The antispasmodics are drugs that are used to relieve, prevent or reduce muscle spasm of smooth muscle, especially of the digestive and urinary tract, and thereby reduce the cramping pain that this spasm causes.
Antispasmodics are also called spasmolytics and from a pharmacological point of view they are parasympatholytic drugs, that is, they block the effect of the parasympathetic system, therefore, they block acetylcholine. The type drug in this group is atropine.
They are drugs capable of blocking some of the muscarinic effects of acetylcholine in the parasympathetic endings of the smooth muscles, the glands, the heart and / or the central nervous system..
Within this group of drugs we can mention those of natural origin, semi-synthetic and synthetic. The natural ones are natural alkaloids among which is atropine (DL-hyoscyamine), which is extracted from the bush called Atropa Belladonna.
Scopolamine (L-hyoscine) is extracted from Hyoscyamus niger and the Jimson weed is extracted from the Datura stramonium, what is a source of L-hyoscyamine.
Semisynthetics differ from the natural compounds from which they are synthesized by their way of distribution and body elimination, as well as the duration of their effects.
Among the antispasmodics of synthetic origin are tertiary amines, whose use is currently restricted for ophthalmological use as pupillary or cycloplegic dilators.
Finally, there are the quaternary antispasmodics, some of which have specific effects on certain subtypes of muscarinic receptors..
Article index
Below is a list of parasympatholytic drugs of which natural ones and some quaternary amines are used as antispasmodics..
The mechanism of action of antispasmodics is to compete with acetylcholine for muscarinic receptors. These receptors are located mainly in the smooth muscle of the gastrointestinal and genitourinary tract, in the glands, in the heart and in the central nervous system..
As this antagonistic effect is competitive, it can be overcome if the concentration of acetylcholine in the vicinity of the receptor is high enough..
Antispasmodic medications are used to treat colic caused by spasmodic contraction of the gastrointestinal smooth muscles, the gallbladder, and the smooth muscles of the genitourinary tract..
Antispasmodic parasympatholytics inhibit the muscarinic cholinergic effect of acetylcholine, thereby inducing non-vascular smooth muscle relaxation or decreasing its activity.
They are used in a wide variety of symptoms related to gastrointestinal and gallbladder motor disorders. These include pylorospasm, epigastric discomfort, and colic that accompany diarrhea..
By relaxing the smooth muscle of the bladder wall, they are used to relieve pain and tenesmus that accompany cystitis..
Although these drugs can experimentally inhibit the contraction of the ureter and bile ducts, biliary or ureteral colic requires narcotic analgesics and generally does not resolve with antispasmodics..
Due to the effect on glandular secretion, they are used in gastric and duodenal ulcers in combination with other specific medications..
The most common side effects are dilated pupils and blurred vision, dry mouth, difficulty swallowing, urinary retention in older men, constipation, vertigo, and fatigue. In the cardiovascular system, tachycardia and a slight increase in blood pressure may occur.
These side effects are the reason that chronic use of these drugs is not well tolerated..
In the case of poisoning, the aforementioned effects are presented, followed by behavioral changes ranging from sedation, delirium, hallucinations, seizures, coma and respiratory depression (large doses), dry and red skin and hyperthermia, especially in children..
The lethal dose of atropine for an adult is around 0.5g and scopolamine 0.2-0.3g..
Pharmacological interactions refer to modifications of the effect of a drug in terms of duration and magnitude of its effects, due to the concomitant or previous use of another drug or substance ingested.
In this sense, antispasmodics that are parasympatholytic or anticholinergic have additive effects with the following drugs:
- amantadine
- antihistamines
- benzodiazepines
- tricyclic antidepressants
- disopyramide
- monoamine oxidase inhibitors
- meperidine
- methylphenidate
- procainamide
- thioxanthines
- quinidine
- nitrates and nitrites
- the primidona
- the orphenadrine
- phenothiazines.
Antispasmodics or anticholinergics in general:
- Increase the depressant effect of alcohol on the central nervous system
- Increase the effect of atenolol and the gastrointestinal bioavailability of diuretics, nitrofurantoin and digoxin
- Increase the pressure in the eye generated by glucocorticoids
- Block the effect of metoclopramide
- Reduce the effect of phenothiazines and levodopa
Antacids reduce the absorption of anticholinergics by mouth. Guanethidine, histamine, and reserpine block the inhibition produced by anticholinergics on gastrointestinal secretions.
Contraindications for the use of parasympatholytics are glaucoma, urinary and gastric retention, and surgical abdomen pictures in the process of diagnosis. In cases of glaucoma, when the pupil needs to be dilated for some ophthalmological process, sympathomimetic drugs are used.
Other contraindications include hypersensitivity to the drug or its excipients, tachycardias, and myocardial ischemias..
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