Chapter 5NERVE AGENTSFREDERICK R. SIDELL, M.D.* |
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Nerve agents are extremely toxic chemicals that were first developed in secrecy before and during World War II primarily for military use. Related substances are used in medicine, in pharmacology, and for other purposes, such as insecticides, but they lack the potency of the military agents. Much of the basic knowledge about the clinical effects of nerve agents comes from research performed in the decades immediately following World War II. The military stockpiles of several major powers are known to include nerve agents, and the armamentaria of other countries are thought to contain them as well (see Chapter 4, The Chemical Warfare Threat and the Military Healthcare Provider). Because of the possibility of nerve agent use in future conflicts, military medical personnel should have some knowledge of these agents, their effects, and the proper therapy for treating casualties. Possibly the earliest recorded use of a substance that works, like nerve agents, by inhibiting cholinesterase (ChE) is by native tribesmen of western Africa who used the Calabar bean as an “ordeal poison” in witchcraft trials.1,2 An extract, “the elixir of the Calabar bean,” was later used medicinally,3 and in 1864, the active principle was isolated by Jobst and Hesse and called physostigmine.1 Vee and Leven independently isolated this same substance in 1865 and named it eserine,1 hence its dual nomenclature. The first organophosphorus ChE inhibitor was probably tetraethyl pyrophosphate (TEPP), synthesized by Wurtz and tasted (with no ill results) by Clermont in 1854.4 During the next 80 years, chemists (such as Michaelis, Arbusow, and Nylen) made numerous advances in organophosphorus chemistry, but generally they did not realize the toxicity of the substances with which they were working.4 In the early 1930s, interest in both physostigmine-type (reversible) and organophosphorus-type (irreversible) ChE inhibitors increased. (The terms “reversible” and “irreversible” refer to the duration of binding of the compound with the enzyme ChE; see the Mechanism of Action section below.) The reversible type, most of which are carbamates, were developed for treating conditions such as intestinal atony, myasthenia gravis, and glaucoma; for example, treating gastric atony with neostigmine was described in 1931.1 Five organophosphorus compounds are generally regarded as nerve agents. They are commonly known as tabun (North Atlantic Treaty Organization [NATO] military designation, GA), sarin (GB), and soman (GD); and GF and VX (also NATO military designations; these compounds have no common names). The agents in the “G” series allegedly were given the code letter G because they originated in Germany; the “V” allegedly stands for venomous. GF is an old agent, previously discarded by the United States as being of no interest. During the Persian Gulf War, it was believed that Iraq might have GF in its arsenal; however, interest has waned again and GF has retreated to obscurity. Lange and Krueger reported on the marked potency of organophosphorus compounds in 1932 after noting the effects of the vapors of dimethyl and diethyl phosphorofluoridate on themselves.1,4 Shortly thereafter, the German company I. G. Farbenindustrie developed an interest in organophosphorus compounds as insecticides. On 23 December 1936, Gerhard Schrader, who headed the company’s research effort, synthesized what today is known as tabun.5,6 Like Lange and Krueger, he noted the toxicity (miosis and discomfort) of the vapors of the substance in himself. Over a year later, Schrader synthesized a second organophosphorus compound and named it sarin in honor of those who were instrumental in its development and production: Schrader, Ambros, Rudriger, and van der Linde.5 Because the German Ministry of Defense required that substances passing certain toxicity tests be submitted to the government for further investigation, these compounds were examined for possible military use. The potential of tabun and sarin as weapons was soon realized. A large production facility was built in Dyhernfurth and production of tabun was begun in 1942.5,6 Sarin was also produced in Dyhernfurth and possibly at another plant in Falkenhagen.6 Late in World War II, Soviet troops captured the Dyhernfurth facility (then in Germany, now in Poland), dismantled it, and moved it, along with key personnel, to the former Soviet Union, where production of the agents commenced in 1946.6 |
| About 10,000 to 30,000 tons of tabun and
smaller quantities of sarin were produced and put into munitions by the
Germans during World War II, but these weapons were never used.6
Why they were not remains a matter of conjecture. In the waning days of World War II, troops of the United States and the United Kingdom captured some of these munitions, which were being stored at Raubkammer, a German testing facility. The munitions, which contained an agent unknown to scientists in the United Kingdom and the United States, were taken to the two countries for examination. Over a single weekend, a small group of scientists at the U.K. Chemical Defence Establishment, working despite miosis caused by accidental exposure to the agent vapor, elucidated the pharmacology and toxicity of tabun and documented the antidotal activity of atropine.7 Thus, during the latter part of World War II, Germany possessed chemical weapons against which its foes had little protection and no antidotes. Use of these weapons probably would have been devastating and might have altered the outcome of that conflict. The Germans had tested nerve agents on inmates of concentration camps, not only to investigate their intoxicating effects but also to develop antidotes.8 Many casualties, including some fatalities, were reported among the