OBJECTIVETo study the neuromuscular function and its relation with the occurrence of myasthenia in rats poisoned by dimethoate (D), phoxim (P), methomyl (M), M + D and M + P respectively.

METHODSThe stimulation single fiber electromyography(SSFEMG) at different stimulus frequencies(5, 10 and 20 Hz) was used. The whole blood cholinesterase (ChE) activity was measured 1 h before and after poisoning.

RESULTS(1) Myasthenia occurred in 5 out of 9.5 out of 10.5 out of 5, and 8 rats poisoned by D, P, M + D, and M + P, respectively. (2) The average mean consecutive differences(MCD) at 5, 10, and 20 Hz in myasthenic rats were significantly higher than those of poisoned rats without myasthenia and the control ones. (3) SSFEMG changes at 5, 10 and 20 Hz were significantly consistent with the clinical manifestation of myasthenia, especially at 10 Hz and 20 Hz. (4) ChE activity was significantly lower in rats with P or D poisoning while ChE inhibition was of no difference in rats with M, M + D, and M + P poisoning. In the D poisoning and P poisoning groups, there was no significant difference in ChE inhibition between the rats with and without myasthenia.

CONCLUSIONMuscle weakness was associated with neuromuscular transmission dysfunction, but not well correlated with ChE inhibition. The SSFEMG with stimulus frequency at 10 Hz or 20 Hz could be used to detect the neuromuscular dysfunction during myasthenia induced by organophosphate insecticides and their mixtures poisoning.