Carbaryl poisoning and edrophonium: electrophysiologic changes

A case of poisoning with the insecticide carbaryl, a cholinesterase inhibitor, was studied using repetitive stimulation and edrophonium administration. Without edrophonium, repetitive stimulation studies were normal. After 5mg of edrophonium was administered intravenously, a 40% decrement was recorded using 3Hz stimulation and an 80% decrement was seen with 30Hz stimulation. It is concluded that edrophonium is useful in testing for poisoning with cholinesterase inhibitors when other electrophysiologic studies are normal. These changes are most likely due to a recently described fast desensitization of the acetylcholine (ACh) receptor induced by high ACh concentrations.