plant workers at Dyhernfurth; the medical staff there eventually developed antidotal compounds.5 The Allies were unaware of these German experiments until the close of the war, months after the initial U.K. studies.7 Soman was synthesized in 1944 by Richard Kuhn of Germany, again in a search for insecticides.6 Small amounts were produced, but development had not proceeded far by the end of the war. The nerve agent VX was first synthesized by an industrial concern in the United Kingdom in the early 1950s6 and was given to the United States for military development. Other potential nerve agents were synthesized by scientists in the United States and United Kingdom but were not developed for military use. For example, GF, which may have been first synthesized about 1949 by a chemist in another country in the search for other nerve agents, was studied in both the United States and the United Kingdom. It was then discarded for reasons that are not entirely clear. Possible explanations are that it was too expensive to manufacture or that there was no perceived need for an agent with its properties. The manufacturing process for GF is apparently similar to that for GB. During the Persian Gulf War (1990–1991), Iraq was believed to have switched from the manufacture of GB to the manufacture of GF when the precursors of GB, but not those of GF, were embargoed. The United States began to produce sarin in the early 1950s, and VX in the early 1960s, for potential military use; production continued for about a decade.6 The U.S. munitions inventory today contains these two nerve agents in 30- to 45-year-old M55 rockets; land mines; 105-mm, 155-mm, and 8-in. projectiles; 500-lb and 750-lb bombs; wet-eye bombs (one of a family of “eye” bombs, which has liquid chemical [wet] contents); spray tanks; and bulk containers.9 These munitions are stored at six depots within the continental United States (CONUS) and one outside the continent; the locations of these depots are public knowledge.10 The six CONUS depots are near Tooelle, Utah; Umatilla, Oregon; Anniston, Alabama; Pine Bluff, Arkansas; Newport, Indiana; and Richmond, Kentucky; the seventh depot is on Johnston Island in the Pacific Ocean. Sarin has also been used in terrorist attacks. In June 1994, members of a Japanese cult released sarin in an apartment complex in Matsumoto, Japan. Although there were almost 300 casualties, including 7 dead, this event was not well publicized. On 20 March 1995, sarin was released on Tokyo subways. More than 5,500 people sought medical care; about 4,000 had no effects from the agent but 12 casualties died. This incident required a major expenditure of medical resources to triage and care for the casualties. (Also see Chapter 1, Overview: Defense Against the Effects of Chemical and Biological Warfare Agents). Cholinesterase in Tissue Nerve agents are compounds that exert their biological effects by inhibition of the enzyme acetyl-cholinesterase (AChE), according to the current, widely accepted explanation. Some other compounds cause similar effects by the same mechanism and, in a broad sense, can also be considered nerve agents. Acetylcholinesterase belongs to the class of enzymes called esterases, which catalyze the hydrolysis of esters. ChEs, the class of esterases to which AChE belongs, have high affinities for the esters of choline. Although there are several types of choline esters, acetylcholine (ACh), the neurotransmitter of the cholinergic portion of the nervous system, is most relevant to nerve agent activity. The enzyme AChE, found at the receptor sites of tissue innervated by the cholinergic nervous system, hydrolyzes ACh very rapidly: it has one of the highest enzyme turnover numbers (number of molecules of substrate that it turns over per unit time) known.11 A similar enzyme having ACh as its preferred substrate is found in or on erythrocytes (red blood cells, RBCs) and is known as erythrocyte, or true, ChE (RBC-ChE). Butyrocholinesterase (BuChE, also known as serum or plasma cholinesterase, and as pseudocholinesterase), another enzyme of the ChE family, has butyrylcholine as its preferred substrate. Butyrylcholine is present in plasma or serum and in some tissues. BuChE and RBC-ChE are discussed in the Blood Cholinesterases section below. |
| Cholinesterase-Inhibiting Compounds Most ChE-inhibiting compounds are either carbamates or organophosphorus compounds. Among the carbamates is physostigmine (eserine; elixir of the Calabar bean), which has been used in medicine for more than a century.3 Neostigmine (Prostigmin, manufactured by ICN Pharmaceuticals, Costa Mesa, Calif.) was developed in the early 1930s for management of myasthenia gravis; ambenonium was developed later for this same purpose. Pyridostigmine bromide (Mestinon, manufactured by ICN Pharmaceuticals, Costa Mesa, Calif.) has been used for decades for the management of myasthenia gravis. The military of the United States and several other nations also field pyridostigmine bromide (manufactured by Phillips Duphar, Holland), known as PB or NAPP (nerve agent pyridostigmine pretreatment), as a pretreatment, or antidote-enhancing substance, to be used before exposure to certain nerve agents (see Chapter 6, Pretreatment for Nerve Agent Exposure). Today these carbamates are mainly used for treating glaucoma and myasthenia gravis. Other carbamates, such as Sevin (carbaryl, manufactured by Techne, St. Joseph, Mo.), are used as insecticides. Most commonly used insecticides contain either a carbamate or an organophosphorus compound. The organophosphorus insecticide malathion has replaced parathion, which was first synthesized in the 1940s. The organophosphorus compound diisopropyl phosphorofluoridate (DFP) was synthesized before World War II and studied by Allied scientists before and during the war, but was rejected for use as a military