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Neurotoxic Disorders of Organophosphorus Compounds and Their Managements

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Neurotoxic Disorders of Organophosphorus Compounds and Their Managements Arch Iranian Med 2008; 11 (1): 65 – 89 Review Article Neurotoxic Disorders of Organophosphorus Compounds and Their Managements Mahdi Balali-Mood MD PhD•*, Kia Balali-Mood PhD** Organophosphorus compounds have been used as pesticides and as chemical warfare nerve agents. The mechanism of toxicity of organophosphorus compounds is the inhibition of acetylcholinesterase, which results in accumulation of acetylcholine and the continued stimulation of acetylcholine receptors. Therefore, they are also called anticholinesterase agents. Organophosphrus pesticides have largely been used worldwide, and poisoning by these agents, particularly in developing countries, is a serious health problem. Organophosphorus nerve agents were used by Iraqi army against Iranian combatants and even civilian population in 1983 – 1988. They were also used for chemical terrorism in Japan in 1994 – 1995. Their use is still a constant threat to the population. Therefore, medical and health professionals should be aware and learn more about the toxicology and proper management of organophosphorus poisoning. Determination of acetylcholinesterase and butyrylcholinesterase activity in blood remains a mainstay for the fast initial screening of organophosphorus compounds but lacks sensitivity and specificity. Quantitative analysis of organophosphorus compounds and their degradation products in plasma and urine by mass spectrometric methods may prove exposure but is expensive and is limited to specialized laboratories. However, history of exposure to organophosphorous compounds and clinical manifestations of a cholinergic syndrome are sufficient for management of the affected patients. The standard management of poisoning with organophosphorous compounds consists of decontamination, and injection of atropine sulfate with an oxime. Recent advances on treatment of organophosphorus pesticides poisoning revealed that blood alkalinization with sodium bicarbonate and also magnesium sulfate as adjunctive therapies are promising. Patients who receive prompt proper treatment usually recover from acute toxicity but may suffer from neurologic complications. Archives of Iranian Medicine, Volume 11, Number 1, 2008: 65 – 89. Keywords: Neurotoxic disorders • anticholinesterase agents • chemical warfare agents • organophosphate pesticides • poisoning Introduction the continued stimulation of acetylcholine rganophosphorus (OP) compounds have receptors. Therefore, they are also called been used as pesticides and also as anticholinesterase agents. Carbamates are also O chemical warfareArchive nerve agents.1 The ofanticholinesterase SID agents, but they are less toxic mechanism of toxicity of OP compounds is the than OP pesticides, thus are mainly used as home inhibition of acetylcholinesterase (AChE), insecticides and herbicides.2 resulting in an accumulation of acetylcholine and OP compounds are not ideal pesticides because of the lack of target vector selectivity, and severe Authors’ affiliations: *Medical Toxicology Center, Imam Reza Hospital, Mashhad University of Medical Sciences, Iran, toxicity and even death in humans and domestic **Structural Bioinformatics and Computational Biochemistry Unit, animals. Their toxicities have been recognized Department of Biochemistry, South Parks Road, The University of since the 1930s, when they were also developed Oxford, Oxford OX1 3QU, UK. 3 •Corresponding author and reprints: Mahdi Balali-Mood MD for use as chemical warfare agents. A few OP PhD, Medical Toxicology Center, Imam Reza Hospital, Mashhad compounds (glyphosate, merphos) are used as 91735-348, Iran. herbicides, but they are structurally different from Tel.: +98-511-859-8973, Fax: +98-511-859-1057, E-mail: [email protected]. the OP pesticides and their AChE-inhibiting power Accepted for publication: 6 June 2007 is very weak.4 OP nerve agents are more toxic than Archives of Iranian Medicine, Volume 11, Number 1, January 2008 65 www.SID.ir Neurotoxic effects of organophosphorus compounds and their managements OP pesticides and because of their high toxicity, developed and stockpiled as chemical warfare they are also called lethal agents.5 nerve agents during the last century.19 But its An important step in the synthesis of OP human exposure until the Iraq-Iran war was compounds was made by von Hofman, who restricted to one prospective study with VX and synthesized methylphosphor chloride in 1873. sarin at low levels and to case reports of accidental Michaelis in 1903 introduced a compound with P- exposure to sarin and soman.6 CN bond, which led to the synthesis of a number The first use of nerve agents in the war of insecticides and the nerve agent tabun. Lang and