Antimuscarinics
Examples
Hyoscine butylbromide (Buscopan)
Mebeverine
Dicycloverine
Mebeverine
Dicycloverine
Indications
Symptomatic relief of gastrointestinal disorders characterised by smooth muscle spasm; bowel colic (e.g. Irritable bowel syndrome)
Contraindications
Contraindications
- Paralytic ileus
- Pyloric stenosis
- Patients susceptible to acute angle closure glaucoma; an adverse effect of muscarinic antagonists is a rise in intraocular pressure, this may precipitate acute angle closure glaucoma.
- Patients with bladder outflow obstruction and prostatic hyperplasia; an adverse effect of muscarinic receptor antagonists is relaxation of the bladder which causes urinary retention. In a patient with any degree of bladder outflow obstruction there is a risk of developing acute urinary retention.
- Patients with myasthenia gravis; administration of muscarinic receptor anagonists may cause deterioration in symptoms.
- Patients with underlying heart disease, particularly arrhythmias, as these can be worsened.
Mechanism
Parasympathetic stimulation causes the release of acetylcholine from the post ganglionic parasympathetic nerve endings, which then acts on cholinergic (muscarinic) receptors, specifically M3 receptors, causing parasympathetic effects.
Muscarinic antagonists inhibit smooth muscle constriction in the gut wall, producing muscle relaxation and reduction spasm.
Muscarinic antagonists inhibit smooth muscle constriction in the gut wall, producing muscle relaxation and reduction spasm.
Administration
In gastrointestinal conditions, muscarinic antagonists are administered orally.
Hyoscine is also available to give parenterally.
Hyoscine is also available to give parenterally.
Adverse Reactions
Muscarinic receptor antagonists are highly polar molecules and are usually administered by aerosol inhalation. As a result they are not well absorbed into the systemic circulation and hence have little action at muscarinic repceptors other than those found in the bronchi. This results in few unwanted effects.
Antagonism of the parasympathetic pathway at muscarinic receptors may lead to adverse effects on other glandular and smooth muscle (note antagonists are not selective and will block all three types of muscarinic receptor - M1, M2 and M3). If the parasympathetic pathway in these tissues is blocked this will result in unopposed sympathetic activity on the tissues, causing classic anti-cholinergic side effects:
Antagonism of the parasympathetic pathway at muscarinic receptors may lead to adverse effects on other glandular and smooth muscle (note antagonists are not selective and will block all three types of muscarinic receptor - M1, M2 and M3). If the parasympathetic pathway in these tissues is blocked this will result in unopposed sympathetic activity on the tissues, causing classic anti-cholinergic side effects:
- Dilated pupils
- Blurred vision
- Dry mouth
- Nausea
- Constipation
- Headache
- Urinary retention / difficulty micturating
- Tachycardia
- Hypotension
- Drowsiness/Confusion
Interactions
Care must be taken if prescribing other anti-cholinergic drugs. This may potentiate the effects of the muscarinic antagonists.
Education
Patients should be alerted to potential side effects that may affect their ability to perform at skilled tasks such as driving and operating heavy machinery.
Pharmacokinetics
None.
Other Systems