agent. For a period of time, this compound was used topically for treatment of glaucoma but later was rejected as unsuitable because it was found to produce cataracts. It has been widely used in pharmacology as an investigational agent. Mechanism of Action Nerve agents inhibit ChE, which then cannot hydrolyze ACh. This classic explanation of nerve agent poisoning holds that the intoxicating effects are due to the excess endogenous ACh. This explanation, however, may not account for all nerve agent effects. Research suggests that other nerve agent actions may contribute to toxicity. For example, ChE inhibitors inhibit enzymes other than ChE; the effect of this inhibition of additional enzymes on nerve agent toxicity may be significant.12 Concentrations of ChE inhibitors that are severalfold higher than lethal concentrations produce direct effects on receptor sites by blocking conductance through the ion channel or by acting as agonists at the channel complex.13 While these findings offer hope that better means of therapy will be developed in the future, their relevance to clinical effects is not clear at this time. A detailed discussion of the chemistry of ChE inhibition is beyond the scope of this chapter and can be found in most textbooks of pharmacology (eg, see Koelle11). The relevant aspects are summarized here. The efferents of the human nervous system can be subdivided according to the neurotransmitter released. The adrenergic nervous system, for which the neurotransmitter is adrenaline (epinephrine) or, more correctly, noradrenaline (norepinephrine), comprises one large subsection. Other, less prominent efferent tracts have g-aminobutyric acid (GABA), dopamine, or some other substance as the neurotransmitter. The cholinergic nervous system, a second major subdivision, has acetylcholine as the neurotransmitter. Acetylcholine is the neurotransmitter of the neurons to skeletal muscle, of the preganglionic autonomic nerves, and of the post-ganglionic parasympathetic nerves. Exogenous ACh causes stimulation of the muscles and other structures innervated by these fibers. This portion of the cholinergic nervous system can be further subdivided into the muscarinic and nicotinic systems, because the structures that are innervated have receptors for the alkaloids muscarine (mAChR) and nicotine (nAChR), respectively, and can be stimulated by these compounds. Muscarinic sites are innervated by postganglionic parasympathetic fibers. These sites include glands (eg, those of the mouth and the respiratory and gastrointestinal systems), the musculature of the pulmonary and gastrointestinal systems, the efferent organs of the cranial nerves (including the heart via the vagus nerve), and other structures. Nicotinic sites are at the autonomic ganglia and skeletal muscles. |
The production of a response in an organ to a neuromediated impulse consists of several stages. First, the impulse travels down a nerve to the axonal terminal, or presynaptic area, creating an action potential. (This action potential consists of a change in the resting potential of the polarized nerve membrane.) At the prejunctional area, the action potential stimulates the release of the neurotransmitter ACh from storage in synaptic vesicles. The ACh diffuses across the synaptic cleft and combines with specialized areas—the receptor sites—on the postsynaptic membrane to produce a postsynaptic potential, which may be either a depolarization or a hyperpolarization of the membrane. The postsynaptic activity thus initiated is a contractile response in muscle or secretion in a gland. (Events in the central nervous system [CNS] are less clear.) Following each impulse, the neurotransmitter is destroyed to prevent further postsynaptic potentials (Figure 5-1). |
In the cholinergic portion of the
nervous system, ChE hydrolyzes the neurotransmitter ACh to terminate its
activity at the receptor site (Figure 5-2). Acetylcholine attaches to
two sites on the ChE enzyme: the choline moiety to the anionic site and
the acetyl group to the esteratic site. The choline splits off, leaving
the acetylated esteratic site, which then reacts very quickly with water
to form acetic acid and regenerated, or reactivated, enzyme.
If AChE were absent from the site, or if it were unable to function, ACh would accumulate and would continue to produce postsynaptic action potentials and activity in the organ. The nerve agents and other ChE-inhibiting substances produce biological activity by disabling (or inhibiting) AChE, an action that leads to an accumulation of ACh. The biological activity, or toxicity, of ChE inhibitors is due to this excess endogenous ACh, which is not hydrolyzed. The compounds in the two major categories of AChE inhibitors, carbamates and organophosphorus compounds, also attach to the ChE enzyme. There are some differences, however, between them and the natural substrate ACh. Carbamates attach to both the esteratic and the anionic sites. A moiety of the carbamate is immediately split off, leaving the enzyme carbamoylated at the esteratic site. Instead of hydrolysis occurring at this site within microseconds, as it does with the acetylated enzyme, hydrolysis does not occur for minutes to hours, and the enzyme remains inactive or inhibited for about 1 hour after reacting with physostigmine and 4 to 6 hours after reacting with pyridostigmine. Most organophosphorus compounds combine with the ChE enzyme only at the esteratic site, and the stability of the bond (ie, the interval during which the organophosphorus compound remains attached) depends on the structure of the compound. Hydrolytic cleavage of the compound from the enzyme may occur in several hours if the alkyl groups of the organophosphorus compound are methyl or ethyl, but if the alkyl groups are larger, cleavage may not occur. Thus, the phosphorylated form of the enzyme