Mechanism of action and main effects
The main representatives of agents that inhibit cholinergic transmission in the central nervous system are trihexyphenidyl (also known as benzhexol) and biperiden. In their pharmacological properties, these drugs are very similar to each other.
The site of action of these medicinal products is the brain structures. They perform a selective blockade of $M_1$ muscarinic receptors in the neostriatum. This action determines their specific pharmacological profile. Central anticholinergics produce a moderate antiparkinsonian effect, which manifests itself extremely unevenly:
- Drugs effectively reduce tremor.
- They reduce muscle rigidity (increased muscle tone).
- They have virtually no effect on bradykinesia (slowness of movement).
Clinical application
Drugs of this group, such as biperiden, are actively used in clinical practice for various forms of extrapyramidal disorders. The main indications include:
- Parkinson's disease: it is most appropriate to prescribe the drugs primarily in the early stages of the disease development.
- Parkinsonism syndrome: used regardless of the etiological factors (various etiological forms) that caused this syndrome.
- Drug-induced parkinsonism: this is one of the most specific indications. Central anticholinergics are used for the prevention and rapid relief of extrapyramidal disorders provoked by the intake of neuroleptics (antipsychotic drugs).
Adverse reactions
The side effects of these drugs are directly related to their mechanism of action and are classified depending on the localization of the blocked receptors:
Central effects occur due to the blockade of M-cholinergic receptors directly in the brain. They manifest as various cognitive impairments, such as memory decline and decreased concentration. Elderly patients are at a special risk group as they have the highest sensitivity to such central adverse reactions.
Peripheral effects are caused by the undesirable inhibition of peripheral receptors:
- Gastrointestinal tract: xerostomia (dry mouth) and constipation are observed.
- Ophthalmology: accommodation impairment, up to complete cycloplegia (inability to focus vision on close objects).
- Cardiovascular system: tachycardia (rapid heartbeat) is noted.
- Urinary system: urinary retention may occur.
Contraindications and overdose
The prescription of central anticholinergics is strictly limited in a number of comorbid conditions due to the risk of worsening the patient's condition.
Absolute contraindications:
- Glaucoma (there is a high risk of a critical increase in intraocular pressure).
- Prostatic hypertrophy (provokes the risk of acute urinary retention).
- Lactation period (breastfeeding).
- Individual hypersensitivity to the drug components.
In case of exceeding the therapeutic dose, a picture of acute intoxication develops. Symptoms of overdose include pronounced psychomotor agitation and hallucinations, which together form the classic clinical picture of anticholinergic delirium.
Briefly: answers to topical questions
Which drug groups are not recommended to be combined with central anticholinergics due to the risk of exacerbating anticholinergic syndrome?
Sources do not establish a general ban on combining central anticholinergics with neuroleptics or tricyclic antidepressants. It is indicated that anticholinergic delirium occurs when prescribing high doses of drugs with pronounced atropine-like action. Provoking drugs include:
- neuroleptics: chlorpromazine, levomepromazine, clozapine;
- tricyclic antidepressants: amitriptyline, imipramine, clomipramine, maprotiline;
- antiparkinsonian agents: trihexyphenidyl, biperiden, diphenhydramine.
For neuroleptics and tricyclic antidepressants, sources indicate blockade of m-cholinergic receptors and side effects such as dry mouth, tachycardia, urinary retention, constipation, mydriasis, and cycloplegia.
How does the effect on parkinsonism symptoms differ between central anticholinergics and levodopa?
The effect of these drug groups on parkinsonism symptoms differs in the degree of effect expression and the spectrum of action.
| Characteristic | Central anticholinergics | Levodopa |
|---|---|---|
| General efficacy | Exert a moderate antiparkinsonian effect | Is the "gold standard" of treatment, providing improvement in nearly 100% of patients |
| Effect on tremor and muscle rigidity | Predominantly reduce these symptoms | Causes overall clinical improvement |
| Effect on bradykinesia | Have little effect on bradykinesia | Causes overall clinical improvement |
To remember the peripheral side effects of M-anticholinergics: everything "dries out" (xerostomia, constipation), fluid is "retained" (urinary retention), the heart "speeds up" (tachycardia), and the eyes become "defocused" (cycloplegia).
Class and exam questions
Why do central anticholinergics have little effect on slowness of movement?
The pharmacological profile of these drugs is such that they predominantly relieve tremor and muscle rigidity, but have a minimal effect on bradykinesia.
Can biperiden be prescribed to patients with glaucoma?
No, this is a strict contraindication. Blockade of peripheral M-cholinergic receptors carries the risk of a dangerous increase in intraocular pressure.
What is the role of these drugs in antipsychotic treatment?
They are the drugs of choice for the prevention and relief of extrapyramidal disorders (drug-induced parkinsonism) caused by the intake of antipsychotic drugs.
What happens in case of trihexyphenidyl overdose?
So-called anticholinergic delirium occurs, the main manifestations of which are pronounced psychomotor agitation and hallucinations.
What else to study on this topic
- Comparison of the pharmacodynamics of trihexyphenidyl and levodopa
- Features of the use of anticholinergics in elderly patients
- Mechanism of development of anticholinergic delirium
- Differential diagnosis of etiological forms of parkinsonism syndrome
- The role of pramipexole in the treatment of extrapyramidal disorders
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