CHOLINERGIC TRANSMISSION

The so-called central nervous system consists of the brain and the spinal cord. All other nerves in the body comprise the peripheral nervous system. Efferent nerves carry messages from the central nervous system to all parts of the body (the periphery). Afferent nerves carry information such as pain intensity from the periphery to the central nervous system. There are two types of efferent nerves: somatic, which go to skeletal muscles, and autonomic, which go to smooth muscles, glands and the heart. Messages in the form of electrical activity are conducted along the nerve fibers or axons. Between the terminus of the axon and the muscle or gland that the nerve controls (innervates), there is a gap called the synapse or synaptic cleft. When the conducted electrical impulse (action potential) reaches the nerve terminus, it provokes the release of chemicals called neurotransmitters. These chemicals diffuse across the synaptic cleft and react with a specialized structure (receptor) on the postjunctional membrane. The receptor is then said to be activated or excited, and its activation triggers a series of chemical events resulting ultimately in a biological response such as muscle contraction. The processes involving neurotransmitter release, diffusion and receptor activation are referred to collectively as transmission. There are many types of transmission, and they are named for the specific neurotransmitter involved. Thus, cholinergic transmission involves the release of the neurotransmitter, acetylcholine, and its activation of the postsynaptic receptor. Things that bind to and activate receptors are called agonists. Thus, acetylcholine is the endogenous agonist for all cholinergic receptors.

After leaving the central nervous system, somatic nerves to skeletal muscles have only one synapse, namely, that between the nerve terminus and the muscle it innervates. The neurotransmitter at that synapse is acetylcholine. Thus, this myo-(for muscle)-neural junction is one site of cholinergic transmission. The postjunctional receptor is called the motor end plate. Autonomic nerves, in contrast to somatic nerves, have an additional synapse between the central nervous system and the innervated structure (end organ). These synapses are in structures called ganglia, and these are nerve-to-nerve junctions instead of nerve-to-end organ junctions. Like somatic nerves, however, autonomic nerves also have a final nerve-to-end organ synapse. The neurotransmitter in autonomic ganglia is also acetylcholine; hence, this represents another site of cholinergic transmission. The motor end plate and the ganglionic receptors can also be activated by exogenously added nicotine. Thus, nicotine is an agonist for this particular subfamily of cholinergic receptors which are called nicotinic, cholinergic receptors.

There are two anatomically and functionally distinct divisions of the autonomic nervous system: the sympathetic division and the parasympathetic division. The preganglionic fibers of the two divisions are functionally identical, and they innervate nicotinic, cholinergic receptors in ganglia to initiate action potentials in the postganglionic fibers. Thus, all ganglia are created pretty much equal. Only the postganglionic fibers of the parasympathetic division, however, are cholinergic. The postganglionic fibers of the sympathetic division generally, but not always, secrete norepinephrine. The cholinergic receptors innervated by the postganglionic fibers of the parasympathetic division of the autonomic nervous system can also be activated by exogenously added muscarine, an agonist found in small amounts in the poisonous mushroom, Amanita muscaria (below). These constitute a second subset of cholinergic receptors which are called muscarinic, cholinergic receptors.

Amanita muscaria L.ex Fries also known as the Fly agaric

Reproduced with permission from L. Woodward, Poisonous Plants, a Color Field Guide, Hippocrene Books, Inc., New York, 1985.

 

Although the receptors in ganglia and the motor end plate both respond to nicotine, they actually constitute two distinct subgroups of nicotinic receptors. Each of the three families of cholinergic receptors can be blocked by specific receptor antagonists to prevent their activation by endogenous acetylcholine or added agonists. Thus, specific blockers are known for cholinergic, muscarinic receptors innervated by postganglionic fibers of the parasympathetic division of the autonomic nervous system, for cholinergic, nicotinic receptors in both sympathetic and parasympathetic ganglia, and for cholinergic nicotinic receptors at the myoneural junction (motor end plates) of the somatic nervous system. When these receptors are blocked, the on-going biological activity associated with their normal and continuous activation is lost. For example, blockade of the motor end plate leads to generalized, flaccid paralysis.

