Organophosphate toxicity

Background

• Highly lipid soluble: absorbed via dermal, gastrointestinal or respiratory routes
• Binds acetylcholinesterase → accumulation of acetylcholine at receptor sites → cholinergic crisis
• Used as insecticides (malathion) and chemical warfare (sarin, VX)
• Over 100 regularly used organophosphate compounds today.

Clinical Features

• Sx caused by acetylcholine buildup in CNS and PNS.
• CNS Sx = headache, confusion, coma, vertigo
• Muscarinic Receptors
  • SLUDGE(M) = Salivation, Lacrimation, Urination, Diarrhea, GI pain, Emesis, Miosis
• Nicotinic Receptors (NMJ)
  • Muscle weakness, fasciculations, paralysis
• Common causes of death in OP toxicity
  • Killers B's = Bradycardia, Bronchorrhea, Bronchospasm

Intermediate Syndrome

• Syndrome that occurs 24-96 hours after acute cholinergic crisis.
• Proximal muscle weakness, cranial nerve palsies
• Can last for days - weeks
• May require mechanical ventilation

Diagnosis

Clinical diagnosis.

Blood tests such as RBC and plasma pseudocholinesterase levels are available, but little clinical utility.

Work-up

• CBC
  • May show leukocytosis
• Comprehensive Metabolic Panel
• CXR
  • Pulmonary edema in severe cases
• ECG
  • Ventricular dysrhythmias, torsades, QT prolongation, AV block

Differential Diagnosis

Weakness

• Neuromuscular weakness
  • Upper motor neuron:
    • CVA
    • Hemorrhagic stroke
    • Multiple sclerosis
    • Amyotrophic Lateral Sclerosis (ALS) (upper and lower motor neuron)
  • Lower motor neuron:
    • Spinal and bulbar muscular atrophy (Kennedy's syndrome)
  • Spinal cord disease:
    • Infection (Epidural abscess)
    • Infarction/ischemia
    • Trauma (Spinal Cord Syndromes)
    • Inflammation (Transverse Myelitis)
      • Acute flaccid myelitis
    • Degenerative (Spinal muscular atrophy)
    • Tumor
  • Peripheral nerve disease:
    • Guillain-Barre syndrome
    • Toxins (Ciguatera)
    • Tick paralysis
    • Diabetic peripheral neuropathy
  • Neuromuscular junction disease:
    • Myasthenia gravis crisis
    • Botulism
    • Organophosphate toxicity
    • Lambert-Eaton myasthenic syndrome
  • Muscle disease:
    • Rhabdomyolysis
    • Dermatomyositis
    • Polymyositis
    • Alcoholic myopathy
• Non-neuromuscular weakness
  • Can't miss diagnoses:
    • ACS
    • Arrhythmia/Syncope
    • Severe infection/Sepsis
    • Hypoglycemia
    • Periodic paralysis (electrolyte disturbance, K, Mg, Ca)
      • Hypokalemic periodic paralysis
      • Thyrotoxic periodic paralysis
    • Respiratory failure
  • Emergent Diagnoses:
    • Symptomatic Anemia
    • Severe dehydration
    • Hypothyroidism
    • Polypharmacy
    • Malignancy
    • Aortic disease - occlusion, stenosis, dissection
  • Other causes of weakness and paralysis
    • Acute intermittent porphyria (ascending weakness)

Chemical weapons

• Blister chemical agents (Vesicants)
  • Lewisite (L)
  • Sulfur mustard (H)
  • Phosgene oxime (CX)
• Pulmonary chemical agents (Choking agents)
  • Ammonia toxicity
  • Methyl isocyanate
  • Methyl bromide
  • Hydrochloric acid
  • Chlorine gas
  • Phosgene
• Incendiary agents
  • Agent orange
  • White phosphorus
• Cyanide chemical weapon agents (Blood agents)
  • Prussic acid (AKA hydrogen cyanide, hydrocyanic acid, or formonitrile)
• Nerve Agents (organophosphates)
  • Acetylcholinesterase inhibitors
  • Household and commercial pesticides (diazinon and parathion)
  • G-series (sarin, tabun, soman)
  • V-series (VX)
• Lacrimating or riot-control agents
  • Pepper spray
  • Chloroacetophenone
  • CS

