Vestibular Suppressants

Vestibular suppressants reduce vertigo, nausea and vomiting in acute vestibular disorders and motion sickness. They act on the central pathways of the vestibular system and are useful only for a short period; prolonged use delays central compensation and rehabilitation. They do not treat the cause.

Mechanism. Antihistamines (promethazine, dimenhydrinate, meclizine, cyclizine) block H1 and muscarinic receptors in the vestibular nuclei and vomiting centre; phenothiazines (prochlorperazine) block dopamine D2 receptors; cinnarizine blocks calcium channels in vestibular hair cells; benzodiazepines (diazepam, lorazepam) enhance GABA-A inhibition; hyoscine (scopolamine) blocks muscarinic receptors. Betahistine is a histamine H3 antagonist with H1 agonism that improves inner ear blood flow; it is not a suppressant and is used for prevention.

Pharmacokinetics. Oral drugs reach effect within 30 to 60 minutes; prochlorperazine and promethazine also given by injection or buccal route when vomiting. Half-lives: prochlorperazine 6 to 8 hours, promethazine 10 to 14 hours, cinnarizine 3 to 4 hours, betahistine about 3 to 4 hours (active metabolite), diazepam up to 48 hours (active metabolite).

• Prochlorperazine: strong antiemetic; dystonia risk.
• Cinnarizine and dimenhydrinate: antihistamines for vertigo and motion sickness; sedating.
• Promethazine and cyclizine: sedating, anticholinergic.
• Meclizine: less sedating antihistamine.
• Hyoscine patch: motion sickness prevention.
• Diazepam, lorazepam: very short use in severe attacks.
• Betahistine: preventive treatment in Meniere disease, with limited evidence.
• Ondansetron: for nausea only; does not suppress vertigo.

Regimens.

• Acute vestibular syndrome (vestibular neuritis, labyrinthitis, acute Meniere attack): prochlorperazine 5 to 10 mg orally three times daily, or 12.5 mg by deep intramuscular injection, or 3 to 6 mg buccal twice daily; or cinnarizine 15 to 30 mg three times daily; or dimenhydrinate 50 mg every 4 to 6 hours (maximum 400 mg daily). Limit to 24 to 72 hours, then stop and begin vestibular exercises.
• Severe attack with vomiting: promethazine 25 mg intramuscularly or intravenously; or diazepam 2 to 5 mg once.
• Motion sickness: hyoscine patch 1.5 mg behind the ear applied 6 hours before travel, lasting 72 hours; or cinnarizine 30 mg 2 hours before travel, then 15 mg every 8 hours; or promethazine 25 mg the night before.
• Meniere disease (prevention): betahistine 16 mg three times daily with food, up to 24 mg three times daily; review at 3 months.
• Benign paroxysmal positional vertigo: canalith repositioning manoeuvre (Epley); drugs are not needed.

Selection. Do not treat vertigo with drugs until a central cause (cerebellar or brainstem stroke, tumour, multiple sclerosis, migraine with brainstem signs) has been excluded in patients with new severe headache, double vision, speech or limb weakness, inability to walk, or vascular risk. Use the shortest course and stop once the patient can mobilise.

Contraindications. Prochlorperazine and cinnarizine: Parkinson disease, depressed consciousness, bone marrow depression, children under 2 years; antihistamines and hyoscine: closed-angle glaucoma, urinary retention, paralytic ileus; benzodiazepines: respiratory depression, myasthenia, sleep apnoea; betahistine: phaeochromocytoma, active peptic ulcer, with caution in asthma.

Effects. Drowsiness, dry mouth, blurred vision, constipation, urine retention, confusion in older adults; extrapyramidal reactions (acute dystonia in young adults and children) and QT prolongation with prochlorperazine; weight gain and depression with cinnarizine; headache, nausea, and rash with betahistine; dependence and falls with benzodiazepines.

Interactions. Alcohol and other sedatives (additive); anticholinergic drugs; QT-prolonging drugs (macrolides, antipsychotics, ondansetron); levodopa (opposed by prochlorperazine); monoamine oxidase inhibitors with betahistine.

Monitoring. Review at 3 days; stop at the first improvement; screen for red flags at each contact; electrolytes and ECG if QT prolonging combinations are used.

Special groups. Pregnancy: cyclizine, promethazine and meclizine are used when needed; avoid benzodiazepines. Breastfeeding: short courses acceptable, with sedation watch. Children: promethazine and prochlorperazine avoided under 2 years; dystonia risk. Older adults: avoid anticholinergic drugs and benzodiazepines; falls and delirium.

Region: dizziness in East Africa is often caused by hypertension, anaemia, malaria, hypoglycaemia, ototoxic drugs (gentamicin, quinine) and raised intracranial pressure, so measure blood pressure and glucose and review drug history before labelling vertigo as vestibular.