Physiology, Anticholinergic Reaction

Excerpt

Acetylcholine (ACh) is a neurotransmitter that acts on the central nervous system (CNS), the autonomic nervous system (ANS), and at the neuromuscular junction (NMJ). Generally, ACh receptors at the NMJ are nicotinic type while in the CNS and ANS they are usually muscarinic type. As a reminder, these receptors are functionally and structurally different; nicotinic ACh receptors are ligand-gated ion channels, whereas muscarinic ACh receptors are G-protein coupled receptors. Processes that enhance ACh function are termed “cholinergic” while processes that inhibit the action of ACh at its receptors are termed “anticholinergic.” Anticholinergic effects are most commonly the result of medication. These medications should be more appropriately termed "antimuscarinics," as they usually block muscarinic but not nicotinic receptors. At least 600 drugs/medicinal products are recognized to have anticholinergic activity, and the most common of these are responsible for a significant amount of poisoning admissions. Many also contribute to the development of an anticholinergic reaction: a constellation of symptoms resulting from the antagonism of muscarinic receptors throughout the body. The features of the anticholinergic reaction are deducible from an understanding of the normal function of muscarinic receptors at various organs, and the following mnemonic summarizes these effects:

1. Mad as a hatter (delirium)

2. Blind as a bat (ocular symptoms)

3. Dry as a bone (anhidrosis/dry mouth/dry skin)

4. Hot as a hare (fever)

5. Bloated as a toad (constipation)

6. The heart runs alone (tachycardia)

7. Full as a flask (urinary retention)

8. Red as a beet (cutaneous vasodilation)

Clinically the most significant feature is delirium, particularly in the elderly, who are most likely to be affected by the anticholinergic reaction.