Autonomic Pharmacology — Organophosphate Poisoning

Organophosphate posisoning

SOURCE – insecticides, exposure to nerve agents (most commonly seen in farming settings)

PATHOPHYSIOLOGY

• Organophosphates form a covalent bond with acetylcholinesterase, inhibiting the metabolism of acetylcholine.
• This results in high levels of Ach at the neuromuscular junction (both nicotinic and muscarinic receptors), causing a cholinergic crisis

SYMPTOMS

• SLUDGE (salivation, urination, defaecation, gastrointestinal motility, emesis)
• CNS – confusion, anxiety, tremors, seizures
• CVS – bradycardia, hypotension
• RESP – respiratory impairment due to muscle weakness, difficulty clearing secretions

MANAGEMENT

• Remove the patient from the source of poisoning
• PPE for staff – mask, goggles, neoprene gloves and gowns rather than latex as hydrocarbons can penetrate latex
• ABC’s and resuscitation as required
• Decontamination –
  • activated charcoal / gastric lavage / whole bowel irrigation unlikely to be effective due to rapid absorption of organophosphates.
  • Clothes should be removed and discarded and the patient bathed with soap and water to remove organophosphates from skin.
  • Irrigate the eyes if ocular exposure has occurred.
• Symptom control
  • Anti muscarinic agents –
    • atropine in large doses will reduce the antimuscarinic effects by competitively antagonizing Ach at the receptor.
    • ETG suggests 1.2-3mg boluses, aiming SBP>80mmHg, HR >80bpm and absence of wheeze, doubling the dose every 5 mins until atropinisation is achieved, followed by an infusion.
    • Glycopyrrolate can be used to reduce secretions post initial atropinisation, however does not cross the BBB so will have no effect on the central cholinergic effects
  • There is no antinicotinic agent so supportive care only for muscle weakness.
  • Benzodiazepines for seizure control
• Definitive treatment
  • Cholinesterase regenerator (an oxime such as pralidoxime) may reactive AchE and prevent the organophosphate-acetylocholinesterase complex ageing and becoming an irreversible bond if given within 48 hrs