may remain indefinitely; in this case, return of enzymatic activity occurs only with the synthesis of new enzyme. Since most of these compounds attach to the esteratic site on AChE, a second binding compound cannot attach on that site if the site is already occupied by a molecule. Thus a previously administered ChE inhibitor will, in a manner of speaking, protect the enzyme from a second one.14,15 This activity forms the pharmacological basis for administering a carbamate (pyridostigmine) before expected exposure to some nerve agents to provide partial protection (lasting 6–8 h) against the more permanently bound nerve agents. (This mode of protection is described in more detail in Chapter 6, Pretreatment for Nerve Agent Exposure). Because of the different lengths of time required for carbamates and organophosphorus compounds to be hydrolyzed from the enzyme, they are sometimes referred to, respectively, as reversible and irreversible inhibitors. After inhibition by irreversibly bound inhibitors, recovery of the enzymatic activity in the brain seems to occur more slowly than that in the blood ChE.16,17 However, one individual severely exposed to sarin was alert and functioning reasonably well for several days while ChE activity in his blood was undetectable (Exhibit 5-1).18 This case study and other data suggest that tissue function is restored at least partially when ChE activity is still quite low. |
Blood Cholinesterases To review, there are two forms of ChE in the blood: BuChE, which is found in plasma or serum, and RBC-ChE, which is associated with erythrocytes. Neither enzyme is identical to the tissue enzyme with the corresponding substrate specificity (butyrylcholine and ACh, respectively). However, because blood can be withdrawn, the activities of each of these enzymes can be assayed by standard, relatively simple laboratory techniques, whereas tissue enzyme is unavailable for assay. The measurements obtained from the blood assay can be used as an approximation of tissue enzyme activity in the event of a known or possible exposure of an animal, such as man, to an AChE inhibitor. Persons who are occupationally exposed to ChE-inhibiting substances are periodically monitored for asymptomatic exposure by assays of blood-ChE activity. Those at risk include crop sprayers and orchard workers who handle ChE-inhibiting insecticides, and chemical agent–depot workers or laboratory scientists who handle nerve agents. To be meaningful, such monitoring must include knowledge of physiological variation in the blood enzymes. Individuals who work with or around nerve agents, such as laboratory investigators and depot or storage-yard personnel, have their RBC-ChE activity monitored periodically. Before the individuals begin work, two measures of RBC-ChE, drawn within 14 days but not within 24 hours of each other, are averaged as a “baseline.” At periodic intervals, the frequency of which depends on the individuals’ jobs, blood is drawn for measuring cholinesterase activity (for further discussion, see Chapter 17, Healthcare and the Chemical Surety Mission). If the activity is 75% or more of their baseline, no action is taken. If the activity is below 75% of their baseline, they are considered to have had an asymptomatic exposure and they are withdrawn from work. Investigations are undertaken to find how they were exposed. Although asymptomatic, they are not permitted to return to a work area around nerve agents until their RBC-ChE activity is higher than 80% of their baseline activity.19 If an individual has symptoms from a possible nerve agent exposure or if an accident is known to have occurred in his area, his RBC-ChE activity is immediately measured and the criteria noted above, as well as signs and symptoms, are used for exclusion from and return to work. The values of 75% and 80% were selected for several reasons, including (a) the normal variation of RBC-ChE in an individual with time (discussed below), (b) laboratory reproducibility in analysis of RBC-ChE activity, and (c) the lower tolerance to nerve agents with a low RBC-ChE as demonstrated in animals (discussed below). This topic is also discussed in Chapter 14, Pesticides, in Occupational Health: The Soldier and the Industrial Base, another volume in the Textbook of Military Medicine series. |
| Butyrocholinesterase The enzyme BuChE is present in blood and throughout tissue. Its physiological role in man is unclear20; however, it may be important in canine tracheal smooth muscle,21 the canine ventricular conducting system,22 and rat atria.23 BuChE is synthesized in the liver and has a replacement time of about 50 days. Its activity is decreased in parenchymal liver disease, acute infections, malnutrition, and chronic debilitating diseases, and is increased in the nephrotic syndrome.20 This enzyme has no known physiological function in blood, but may assist in hydrolyzing certain cho-line esters. Persons who have a prolonged paralysis caused by succinylcholine, a muscle relaxant, usually are found to have low BuChE activity.20 The structure of BuChE is determined by two autosomal alleles. The frequency of occurrence of the gene responsible for abnormal ChE is about 1 in 2,000 to 1 in 4,000 people. Thus, about 96% of the population have the usual phenotype, close to 4% have the heterozygous phenotype, and about 0.03% have the homozygous abnormal phenotype.20 In addition to having low BuChE activity, which results from this genetic abnormality, in the usual assay, persons with abnormal ChE have low dibucaine numbers (the enzyme activity in an assay in which dibucaine is used as the ChE substrate). The mean dibucaine number for the normal phenotype is about 79%, that for the heterozygote is 62%, and that for the homozygous abnormal phenotype is 16%.24 The relationship of BuChE activity and succinylcholine can be somewhat