Anticholinergic Antiemetics

Anticholinergic antiemetics block muscarinic M1 receptors in the vestibular nuclei and vomiting centre, and reduce gut secretions and motility. They are most useful for motion sickness, vestibular nausea, malignant bowel obstruction and excess respiratory secretions in palliative care.

Classification. Tertiary amines that cross the blood-brain barrier (hyoscine hydrobromide) and quaternary amines that do not (hyoscine butylbromide, glycopyrronium).

Pharmacokinetics. Hyoscine hydrobromide has low oral bioavailability and a short half life; transdermal delivery gives steady levels over 72 hours. Quaternary agents act peripherally, with little central antiemetic effect.

• Hyoscine hydrobromide 300 microgram orally 30 minutes before travel, then 300 microgram every 6 hours; maximum 900 microgram daily. Transdermal patch 1 mg over 72 hours, applied behind the ear 5 to 6 hours before travel.
• Hyoscine butylbromide 20 mg subcutaneously or intravenously every 4 to 6 hours, or 60 to 120 mg over 24 hours by continuous subcutaneous infusion in malignant obstruction; maximum 100 mg daily by injection.
• Glycopyrronium 200 microgram subcutaneously every 8 hours.

Differences. Only hyoscine hydrobromide treats motion sickness and vestibular causes. The quaternary agents reduce colic and secretions without sedation.

• Motion sickness and vertigo: hyoscine hydrobromide before the journey.
• Malignant bowel obstruction: hyoscine butylbromide or glycopyrronium with cyclizine or haloperidol; add octreotide when vomiting persists.
• Terminal respiratory secretions: hyoscine butylbromide or glycopyrronium.

Selection. Ineffective for chemotherapy-induced or opioid-induced vomiting.

Contraindications. Narrow-angle glaucoma, myasthenia gravis, paralytic ileus or non-malignant obstruction, urinary retention, tachyarrhythmia, megacolon.

Adverse effects. Dry mouth, blurred vision, drowsiness, urinary retention, tachycardia, confusion and hallucinations in older adults. Withdrawal after removing the patch can cause nausea, dizziness and headache. Patch contact then touching the eye causes unilateral mydriasis.

Interactions. Additive anticholinergic effects with tricyclic antidepressants, antipsychotics and sedating antihistamines. Anticholinergics oppose metoclopramide and domperidone.

Special groups.

• Pregnancy: use only when clearly needed.
• Breastfeeding: may reduce milk supply.
• Children: avoid the patch under 10 years.
• Older adults: avoid; delirium and falls.
• Kidney and liver impairment: reduce dose and watch for accumulation.

Monitoring. Mental state, bladder emptying, heart rate and bowel sounds.

Referral and escalation. Acute confusion, urinary retention, or vomiting with abdominal distension and absent bowel sounds needs review for obstruction.