Acute exogenous poisoning with organophosphorus compounds
Organic phosphorus compounds (OPC) are most commonly found in pesticides. They can be phosphoric acid esters or thiophosphoric acid esters and are common in the world. They have also been used as chemical warfare nerve agents and in terrorist attacks.
The main mechanism of toxicity is through inhibition of acetylcholinesterase in neuronal synapses leading to excess acetylcholine and overstimulation in the peripheral nervous system.
Image: “Cholinergic synapse” by Smedlib , based on original work by Pancrat, CC BY-SA 4.0, via Wikimedia Commons.
Acute symptoms of OPC toxicity are:
- Muscarinic symptoms - bronchospasm, bronchorrhea, bradycardia, diarrhea, urination, miosis, emesis and salivation
- Nicotinic symptoms - paralysis, fasciculations
- CNS toxicity (rarer) - seizures, altered mental status, apnea
- Treat fast!!!
There are also chronic symptoms of OPC toxicity, these are the so called organophosphorus induced delayed neuropathy (OPIDN), characterised by bilateral paralysis of the distal muscles (mostly lower extremities), 7-10 days after ingestion.
Diagnosis of OPC toxicity is through the measurement of serum cholinesterase activity, which decreases. Additional tests are FBCs, U&Es, and ECG (expect prolonged QT interval, elevation of ST, sinus tachy, sinus brady and in rare cases complete heart block may all occur).
Treatment is ABC, oral activated charcoal is a reasonable intervention after GI poisoning if ingestion was within the last hour. Atropine, pralidoxime (2-PAM), and benzodiazepines, are the mainstays of pharmaceutical treatment, beginning with the adequate use of atropine. Large doses of atropine are often needed, start at 2-5mg IV dose and then work up! This helps to prevent reflex bradycardia and control heart rate. In severe cases, doses far above 16mg may be required (no maximum: titrate until bronchial secretions are dry).