Acute Poisoning: Organophosphate & Paracetamol
Organophosphate Poisoning
Pathophysiology: organophosphate compounds irreversibly inhibit acetylcholinesterase, causing acetylcholine accumulation at muscarinic and nicotinic synapses, producing a characteristic cholinergic toxidrome.
Clinical features (mnemonics SLUDGE/DUMBELS): Salivation, Lacrimation, Urination, Defecation, GI distress, Emesis; or Diarrhea, Urination, Miosis, Bradycardia/Bronchorrhea/Bronchospasm, Emesis, Lacrimation, Salivation — bronchorrhea and bronchospasm being the most immediately life-threatening muscarinic manifestations. Nicotinic effects: fasciculations, weakness, and, in severe cases, respiratory muscle paralysis. CNS: confusion, seizures, coma.
Antidote protocol:
- Atropine — competitive muscarinic antagonist, titrated aggressively to effect (drying of secretions/resolution of bronchorrhea, not a fixed heart rate/pupil size target); initial dose 0.02–0.05 mg/kg IV, doubled and repeated every 5–10 minutes until adequate atropinization — addresses only muscarinic effects, no effect on nicotinic toxicity
- Pralidoxime (2-PAM) — reactivates acetylcholinesterase by displacing the organophosphate, effective only before “aging” occurs (permanent enzyme-bond stabilization within hours) — give as early as possible, ideally alongside atropine. Addresses both muscarinic and nicotinic toxicity, a genuinely complementary addition to atropine
- Supportive care: aggressive airway management/suction, mechanical ventilation for respiratory failure, benzodiazepines for seizures, thorough decontamination (clothing removal, skin washing) to prevent ongoing absorption and protect healthcare workers
Paracetamol (Acetaminophen) Overdose
Pathophysiology: at therapeutic doses, paracetamol is metabolized via glucuronidation/sulfation; a small fraction via CYP450 to toxic NAPQI, normally detoxified by glutathione conjugation. In overdose, glucuronidation/sulfation saturate, diverting more through CYP450, generating NAPQI faster than glutathione can detoxify — causing centrilobular hepatocellular necrosis.
| Stage | Timing | Features |
|---|---|---|
| I | 0–24 hours | Often asymptomatic or mild nausea/vomiting/malaise, without significant biochemical hepatotoxicity yet — dangerous false-reassurance period |
| II | 24–72 hours | RUQ pain; rising transaminases (biochemical hepatotoxicity apparent) |
| III | 72–96 hours | Peak hepatotoxicity; most severe cases progress to fulminant hepatic failure (coagulopathy, encephalopathy, jaundice, renal impairment) |
| IV | 4 days – 2 weeks | Resolution/recovery in survivors, or continued deterioration/death in the most severe cases |
Rumack-Matthew Nomogram: plasma paracetamol concentration vs time since ingestion (starting from 4 hours post-ingestion), used to determine need for NAC after a single acute overdose of known timing. Not valid for staggered/chronic ingestion, modified-release formulations, or unknown ingestion timing — a lower threshold for empirical NAC applies in these scenarios.
Antidote — N-Acetylcysteine (NAC): replenishes hepatic glutathione stores, restoring NAPQI detoxification capacity, plus direct antioxidant/hepatoprotective properties. Efficacy is highest, hepatotoxicity risk essentially eliminated, when given within 8–10 hours of ingestion. Still beneficial (reduced efficacy) when started later, and given in established hepatotoxicity/fulminant hepatic failure given benefit even at this late stage. May be given via traditional IV three-bag protocol or an oral/enteral regimen — IV generally preferred with significant vomiting, altered mental status, or established hepatic failure.