Anticholinergic

Anticholinergic (anticholinergic agent) is a group of substances that blocks the action of the neurotransmitter acetylcholine (ACh) at synapses in the central and the peripheral nervous system, and, in broad terms, neuromuscular junction.[1][2]

Not to be confused with Anti-cholinesterase.

These agents inhibit parasympathetic nerve impulses by selectively blocking the binding of the neurotransmitter acetylcholine 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, and many other parts of the body;[3] cholinergic process otherwise by enhancing ACh function.[3]

In broad terms, anticholinergics are divided into two categories in accordance with their specific targets in the central, peripheral nervous system and neuromuscular junction:[3] antimuscarinic agents, and antinicotinic agents (ganglionic blockers, neuromuscular blockers).[4]

In strict terms, anticholinergic only comprises antimuscarinic which competitively inhibits binding of the neurotransmitter acetylcholine to muscarinic acetylcholine receptors though.[3][5]; anticholinergic agents do not antagonize the binding at nicotinic acetylcholine receptors at the neuromuscular junction, for example.[5][3] (See: Cholinergic crisis § treatment and Cholinesterase inhibitor)

Medical uses

Anticholinergic drugs are used to treat a variety of conditions:

Anticholinergics generally have antisialagogue effects (decreasing saliva production), and most produce some level of sedation, both being advantageous in surgical procedures.[6][7]

Until the beginning of the 20th century anticholinergic drugs were used widely in psychiatric disorders.[8]

Physiological effects
  • delirium
  • ocular symptoms--mydriasis, pupil dilation, and acute angle-closure glaucoma in those with shallow anterior chamber. (Eye drop form)[9][10][11]
  • anhidrosis/dry mouth/dry skin
  • fever
  • constipation
  • tachycardia
  • urinary retention
  • cutaneous vasodilation

[3]

Clinically the most significant feature is delirium, particularly in the elderly, who are most likely to be affected by the toxidrome.[3]

Side effects

Long-term use may increase the risk of both cognitive and physical decline.[12][13] It is unclear whether they affect the risk of death generally.[12] However, in older adults they do appear to increase the risk of death.[14]

Possible effects of anticholinergics include:

Possible effects in the central nervous system resemble those associated with delirium, and may include:

  • Confusion
  • Disorientation
  • Agitation
  • Euphoria or dysphoria
  • Respiratory depression
  • Memory problems[16]
  • Inability to concentrate
  • Wandering thoughts; inability to sustain a train of thought
  • Incoherent speech
  • Irritability
  • Mental confusion (brain fog)
  • Wakeful myoclonic jerking
  • Unusual sensitivity to sudden sounds
  • Illogical thinking
  • Photophobia
  • Visual disturbances
    • Periodic flashes of light
    • Periodic changes in visual field
    • Visual snow
    • Restricted or "tunnel vision"
  • Visual, auditory, or other sensory hallucinations
    • Warping or waving of surfaces and edges
    • Textured surfaces
    • "Dancing" lines; "spiders", insects; form constants
    • Lifelike objects indistinguishable from reality
    • Phantom smoking
    • Hallucinated presence of people not actually there
  • 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.[17]

Older patients are at a higher risk of experiencing CNS side effects.

Toxicity

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.[18]

Pharmacology

Anticholinergics are classified according to the receptors that are affected:

Examples

Examples of common anticholinergics:

Plants of the family Solanaceae contain various anticholinergic tropane alkaloids, such as scopolamine, atropine, and hyoscyamine.

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.

Recreational uses

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 recreational drug use. Anticholinergic drugs are usually considered the least enjoyable by many recreational drug users. In the context of recreational use, anticholinergics are often called deliriants.[20]

Plant sources

The most common plants containing anticholinergic alkaloids (including atropine, scopolamine, and hyoscyamine among others) are:

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.[28] 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.[29]

References
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