Benztropine

Benztropine is a centrally acting anticholinergic used for acute drug-induced dystonia, antipsychotic-induced parkinsonism and Parkinson disease tremor. It acts within minutes when given by injection, but carries anticholinergic toxicity, cognitive harm in older adults, and worsening of tardive dyskinesia.

Mechanism. Blocks muscarinic acetylcholine receptors in the striatum, restoring the balance between cholinergic and dopaminergic activity. It also inhibits dopamine reuptake and has antihistamine action.

Pharmacokinetics. Oral onset in about 1 hour. Injection acts within 15 minutes. Duration is 6 to 10 hours, and it is metabolised in the liver. Its half-life is poorly defined and long.

Emergency use. Acute dystonia from antipsychotics or metoclopramide:

• Benztropine 1 to 2 mg intramuscularly or slowly intravenously. Repeat after 30 minutes if needed. Then give 1 to 2 mg orally twice daily for 2 to 3 days to prevent recurrence.

First-line and alternative use.

• Drug-induced parkinsonism, oral benztropine: 1 to 4 mg once or twice daily. Maximum 6 mg daily.

• Parkinson disease tremor, oral: 0.5 to 1 mg at bedtime, increased by 0.5 mg every 5 to 6 days to a usual 1 to 2 mg daily. Maximum 6 mg daily.

• Prophylaxis during antipsychotic therapy is not routine; review need regularly and stop after 1 to 3 months where possible.

Kidney, liver, weight and age.

• Kidney and liver: use caution; reduce the dose in significant impairment.

• Older adults: start 0.5 mg once or twice daily; avoid in dementia.

Contraindications. Narrow-angle glaucoma, myasthenia gravis, obstructive gastrointestinal disease (paralytic ileus, megacolon), urinary retention or obstructive prostatic enlargement, tardive dyskinesia, and hypersensitivity.

Serious adverse effects. Anticholinergic toxicity (hot, dry, red skin, delirium, tachycardia, urinary retention, ileus), heat stroke from reduced sweating, acute angle-closure glaucoma, and psychosis.

Common adverse effects. Dry mouth, blurred vision, constipation, urinary hesitancy, tachycardia, drowsiness, confusion and memory impairment.

Interactions. Other anticholinergics (tricyclic antidepressants, antipsychotics, antihistamines, antispasmodics) add toxicity. It opposes cholinesterase inhibitors. Alcohol and sedatives add drowsiness.

Common prescribing errors. Prescribing long term by habit with antipsychotics. Using it in older adults with cognitive impairment. Giving it with other anticholinergics.

Special groups.

• Pregnancy: avoid unless needed.

• Breastfeeding: may suppress lactation; avoid.

• Regional: acute dystonia is a frequent presentation wherever depot haloperidol and fluphenazine are used, and heat intolerance matters in hot climates.

Formulations. Injection 1 mg/mL (2 mL ampoules). Tablets 0.5, 1 and 2 mg. Give with or after food.

Monitoring.

• Resolution of dystonia within 20 to 30 minutes of injection.

• Cognition, bowel and bladder function, intraocular pressure risk, temperature in hot weather, and need for continued use.

Stopping. Taper over 1 to 2 weeks to avoid rebound parkinsonism or cholinergic rebound.

Referral and escalation. Refer for persistent movement disorder despite a lower antipsychotic dose or change of antipsychotic, or for tardive dyskinesia.