A 35-year-old farmer is brought unconscious with miosis, excessive salivation, bradycardia, bronchoconstriction, and muscle fasciculations. Plasma cholinesterase is 15% of normal. The treatment includes atropine titrated to dry secretions PLUS oxime (pralidoxime). Pralidoxime is effective ONLY if given within what time frame after organophosphorus (OP) exposure, and why?
- A Within 48–72 hours; after this period the OP-AChE complex undergoes 'aging' (dealkylation) making it irreversible ✓
- B Within 6 hours; plasma cholinesterase regenerates spontaneously after this time
- C Any time; pralidoxime works regardless of aging
- D Within 24 hours; only muscarinic effects persist beyond this
Explanation
After organophosphorus compounds bind to acetylcholinesterase (AChE), the OP-enzyme complex undergoes a time-dependent irreversible process called 'aging' (dealkylation of one of the alkyl groups from the phosphorylated enzyme). Once aging occurs, pralidoxime (2-PAM) — which works by nucleophilically attacking the phosphoryl bond and regenerating free AChE — can no longer reactivate the enzyme. The aging half-life varies by compound: soman ages within minutes, parathion within hours, while some agents age over 24–48 hours. As a practical guideline, pralidoxime should be administered within 24–48 hours of exposure for most common agricultural OP compounds. After aging, only supportive care and atropine (for muscarinic effects) are useful.
Reference: The Essentials of Forensic Medicine and Toxicology (Narayan Reddy), 34th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.