Electromyography in relation to end-plate acetylcholinesterase in rats poisoned by different organophosphates

Organophosphate (OP) poisoning produces various forms of acute, subacute, or delayed neurotoxicity. We investigated in vivo the relationship between clinical, histochemical and electromyographic (EMG) parameters in rats at various stages of poisoning by paraoxon or fenthion. Paraoxon is acutely toxic, whereas fenthion produces more sustained AChE inhibition. Fenthion has been involved in a subacute type of OP-related neurotoxicity in patients, the so-called intermediate syndrome. The animals underwent serial EMGs, with single and repetitive nerve stimulation, and concomitant contralateral muscle biopsies to determine the end-plate acetylcholinesterase (AChE) activity. Repetitive activity (RA) after single nerve stimulation and decrements on repetitive nerve stimulation (RNS) were the major EMG findings in either type of poisoning, occurring in the initial and later stages of the poisoning, respectively. RA was highly correlated to fasciculations in acute, but not in prolonged intoxication. Amplitude decrements provoked by RNS occurred only in weak rats with severe end-plate AChE inhibition. The smallest amplitude occurred either at the second response with gradual improvement in the subsequent responses (decrement-increment phenomenon), or the amplitude decrease progressed up to the last response (decrement phenomenon). The decrement-increment phenomenon preceded the decrement phenomenon and occurred at a slightly less severe degree of AChE inhibition. Various types of impairment of neuromuscular transmission coexist, probably to a different extent at distinct stages of anticholinesterase poisoning.