Chapter 6  ·  Module 3  ·  Visual Summary
Organophosphate Toxicology and Cholinergic Toxidrome Management
Phosphorylation, aging, the full toxidrome, antidote pharmacology, and carbamate comparison
Organophosphate Inhibition and the Aging Phenomenon
Mechanism of Inhibition
  • Organophosphate attacks the active site serine of acetylcholinesterase
  • Forms a covalent phosphoryl-enzyme bond
  • Bond is hydrolyzed millions of times more slowly than normal
  • Acetylcholine accumulates at all cholinergic synapses
  • Both muscarinic and nicotinic receptors continuously stimulated
The Aging Reaction
  • After phosphorylation, a secondary dealkylation occurs spontaneously
  • Converts the adduct to a form resistant to oxime reactivation
  • Once aged: permanently inhibited, no antidote can restore the enzyme
  • Rate of aging is compound-specific — determines pralidoxime window
  • Recovery requires synthesis of new enzyme (days to weeks)
Aging Half-Lives — Window for Pralidoxime (relative scale)
Soman (GD)
2–6 minutes
Sarin (GB)
3–5 hours
VX
36–48 hours
Pesticide organophosphates
12–36+ hours
The Cholinergic Toxidrome — Three Components
Muscarinic (SLUDGE)
  • Salivation
  • Lacrimation
  • Urination
  • Defecation / diarrhea
  • Gastrointestinal cramping
  • Emesis
  • Miosis
  • Bronchospasm — life-threatening
  • Bronchorrhea — life-threatening
  • Bradycardia — life-threatening
  • Diaphoresis
Nicotinic (Neuromuscular Junction & Ganglia)
  • Fasciculations (early)
  • Muscle weakness (progresses)
  • Flaccid paralysis (severe)
  • Respiratory muscle failure
  • Ganglia: initial tachycardia and hypertension
  • Progresses to bradycardia as parasympathetic dominates
  • Not reversed by atropine
  • Addressed by pralidoxime
Central Nervous System
  • Anxiety, agitation
  • Cognitive impairment
  • Seizures
  • Loss of consciousness
  • Central respiratory depression
  • Coma in severe poisoning
  • Atropine crosses blood–brain barrier: treats this component
  • Pralidoxime does NOT cross blood–brain barrier
1 Neuromuscular Junction Blockade Depolarizing paralysis of diaphragm and intercostal muscles
2 Bronchospasm Muscarinic receptor-mediated airway smooth muscle contraction
3 Bronchorrhea Muscarinic receptor-mediated gland hypersecretion flooding the airways
4 Central Respiratory Depression Central nervous system acetylcholinesterase inhibition suppressing the medullary respiratory center

Atropine addresses components 2, 3, and 4 (muscarinic and central nervous system). Mechanical ventilation is the only intervention that addresses all four simultaneously.

The Antidote Triad — Three Targets, Three Drugs
Atropine Competitive muscarinic receptor antagonist (all subtypes). Blocks bronchospasm, bronchorrhea, bradycardia, and other parasympathetic effects. Crosses the blood–brain barrier (tertiary amine). Titrate to secretion drying, NOT to heart rate.
Does NOT reverse neuromuscular junction paralysis. Does NOT reactivate acetylcholinesterase. Large doses required in severe poisoning.
Pralidoxime (2-PAM) Oxime nucleophile that reactivates phosphorylated acetylcholinesterase by displacing the phosphoryl group. Time-dependent: ineffective after aging. Addresses nicotinic effects at the neuromuscular junction. Does NOT cross the blood–brain barrier (quaternary compound).
Give as early as possible. Continue 24–48 hours in pesticide poisoning. Not indicated for carbamate poisoning.
Benzodiazepines Enhance gamma-aminobutyric acid type A receptor chloride conductance. Suppress seizures by hyperpolarizing neurons. Remain effective even after muscarinic blockade is complete, because late-phase seizures are glutamate-driven. Diazepam, lorazepam, or midazolam.
Treat promptly: delay allows progression to self-sustaining status epilepticus. Use for prophylaxis in moderate to severe poisoning, not only for active seizures.
Organophosphates vs. Carbamates — Key Differences
Feature Organophosphates Carbamates
Mechanism Phosphorylation of active site serine Carbamylation of active site serine
Aging Yes — adduct becomes permanently resistant to pralidoxime over time No — carbamyl adduct spontaneously hydrolyzes (30–120 min half-life)
Enzyme Recovery Only with pralidoxime (before aging) or synthesis of new enzyme Spontaneous, without antidote
Pralidoxime Indicated — give early and continue 24–48 hours Not indicated — may be harmful; enzyme recovers on its own
Atropine Required — titrate to secretion drying Required — same titration principle; dose requirements typically lower
Clinical Course Potentially prolonged; redistribution from tissue stores can cause relapse Shorter and more self-limited; symptoms typically resolve in 6–24 hours
Toxidrome Full cholinergic toxidrome Full cholinergic toxidrome (identical presentation)