As shown in the diagram below, there are some anomalous fibers in the sympathetic division of the autonomic nervous system. For example, the sympathetic postganglionic nerves that go to sweat glands are cholinergic instead of adrenergic, like most other sympathetic fibers, and they innervate mucarinic receptors. The sympathetic nerve to the adrenal gland innervates a receptor that is nicotinic like all autonomic ganglia, but there is no postganglionic fiber. The gland itself is analogous to a postganglionic sympathetic fiber, but, instead of secreting a neurotransmitter, it secretes epinephrine and norepinephrine into the blood stream, where they function as hormones. These hormones activate adrenergic receptors throughout the body. Finally, not shown in the diagram, are nicotinic and muscarinic receptors in the central nervous system; their functions are incompletely understood. All of the stories in this section involve poisons or diseases that affect cholinergic transmission at one or more of the various sites described.

In the diagram above, ACh stands for acetylcholine, E for epinephrine, NE for norepinephrine. The muscarinic sites for cholinergic transmission are at the terminus of the postganglionic parasympathetic fiber. The nicotinic sites are at the terminus of the somatic motor nerve fibers and in all autonomic ganglia including the terminus of the preganglionic sympathetic fiber to the adrenal gland. Both muscarinic and nicotinic cholinergic transmission occur in the central nervous system (not shown). As adapted from: Fleming, W. W., Chapt. 11. Introduction to the Autonomic Nervous System. In Craig. C. R. and Stitizel, R. E. Modern Pharmacology with Clinical Applications, 5th ed. Little Brown and Company, Boston, 1997.

 


 

The story, "Something a Little Unusual" can be found in Roueché, B., "The Medical Detectives, Truman Talley Books/Plume, New York, New York, 1991.

Also found in Rouché, B., Annals of Epidemiology, Little Brown and Company, Boston, 1967. (Out of Print)

Also found in Rouché, B., The Orange Man and Other Narratives of Medical Detection, Little Brown and Company, 1971. (Out of Print)


 

"Something a Little Unusual" Addendum

On occasions when a single patient has presented with the signs and symptoms described in this story, it has been understandably misdiagnosed as some form of schizophrenia, primarily because of the hallucinations and "word salad" speech consisting of a jumble of meaningless and unrelated words. Contemporaneous, sudden onset of schizophrenia in two members of the same family, however, would be beyond the realm of unlikely and into the kingdom of the impossible.

Two other misdiagnoses were entertained; one by the family, nicotine poisoning, and one by the examining physician, botulism. interestingly, both of these affect cholinergic transmission. It is true as it says in the story that nicotine poisoning "could not result from superficial contact" with tobacco plants. Many years ago, however, nicotine was widely used as one of the first commercially available insecticides under the Black Flag logo. Concentrations as high as 40% solutions of nicotine salts or the free alkaloid were used. The free alkaloid in particular is rapidly absorbed through the intact skin, and superficial contact with these preparations resulted in many poisonings and deaths. Nicotine poisoning is complicated because it first stimulates and then blocks cholinergic transmission at all nicotinic synapses, namely ganglia and the myoneural junction. Botulism has the potential to block cholinergic transmission at both nicotinic and muscarinic sites.

The substance that was responsible for these poisonings, l-hyoscyamine, has effects that so closely resemble those of atropine, a widely used drug, that we are on firm ground in using the simpler and better known name. Another, very closely related drug, scopalamine, is found in some antimotion sickness remedies, especially the transdermal patches.

In addition to Datura strammonium (below), the Solanaceae family includes Atropa belladonna. Belladonna in Spanish means "beautiful lady". In some societies many years ago, widely dilated pupils were apparently considered a sign of beauty. Extracts of belladonna could be dropped in the eyes to produce that effect without the other disturbing signs and symptoms. How the beautiful lady was able to get through her dinner date, while being virtually blind, however, is a matter for conjecture.

Datura stramonium L., also known as Jimsonweed, Jamestown weed and Apple of Peru is widely distrubuted in the US from Florida to Texas, north into Canada and in the far western states. The atropine-like alkaloids are found in all parts of the plant.

Drawing in ink and watercolor by Louise Glass, Piermont, NH. The original is the property of Dartmouth College and hangs in Dana Biomedical Library.