different, however. One author25 reports on an individual whose BuChE activity was 3-fold higher than normal. His dibucaine number was normal, and he was found to be relatively resistant to succinylcholine. His sister and daughter also had high BuChE activities. The author of this report suggests that this abnormality is autosomal dominant and that it represents another genetic abnormality of BuChE. Erythrocyte Cholinesterase RBC-ChE is synthesized with the erythrocyte, which has an average life of 120 days. The activity of this enzyme is decreased in certain diseases involving erythrocytes (such as pernicious anemia) and is increased during periods of active reticulocytosis (such as recovery from pernicious anemia) because reticulocytes have higher ChE activity than do mature cells. No other disease states are known to affect RBC-ChE activity,20 but one report26 describes three members of one family who had decreased RBC-ChE activity, suggesting that differences in this enzyme are genetic. The physiological role of the enzyme in (or on the stroma of) the erythrocyte is unknown. Recovery of RBC-ChE activity after irreversible inhibition takes place only with the synthesis of new erythrocytes, or at a rate of approximately 1% per day. Variation in Cholinesterase Activities Butyrocholinesterase. In longitudinal studies27,28 lasting 3 to 250 weeks, the coefficient of variation (standard deviation divided by the mean) for an individual’s BuChE activity ranged from 5% to 11.8% in men and women. Of the ranges (range is defined as the difference between the highest and lowest activities divided by the mean) for individuals in the study, the lowest was 24% and the highest was 50% over 1 year.28 BuChE activity does not vary with age in women29,30 until the age of 60 years, when higher BuChE activities are seen.30 BuChE activities in men have been reported in some studies to increase with age and in other studies to decrease with age.20 In matched age groups, BuChE activity was higher in men than in women,20,30 and higher in women not taking oral contraceptives than in those taking them.30–32 Erythrocyte Cholinesterase. RBC-ChE activity is more stable than the activity of the BuChE.28,33,34 In a study28 that lasted 1 year, the coefficients of variation were 2.1% to 3.5% in men and 3.1% to 4.1% in women, with ranges of 7.9% to 11.4% in men and 12.0% to 15.9% in women. This variation was less than that observed for the hematocrits of these individuals. It is unclear whether age affects RBC-ChE activity. In one study,29 RBC-ChE activity was unchanged with age, while in another,30 enzyme activity increased with age from the third to the sixth decades in men, with a less marked increase through the fifth decade in women. |
| Inhibition of Blood Cholinesterases Some ChE-inhibiting substances inhibit BuChE preferentially, and some inhibit RBC-ChE preferentially. Large amounts of ChE inhibitors will completely inhibit both enzymes. The blood enzymes appear to act as buffers for the enzymes in the tissue. There is little inhibition of tissue enzyme until much of the blood enzyme is inhibited. The RBC-ChE appears to be more important than the plasma enzyme in this regard. In two studies,35,36 a small dose of DFP in humans inhibited about 90% of the plasma enzyme activity but only 15% to 20% of RBC-ChE activity. Symptoms correlated with depression of RBC-ChE, but not with depression of BuChE (see the Central Nervous System and Behavior section below). In humans, some pesticides, such as parathion,37–39 systox,37 and malathion,20 also preferentially inhibit the plasma enzyme, while others, such as dimefox39 and mevinphos,40 initially bind with the RBC enzyme. In animals, there appears to be a species difference, inasmuch as parathion preferentially inhibits RBC-ChE in rats and the plasma enzyme in dogs.20 The nerve agent VX preferentially inhibits RBC-ChE; in two studies,41,42 a small amount caused a 70% or greater decrease in the activity of this enzyme, whereas the activity of BuChE was inhibited by no more than 20%. Sarin also preferentially inhibits the RBC-ChE; 80% to 100% inhibition of RBC-ChE activity was observed in two studies,35,43 while BuChE was inhibited by 30% to 50%. Therefore, estimation of the RBC-ChE activity provides a better indicator of acute nerve agent exposure than does estimation of the plasma enzyme activity. When the blood enzymes have been irreversibly inhibited, recovery of ChE activity depends on production of new plasma enzymes or production of new erythrocytes. Hence, complete recovery of BuChE activity that has been totally inhibited by sarin will occur in about 50 days, and recovery of the RBC-ChE, in 120 days (about 1% per day).44 In humans, after inhibition by VX, the RBC-ChE activity seems to recover spontaneously at the rate of about 0.5% to 1% per hour for a few days, but complete recovery depends on erythrocyte production.41,42 Time Course of Inhibition. After very large amounts of nerve agent (multiple LD 50 s [ie, multiples of the dose that is lethal to 50% of the exposed population]) are placed on the skin, signs and symptoms occur within minutes, and inhibition of blood ChE activities occurs equally quickly. However, with smaller amounts of agent, the onset is not so rapid. In studies in which small amounts of VX were applied on the skin of humans, the onset of symptoms and the maximal inhibition of blood ChE activity were found to occur many hours after application of the agent. In one study42 in which equipotent amounts of VX were applied to the skin in different regions, the time to maximal inhibition was 5 hours for the head and neck, 7 hours for the extremities, and 10 hours for the torso. In a similar study,45 the average time from placing VX on the skin to the onset of nausea and vomiting and maximal drop of blood ChE activity was 10.8 hours.
