## Structural Basis of CNS Penetration **Key Point:** The critical difference between physostigmine and neostigmine lies in their chemical structure and resulting lipophilicity, which determines blood-brain barrier (BBB) penetration. ### Physostigmine - Tertiary amine (lipophilic) - Crosses the BBB readily - Can treat both peripheral AND central cholinergic deficiency (e.g., anticholinergic poisoning with CNS manifestations, atropine overdose with confusion) ### Neostigmine - Quaternary ammonium compound (hydrophilic, ionized) - Cannot cross the BBB - Restricted to peripheral effects only - Useful for myasthenia gravis (neuromuscular junction) and glaucoma (eye drops) ## Comparison Table | Feature | Physostigmine | Neostigmine | |---------|---------------|-------------| | Chemical class | Tertiary amine | Quaternary ammonium | | Lipophilicity | High | Low | | BBB penetration | Yes | No | | CNS effects | Present | Absent | | Duration | 30 min–2 hrs | 2–8 hrs | | Clinical use | Anticholinergic toxicity, glaucoma | Myasthenia gravis, postoperative ileus | **High-Yield:** Physostigmine is the drug of choice for atropine/anticholinergic overdose WITH CNS symptoms (confusion, hallucinations, seizures) because it crosses the BBB. Neostigmine cannot treat central anticholinergic effects. **Clinical Pearl:** In anticholinergic poisoning, if the patient has only peripheral signs (dry mouth, tachycardia, mydriasis), either drug works; but if confusion or agitation is present, physostigmine is mandatory. [cite:KD Tripathi 8e Ch 6]
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