 

As indicated in the story, the hallucinatory effects of atropine and its relatives are often sought out by recreational drug users. This may have been true even in ancient times. There are allegations that preparations were used by the Vikings to psych themselves for raids on England, and by practitioners of witchcraft. Medieval witches may have confessed to deeds that occurred only in their hallucinations. A case has been made, however, for the involvement of ergot alkaloids, which sometimes have effects like LSD, in the incidents in Salem, Massachusetts. These incidents served as the basis for the Arthur Miller drama, "The Crucible". In order to obtain the desired hallucinatory effects of atropine, however, the user must put up with an astonishing array of unpleasant "side" effects. For a fascinating account of the very early history of this drug see Jennings, RE, Journal of Pediatrics 6:657, 1935.

Atropine is the prototype blocker of muscarinic receptors. So much so, that it has become part of the definition, namely, that if a biological response is blocked by atropine, it necessarily involves muscarinic receptors. In blocking the receptor, atropine is not producing any unique biological effects. The resulting signs and symptoms are due solely to an interruption of normal cholinergic, muscarinic transmission at the various end organs. The blockade is competitive and reversible. If the concentration of acetylcholine in the vicinity of the blocked receptor can be increased, acetylcholine will displace atropine and transmission will be restored. That is what happens, albeit, much more slowly during spontaneous recovery from atropine poisoning.

A mnemonic device for recalling some of the signs and symptoms of atropine poisoning are the following similes:

Mad as a hatter.

Blind as a bat.

Dry as a bone.

Red as a beet.

Hot as a pistol.

Mad as a hatter, of course, makes reference to the apocryphal senario of the mad tea party in Alice in Wonderland. Hatters were made "mad", however, by exposure to mercury salts, not atropine. We must assume in the case of atropine that there are muscarinic receptors in the brain that play important roles in cognitive function. The exact location of these receptors is not known, but measures to increase brain acetylcholine levels in atropine poisoning lead to at least partial restoration of normal cognition. This drug-induced delirium is one of the most dangerous aspects of atropine poisoning in adults since it may result in self-destructive acts, such as leaping out of windows or pulling out intravenous tubing.

Blind as a bat refers to three effects on vision. Cholinergic tone to the sphincter iridis muscle of the iris tends to contract the pupil. Interruption of that tone leads to pupillary dilation (mydriasis). Unrelenting mydriasis results in photophobia. That is why ophthalmologists tell you to bring dark glasses when they intend to dilate the pupils with drugs related to atropine.

The smooth muscles of the iris. The sphincter muscle is innervated by cholinergic fibers to muscarinic receptors. Its contraction under the influence of an agonist results in miosis, and its blockade by atropine results in mydriasis. The radial muscle is innervated by adrenergic fibers to an alpha-1 receptor. Its contraction by an agonist results in mydriasis and its blockade results in miosis.

Dilated pupils, however, do not result in blurred vision. That is the result of a less obvious third effect in the eye, which is mediated by interruption of cholinergic tone to the ciliary muscle which controls the angle of refraction of the lens. Contraction of the ciliary muscle allows the lens to round up by its own elasticity to accommodate for near vision. Blockade of that tone to relax the ciliary muscle leaves one permanently focused in far vision (cycloplegia). As a result, near vision is blurred.

 

Diagram of eye depicting major pathway for outflow of aqueous humor (heavy arrow) which drains at Schlemm's canal. If the angle between the iris and the cornea is particularly narrow, a physical impediment may exist to drainage and the intraocular pressure is increases. This is exacerbated during mydriasis because the iris contracts toward that angle (narrow angle glaucoma) to further impede outflow. The ciliary muscle (shown in cross section) is actually a sphincter. When it contracts in response to activation of its muscarinic receptors, it lengthens and relaxes the tension of the suspensory ligaments on the lens. The lens rounds of its own elasticity to accomodate to near vision. Under the influence of excess acetylcholine, however, the muscle goes into ciliospasm, and cannot reaccomodate for far vision. This occurs in anticholinesterase poisoning. Hoover, D. B., Chapt. 14. Directly and Indirectly Acting Cholinomimetics. In Craig. C. R. and Stitizel, R. E. Modern Pharmacology with Clinical Applications, 5th ed. Little Brown and Company, Boston, 1997

