Organophosphate insecticide poisoning

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"organophosphate poisoning" AND management

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Organophosphate Insecticide Poisoning

Overview

Organophosphates (OPs) are the most clinically significant class of insecticides worldwide. Since 1945, approximately 15,000 individual OP compounds have come into use. Globally, OP poisoning causes more than 150,000 deaths per year, with self-poisoning being the most common cause in low- and middle-income countries. Common agents include parathion, malathion, diazinon, acephate, chlorpyrifos, and dichlorvos. Chemical warfare nerve agents (sarin, soman, tabun, VX) are also organophosphate derivatives.

Mechanism of Toxicity

OPs inhibit acetylcholinesterase (AChE) and pseudocholinesterase (butyrylcholinesterase) by phosphorylating the serine hydroxyl group at the enzyme's active site. This prevents hydrolysis of acetylcholine (ACh), causing ACh to accumulate at:

Receptor Site	Location	Effect
Muscarinic (M)	Postganglionic parasympathetic	SLUDGE/DUMBELS syndrome
Nicotinic (N) - ganglionic	Sympathetic and parasympathetic ganglia	Tachycardia, hypertension
Nicotinic (N) - NMJ	Skeletal muscle	Fasciculations → paralysis
CNS	Brain/spinal cord	Seizures, coma

OPs are lipid soluble and absorbed via dermal, GI, and respiratory routes, allowing fat deposition and potential delayed toxicity.

Aging

A critical phenomenon is "aging" - the irreversible conformational change that occurs when OP remains bound to AChE for a prolonged period. Once aging occurs, the enzyme cannot be reactivated by oximes (pralidoxime). Aging speed varies: soman ages within minutes; many agricultural OPs take hours to days.

Clinical Features

1. Acute Cholinergic Crisis (onset: minutes to hours)

Muscarinic effects - remembered by the mnemonics SLUDGE or DUMBELS:

SLUDGE	DUMBELS
Salivation	Defecation
Lacrimation	Urination
Urination	Miosis
Defecation	Bradycardia/Bronchospasm/Bronchorrhea
GI cramps	Emesis
Emesis	Lacrimation
	Salivation/Sweating

Additional muscarinic effects: miosis (pinpoint pupils), bradycardia, bronchorrhea, bronchospasm.

Nicotinic effects (at NMJ and ganglia):

Muscle fasciculations, cramps, weakness
Tachycardia, hypertension (via ganglionic stimulation - can override bradycardia)
Diaphoresis
Progression to paralysis, areflexia

CNS effects:

Anxiety, restlessness, confusion
Seizures (NMDA receptor involvement + ACh accumulation)
Coma, respiratory depression

Respiratory failure - the main cause of death - results from a combination of:

Bronchospasm and bronchorrhea
Respiratory muscle paralysis (diaphragm)
Pulmonary edema (inflammatory mediators + increased vascular permeability)
CNS respiratory depression

A characteristic garlic-like or hydrocarbon odor may be present and aid diagnosis.

2. Intermediate Syndrome (24-96 hours after acute phase)

Occurs in up to 40% of patients following ingestion. Described by Senanayake and Karalliedde. Characterized by:

Weakness/paralysis of proximal limb muscles, neck flexors, cranial nerve-innervated muscles, and respiratory muscles
Respiratory paralysis may be fatal
No cholinergic excess signs at this stage
Does NOT respond to atropine or pralidoxime
Resolves within 2-3 weeks (up to 7 days in some cases)
EMG may assist diagnosis

3. Organophosphate-Induced Delayed Polyneuropathy (OPIDN) - 2-5 weeks after exposure

Distal symmetrical sensorimotor polyneuropathy (predominantly motor)
Caused by inhibition of neuropathy target esterase (NTE), not AChE
Progresses to muscle atrophy
In TOCP poisoning: signs of corticospinal damage (upper motor neuron) become apparent as neuropathy resolves
Does NOT respond to atropine or pralidoxime
Recovery is variable

4. Chronic Low-Level Exposure

Vague confusion, mild visual disturbances, chronic abdominal cramps, nausea, diarrhea
Symmetrical sensorimotor axonopathy in agricultural workers
Cognitive dysfunction, mood changes, autonomic dysfunction
Children are at greater risk due to smaller body size and lower baseline cholinesterase activity

Differential Diagnosis

Carbamate pesticide poisoning (similar syndrome, but aging does NOT occur and recovery is faster)
Carbamate medications (rivastigmine, physostigmine)
Nicotine toxicity
Cholinomimetics (pilocarpine, bethanechol)
Nerve agent exposure
Bacterial/viral gastroenteritis (milder cases)
Inferior MI with pulmonary edema (exaggerated vagal response)
Myasthenia gravis (for delayed neuropathy presentation)

Diagnosis

Diagnosis is primarily clinical - do not delay treatment waiting for labs.

Key clinical pointers:

History of OP exposure
Cholinergic toxidrome (SLUDGE/DUMBELS)
Pinpoint pupils + altered mental status + diaphoresis + respiratory distress

Laboratory:

Test	Clinical Role
Plasma butyrylcholinesterase	Decreases first in acute toxicity; easier to assay; less specific
RBC acetylcholinesterase	More accurate marker of synaptic cholinesterase inhibition; takes longer to recover (up to 12 weeks)
ABG / lactate	Assess respiratory failure, metabolic acidosis (higher mortality)
BMP, glucose	Both hyperglycemia and hypoglycemia associated with increased mortality
CBC, renal/hepatic panel	Evaluate end-organ function
ECG	QTc prolongation, bradycardia, arrhythmias

Note: Baseline cholinesterase activity varies widely between individuals, and there is poor standardization of normal ranges between laboratories.

