Antihistamine and Anaphylaxis Management

Introduction to Antihistamine Choice

The American Academy of Allergy, Asthma, and Immunology recommends second-generation H1 antihistamines, such as cetirizine 10mg daily or loratadine 10mg daily, over first-generation agents like diphenhydramine for ongoing management due to their less sedating effects and 24-hour relief compared to diphenhydramine 1
Second-generation antihistamines have fewer anticholinergic effects and cognitive impairment risks, making them a preferred choice for patients, especially the elderly 1

The American Academy of Pediatrics recommends immediate administration of intramuscular epinephrine as the first-line treatment for anaphylaxis, with an adult dose of 0.3-0.5 mg of 1:1000 concentration epinephrine IM in the mid-outer thigh (vastus lateralis muscle) 2
The American Heart Association suggests activating emergency medical services (EMS) for emergency department evaluation and monitoring 3
Epinephrine is the only appropriate first-line medication for treating anaphylaxis, while norepinephrine has no established role in the initial management of anaphylactic reactions, as recommended by the American Academy of Pediatrics 2

Patients should be placed in a recumbent position with legs elevated if tolerated, and not allowed to stand, walk, or run as this can worsen shock, as advised by the American Academy of Pediatrics 2
Patients should be placed in a supine position or Trendelenburg position if hypotensive, with legs elevated to improve venous return, and oxygen should be administered (8-10 L/min) via face mask for respiratory distress, as recommended by the American Heart Association 4
Supplemental oxygen should be provided for respiratory symptoms, as part of the supportive care for anaphylaxis patients, as recommended by the American Thoracic Society 4

The American Academy of Allergy, Asthma, and Immunology recommends administering IV fluids (normal saline) if signs of hypotension develop, and considering the use of H1 antihistamines such as diphenhydramine 25-50 mg IV/PO, corticosteroids such as prednisone 40-60 mg PO or methylprednisolone 60-80 mg IV, and H2 blockers such as ranitidine 50 mg IV or famotidine 20 mg IV for better symptom control 5
Corticosteroids may help prevent protracted or biphasic anaphylaxis, and may be particularly beneficial for patients with asthma or those recently treated with corticosteroids, as stated by the Journal of Allergy and Clinical Immunology 6
H1 antihistamines, such as diphenhydramine, can be used for isolated mild symptoms, but are not life-saving interventions 7, 5, 8
H2 antihistamines, such as ranitidine, can be added for better symptom control, but have no evidence of benefit in preventing biphasic anaphylaxis 7, 5

For elderly patients, it is crucial to be cautious with antihistamine selection due to the increased risk of anticholinergic effects 1
First-generation antihistamines like diphenhydramine can cause significant sedation, cognitive impairment, and anticholinergic effects, especially with prolonged use, highlighting the need for careful consideration in treatment plans 1
Pregnant women with a rash can be treated with chlorphenamine (4-12 mg orally) or cetirizine/lomatadine (10 mg daily), as recommended by the British Journal of Dermatology guidelines 9
Patients with mastocytosis require first-line therapy with epinephrine and fluids, and may require more aggressive and prolonged treatment, as recommended by the British Journal of Anaesthesia 10

Patients with asthma are at higher risk for fatal anaphylaxis and require prompt epinephrine administration, according to the American Academy of Pediatrics 2
Adolescents are also at higher risk for fatal anaphylaxis and should be ensured to carry and know how to use epinephrine autoinjectors, as recommended by the American Academy of Pediatrics 2

Patients should be observed until signs and symptoms have fully resolved, and extended observation or hospital admission should be considered for severe initial reactions or patients requiring multiple epinephrine doses, as recommended by the American College of Emergency Physicians 4
The Journal of Allergy and Clinical Immunology recommends monitoring for at least 4-6 hours after symptom resolution, and considering extended observation for severe reactions requiring multiple epinephrine doses 5
Patients should be educated about the possibility of biphasic reactions, which can occur up to 72 hours later, with a mean time of 11 hours 4
Epinephrine autoinjectors should be prescribed upon discharge, as suggested by the Journal of Allergy and Clinical Immunology and the Annals of Oncology 4, 6
Referral to an allergist/immunologist is recommended for patients with recurrent allergic reactions 7, 5
Patients should be educated on trigger avoidance and proper use of emergency medications, and provided with identification (e.g., medical alert bracelet) for patients at risk, as suggested by the Journal of Allergy and Clinical Immunology and the Annals of Oncology 6, 11