What is the treatment for an acute allergic reaction?

Treatment of Acute Allergic Reaction

Intramuscular epinephrine is the first-line treatment for acute allergic reactions and must be administered immediately—all other therapies are adjunctive and should never delay or replace epinephrine. 1, 2

Immediate Management: Epinephrine Administration

Dosing and Route

• Adults and children ≥30 kg: 0.3-0.5 mg (0.3-0.5 mL of 1:1000 concentration) intramuscularly 1, 3
• Children <30 kg: 0.01 mg/kg (maximum 0.3 mg) intramuscularly 1, 3
• Injection site: Anterolateral thigh (vastus lateralis)—never inject into buttocks, digits, hands, or feet 1, 3
• Repeat dosing: Every 5-15 minutes if symptoms persist or progress 1, 2

Critical Action Sequence

The following steps should occur simultaneously, not sequentially 1, 2:

1. Eliminate allergen exposure (if ongoing) 1
2. Inject epinephrine intramuscularly 1, 2
3. Call for help (911 in community, resuscitation team in hospital)—but do not delay epinephrine to summon help 1
4. Position patient supine with lower extremities elevated (if tolerated and primarily cardiovascular presentation) 1, 2
5. Administer supplemental oxygen 1, 2
6. Establish IV access and begin fluid resuscitation with normal saline for hypotension or incomplete response to epinephrine 1, 2

Adjunctive Therapies (After Epinephrine)

Critical pitfall: Antihistamines are the most common reason for not using epinephrine and may place patients at significantly increased risk for life-threatening progression. 1 Never substitute antihistamines for epinephrine. 1

H1 Antihistamines

• Diphenhydramine: 1-2 mg/kg (maximum 50 mg) IV or oral 2, 4
• Useful only for relieving itching and urticaria—does not relieve stridor, shortness of breath, wheezing, GI symptoms, or shock 1
• Second-generation options (cetirizine 10 mg) may be used to avoid sedation 1

H2 Antihistamines

• Minimal evidence supports their use; often given empirically but not proven effective 1

Bronchodilators

• Albuterol for bronchospasm: 4-8 puffs (children) or 8 puffs (adults) via MDI, or 1.5 mL (children) or 3 mL (adults) nebulized 2
• Administer only after initial epinephrine treatment 1

Corticosteroids

• No role in acute treatment due to 4-6 hour onset of action 1
• Not proven to prevent biphasic reactions despite widespread empiric use 1
• If given, discontinue within 2-3 days as all biphasic reactions occur within 3 days 1

Fluid Resuscitation

• Large-volume normal saline for patients with orthostasis, hypotension, or incomplete response to IM epinephrine 1, 2
• Anaphylaxis can shift up to 35% of intravascular volume into extravascular space within minutes 1

Special Circumstances

Refractory Anaphylaxis

• Repeat IM epinephrine every 5-15 minutes 1
• Consider IV epinephrine infusion (1:10,000 concentration) for protracted anaphylaxis unresponsive to IM dosing 1
• Adult IV dosing: 1-3 mg slowly over 3 minutes, then 3-5 mg over 3 minutes, then 4-10 mg/min infusion 1
• Add vasopressors if hypotension persists despite epinephrine and IV fluids 1

Patients on Beta-Blockers

• May be resistant to epinephrine with refractory hypotension and bradycardia 1
• Glucagon: 1-5 mg IV over 5 minutes (adults) or 20-30 μg/kg up to 1 mg (children), followed by 5-15 μg/min infusion 1
• Glucagon has inotropic/chronotropic effects not mediated through β-receptors 1

Bradycardia

• Atropine IV for patients with bradycardia 1

Observation and Monitoring

Duration

• Minimum 4-6 hours observation in a medical facility for all patients receiving epinephrine 1, 2, 5
• Extended observation (up to 6 hours or hospital admission) for severe anaphylaxis requiring >1 dose of epinephrine 1

Biphasic Reactions

• Occur in 10.3% of patients, with mean onset at 11 hours (range up to 72 hours) 1
• Risk factors: severe initial reaction, requiring >1 dose of epinephrine, wide pulse pressure, unknown trigger 1
• Early epinephrine administration may reduce biphasic reaction risk 1

Discharge Planning

All patients must receive 1, 2:

1. Epinephrine auto-injector prescription with proper training on self-administration 1, 2
2. Anaphylaxis emergency action plan 2
3. Referral to allergist-immunologist for diagnostic testing, trigger identification, and long-term management 1
4. Medical alert identification (e.g., MedicAlert jewelry) 1

High-Risk Populations

Patients at increased risk for severe or fatal reactions 1, 2, 5:

• Adolescents and young adults 2
• History of previous anaphylaxis 2, 5
• Coexisting asthma 1, 5
• Peanut/tree nut allergy 5
• Underlying cardiovascular disease 5
• Mast cell disorders 5

Common Pitfalls to Avoid

• Delaying epinephrine while administering antihistamines—this is the most dangerous error 1, 4
• Failing to recognize that generalized urticaria after known allergen exposure warrants epinephrine, even without respiratory/cardiovascular symptoms 1, 4
• Withholding epinephrine due to fear of cardiovascular side effects—there are no absolute contraindications to epinephrine in anaphylaxis 1, 3
• Injecting epinephrine subcutaneously or into incorrect sites (buttocks, arms) rather than IM into anterolateral thigh 1, 3
• Inadequate observation periods leading to missed biphasic reactions 1
• Discharging patients without epinephrine auto-injector and proper training 1, 2