When signs and symptoms occur in one or both eyes in the absence of other effects of interruption of parasympathetic transmission, they may be misinterpreted as an intracranial aneurysm, and result in unnecessary and extensive neurosurgical workup. A laboratory technician, who rubbed her eye while weighing out atropine powder for an experiment, narrowly escaped a lumbar puncture. A more careful questioning about her occupation led to an explanation for the single widely dilated pupil. During periods of drought in the midwest, the corn crop may be damaged, but jimpson weed will continue to fluorish. Mechanical corn pickers in such fields can fill the air with dust and particles of jimpson weed. A little of this dust in the eye can produce "cornpicker's pupil".

Dry as a bone is primarily manifested by a blockade of cholinergic tone to the salivary glands resulting in decreased salivation, dry mouth, intense thirst and difficulty in swallowing. The skin is also dry because of a blockade of sweating. This is a rare exception in which the sympathetic nerves to sweat glands are cholinergic and innervate muscarinic receptors.

Red as a beet describes the marked flush noted in some of the Mason family. This is an exceptional effect of atropine that cannot be ascribed to blockade of muscarinic receptors. The mechanism of this anomalous response is unknown.

Hot as a pistol refers to elevated body temperature, which may be especially pronounced in young children, and may reach life-threatening levels. In part, this fever is due to a blockade of sweating, but other effects mediated through the central nervous system cannot be ruled out. Both Mason and Mrs. Smart had temperatures of 99° F.

The mnemonic device by no means includes all of the effects of atropine, nor even all of the important ones. Less obvious, but potentially dangerous, is muscarinic blockade of the detrusor muscle of the bladder resulting in prolonged urinary retention. Atropine tends to dilate the bronchi, and muscarinic blockers are sometimes used in the management of bronchial asthma. At one time atropine-containing cigarettes were marketed for that purpose. Could any remedy be more irrational? The branch of the vagus nerve that goes to the sinoatrial node of the heart tends to slow the rate of beating; thus, its blockade by atropine results in a rapid heart rate (tachycardia). Mason's pulse was 30 beats faster than normal. In the anecdote about the Bacon rebellion, the soldiers did not remember anything about the period after ingestion of jimpson weed. Amnesia is a frequent effect, which is assumed to involve central nervous system muscarinic receptors. This effect may be an incidental benefit in surgical procedures. In large doses atropine sometimes triggers coma or convulsions.

Nausea, and particularly, vomiting are important autoprotective measures in many different kinds of poisoning. So many different poisons elicit the responses, that the signs are not specific enough to be helpful in making a diagnosis. Atropine poisoning is something of an exception in that these are relatively rarely seen. True vertigo is also rarely described, and none of these signs are known to be associated with muscarinic receptors. Many people describe faintness or dizziness incorrectly as vertigo. In true vertigo the external world is perceived as spinning. This experience is often accompanied by nystagmus in which the pupils of the eye rhythmically oscillate as if one were watching telephone poles going by from a train window. This is called horizontal nystagmus, but vertical forms also occur under some circumstances.

Two drugs were used to try to alleviate some of the symptoms, trimethobenzamide, an antiemetic agent to prevent nausea and vomiting, and oral pilocarpine, supposedly to antagonize the effects of atropine on vision. The former is rational, but the similar, metoclopramide, is more effective. The latter, however, is not optimal therapy. Pilocarpine is in the same class of drugs as muscarine; it is a muscarinic agonist. Since the binding of atropine by muscarinic receptors is competitive and reversible, it can be antagonized by muscarinic agonists. The drug is a rational choice; it is the route of administration that is not. Pilocarpine is commercially available as ophthalmic drops. Putting pilocarpine directly in the eye can reverse the effects of atropine, while avoiding the possibility of unwanted effects on other parts of the body. It is true that oral pilocarpine has been used to relieve the dry mouth that sometimes follows head/neck radiation treatments, but better approaches are available.