Management

Decontamination

• Providers should wear appropriate PPE during decontamination.
  • Neoprene or nitrile gloves and gown (latex and vinyl are ineffective)
• Dispose of all clothes in biohazard container
• Wash patient with soap and water

Supportive Care

• IVF, O2, Monitor
• Aggressive airway management is of utmost importance.
  • Intubation often needed due to significant respiratory secretions / bronchospasm.
  • Use nondepolarizing agent (Rocuronium or Vecuronium)
  • Succinylcholine is absolutely contraindicated
• Benzodiazepines for seizures

Antidotes

• Dosing with atropine and pralidoxime are time dependent and provides ability to reverse symptoms while awaiting agent metabolism
• For exposure to nerve agents, manufactured IM autoinjectors are available for rapid administration:
  • Mark 1
    • Contains 2 separate cartridges: atropine 2 mg + 2-PAM 600 mg
    • Being phased out with newer kits
  • DuoDote
    • Single autoinjector containing both medications
    • Same doses as Mark 1: atropine 2 mg + 2-PAM 600 mg

Antidotes

Atropine

• First-line antidote — muscarinic antagonist; treats bronchorrhea, bronchospasm, bradycardia, and secretions
• Does NOT reverse nicotinic symptoms (weakness, fasciculations, paralysis)
• Starting dose: Atropine 1-2 mg IV (double q5min until atropinization) IV — May need 100+ mg in first 24h; endpoint is drying of secretions
• Pediatric: Atropine 0.02-0.05 mg/kg IV (min 0.1 mg), double q5min IV
• Doubling protocol: If inadequate response after 5 minutes, double the dose (1 → 2 → 4 → 8 → 16 mg...) until atropinization is achieved
• Massive doses may be required — total doses of 100+ mg in the first 24 hours have been reported
• Endpoints of adequate atropinization (goal of therapy):
  • Drying of bronchial secretions (most important endpoint)
  • Heart rate >80 bpm
  • Systolic BP >80 mmHg
• Do NOT target: Fully dilated pupils, absent bowel sounds, or HR >150 — these indicate atropine toxicity
• After initial atropinization: Consider atropine infusion (10-20% of loading dose per hour) to maintain effect
• Optimize oxygenation before giving atropine to reduce risk of dysrhythmias (though in resource-limited settings, do not withhold atropine waiting for oxygen)

Pralidoxime

• AKA 2-PAM
• Oxime that reactivates phosphorylated AChE → primarily reverses nicotinic symptoms (weakness, fasciculations, respiratory muscle paralysis)
• Must give atropine BEFORE pralidoxime to prevent worsening of muscarinic symptoms
• Must be given before aging occurs (see aging table above)
• Pralidoxime 1-2 g IV over 15-30 min, then 8-10 mg/kg/hr infusion (or repeat bolus in 1 hr) IV
• Pediatric: Pralidoxime 20-50 mg/kg IV, then 5-10 mg/kg/hr infusion IV
• Continue until clinical improvement or patient is off ventilator
• Controversies:
  • Evidence for benefit of pralidoxime is inconsistent; several meta-analyses have not shown clear mortality benefit when added to atropine
  • However, per AHA 2023 guidelines and expert consensus, oximes should still be given for significant OP poisoning, particularly when fasciculations, weakness, or paralysis are present
  • Efficacy depends on timing (before aging), dose, and the specific OP compound involved
• Caution: Administer slowly — rapid IV push can cause hypertensive crisis, cardiac arrest

Disposition

• Minimal exposure only requires decon and 6-8hr obs

See Also

• Toxidromes
• Atropine

References

• World Health Organization. Environmental Health Criteria No 63. Organophosphorus Pesticides: A General Introduction. World Health Organization, Geneva 1986
• Management of acute organophosphorus pesticide poisoning. Eddleston et al. Lancet. 2008 February 16; 371(9612): 597–607.
• Medical treatment of acute poisoning with organophosphorus and carbamate pesticides. Milan Jokanovi´c, Toxicology Letters 190 (2009) 107–115
• Tina Elersek and Metka Filipic (2011). Organophosphorous Pesticides - Mechanisms of Their Toxicity, Pesticides - The Impacts of Pesticides Exposure, Prof. Margarita Stoytcheva (Ed.), ISBN: 978-953-307-531-0, InTech, DOI: 10.5772/14020.