Hyoscine

Emma Farrell

UNDER REVIEW (September 2016)

Mechanism of Action:

Peripherally acting muscarinic receptor antagonist (antimuscarinic) that competitively inhibits the action of acetylcholine (ACh) at muscarinic receptors. Muscarinic AChRs are seven-transmembrane G-protein-coupled receptors which are activated by the endogenous neurotransmitter, ACh. They are generally associated with the parasympathetic nervous system, and also cholinergic transmission in the CNS. Hence, antimuscarinics block the effects of parasympathetic nerve stimulation and so dilate pupils, reduce intestinal motility and smooth muscle contraction and reduce secretions from glands (sweat, salivary, bronchial).

Lecture and CAL materials:

Drug specifics

Alternative drug name Scopolamine, Hyoscine Butylbromide, Buscopan ®
Effects Antispasmodic that decreases intestinal motility and secretions which is helpful in treating motility disorders (e.g. management of discomfort and spasm in irritable bowel syndrome and diverticular disease). Their value has not been established and the response varies. Hyoscine butylbromide is advocated as a gastro-intestinal antispasmodic (Buscopan), but it is relatively poorly absorbed. Hyoscine injection is useful in endoscopy and radiology. Other separate indications for antimuscarinic drugs include bradyarrhythmias, relieving bronchoconstriction in asthma and airways disease, motion sickness, parkinsonism, urinary incontinence, mydriasis (dilating pupils, usually prior to fundoscopy) and cycloplegia, premedication and as an antidote to organophosphorus poisoning, and reducing airways secretions in palliation of the terminally ill
Adverse actions Predictable antimuscarinic effects i.e. dry mouth, blurred vision (dilatation of the pupils with loss of accommodation), tachycardia, constipation, difficulty with micturition and urine retention, confusion, flushing and dryness of the skin. Drowsiness, especially in the elderly. Drowsiness may affect performance of skilled tasks (e.g. driving) and may persist for up to 24 hours or longer after removal of patch. Very rarely the pupillary dilatation may precipitate angle-closure glaucoma.
Dose Delivered as tablets for intestinal symptoms, a patch for motion sickness or as an injection for premedication.
Interactions Contraindicated in myasthenia gravis, a condition characterized by autoantibodies to acetylcholine receptors. May enhance the effects of alcohol. Should be avoided in those with poor urinary flow. Antimuscarinics like hyoscine should be used with caution in the elderly, paralytic ileus, pyloric stenosis and prostatic enlargement.
Contraindications not specified
Comments Hyoscine is one of the most used antimuscarinic agents. It is also used as a transdermal antiemetic in preventing nausea and treating motion sickness. The symptoms associated with motion sickness are due to overstimulation of the semicircular canals (vestibular system), which might activate the parasympathetic nervous system – hyoscine helps to counteract this overactivity, being an antagonist of acetylcholine, the neurotransmitter of the parasympathetic nervous system. Hyoscine butylbromide is does not readily cross the blood-brain barrier. This reduces the blocking of muscarinic receptors in the central nervous system, and avoids risk of side effects eg confusion and memory impairment. It is also used as an adjunct for anaesthesia because it decreases respiratory secretions. OTHER ANTICHOLINERGIC (ANTIMUSCARINIC) DRUGS. Those used for gastro-intestinal smooth muscle spasm include the tertiary amines atropine sulphate and dicycloverine hydrochloride (dicyclomine hydrochloride) and the quaternary ammonium compounds propantheline bromide and hyoscine butylbromide. The quaternary ammonium compounds are less lipid soluble than atropine and are less likely to cross the blood–brain barrier; they are also less well absorbed. Atropine and the belladonna alkaloids are outmoded treatments, any clinical virtues being outweighed by atropinic side-effects. Dicycloverine hydrochloride has a much less marked antimuscarinic action.
Contributors