Anticholinergic drugs are known for their wide range of adverse effects, which result from the inhibition of acetylcholine at muscarinic receptors. These effects are categorized into central and peripheral based on their site of action. Central adverse effects are caused by excessive blockade of cholinergic receptors in the central nervous system (CNS), while peripheral adverse effects result from cholinergic inhibition in exocrine glands, smooth muscles, and other end-organs regulated by the parasympathetic nervous system.
Central adverse effects include headache, memory impairment, cognitive dysfunction, and behavioral disturbances such as anxiety and insomnia. These effects are more common at lower doses of anticholinergics, especially in drugs that cross the blood-brain barrier, such as scopolamine and atropine. At higher doses, these central effects become more severe, manifesting as agitation, confusion, delirium, and even seizures. The likelihood of CNS effects increases with drugs that have a higher affinity for crossing the blood-brain barrier, like tertiary amines (e.g., scopolamine), while quaternary amines like glycopyrrolate are less likely to cause CNS side effects due to their inability to easily cross the barrier.
Peripheral adverse effects of anticholinergics are widespread due to the inhibition of acetylcholine in various organs. These include hyperthermia due to reduced sweating (anhidrosis), tachycardia, flushing, and arrhythmias as a result of the drug's cardiovascular actions. In the gastrointestinal system, anticholinergic drugs can lead to constipation, reduced gut motility, vomiting, and dry mouth due to the inhibition of salivary and GI secretions. Additionally, urinary retention can occur, particularly in patients with prostatic hypertrophy or bladder outlet obstruction, as the detrusor muscle's ability to contract is reduced. Ocular effects, such as blurred vision, mydriasis (pupil dilation), and increased risk of narrow-angle glaucoma, are also common due to the inhibition of parasympathetic control of the eye. Lastly, diminished muscle contraction and reduced sweating contribute to the risk of hyperthermia and heat-related complications, particularly in hot environments.
