Anaphylaxis Management: Adjunctive Therapy
Diphenhydramine (Option B) should be initiated immediately as the next treatment for this patient with anaphylaxis. 1, 2
Rationale for Diphenhydramine
H1-antihistamines like diphenhydramine are recommended as second-line adjunctive therapy in anaphylaxis management, administered simultaneously with or immediately following epinephrine. 1, 2 The NIAID expert panel specifically recommends diphenhydramine at 1-2 mg/kg per dose (maximum 50 mg) IV or oral for food-induced anaphylaxis. 1, 2
Mechanism and Benefits
Diphenhydramine provides symptomatic relief of urticaria and pruritus that commonly occur during anaphylaxis. 1 While antihistamines do not reverse the life-threatening cardiovascular or respiratory manifestations of anaphylaxis (which require epinephrine), they effectively address the cutaneous symptoms this patient is experiencing. 1
The combination of H1 and H2 antihistamines is emerging as important in preventing severe cardiac deficits during anaphylaxis. 1 The NIAID guidelines recommend continuing H1 antihistamines (diphenhydramine every 6 hours) along with H2 antihistamines (ranitidine twice daily) for 2-3 days post-discharge to prevent biphasic reactions. 1
FDA labeling explicitly indicates diphenhydramine injectable form for "amelioration of allergic reactions...in anaphylaxis as an adjunct to epinephrine and other standard measures after the acute symptoms have been controlled." 3
Why Not the Other Options
Benztropine (Option A) - Incorrect
- Benztropine is an anticholinergic agent used for extrapyramidal symptoms and Parkinson's disease, not anaphylaxis. It has no role in allergic reaction management and would be inappropriate in this acute setting.
Lorazepam (Option C) - Incorrect
- Lorazepam is a benzodiazepine with no direct therapeutic benefit in anaphylaxis. While anxiety may accompany anaphylaxis, benzodiazepines do not address the underlying pathophysiology and could potentially mask worsening symptoms or cause respiratory depression in a patient already in respiratory distress.
Nebulized Racemic Epinephrine (Option D) - Less Appropriate
- While bronchodilators like albuterol are recommended for wheezing or bronchospasm in anaphylaxis, nebulized racemic epinephrine is not standard therapy. 2 The patient has already received systemic epinephrine (the correct first-line treatment). If bronchodilation is needed, albuterol via MDI (8 puffs) or nebulizer would be the appropriate bronchodilator choice. 2
Complete Management Algorithm
After epinephrine and methylprednisolone have been administered, the treatment sequence should include:
- Diphenhydramine 1-2 mg/kg IV (maximum 50 mg) 1, 2
- Bronchodilators (albuterol) for persistent wheezing - 8 puffs via MDI or 3 mL via nebulizer every 20 minutes 2
- Supplemental oxygen for respiratory distress 2
- IV fluid boluses (10-20 mL/kg) for hypotension - this patient's BP of 85/50 indicates need for volume resuscitation 2
- Repeat epinephrine every 5-15 minutes if symptoms persist 2
Critical Monitoring Points
All patients receiving epinephrine must be observed for 4-6 hours minimum, with longer observation (up to 12 hours) for severe reactions like this one. 2 This patient's moderate respiratory distress, hypotension, and angioedema warrant extended monitoring.
The evidence for antihistamines and corticosteroids preventing biphasic reactions is low-certainty, but their use remains standard practice for symptom control. 1 The 2020 JACI practice parameter acknowledges that while high-quality evidence is lacking, antihistamines and glucocorticoids are "fairly embedded into common practice" and may provide benefit for treating urticaria and pruritus during anaphylaxis. 1
Antihistamines should never delay or replace epinephrine administration - epinephrine remains the only first-line medication for anaphylaxis. 1, 2 However, in this case, epinephrine has already been appropriately administered.