Antihistamines Flashcards
List two non-sedating antihistamines.
Cetirizine and loratadine
What are the hallmark features of the non-sedating antihistamines in overdose?
Mild CNS depression, anticholinergic symptoms +/- hypotension, rarely an association with QT prolongation
Discuss the toxic mechanism of non-sedating antihistamines.
Mild lipophilia, less able to cross the BBB. Selective competitive reversible inhibitors of peripheral histamine-1 receptors. Some anticholinergic effects and cardiac potassium channel blockade (QT interval prolongation) can be seen too. In overdose some of the selective peripheral nature of the drugs may be lost.
What differentiates the sedating from the non-sedating antihistamines?
Non-sedating antihistamines have lower affinity for central H1, muscarinic (M1), alpha-1 adrenergic and serotonergic receptors
What is one prophylactic drug that can be given if the QT interval starts prolonging in a non-sedating antihistamine overdose?
Magnesium
Name three sedating antihistamines.
Promethazine, doxylamine, diphenhydramine, cyproheptadine
What are the prominent features of sedating antihistamine overdose?
Sedation, anticholinergic effects, orthostatic hypotension and in massive doses - cardiovascular toxicity/QT/QRS prolongation and hypotension. Some lower seizure threshold.
What is the mechanism of action of sedating antihistamines?
Competitive inhibition of histamine (H1)-receptors. Also antagonism at muscarinic (M1), alpha-1 adrenergic and serotonergic (5-HT) receptors. In massive ingestions - sodium/potassium channel block in cardiac myocytes
What antihistamine is known to cause conduction abnormalities secondary to fast sodium channel blockade?
Diphenhydramine
What drugs would you choose to manage hypotension due to antihistamine overdose? Why?
Fluids and then noradrenaline for its alpha-1 adrenergic agonist activity
