• Administer supplemental oxygen 1, 4
• Establish intravenous access 4
• Fluid resuscitation with crystalloids: 10-20 mL/kg bolus (0.5-1 L in adults), repeated as needed up to 20-30 mL/kg based on clinical response 1, 2, 4

Management of Refractory Anaphylaxis

If symptoms persist after 2-3 doses of intramuscular epinephrine:

• Intravenous epinephrine bolus: 20 μg for Grade II reactions, 50-100 μg for Grade III reactions, 1 mg for Grade IV (cardiac arrest) reactions 4
• Epinephrine infusion: 0.05-0.1 μg/kg/min for persistent symptoms after multiple boluses 1, 4
• Alternative vasopressors (norepinephrine, vasopressin, phenylephrine, metaraminol) for refractory hypotension 1, 4
• Glucagon 1-2 mg IV for patients on beta-blockers who may have reduced response to epinephrine 1, 5, 2

Second-Line Adjunctive Treatments (ONLY After Epinephrine)

These medications should never be given before or instead of epinephrine, as this is the most common reason for treatment failure and progression to life-threatening reactions. 5

H1 Antihistamines

• Diphenhydramine: 25-50 mg IV/oral (adults) or 1-2 mg/kg (children, maximum 50 mg) 5, 2
• Continue every 6 hours for 2-3 days after discharge 5
• Purpose: Relieves itching and urticaria only; has much slower onset than epinephrine 5

H2 Antihistamines

• Ranitidine: 1-2 mg/kg per dose (maximum 75-150 mg) IV/oral 5, 2
• Famotidine: Alternative H2 blocker 5
• The combination of H1 and H2 antihistamines works better than either alone 1, 5

Corticosteroids

• Prednisone: 1 mg/kg orally (maximum 60-80 mg) 5, 2
• Purpose: May prevent recurrent or protracted anaphylaxis, though evidence is limited 5
• Should only be given after adequate resuscitation with epinephrine and fluids 1

Post-Anaphylaxis Observation

• Minimum observation period: 6 hours in a monitored area from onset of reaction 1, 4
• Extended observation: Consider for high-risk patients (severe initial presentation, required >1 dose epinephrine, history of biphasic reactions, poor access to emergency care) 4, 6
• ICU admission: Most patients with Grade III-IV reactions require intensive care 1, 4

Important: Biphasic reactions (recurrence without re-exposure) can occur, though mandatory observation periods beyond 6 hours are not necessary as these reactions are unpredictable and may occur outside typical observation windows. 7

Diagnostic Testing

Serum tryptase sampling can confirm anaphylaxis when diagnosis is uncertain:

• First sample: 1 hour after reaction onset 1, 4
• Second sample: 2-4 hours after onset 1, 4
• Baseline sample: At least 24 hours post-reaction for comparison 1, 4

Discharge Planning and Long-Term Management

Before discharge, ensure:

• Prescribe epinephrine auto-injector (2 devices): 0.1 mg for infants >7.5 kg (where available), 0.15 mg for 10-25 kg, 0.3 mg for ≥25 kg 4
• Provide hands-on training on auto-injector use and have patient demonstrate proper technique 2, 8
• Supply written anaphylaxis emergency action plan 2, 6
• Recommend medical alert identification (bracelet/necklace) 1
• Refer to allergist for trigger identification and long-term management 2, 6
• Educate on allergen avoidance specific to identified or suspected triggers 1, 2

Common Pitfalls to Avoid

1. Using antihistamines as primary treatment instead of epinephrine - This is the most common error and significantly increases risk of death 5, 2
2. Delaying epinephrine to give other medications - Fatal reactions are associated with delayed epinephrine administration 2, 4
3. Administering epinephrine subcutaneously or in the deltoid - Intramuscular thigh injection is mandatory for optimal absorption 4, 3
4. Withholding epinephrine due to patient comorbidities - There are no absolute contraindications to epinephrine in anaphylaxis, even in elderly patients with cardiovascular disease 4
5. Failing to prescribe two epinephrine auto-injectors at discharge - Patients frequently require repeat dosing 2, 8

Special Populations

Pregnant women: Follow the same treatment algorithm; epinephrine remains first-line therapy. Position with left uterine displacement to avoid aortocaval compression. 1, 4

Patients on beta-blockers: May have reduced response to epinephrine and increased risk of severe anaphylaxis; have glucagon readily available (20-30 μg/kg in children, 1-5 mg in adults). 1, 5, 2