occurred in February 1984 in Majnoon Island by von Kreuger synthesized compounds with P-F the Iraqi army against the Iranian troops. Among linkage in 1932. Schrader developed sarin and the chemical warfare agents applied by the Iraqi tabun in 1937 and in 1944 Germans developed army, the nerve agent tabun was found in the soman. Britons developed VX in 1952.6, 7 environmental samples and in the postmortem Over the past five decades, numerous examination of the patients who died soon after the structurally different OP compounds have been exposure. More than 300 patients died within 30 synthesized.2, 8 Presently, more than 100 different min of exposure in the field and several thousands OP compounds are used worldwide as were poisoned by tabun.18 Toxicological analyses insecticides.9 The advantage of a lower of the blood, urine, skin, and gastric juice of the environmental stability, compared with chemical war gas victims revealed tabun and sulfur organochlorine compounds and a high mustard.18 Later in 1987 and 1988, particularly effectiveness against different insect species is during the Halabjah massacre, another nerve agent accompanied by the disadvantage of high (sarin) was also identified.1, 20 mammalian toxicity.10 The widespread use and A confirmed terrorism attack with sarin easy accessibility of these compounds result in a occurred in a residential area of the city of huge number of intoxications. It was estimated that Matsumoto, Japan, on June 27, 1994. About 600 between 750,000 and 3,000,000 human OP residents and rescue staff were poisoned; 58 were intoxications occurred worldwide,9, 11 resulting in admitted to hospital and 7 died.21 On March 20, several thousands of fatalities annually.12 1995, terrorists released sarin at several points in Occupational and accidental OP exposure are the the Tokyo subway, which killed 11 and poisoned main causes for mild poisonings,13 – 15 whereas more than 5,500 people.22, 23 severe cases are mostly due to suicidal attempts Based on the above information, OP and self-poisoning. 16, 17 compounds have induced tragedies with lots of In some parts of the developing world, pesticide human morbidities and mortalities. OP pesticides poisoning causes more deaths than infectious are still used in most parts of the world and diseases.11 Use of pesticides is poorly regulated and unfortunately are easily available in some often dangerous; their easy availability also makes developing countries. Thus occupational and them a popular method of self-poisoning. In 1985, accidental exposure and even intentional ingestion the UN Food and Agriculture Organization (FAO) are common and can induce health problems. In produced a voluntary code of conduct for the spite of the establishment of organization for pesticide industry in an attempt to limit the harmful prohibition of chemical weapons (OPCW) and its effects of pesticides.16 Unfortunately, the lack of active role in chemical warfare agents (CWA) adequate governmental resources in the developing control, OP nerve agents are still a big threat to the Archive16 of SID world makes this code ineffective. The World population worldwide as a chemical war or Health Organization (WHO) has recommended terrorism agents. Therefore, medical and health that access to highly toxic pesticides be restricted, professionals should be aware and learn more but it has not been applied in some developing about the toxicology and proper management of countries such as the Islamic Republic of Iran. OP OP poisoning. pesticides are still easily available in this country, thus their exposure either as occupational or Definition, classification, and chemical intentional oral ingestion are common and induce structures health problems as well as environmental OP compounds including organophosphates are 1, 18 chemically derived from phosphoric, phosphonic, hazards. 4 Several highly toxic OP compounds were phosphinic, and thiophosphoric acids. 66 Archives of Iranian Medicine, Volume 11, Number 1, January 2008 www.SID.ir M. Balali-Mood, K. Balali-Mood Figure 1. General chemical structure of an organophosphate. R1 and R2 are alkyl-, alkoxy-, alkylthio-, or amido-groups. X is the acyl residue (labile fluorine-, cyano-, substituted- or branched- aliphatic, aromatic, or heterocyclic groups). Organophosphates are usually esters, amides, or thiol derivatives of phosphoric, phosphonic, or phosphinic acids, which have the general structural Figure 2. Chemical structure of nerve agents. formula as shown in Figure 1.24 OP compounds are divided into two main ing organophosphate compounds. The principal G groups; pesticides and chemical warfare nerve agents; GA, GB, and GD have common names of agents. Very few OP compounds such as tabun, sarin, and soman, respectively. The other G glyphosate and merphos have been used as agents and V agents do not have common names. herbicides.
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