Hyoscine Hydrobromide

Hyoscine hydrobromide (scopolamine) is a tertiary antimuscarinic that crosses the blood brain barrier and acts at the vestibular nuclei, vomiting centre and secretory glands. It is used for motion sickness, drooling, respiratory secretions in palliative care and postoperative nausea, but central anticholinergic effects (sedation, delirium) are common.

Classification. Antimuscarinic; antiemetic; antisialogogue.

Mechanism. Competitive blockade of muscarinic receptors. In the vestibular system it reduces cholinergic transmission to the vomiting centre. Peripheral blockade reduces salivary, bronchial and gastric secretions.

Pharmacodynamics. Sedative and amnestic at central doses. Antisecretory effects occur at low doses, within 10 to 30 minutes of injection.

Pharmacokinetics. Oral absorption is incomplete (about 10% bioavailable). The transdermal patch releases about 1 mg over 72 hours, reaching steady levels after about 6 hours. Half-life 4.5 hours.

Metabolism. Extensive hepatic metabolism, with less than 5% excreted unchanged in urine.

Adult doses unless stated.

Motion sickness.

• Oral: 150 to 300 micrograms 30 minutes before travel, then every 6 hours if needed; maximum 900 micrograms daily.

• Transdermal patch: one patch (about 1 mg over 72 hours) behind the ear at least 5 to 6 hours before travel; replace every 72 hours for long journeys.

Postoperative nausea and vomiting. One transdermal patch applied the evening before surgery, or at least 4 hours before, removed 24 hours after surgery.

Drooling and sialorrhoea. One transdermal patch every 72 hours, alternating ears.

Respiratory secretions at end of life (palliative care). 400 micrograms SC every 4 hours as needed, or 1.2 to 2.4 mg over 24 hours by continuous subcutaneous infusion, with specialist advice. A patch (1 patch every 72 hours) is an option where injections are unsuitable.

Children.

• Oral motion sickness: age 4 to 9 years 75 to 150 micrograms; age 10 to 11 years 150 to 300 micrograms; age 12 years and over adult doses; same intervals.

• Patch: age 10 years and over only.

Kidney and liver adjustment. Use lower doses and monitor in kidney or liver impairment.

Contraindications. Narrow-angle glaucoma; myasthenia gravis; paralytic ileus; pyloric stenosis; prostatic enlargement with urinary retention; severe ulcerative colitis or toxic megacolon; porphyria; hypersensitivity.

Common adverse effects. Dry mouth, drowsiness, blurred vision, dizziness and mydriasis. Skin reaction at the patch site.

Serious adverse effects.

• Delirium, hallucinations, agitation and memory loss, particularly in older adults, children and with patch use.

• Acute urinary retention and acute angle-closure glaucoma.

• Tachycardia and arrhythmia.

• Hyperthermia in hot environments.

• Withdrawal syndrome after patch removal following 3 or more days of use: nausea, vomiting, headache, dizziness and imbalance.

• Eye exposure from transfer of patch contents on fingers.

Major interactions.

• Other anticholinergics (tricyclic antidepressants, antihistamines, antipsychotics, amantadine, cyclizine): additive effects.

• CNS depressants, alcohol and opioids: additive sedation.

• Cholinesterase inhibitors (donepezil, rivastigmine, neostigmine): opposed effects.

• Metoclopramide and domperidone: prokinetic effect reduced.

• Potassium chloride tablets: increased risk of gastrointestinal ulceration.

• Reduced absorption of some drugs through slowed gastric emptying.

Special groups.

• Pregnancy: avoid; neonatal respiratory depression and withdrawal reported.

• Breastfeeding: avoid; infant drowsiness and reduced milk supply.

• Children: patch not for under 10 years; high risk of delirium.

• Older adults: avoid where possible because of delirium and falls.

• Kidney impairment: use lower doses.

• Liver impairment: use lower doses.

Formulations. Tablets 150 micrograms and 300 micrograms; injection 400 micrograms in 1 mL and 600 micrograms in 1 mL; transdermal patch 1.5 mg (releasing about 1 mg over 72 hours). Availability varies by market.

Patch administration.

• Apply to clean, dry, hairless skin behind one ear.

• Wash hands thoroughly after handling, and avoid touching the eyes.

• Replace every 72 hours on the other ear; remove after 72 hours.

• Fold the used patch in half and dispose of it safely.

• Remove before MRI if the patch has a metal backing.

Baseline. Eye pressure history, urinary symptoms, cognitive status, and review of anticholinergic burden.

Monitoring.

• Sedation, delirium and visual disturbance.

• Urinary retention and bowel function.

• Temperature in hot environments.

• Skin reaction under the patch.

• Review need at 72 hours.

Therapeutic drug monitoring. Not applicable.

Stopping. After 3 or more days of continuous patch use, taper by replacing the patch every 96 hours or removing it gradually to limit withdrawal. After short use, stop without a taper.

Referral and escalation. Eye emergency for painful red eye with halos (angle-closure glaucoma). Urgent review of acute confusion, urinary retention or arrhythmia; physostigmine is a specialist antidote for severe central anticholinergic toxicity.