Hayfever (seasonal allergic rhinitis)

Question 1

Antihistamines (H1-receptors antagonists)

Common indications

Answer 1

1. As a first-line treatment for allergies particularly hayfever (seasonal allergic rhinitis)
2. To aid relief to itchiness (pruritus) and hives (urticaria) due for example insect bites, infection (chickenpox) and drug allergies
3. As an adjunctive treatment in anaphylaxis, after administration of adrenaline and other life-saving measures
4. Other drugs in this class may be used for N&V

Question 2

Antihistamines (H1-receptor antagonist)

MOA

Answer 2

• H2+H1 have different effects
• Histamine is release from storage granules in mast cells as a result of antigen binding to IgE on the cell surface
• Mainly via H1-R, histamine induces the features of immediate-type (type 1) hypersensitivity: increased capillary permeability causing oedema formation (wheal), vasodilation causing erythema (flare) and itch due to sensory nerve stimulation
• When histamine is released in the nasopharynx, as in hayfever, it causes nasal irritation, sneezing, rhinorrhea, congestion, conjunctivitis and itch
• In the skin it causes urticaria
• Anaphylaxis = widespread histamine release. Produces generalised vasodilation and vascular leakage, with consequent hypotension
• Antihistamines antagonise H1 receptors, blocking the effects of excess histamine

Question 3

Antihistamine H1-receptor antagonist

Adverse effects

Answer 3

• The first-gen antihistamines (e.g. chlorphenamine) cause sedation
• This is because H1-receptors has a role in the brain in maintaining wakefulness
• Newer ‘second-gen’ antihistamines do not cross the blood-brain barrier so tend not to have this effect
• In general, they have fewer side effects the 1st gen

Question 4

Antihistamines

Warnings

Answer 4

• Commonly used antihistamines, including those mentioned above, are safe in most patients
• Sedating antihistamines (e.g. chlorphenamine) should be avoided in severe liver disease, as they may precipitate hepatic encephalopathy