Management

Treatment has four goals:

Decontamination
Supportive care (respiratory stabilization priority)
Reversal of ACh excess (atropine)
Reactivation of AChE (pralidoxime/oximes)

Step 1 - Decontamination

Remove patient from exposure source immediately
Remove all clothing (can contain up to 70% of dermal exposure)
Copious water irrigation of skin, eyes, mucous membranes
Healthcare workers must use PPE (gloves, gown, eye protection) to prevent secondary contamination
For GI ingestion: activated charcoal (1 g/kg) if airway is protected and within 1 hour of ingestion
No role for enhanced elimination, hemodialysis, or hemoperfusion

Step 2 - Airway and Supportive Care

Early intubation is critical - do not wait for complete respiratory failure
Succinylcholine has prolonged effect (cholinesterase inhibition), so prefer rocuronium for RSI
Mechanical ventilation for respiratory failure
Benzodiazepines for seizures (first-line; barbiturates second-line)
Aggressive IV fluid resuscitation
Treat cardiac arrhythmias (QT prolongation risk)

Step 3 - Atropine (Anticholinergic - muscarinic blockade)

Atropine is the primary antidote and life-saving treatment. It competes with ACh at muscarinic receptors only (does NOT reverse nicotinic effects/muscle weakness).

Dosing (Adults):

Initial: 2-4 mg IV every 5-10 minutes until secretions dry (not pupil dilation)
Severe poisoning: may require hundreds of milligrams over hours
Endpoint: Drying of bronchial secretions and reduction of bronchospasm - NOT pupil size or heart rate

Dosing (Children):

0.02-0.05 mg/kg IV

The Harriet Lane Handbook confirms atropine is used in combination with pralidoxime for OP poisoning.

Step 4 - Pralidoxime (2-PAM) - Oxime / Cholinesterase Reactivator

Pralidoxime reactivates phosphorylated cholinesterase before aging occurs. It reverses both muscarinic AND nicotinic effects (muscle weakness - the one thing atropine cannot address).

Key points:

Most effective when given within 24-48 hours of exposure (generally ineffective after 36-48 hours once aging is complete)
Must be given with atropine
Does NOT work for carbamate poisoning

Dosing (Adults):

IV intermittent: 1-2 g IV x1; may repeat in 1-2 hours, then every 10-12 hours PRN
IV continuous infusion preferred: 250-400 mg/hr

Dosing (Children):

IV: 20-50 mg/kg/dose (max 2000 mg) x1; may repeat in 1-2 hours, then Q10-12 hours
IV continuous: 20-50 mg/kg loading dose, then 10-20 mg/kg/hr (max 500 mg/hr)

Auto-injector (Duodote/ATNNA): 600 mg pralidoxime + 2.1 mg atropine per device, for field/EMS use in nerve agent and insecticide mass casualty events.

Caution: Rapid IV infusion of pralidoxime can cause muscle rigidity, laryngospasm, and tachycardia.

Benzodiazepines for Seizures

Diazepam or lorazepam IV - first-line
Phenytoin is NOT effective for OP-induced seizures
Barbiturates are second-line

Prognosis and Complications

Phase	Timing	Reversibility
Acute cholinergic crisis	Minutes-hours	Reversible with atropine + pralidoxime
Intermediate syndrome	24-96 hours	Supportive only; resolves 2-7 weeks
Delayed polyneuropathy	2-5 weeks	Variable; may be permanent

Predictors of poor prognosis:

Metabolic acidosis on presentation
Hyperglycemia or hypoglycemia
Severity of AChE inhibition
Aspiration pneumonia/lipoid pneumonia (from hydrocarbon solvents in insecticide preparations)
Delayed administration of antidotes

Key Summary Points

OPs are irreversible AChE inhibitors (unlike carbamates which are reversible)
Clinical triad of severe poisoning: altered mental status + pinpoint pupils + excessive secretions
Atropine (titrated to dry secretions) + pralidoxime (before aging) are the antidotes
Respiratory failure is the primary cause of death
Intermediate syndrome at 24-96 hours does not respond to antidotes - mechanical ventilation may be needed
Delayed neuropathy can emerge weeks later, caused by NTE inhibition
Children are more vulnerable due to lower baseline cholinesterase levels
Pralidoxime is largely ineffective after 36-48 hours once enzyme aging is complete

Sources:

Rosen's Emergency Medicine: Concepts and Clinical Practice, 9th Ed. (Chapter 152)
Tintinalli's Emergency Medicine: A Comprehensive Study Guide, 9th Ed. (Chapter 201)
Adams and Victor's Principles of Neurology, 12th Ed. (Chapter on Phosphorus and Organophosphate Poisoning)
The Harriet Lane Handbook, 23rd Ed. (Pralidoxime Chloride + Atropine dosing)
Katzung's Basic and Clinical Pharmacology, 16th Ed.

Recent Evidence Note: A 2025 systematic review (PMID: 38797902) examined glycopyrrolate as an alternative to atropine in OP/carbamate poisoning. Glycopyrrolate does not cross the blood-brain barrier, which may reduce CNS side effects, but its role remains investigational and atropine remains the standard of care.

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