Six patients who developed extrapyramidal manifesta tions following poisoning with the organophosphorus (OP) insecticide fenthion are reported. The extrapyramidal fea tures, in order of frequency, were dystonia, rest tremor, cog-wheel rigidity, and choreo-athetosis. The delay in onset of these signs, following poisoning, varied from 4 to 40 days, and they disappeared spontaneously in about 1 to 4 weeks in those who survived. The human extrapyrami dal system is rich in cholinergic neurons and acetyl cholinesterase (AChE). Inhibition of AChE by fenthion, which has ready access to central neurons on account of its lipid solubility, is postulated as the mechanism under lying the extrapyramidal manifestations.
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