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| Inhalation of nerve agent vapor inhibits
blood ChE activity and produces signs and symptoms of exposure more
rapidly than does dermal contact. Although there is no correlation
between ChE activity and clinical effects after exposure to small
amounts of vapor, both clinical effects and ChE inhibition occur within
minutes. In one study,41 both the maximal inhibition of
RBC-ChE activity and the appearance of signs and symptoms occurred about
1 hour after intravenous administration of small amounts of VX. After
ingestion of VX, the interval was 2 to 3 hours. Relation to Signs and Symptoms. The local signs and symptoms in the eye, nose, and airways caused by small amounts of vapor are due to the direct effect of the vapor on the organ; no correlation between the severity of these effects and the blood ChE activity seems to exist. These early experimental data47–49 indicating the lack of correlation were supported by a retrospective analysis of 62 individuals seen at the Edgewood Arsenal Toxic Exposure Aid Station between 1948 and 1972. Although all individuals had physical signs or definite symptoms (or both) of nerve agent vapor exposure, there was no correlation between local effects from vapor exposure and RBC-ChE activity (Table 5-1).50
Table 5-2 categorizes data from 283 individuals (data are from published sources41,42 and unpublished research) who received VX by various routes; the numbers of subjects, the activity ranges of RBC-ChE, and the numbers and percentages of those who vomited are shown. The degree of inhibition needed to cause vomiting in these 283 people corresponds to that found in experimental data from other sources, which indicate that “to exert significant actions in vivo, an anti-ChE must inhibit from 50% to 90% of the enzyme present.”11(p446) Nerve Agents Molecular models of the nerve agents tabun, sarin, soman, and VX are shown in Figure 5-3. Table 5-3 summarizes the chemical, physical, environmental and biological properties of these compounds. Nerve agents differ from commonly used ChE inhibitors primarily because they are more toxic (ie, a smaller amount is needed to cause an effect on an organism). For example, an in vitro study43 with ChE from human erythrocytes, brain, and muscle showed that sarin had about 10-fold more inhibitory activity than TEPP, 30-fold more than neostigmine, 100-fold more than DFP, and 1,000-fold more than parathion. The vapor or aerosol exposure (the product of concentration [C] and time [t]) needed to cause death in 50% of the exposed population is known as the LCt50 (Exhibit 5-2); the estimated LCt50s for humans for these four agents are as follows:
The estimated percutaneous LD50s for the four compounds are as follows:
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| VX has a much lower LD 50 because it is much less volatile
and remains intact on the skin, whereas the other nerve agents will
evaporate unless covered (eg, by clothing).6,8 Different
sources provide different estimates for these LD 50 and LCt
50 values; however, those noted above seem to be the most commonly
accepted. The four nerve agents are liquid at moderate temperatures; thus, the term “nerve gas” is a misnomer. In their pure state, they are clear, colorless, and, at least in dilute solutions of distilled water, tasteless. Tabun has been reported to have a faint, slightly fruity odor, and soman, to have an ill-defined odor; sarin and VX are apparently odorless. The G agents are volatile; VX has very low volatility. Sarin, the most volatile, is somewhat less volatile than water; tabun and soman are less volatile than sarin. The G agents present a definite vapor hazard; VX is much less likely to unless the ambient temperature is high. |
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Inhalational Exposure to Vapor The effects produced by nerve agent vapor begin in seconds to minutes after the onset of exposure, depending on the concentration of vapor. These effects usually reach maximal severity within minutes after the individual is removed from or protected from the vapor or may continue to worsen if the exposure continues. There is no delay in onset as there is after liquid exposure.
At low Cts, the eyes, nose, airways, or a combination of these organs are usually affected. The eyes and nose are the most sensitive organs; the eyes may be affected equally or unequally. There may be some degree of miosis (with or without associated conjunctival injection and pain) with or without rhinorrhea, or there may be rhinorrhea without eye involvement (Table 5-4). As exposure increases slightly, the triad of eye, nose, and lung involvement is usually seen. The casualty may or may not notice dim vision and may complain of “tightness in the chest.” “Tightness in the chest” may occur in the absence of physical findings. At higher exposures, the effects in these organs intensify. Marked miosis, copious secretions from the nose and mouth, and signs of moderate-to-severe impairment of ventilation are seen. The casualty will complain of mild-to-severe dyspnea, may be gasping for air, and will have obvious secretions. In severe exposures, the casualty may not have time to report the initial effects before losing consciousness, and may not remember them on awakening. One severely exposed individual later recalled that he noticed an increase in secretions and difficulty in breathing, and another said he felt giddy and faint before losing consciousness. In both instances, the casualties were unconscious within less than a minute after exposure to agent vapor. When reached (within minutes) by rescuers, both were unconscious and exhibited convulsive jerking motions of the limbs; copious secretions from the mouth and nose; very labored, irregular, and gasping breathing; generalized muscular fasciculations; and miosis. One developed flaccid paralysis and apnea a minute or two later. The other received immediate, vigorous treatment, and his condition did not progress (personal observation).
