A young adult admitted with delirium, flushed dry skin, dilated pupils and tachycardia after ingesting belladonna seeds is given a cholinesterase inhibitor intravenously. Heart rate falls and the delirium clears. Why was physostigmine chosen instead of neostigmine?

• A Physostigmine has greater affinity for peripheral muscarinic receptors
• B Physostigmine is a tertiary amine that crosses the blood brain barrier ✓
• C Neostigmine is contraindicated in tachycardia
• D Physostigmine directly activates central nicotinic receptors

Explanation

The target symptoms here include central anticholinergic delirium, so the antidote must act inside the brain. Physostigmine is a tertiary amine that penetrates the blood brain barrier and restores central acetylcholine levels by inhibiting the enzyme there. Neostigmine and pyridostigmine are quaternary ammonium compounds that cannot cross the barrier, limiting them to peripheral indications such as myasthenia. Option A is incorrect because both drugs inhibit the same peripheral enzyme equally well; distribution, not potency, decides the choice.

Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.

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