These agents inhibit the parasympathetic nervous system by selectively blocking the binding of ACh to its receptor in nerve cells. The nerve fibers of the parasympathetic system are responsible for the involuntary movement of smooth muscles present in the gastrointestinal tract, urinary tract, lungs, sweat glands, and many other parts of the body.
In broad terms, anticholinergics are divided into two categories in accordance with their specific targets in the central and peripheral nervous system and at the neuromuscular junction: antimuscarinic agents, and antinicotinic agents (ganglionic blockers, neuromuscular blockers).
The term "anticholinergic" is typically used to refer to antimuscarinics which competitively inhibit the binding of ACh to muscarinic acetylcholine receptors; such agents do not antagonize the binding at nicotinic acetylcholine receptors at the neuromuscular junction, although the term is sometimes used to refer to agents which do so.
Anticholinergic drugs are used to treat a variety of conditions:
- Dizziness (including vertigo and motion sickness-related symptoms)
- Extrapyramidal symptoms, a potential side-effect of antipsychotic medications
- Gastrointestinal disorders (e.g., peptic ulcers, diarrhea, pylorospasm, diverticulitis, ulcerative colitis, nausea, and vomiting)
- Genitourinary disorders (e.g., cystitis, urethritis, and prostatitis)
- Insomnia, although usually only on a short-term basis
- Respiratory disorders (e.g., asthma, chronic bronchitis, and chronic obstructive pulmonary disease [COPD])
- Sinus bradycardia due to a hypersensitive vagus nerve
- Organophosphate based nerve agent poisoning, such as VX, sarin, tabun, and soman (atropine is favoured in conjunction with an oxime, usually pralidoxime)
Until the beginning of the 20th century anticholinergic drugs were widely used to treat psychiatric disorders.
- Delirium (often with hallucinations and delusions indistinguishable from reality)
- Ocular symptoms (from eye drops): mydriasis, pupil dilation, and acute angle-closure glaucoma in those with shallow anterior chamber
- Anhidrosis, dry mouth, dry skin
- Urinary retention
- Cutaneous vasodilation
Long-term use may increase the risk of both cognitive and physical decline. It is unclear whether they affect the risk of death generally. However, in older adults they do appear to increase the risk of death.
Possible effects of anticholinergics include:
- Poor coordination
- Decreased mucus production in the nose and throat; consequent dry, sore throat
- Dry-mouth with possible acceleration of dental caries
- Cessation of sweating; consequent decreased epidermal thermal dissipation leading to warm, blotchy, or red skin
- Increased body temperature
- Pupil dilation; consequent sensitivity to bright light (photophobia)
- Loss of accommodation (loss of focusing ability, blurred vision – cycloplegia)
- Increased heart rate
- Tendency to be easily startled
- Urinary retention
- Urinary incontinence while sleeping
- Diminished bowel movement, sometimes ileus (decreases motility via the vagus nerve)
- Increased intraocular pressure; dangerous for people with narrow-angle glaucoma
- Euphoria or dysphoria
- Respiratory depression
- Memory problems
- Inability to concentrate
- Wandering thoughts; inability to sustain a train of thought
- Incoherent speech
- Mental confusion (brain fog)
- Wakeful myoclonic jerking
- Unusual sensitivity to sudden sounds
- Illogical thinking
- Visual disturbances
- Periodic flashes of light
- Periodic changes in visual field
- Visual snow
- Restricted or "tunnel vision"
- Visual, auditory, or other sensory hallucinations
- Rarely: seizures, coma, and death
- Orthostatic hypotension (severe drop in systolic blood pressure when standing up suddenly) and significantly increased risk of falls in the elderly population.
Older patients are at a higher risk of experiencing CNS side effects.
An acute anticholinergic syndrome is reversible and subsides once all of the causative agents have been excreted. Reversible acetylcholinesterase inhibitor agents such as physostigmine can be used as an antidote in life-threatening cases. Wider use is discouraged due to the significant side effects related to cholinergic excess including seizures, muscle weakness, bradycardia, bronchoconstriction, lacrimation, salivation, bronchorrhea, vomiting, and diarrhea. Even in documented cases of anticholinergic toxicity, seizures have been reported after the rapid administration of physostigmine. Asystole has occurred after physostigmine administration for tricyclic antidepressant overdose, so a conduction delay (QRS > 0.10 second) or suggestion of tricyclic antidepressant ingestion is generally considered a contraindication to physostigmine administration.
Anticholinergics are classified according to the receptors that are affected:
- Antimuscarinic agents operate on the muscarinic acetylcholine receptors. The majority of anticholinergic drugs are antimuscarinics.
- Antinicotinic agents operate on the nicotinic acetylcholine receptors. The majority of these are non-depolarising skeletal muscle relaxants for surgical use that are structurally related to curare. Several are depolarizing agents.
Examples of common anticholinergics:
- Antimuscarinic agents
- Antipsychotics (clozapine, quetiapine)
- Certain SSRIs (Paroxetine)
- Dicyclomine (Dicycloverine)
- Propantheline bromide
- Tricyclic antidepressants (28 compounds with numerous trade names)
- Antinicotinic agents
- Bupropion – Ganglion blocker
- Dextromethorphan - Cough suppressant and ganglion blocker
- Doxacurium – Nondepolarizing skeletal muscular relaxant
- Hexamethonium – Ganglion blocker
- Mecamylamine – Ganglion blocker and occasional smoking cessation aid
- Tubocurarine - Nondepolarizing skeletal muscular relaxant
Physostigmine is one of only a few drugs that can be used as an antidote for anticholinergic poisoning. Nicotine also counteracts anticholinergics by activating nicotinic acetylcholine receptors. Caffeine (although an adenosine receptor antagonist) can counteract the anticholinergic symptoms by reducing sedation and increasing acetylcholine activity, thereby causing alertness and arousal.
When a significant amount of an anticholinergic is taken into the body, a toxic reaction known as acute anticholinergic syndrome may result. This may happen accidentally or intentionally as a consequence of either recreational or entheogenic drug use, though many users find the side effects to be exceedingly unpleasant and not worth the recreational effects they experience. In the context of recreational use, anticholinergics are often called deliriants.
- Atropa belladonna (deadly nightshade)
- Brugmansia species
- Datura species
- Garrya species
- Hyoscyamus niger (henbane)
- Mandragora officinarum (mandrake)
Use as a deterrent
Several narcotic and opiate-containing drug preparations, such as those containing hydrocodone and codeine are combined with an anticholinergic agent to deter intentional misuse. Examples include Hydromet/Hycodan (hydrocodone/homatropine), Lomotil (diphenoxylate/atropine) and Tussionex (hydrocodone polistirex/chlorpheniramine). However, it is noted that opioid/antihistamine combinations are used clinically for their synergistic effect in the management of pain and maintenance of dissociative anesthesia (sedation) in such preparations as Meprozine (meperidine/promethazine) and Diconal (dipipanone/cyclizine), which act as strong anticholinergic agents.
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Anticholinergics have antisecretory activities and decrease nasal and bronchial secretions, salivation, lacrimation, sweating and gastric acid production, and can be used to decrease secretions in allergic and inflammatory diseases. Anticholinergics relax smooth muscle in the gastrointestinal tract, bladder and lung and can be used for gastrointestinal, urological or respiratory conditions associated with spasm and dysmotility.
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