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| Dermal Exposure to Liquid The early effects of a drop of nerve agent on the skin and the time of onset of these effects depend on the amount of nerve agent and several other factors, such as the site on the body, the temperature, and the humidity. After a delay during which the individual is asymptomatic, localized sweating occurs at the site of the droplet; less commonly, there are localized fasciculations of the underlying muscle (Table 5-5). Unless the amount of the nerve agent is in the lethal range, the next effects (or perhaps the first effects, if the sweating and fasciculations do not occur or are not noticed) are gastrointestinal: nausea, vomiting, diarrhea, or a combination of these symptoms. The casualty may notice generalized sweating and complain of tiredness or otherwise feeling ill. There may be a period of many hours between exposure and the appearance of symptoms and signs. These signs and symptoms might occur even if the casualty has been decontaminated.46 After large exposures, the time to onset of effects may be much shorter than for smaller exposures and decreases as the amount of agent increases. For instance, two individuals were decontaminated within minutes of exposure to a drop of nerve agent. There was a 15- to 20-minute, asymptomatic interval before the precipitant onset of effects: collapse, loss of consciousness, convulsive muscular jerks, fasciculations, respiratory embarrassment, and copious secretions. Within several minutes, flaccid paralysis and apnea occurred in both (personal observation). Most of the information on the effects of nerve agents on organ systems in humans is derived from studies done in the post–World War II period, from reports of people exposed to pesticides, or from clinical evaluations of accidental exposures of people who worked in nerve agent–research laboratories, manufacturing facilities, or storage areas or depots (Table 5-6). Some organ systems have been studied more intensively than others; for some organ systems there are few human data. For example, for the musculoskeletal system, there is a plethora of data from animal studies and studies in isolated neuromuscular preparations, but study results are difficult to apply to a human clinical situation. The Eye Nerve agents in the eye may cause miosis, conjunctival injection, pain in or around the eye, and dim or blurred vision (or both). Reflex nausea and vomiting may accompany eye exposure. These effects are usually local, occurring when the eye is in direct contact with nerve agent vapor, aerosol, or liquid, but exposure by other routes (such as on the skin) can also affect the eyes. Because eyes often react late in the course of intoxication in the latter case (exposure on the skin), they cannot be relied on as an early indication of exposure. Systemic (such as skin or peroral) exposure to a nerve agent might be large enough to produce moderate symptoms (nausea, vomiting) without miosis. In studies41,42,45 in which VX was placed on the skin, administered intravenously, or given orally, a significant number of subjects experienced nausea, vomiting, sweating, or weakness, but none had miosis. In 47 patients with parathion poisoning, all of the 14 severe cases had miosis, whereas 6 of 11 patients with moderate poisoning and only 5 of 22 patients with mild effects had miosis.51 On the other hand, a vapor or aerosol exposure might cause miosis without other signs or symptoms and an exposure in one eye will cause miosis in that eye (a local effect because of a mask leak in one eyepiece or similar causes) without affecting the other eye. If the eye exposure is not associated with inhalation of the nerve agent, there is no good correlation between severity of the miosis and inhibition of RBC-ChE activity. The latter may be relatively normal or may be inhibited by as much as 100% (see Table 5-1), so the severity of the miosis cannot be used as an index of the amount of systemic absorption of agent or amount of exposure. On the other hand, an early study52 demonstrated a relationship between the Ct of sarin and pupil size at the time of maximal miosis, and the investigator suggested that the pupil size might be used as an index of the amount of exposure. Unilateral miosis is sometimes seen in workers handling nerve agents or insecticides and usually occurs because of a small leak in the eyepiece of the protective mask. Again, the RBC-ChE may or may not be inhibited (see Table 5-1). The unilateral miosis has no prognostic medical significance. However, there may be problems with judging distances (that is, depth perception). This impairment may cause difficulty in activities such as driving a car or piloting an airplane, which require stereo-visual coordination (the Pulfrich stereo effect).20 |
The onset of miosis may be within seconds to minutes of the start of exposure; if the concentration of agent vapor or aerosol is quite low, maximal miosis may not occur until an hour or longer following exposure. The duration varies according to the amount of agent. The pupils may regain their ability to react to normal levels of indoor lighting within several days after exposure, but their ability to dilate maximally in total darkness may not return for as long as 9 weeks (Figure 5-4 and Exhibit 5-3). 18,53 The effects of nerve agents on vision have been studied for decades. Characteristically, an unprotected individual exposed to nerve agent will have the signs discussed above and may complain of dim vision, blurred vision, or both. Light Reduction Dim vision is generally believed to be related to the decrease in the amount of light reaching the retina because of miosis. In a study54 in which miosis was induced in one eye by instillation of sarin, the decrease in visual sensitivity correlated with the reduction in the area of pupillary aperture. Fifty-three subjects accidentally exposed to G agents reported improvements in dim vision before the miosis improved, which suggests that factors other than a small pupil are responsible for the high light threshold.55 In another study,56 however, no change in visual threshold was measured after miosis was induced by instillation of sarin onto the eye; the light threshold increased after systemic administration of sarin vapor with the eyes protected, so that no miosis occurred. The threshold was reduced to normal following systemic administration of atropine sulfate (which enters the CNS), but not after administration of atropine methylnitrate (which does not enter the CNS).57 The authors suggested that the dimness of vision was due to neural mechanisms in the retina or elsewhere in the CNS. |
