Oximes For Organophosphorus Pesticide Poisoning: an Update For Emergency Physicians
Yiu Cheung Chan1
1AED, United Christian Hospital, Hong Kong
Organophosphorus pesticide (OP) poisoning is an important clinical problem, killing an estimated 200,000 people globally in each year. The reported case fatality was generally in range of 15-30%. OP compounds inhibit acetylcholinesterase (Ach) resulting in overstimulating of both muscarinic and nicotinic receptors in the nervous system. Standard management involves decontamination, supportive care, administration of the antidotes: atropine and oxime. The use of atropine is well established while the use of oxime is more controversial.
Oximes reactivate Ach, as shown by vitro studies, animal studies and observational human studies. Pralidoxime is the first and commonest used oxime worldwide. A second oxime, obidoxime, was developed in 1960s. The newer oximes, including trimedoxime, HI-6 and HLö-7, have been developed but not yet into clinical use.
In the 1950’s, the therapeutic use of pralidoxime for occupational parathion poisoning was first reported, its clinical effectiveness seemed clear. However, subsequent studies and experiences on its clinical efficacy yielded conflicting results; from beneficial, equivocal or even harmful. Theoretical and practical attributes to the diversities of clinical outcomes include different toxicities of OP, different efficacies of oxime in reactivate Ach, inadequate oxime dose, late administration or early termination of oxime therapy, the issue of “ageing”, non-Ach mediated toxic effects, and flaws in study design. On the other hand, there are clearly toxic effects of pralidoxime as noted in the RCTs and case reports; severe effects include arrhythmia, pulmonary edema, and respiratory arrest.
The latest Cochrane review and recent meta-analysis on the utility of oximes on OP poisoning conclude that the current evidence is insufficient to indicate whether oximes are harmful or beneficial. The routine use of oxime in the treatment of OP poisoning is not supported. It is unclear whether there is a subgroup of OP poisoned patient who will benefit clinically from oxime administration. Based on our knowledge of OP toxico-kinetic and toxico-dynamic, in vitro and animal data, it is reasonable to consider individually for the use of oxime, benefit−risk ratio is more likely to be favourable when it is given early, to patients with serious poisoning by diethyl OPs.
Besides, local experience on the use of oximes for OP poisoning in Hong Kong will be discussed.