Physostigmine

A reversible acetylcholinesterase inhibitor useful in the treatment of central anticholinergic delirium that is not easily controlled by benzodiazepine sedation and there is high risk for aspiration and excessive sedation. Only for those patients who have an isolated anticholinergic agent toxicity (not the polypharmacy overdose). It can also be considered in the management of a recovering TCA overdose but not in the acute management as you will precipitate seizures.

Contraindications include bradydysrhythmias, intraventricular and AV block and bronchospasm. This is because in excess it produces a cholinergic toxicity (remember organophosphate toxicity).

Administration

• Confirm absence of conduction defect on a 12-lead ECG (QRS >100ms)
• Administer 0.5 – 1 mg IV as a slow push over 5 minutes and repeat every 10 minutes until the desired clinical effects are observed.
• It is rare for more than 4 mg to be required.
• The duration of action is much shorter than most cases of anticholinergic delirium. Delirium may reoccur in 1-4 hours and repeated doses maybe required.
• Paediatric dose = 0.02 mg/kg to a maximum of 0.5mg

Adverse drug reactions: (the cholinergic crisis)

• Seizures following rapid administration
• Bradycardia
• Bronchospasm
• Bronchorrhoea
• Nausea, vomiting and diarrhoea
• If a cholinergic crisis occurs treat with atropine until there is resolution of the bradycardia and drying of the respiratory secretions.

References

• Burns MJ, Linden CH, Graudins A et al.  A comparison of physostigmine and benzodiazepines for the treatment of anticholinergic poisoning. Annals of Emergency Medicine 2000; 35:374-381.
• Suchard JR. Assessing physostigmine’s contraindication in cyclic antidepressant ingestions. Journal of Emergency Medicine 2003; 25(2):185-191.