What is the first-line treatment for an acute allergic reaction in the emergency room?

Acute Allergic Reaction Treatment in the Emergency Room

Intramuscular epinephrine is the only first-line treatment for acute allergic reactions (anaphylaxis) in the emergency room, and must be administered immediately upon recognition—all other medications are adjunctive and have delayed onset of action. 1

Immediate First-Line Treatment: Epinephrine

Dosing and Administration

• Adults and children ≥30 kg: 0.3-0.5 mg (0.3-0.5 mL of 1:1000 solution) intramuscularly 1, 2
• Children <30 kg: 0.01 mg/kg (maximum 0.3 mg) intramuscularly 1, 2
• Injection site: Anterolateral thigh (vastus lateralis muscle)—this provides more rapid plasma concentrations than subcutaneous injection 1
• Repeat dosing: Every 5-15 minutes as needed if symptoms persist or progress 1

Critical Timing Considerations

Delayed epinephrine administration is the primary factor associated with fatal outcomes. In a study of fatal and near-fatal food-induced anaphylaxis in children, 6 of 7 survivors received epinephrine within 30 minutes, whereas only 2 of 6 who died received it within the first hour 1. Failure to respond promptly can result in rapid decline and death within 30-60 minutes 1

Concurrent Initial Actions

While administering epinephrine, simultaneously:

• Eliminate additional allergen exposure 1
• Call for help (summon resuscitation team in hospital, call 911 in community)—but never delay epinephrine to summon help 1
• Position patient supine with lower extremities elevated (if tolerated) 1
• Provide supplemental oxygen 1
• Establish IV access and begin fluid resuscitation if hypotension or orthostasis present 1

Adjunctive Treatments (Only AFTER Epinephrine)

H1 Antihistamines

• Diphenhydramine: 1-2 mg/kg per dose, maximum 50 mg IV or oral 1, 3
• Important limitation: H1 antihistamines only relieve itching and urticaria—they do NOT relieve stridor, shortness of breath, wheezing, GI symptoms, or shock 1
• Never substitute antihistamines for epinephrine, as this is the most common reason for not using epinephrine and places patients at significantly increased risk 3

H2 Antihistamines

• Ranitidine: 1-2 mg/kg per dose, maximum 75-150 mg oral or IV 1, 3
• Evidence: Minimal evidence supports H2 antihistamines in emergency anaphylaxis treatment, but combination of H1 and H2 antihistamines works better than either alone 1

Bronchodilators

• Albuterol nebulizer: 1.5 mL (child) or 3 mL (adult) every 20 minutes or continuously as needed 1
• Alternative: MDI with spacer (child: 4-8 puffs; adult: 8 puffs) 1
• Critical caveat: Albuterol does NOT relieve airway edema (laryngeal edema) and should never be substituted for epinephrine 1

Corticosteroids

• Methylprednisolone: 1 mg/kg IV, maximum 60-80 mg OR 1, 3
• Prednisone: 1 mg/kg oral, maximum 60-80 mg 1, 3
• Purpose: Primarily used to prevent recurrent or protracted anaphylaxis, though clear evidence for this is lacking 3

Special Situations

Refractory Hypotension

• IV epinephrine (1:10,000 solution) for patients not responding to IM epinephrine and fluid resuscitation 1
• Alternative vasopressors may be titrated to effect 1
• Glucagon for patients on beta-blockers: 20-30 μg/kg (child) or 1-5 mg (adult), may repeat or follow with infusion of 5-15 μg/min 1, 3

Patients on Beta-Blockers

• May have reduced response to epinephrine 1
• Glucagon should be available as it bypasses beta-receptor blockade 1, 3
• Important: There are NO absolute contraindications to epinephrine use in anaphylaxis, including patients on beta-blockers 1, 4

Observation and Disposition

Observation Period

• Minimum 4-6 hours after successful treatment 5
• Extended observation warranted for severe reactions, multiple epinephrine doses required, or history of biphasic reactions 1, 5
• Biphasic anaphylaxis occurs in <1% to 20% of patients, typically 1-72 hours after initial episode resolution 1

Discharge Requirements

• Prescribe two epinephrine autoinjectors with proper training on use 1, 3, 5
• Continue diphenhydramine every 6 hours for 2-3 days 1, 3
• Continue H2 antihistamine twice daily for 2-3 days 1, 3
• Continue prednisone daily for 2-3 days 1, 3
• Provide written anaphylaxis emergency action plan 5
• Refer to allergist for trigger identification and consideration of immunotherapy 1, 3

Common Pitfalls to Avoid

Recognition Failures

• Do NOT wait for full diagnostic criteria to be met before administering epinephrine—when anaphylaxis is suspected, it is better to err on the side of caution 1, 6
• Asthmatic patients are at particularly high risk for fatal anaphylaxis; wheezing in an asthmatic having an allergic reaction mandates immediate epinephrine 3

Treatment Errors

• Never inject epinephrine into buttocks, digits, hands, or feet due to risk of tissue necrosis 2
• Do NOT use subcutaneous route when IM is available—IM provides more rapid tissue concentrations 1
• Never allow patient to stand, walk, or run as this can precipitate cardiovascular collapse 5
• Do NOT substitute antihistamines or bronchodilators for epinephrine as first-line treatment 1, 3

High-Risk Populations Requiring Heightened Vigilance

• Adolescents and young adults 5
• Patients with coexisting asthma (especially severe or uncontrolled) 1, 3
• Previous history of anaphylaxis 5
• Cardiovascular disease or older age 1