Antihistamines (H1 receptor antagonists)

Question 1

Antihistamine indication

Answer 1

Hay fever, pruritis, urticaria, anaphylaxis after adrenaline administration, nausea and vomiting

Question 2

Histamine issues

Answer 2

Histamine is released from mast cells due to antigen binding to IgE on cell surface. Histamine induces features of immediate type, type 1, hypersensitivity = increased capillary permeability causing oedema, vasodilation causing erythema and itch due to sensory nerve stimulation. Causes nasal irritation, sneezing rhinorrhoea, congestion, conjunctivitis, itch, urticaria, released in anaphylaxis causing vasodilation and vascular leakage

Question 3

Antihistamine MOA

Answer 3

Antagonises H1 receptor blocking effects of excess histamine

Question 4

Antihistamines adverse effects

Answer 4

sedation in first generation antihistamines like chlorphenamine as crosses BBB

Question 5

Second gen antihistamines

Answer 5

Loratidine, cetirizine, fexofenadine, do not cross bbb so fewer adverse effects like sedation

Question 6

Antihistamine cautions

Answer 6

Avoid sedating histamines in severe liver disease and precipitate hepatic encephalopathy

Question 7

Antihistamine counselling

Answer 7

Tell patients they may feel sleepy or lose concentration so avoid driving or activity needing concentration-first gen