Anticholinergic Drugs in Parkinson Disease

Anticholinergic drugs block muscarinic receptors in the striatum and restore the balance between dopamine and acetylcholine. They improve tremor and dystonia but have little effect on bradykinesia.

Classification. Trihexyphenidyl, benzatropine, procyclidine, orphenadrine.

• Trihexyphenidyl 1 mg daily, increased to 2 mg three times daily; usual 6 to 10 mg daily; maximum 15 mg daily.
• Benzatropine 0.5 to 1 mg at night, increased to a maximum of 6 mg daily; 1 to 2 mg intramuscularly or intravenously for acute dystonia.
• Procyclidine 2.5 mg three times daily, increased to 5 mg three times daily; maximum 30 mg daily; 5 to 10 mg intramuscularly or intravenously for acute dystonia.
• Orphenadrine 150 to 400 mg daily in divided doses.

Differences. Efficacy is similar across the group; trihexyphenidyl has the highest misuse potential.

• Tremor-dominant Parkinson disease in patients under 60 years: when cognition is intact and other drugs fail.
• Drug-induced parkinsonism and acute dystonia: short course alongside review of the dopamine antagonist.
• Sialorrhoea: low doses help.
• Withdrawal: taper over weeks to avoid rebound.

Selection. Avoid in people over 70, with cognitive impairment, glaucoma or prostatic enlargement.

Contraindications. Narrow-angle glaucoma, urinary retention, paralytic ileus, tardive dyskinesia (can worsen), myasthenia gravis, dementia.

Adverse effects. Dry mouth, blurred vision, constipation, urinary retention, tachycardia, confusion, hallucinations, memory loss, delirium. Reduced sweating can cause heat intolerance and hyperthermia in hot climates.

Interactions. Additive anticholinergic effects with tricyclic antidepressants, antihistamines, antipsychotics and antispasmodics. They oppose metoclopramide and domperidone.

Special groups.

• Pregnancy: limited data; avoid if possible.
• Breastfeeding: may reduce milk; avoid.
• Children: used for dystonia only under specialist direction.
• Older adults: avoid.
• Kidney and liver impairment: reduce doses.

Monitoring. Cognition, bladder emptying, bowels, eye symptoms and hydration in hot weather.

Referral and escalation. Acute confusion, urinary retention or hyperthermia. Tremor resistant to drugs needs referral for advanced options.