2. Allergic disease Flashcards
Antihistamine mechanism of action
Block H1 receptors
Types of antihistamines
Sedating
- Chlorphenamine
- Promethazine
Non-sedating
- Acrivastine
- Cetirizine
- Fexofenadine
- Loratidine
- Mizolastine
Antihistamine uses
Hayfever, hives, itching, runny nose, nausea and vomiting, insomnia (due to sedative effects)
Can also be used in anaphylaxis and angioedema
Antihistamine side effects
Sedative effects (greater in sedating antihistamines)
QT prolongation with hydroxyzine
Therefore hydroxyzine is contra-indicated in patients with a prolonged QT interval or risk factors for it.
If ever used should be used at the lowest effective dose for the shortest time
If used in elderly, must half the dose
Antihistamine cautions
Antihistamines have antimuscarinic effects, so cause urinary retention as a side effect. Therefore antihistamines cannot be use in prostatic hypertrophy
Angle-closure glaucoma - as they raise intraocular pressure
Severe liver impairment - sedating antihistamines can precipitate hepatic encephalopathy
What is allergen immunotherapy
When vaccines containing allergens, are administered to help the body build immunity and tolerance
risk of invoking anaphylaxis
Must be given under specialist supervision with CPR available
When should allergen vaccine not be administered
Avoid allergen vaccines in patients taking beta-blockers as adrenaline might be ineffective. As beta-blockers antagonise adrenaline
ACE inhibitors can cause anaphylactoid reasons, therefore allergen vaccines must also be avoided in these patients
Anaphylaxis
A severe, life-threatening allergic reaction
Anaphylaxis treatment
IM adrenaline
Adult: 300/500mcg
Child: 15-30kg 150mcg
Anaphylaxis symptoms
Angioedema
Hives
Light-headedness
Cold clammy skin
what is adrenaline
Adrenaline is a sympathomimetic so causes bronchodilation and raises blood pressure
Adrenaline administration
Inject into the middle of the outer thigh
A second dose can be given after 5-15 mins
Lie down with legs raised to restore blood pressure but if they are having breathing difficult
2 auto-injectors should always be prescribed and carried at all times.
Maintain the same brand as injection technique is different for each brand
Angioedema
Swelling in the skin caused by an allergic reaction
Symptoms of angioedema
Swollen lips, eyes, tongue
Hives
Laryngeal oedema (severe, dangerous as it affects airways and is a sign of anaphylaxis )
Angioedema treatment
Antihistamine + corticosteroid
IM adrenal if severe
