Organophosphates are commonly used pesticide agents and represent a frequent cause of severe accidental and intentional poisoning in children and adolescents, particularly in developing nations.
Pathophysiology
Organophosphates exert their toxic effects by binding to and inhibiting the enzyme acetylcholinesterase (AChE) in the peripheral and central nervous systems.
This inhibition prevents the normal degradation of the neurotransmitter acetylcholine (ACh), leading to an excessive accumulation of ACh at both nicotinic and muscarinic synaptic receptors.
Initially, the bond between the organophosphate and the enzyme is reversible, but over a variable period, it undergoes a conformational change and becomes irreversible, a process referred to as "aging".
Once aging occurs, the enzyme is permanently inactivated, and normal neurological function requires the synthesis of new AChE, which can take weeks to months.
Clinical Manifestations
The severity and onset of symptoms depend upon the route of exposure, the duration of exposure, and the absorbed dose.
Acute organophosphate poisoning is classically characterized by three distinct clinical phases: acute cholinergic crisis, intermediate syndrome, and delayed neuropathy.
The acute cholinergic crisis manifests with a classic toxidrome reflecting muscarinic, nicotinic, and central nervous system (CNS) overstimulation.
Receptor Type
Clinical Features and Toxidromes
Muscarinic Effects
Manifests as the DUMBBELS toxidrome: Diarrhea/defecation, Urination, Miosis, Bronchorrhea/bronchospasm, Bradycardia, Emesis, Lacrimation, and Salivation.
Nicotinic Effects
Characterized by muscle weakness, fasciculations, tremors, hypoventilation (due to diaphragm weakness), hypertension, tachycardia, and dysrhythmias.
Central Nervous System Effects
Includes lethargy, agitation, confusion, seizures, coma, and respiratory depression.
Following the initial cholinergic crisis, some patients may develop an "intermediate syndrome," which is characterized by the delayed onset of respiratory failure requiring mechanical ventilation.
Late complications include delayed polyneuropathy and a range of chronic neuropsychiatric symptoms.
Diagnosis
The diagnosis of organophosphate poisoning is primarily clinical, based on the characteristic cholinergic signs and the presence of a garlic-like odor from the patient or their bodily fluids.
Laboratory confirmation is achieved by demonstrating reduced butyrylcholinesterase activity in plasma or reduced acetylcholinesterase activity in the blood.
Enzyme activity levels measuring of normal values indicate severe poisoning.
Emergency treatment must be initiated immediately based on clinical suspicion and should never be delayed while awaiting the results of cholinesterase assays.
Management
The management of organophosphate toxicity is a medical emergency that involves immediate resuscitation, thorough decontamination, and the expeditious administration of specific antidotes.
Management Step
Specific Interventions
Initial Stabilization (ABCs)
Check airway, breathing, and circulation. Provide oxygen and mechanical ventilation as indicated for respiratory failure or excessive secretions. Avoid using succinylcholine for rapid sequence intubation, as it is metabolized by the same inhibited cholinesterase enzymes and will cause prolonged paralysis.
Decontamination
Immediate dermal and ocular decontamination is critical. Remove all clothing and thoroughly wash the exposed skin with soap and water. Activated charcoal or gastric lavage is generally not beneficial as these liquid agents are absorbed very rapidly.
Atropine Therapy
Atropine acts as an antidote by competitively antagonizing muscarinic acetylcholine receptors. Administer an initial intravenous (IV) or intraosseous (IO) bolus of mg/kg. Assess pupil size, sweating, heart rate, and blood pressure. If there is no improvement, repeat boluses of mg/kg every 5 to 10 minutes. The therapeutic endpoint (atropinization) is reached when the heart rate is appropriate for age, systolic blood pressure is centile, and chest auscultation is clear of bronchorrhea. Once stable, initiate a continuous IV infusion of atropine at of the cumulative bolus dose per hour.
Pralidoxime (PAM) Therapy
Pralidoxime facilitates the reactivation of acetylcholinesterase by breaking the bond between the enzyme and the toxin before "aging" occurs. Administer a loading dose of mg/kg IV over 20-30 minutes. Follow with a continuous infusion of mg/kg/hr in saline. PAM infusion should continue until atropine is no longer required for 12-24 hours and the patient is successfully extubated.
Supportive Care
Provide sedation with benzodiazepines to manage atropine-induced agitation or organophosphate-induced seizures. Continuously monitor fluid balance, electrolytes, and cardiovascular status.