The cholinergic system is a major neuromodulatory neurotransmitter system, interacting with core regions of the brain and affecting learning and memory function. Cholinergic function is severely reduced in patients with pathologic conditions, such as Alzheimer's disease. Muscarinic receptors M1–M5 are located at the postsynaptic level of the central cholinergic synapse. Blockage of certain muscarinic receptors has been shown to affect cognitive functioning. The roles of the individual receptor subtypes has implications for pharmacotherapy; eg, nonselective antimuscarinic/anticholinergic medications may block muscarinic neurotransmission, causing deficits in recall and new learning, while M1-receptor agonists and M2-receptor antagonists may not interfere with cognitive functioning. Studies investigating central nervous system (CNS) depressant effects of drugs have shown that antimuscarinic agents have the potential to cause impairment of CNS functioning. CNS dysfunction may be manifested as changes in memory, disruption of sleep, hallucinations, confusion, and delirium.
Anticholinergic load refers to the cumulative effect of taking multiple drugs with anticholinergic activity. The extent to which a particular drug affects the cholinergic system can be determined by its ability to cross the blood-brain barrier, the integrity of which decreases with age. Elderly individuals are also at high risk for increased anticholinergic load due to reductions in metabolism and elimination as well as pharmacokinetic drug interactions. In addition, elderly people are often prescribed several medications that have direct anticholinergic properties and take over-the-counter medications which may also carry substantial anticholinergic burden. The cumulative effect of these medications in vulnerable elderly patients may precipitate early cognitive declines or further exacerbate cognitive deficits in those already suffering from dementia. Risk of cognitive impairment in patients with overactive bladder can thus be reduced through use of M3-selective antagonists, such as darifenacin, and via alternatives to pharmacotherapy, such as behavioral modification, pelvic floor strengthening, and bladder-retraining exercises.