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Although the dim vision reported by persons exposed to nerve agent vapor is generally ascribed to miosis, the above accounts suggest that more-central neural mechanisms may have equal or greater importance. In the case of the carbamate physostigmine, an increase in light sensitivity (a decreased threshold) after intramuscular administration of the drug has been reported.58 However, carbamates may differ from nerve agents in their effects on vision. Regardless of its cause, reduction in visual sensitivity impairs those who depend on vision in dim light: individuals who watch a tracking screen, monitor visual displays from a computer, or drive a tank in the evening or at night. As a practical matter, anyone whose vision has been affected by exposure to a nerve agent should not be allowed to drive in dim light or in darkness. Visual Acuity Persons exposed to nerve agents sometimes complain of blurred as well as dim vision. In one study,59 visual acuity was examined in six subjects before and after exposure to sarin vapor at a Ct of 15 mg•min/m3 . Near visual acuity was not changed in any of the six after exposure and was worsened after an anticholinergic drug (cyclopentolate) was instilled in the eyes. Far visual acuity was unchanged after sarin exposure in five of the six subjects and was improved in the sixth, who nonetheless complained that distant vision was blurred after sarin. Two presbyopic workers who were accidentally exposed to sarin had improved visual acuity for days after exposure. As the effects of the agent decreased, their vision returned to its previous state; in each case, this took about 35 days.53 The author suggested, as others have previously, that miosis accounted for the improvement in visual acuity (the pinhole effect). |
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| Eye Pain Eye pain may accompany miosis, but the reported incidence varies. A sharp pain in the eyeball or an aching pain in or around the eyeball is common. A mild or even severe headache (unilateral if the miosis is unilateral) may occur in the frontal area or throughout the head. This pain is probably caused by ciliary spasm and is worsened by looking at bright light, such as the light from a match a person uses to light a cigarette (the “match test”). Sometimes this discomfort is accompanied by nausea, vomiting, and malaise. Local instillation of an anticholinergic drug such as atropine or homatropine usually brings relief from the pain and systemic effects (including the nausea and vomiting), but because these drugs cause blurring of vision, they should not be used unless the pain is severe.59 The Nose Rhinorrhea is common after both local and systemic nerve agent exposure. It may occur soon after exposure to a small amount of vapor and sometimes precedes miosis and dim vision, or it may occur in the absence of miosis. Even a relatively small exposure to vapor may cause severe rhinorrhea. One exposed worker compared the nasal secretions to the flow from a leaking faucet, and another said that they were much worse than those produced by a cold or hay fever (personal observation). Rhinorrhea also occurs as part of an overall, marked increase in secretions from glands (salivary, pulmonary, and gastrointestinal) that follows a severe systemic exposure from liquid on the skin and, under this circumstance, becomes a secondary concern to both the casualty and the medical care provider. Pulmonary System After exposure to a small amount of nerve agent vapor, individuals often complain of a tight chest (difficulty in breathing), which is generally attributed to spasm or constriction of the bronchiolar musculature. Secretions from the goblet and other secretory cells of the bronchi also contribute to the dyspnea. Exposure to sarin at a Ct of 5 to 10 mg•min/m3 will produce some respiratory discomfort in most individuals, with the discomfort and severity increasing as the amount of agent increases. Several decades ago, investigators attempted to characterize pulmonary impairment caused by exposure to nerve agents by performing pulmonary function studies (such as measurements of vital capacity and maximal breathing capacity) on subjects exposed to small amounts of sarin vapor (the Ct values for sarin ranged up to 19.6 mg•min/m3 ).60 Some observers found increases in airway resistance61 and other changes, while other researchers did not.62 Although these studies yielded conflicting results, clinical practitioners have found that the inhalation of nerve agent vapor or aerosol causes dyspnea and pulmonary changes that usually are audible on auscultation. These changes are noticeable after low Ct exposures (5–10 mg•min/m3 ) and intensify as the Ct increases. The pulmonary effects begin within seconds after inhalation. If the amount inhaled is large, the effects of the agent include severe dyspnea and observable signs of difficulty with air exchange, including cyanosis. If the amount of the inhaled agent is small, a casualty may begin to feel better within minutes after moving into an uncontaminated atmosphere, and may feel normal in 15 to 30 minutes. It was not uncommon, for example, for individuals who had not received atropine or other assistance to arrive at the Edgewood Arsenal Toxic Exposure Aid Station about 15 to 20 minutes after exposure and report that their initial, severe trouble in breathing had already decreased markedly (personal observation). If the exposure was larger, however, relief was likely to come only after therapeutic intervention, such as administration of atropine. Attempts to aid ventilation in severely poisoned casualties can be greatly impeded by constriction of the bronchiolar musculature and by secretions. One report63 mentions thick mucoid plugs that hampered attempts at assisted ventilation until the plugs were removed by suction. Atropine may contribute to the formation of this thicker mucus because it dries out the thinner secretions. A severely poisoned casualty becomes totally apneic and will die as a result of ventilatory failure, which precedes collapse of the circulatory system. Many factors contribute to respiratory failure, including obstruction of air passages by broncho-constriction and secretions; weakness followed by flaccid paralysis of the intercostal and diaphragmatic musculature, which is needed for ventilation; and a partial or total cessation of stimulation to the muscles of respiration from the CNS, indicating a defect in central respiratory drive. Older data on the relative contributions of each of these factors in causing death were summarized in a report64 describing original studies in nine species. The authors concluded that central respiratory failure appeared to dominate in most species, but its overall importance varied with the species, the agent, and the amount of agent. For example, under the circumstances of the studies, failure of the central respiratory drive appeared to be the major factor in respiratory failure in the monkey, whereas bronchoconstriction appeared early and was severe in the cat. The authors of another report65 suggest that the presence of anesthesia, which is used in studies of nerve agent intoxication in animals, and its type and depth are also factors in establishing the relative importance of central and peripheral mechanisms. In another study,66 bronchoconstriction seen in the dog after intravenous sarin administration was quite severe compared with that found in the monkey (however, the dog is known to have thick airway musculature). Differences in circulatory and respiratory effects were seen between anesthetized and unanesthetized dogs given sarin.67 Convulsions and their associated damage were not seen in the anesthetized animals. In this study, there were no significant differences in the cardiovascular and respiratory effects when the agent was given intravenously, percutaneously, or by inhalation. In a study68 of rabbits poisoned with sarin, bronchoconstriction appeared to be a minor factor, while neuromuscular block (particularly at the diaphragm) and central failure were the primary factors in respiratory failure. In a recent review69 descr |
