Allergies

Question 1

Antihistamine (H1-receptor antagonists)

Common indications

Answer 1

1. As a first-line treatment for allergies, particularly hayfever (seasonal allergic rhinitis)
2. To aid relief of pruritus and uticaria- 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

Antihistamine (H1-receptor antagonists)

MOA

Answer 2

• The term ‘antihistamine’ is generally used to mean an antagonist of the H1-R.
• H2-receptor antagonists have different uses and are discussed separately
• Histamine is released from storage granules in mast cells as a result of Ag binding to IgE on the cell surfac.e
• 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 release in the nasopharynx, as in hayfever, it causes nasal irritation, sneezing, rhinorrheoa, congestion, conjuctivitis and itch
• In the skin, it causes urticaria
• Widespread histamine release, as in anaphylaxis, produces generalised vasodilation and vascular leakage, with consequent hypotension
• Antihistamines work in these conditions by antagonism at the H1-R, blocking the effects of excess histamine
• In anaphylaxis, their effect is too slow to be life-saving, so adrenaline is the more important first-line treatment

Question 3

Antihistamine (H1-receptor antagonists)

Important adverse effects

Answer 3

• The first-generation antihistamine (e.g. chlorphenamine) cause sedation
• This because histamine, via H1-receptors, has a role in the brain in maintaining wakefulness
• Newer 2nd-gen antihistamines (loratidine) do not cross the BBB so tend not to have this effect
• They have few adverse effects

Question 4

Antihistamine (H1-receptor antagonists)

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

Question 5

Antihistamine (H1-receptor antagonists)

Communication

Answer 5

• As appropriate, explain that you are offering a treatment to help relieve their allergic symptoms or their itchy rash/hives
• In hayfever, the tablets should improve sneezing, itchiness and runniness, but tend not to help with nasal congestion
• In the cases of cetirizine and loratidine, you can say that you do not anticipate any side effects
• For chlorphenamine, you should mention that it may make them feel sleepy or lose concentration
• They should therefore avoid taking it if they need to drive or carry out any other activity that requires concentration
• They should also avoid combining it with alcohol, which may exacerbate the effects

Question 6

See Addison’s disease for corticosteroids