Which organophosphorus compounds are associated with poisoning?

Organophosphorus Compounds Causing Poisoning

Organophosphorus compounds causing poisoning include pesticides (malathion, dichlorvos, chlorpyrifos, diazinon, profenofos, azinphos-methyl, methamidophos, terbufos, dimethoate, fenitrothion, isoxathion, pyridaphenthion, trichlorfon), nerve agents (tabun, sarin, soman, VX), and some medications, all of which irreversibly inhibit acetylcholinesterase leading to potentially fatal cholinergic toxicity. 1

Pesticide Organophosphates

Commonly Implicated Agricultural Compounds

• Malathion, dichlorvos, chlorpyrifos, and azinphos-methyl are among the most frequently used OP pesticides in the United States and globally, licensed before human toxicity evaluation requirements were established 1
• Metacid (methyl parathion) and Nuvan (dichlorvos) are the most commonly ingested OP pesticides in deliberate self-poisoning cases, particularly in Nepal and South Asia 2
• Profenofos, dimethoate, and chlorpyrifos represent other frequently encountered compounds in poisoning cases 3, 2
• Fenitrothion, malathion, isoxathion, pyridaphenthion, and trichlorfon have been documented in human suicide attempts with varying toxicity profiles 4
• Terbufos and methamidophos are highly hazardous OP pesticides imported in large quantities into developing countries, targeted for phaseout by the Rotterdam Convention 1

Global Burden

• Over 40 OP pesticides are classified by the US EPA and WHO as moderately or highly hazardous to human health 1
• OP pesticides account for approximately 200,000 deaths annually worldwide, with 99% occurring in developing countries 1
• These compounds represent 110,000 pesticide self-poisoning deaths per year globally, accounting for 13.7% of all suicides on average 1

Nerve Agent Organophosphates

Military/Terrorist Compounds

• Tabun (GA), sarin (GB), and soman (GD) are fluorinated cyanide-containing organophosphates known as "G-agents," originally synthesized as nerve gas agents during the 1930s-1940s 1
• VX is a sulfur-containing organophosphate ("V-agent") that is less volatile, more persistent, and constitutes primarily a liquid contact hazard 1
• VX has an LCt50 500 times less than hydrogen cyanide, making it among the most lethal compounds known to mankind 1
• Nerve agents are colorless, odorless volatile liquids whose vapors are heavier than air and gravitate to the ground 1

Mechanism of Toxicity

Acute High-Level Exposure Effects

• Irreversible inhibition of acetylcholinesterase (AChE) leads to accumulation of acetylcholine and cholinergic syndrome 1, 2
• Muscarinic effects include narrowed pupils (miosis), excessive salivation, bronchoconstriction, bronchorrhea, bradycardia, hypersalivation, lacrimation, urination, diarrhea, vomiting, and diaphoresis 1
• Nicotinic effects include tachycardia, mydriasis, muscle fasciculations progressing to depolarizing neuromuscular blockade and paralysis 1
• CNS effects include mental confusion, altered mental status, central apnea, convulsions, tremors, seizures, and in severe cases, death 1
• Delayed polyneuropathy has been described with high exposures 1

Chronic Low-Level Exposure Effects

• Neurodevelopmental toxicity occurs at exposure levels below those causing AChE inhibition, particularly affecting fetal brain development 1
• The US EPA concluded in 2016 that neurodevelopmental effects occur at chlorpyrifos exposure levels insufficient to cause acetylcholinesterase inhibition 1
• AChE inhibition is uninformative regarding neurodevelopmental effects in children, as toxic effects from chronic low-level exposure occur at concentrations too low to inhibit cholinesterase 1

Clinical Presentation Patterns

Route-Dependent Manifestations

• Vapor/respiratory exposure produces early respiratory symptoms (shortness of breath, wheezing, bronchorrhea) as muscarinic effects, followed by progressive muscle weakness leading to cardiorespiratory collapse within minutes 1
• Dermal exposure leads to early local muscle twitching (nicotinic effects), progressing to fasciculations, weakness, and delayed complete paralysis with more gradual respiratory symptom progression 1
• Respiratory failure ultimately develops and leads to death in either exposure route 1

Severity Assessment

• Pseudocholinesterase levels are traditionally used but do not correlate well with clinical outcomes; LC-MS/MS quantification of actual OP compound levels better predicts morbidity and mortality 3
• Maintaining AChE activity above 15-20% is sufficient to restore respiratory function and alleviate symptoms to baseline levels 5

Critical Clinical Pitfall

The absence of overt acute poisoning symptoms does not indicate absence of neurologic damage, particularly in pregnant women and children exposed to chronic low-level OP pesticides, as developmental deficits may not manifest until months or years later 1