Instead of adding an exogenous muscarinic agonist, one can take advantage of the natural agonist, acetylcholine. In the body acetylcholine is rapidly broken down by enzymes called cholinesterases. This is one of the ways in which cholinergic transmission is terminated or prevented from becoming excessive. Physostigmine is one of the many known inhibitors of cholinesterase, and one that is capable of getting into the brain. Moreover, it is short-acting so that repeated doses can be given to titrate the patient to a desired level of cholinergic transmission. When cholinesterase is inhibited, acetylcholine accumulates in the synapse and competes with atropine for the muscarinic receptor, and the signs and symptoms of muscarinic blockade are reversed. The danger of over-doing it with physostigmine is that cholinergic transmission may become exaggerated at nicotinic sites, ganglia and the myoneural junction. In the absence of more specific therapy, diazepam (ValiumR) can be used to control the delirium and convulsions. Urinary retention can be managed by catheterization, and alcohol sponges or ice blankets can help lower body temperature.

According to a leading textbook on poisonous plants, livestock poisoning by jimpson weed and related species is not uncommon, and losses have been reported in the world literature for all classes of domestic animals including ostriches.

Although atropine poisoning represents a bizarre and distressing syndrome, it is only rarely fatal. The common therapeutic dose as in preparation for some surgical procedures is 0.5 to 1.0 mg, but patients have survived the ingestion of 1 gm (5,000 therapeutic doses). For most drugs a 10-fold overdose can be dangerous. There are differences in sensitivity to most drugs in the human population. The difference in atropine content between Mason's and Clayton's tomatoes was only about 3-fold; that is not particularly large in terms of variations in sensitivity from one individual to another.

It was a quiet Saturday morning in the late 1950's, when the commanding officer of the USAREUR Medical Laboratory in Landstuhl, Germany appeared at the door of the Chemistry Section carrying a salt shaker. It had been confiscated from the cafeteria of the Voice of America broadcasting studios in Munich. No other information was forthcoming. "Run it through a tox screen." I gave it to our best technician and retired to my office. Twenty minutes later there was the sound of glass shattering in the lab. The technician, with pupils wildly dilated and mumbling incoherently, was trying to find a large jar of activated charcoal on the reagent shelf. Activated charcoal is used by some as an adsorbent in chemical poisonings. It took three MPs to subdue the technician enough to get him to the emergency room. For reasons best known to himself, he had dissolved some of the salt in water and consumed it. Acidic and alkaline solutions of the salt were prepared and put in a recording spectrophotometer. The absorption spectra were unambiguous. Unfortunately, it was too late. His attending physicians had made the decision to intervene, and acting with imperfect knowledge about cholinergic transmission, they had given him&emdash;more atropine. Much explaining all around remained to be done.

Additional reading:

Brown, J. H. and P. Taylor. Chapter 7, Muscarinic receptor agonists and antagonists. In Goodman and Gilman's The Pharmacological Basis of Therapeutics, 9th ed. J. G. Hardman and L. E. Limbird, Editors-in-Chief, McGraw-Hill, New York, 1996.

Eglen, R. M., S. S. Hegde and N. Watson. Muscarinic receptor subtypes and smooth muscle function. Pharmacol. Rev. 48:531-565, 1996.


 

The story, "The Dead Mosquitoes" can be found in Roueché, B., The Medical Detectives, Truman Talley Books/Plume, New York, New York, 1991.

Also found in Rouché, B., The Orange Man and Other Narratives of Medical Detection, Little Brown and Company, 1971. (Out of Print)

The story, "The Fumigation Chamber" can be found in Roueché, R., The Medical Detectives, Truman Talley Books/Plume, New York, New York, 1991. That version, however, does not include a short postlude titled "Department of Amplification" published in the February 29, 1988 issue of The New Yorker. The original story was published in the January 4,1988 issue.


 

"The Dead Mosquitoes" Addendum

The function of cholinesterase and the concept of inhibitors of that enzyme were introduced in the Addendum to the preceding story. It turns out that there are different types of cholinesterase and different families of cholinesterase inhibitors. Pseudocholinesterase (also known as plasma cholinesterase or butyrlcholinesterase) is found in blood plasma . Since acetylcholine does not normally circulate in plasma, the physiological function of this enzyme is unknown. Rare individuals have been born with a genetic defect in which plasma cholinesterase is very low or even undetectable. These people appear completely normal unless they are challenged by a particular drug, succinylcholine, a close chemical relative of acetylcholine, which is used to relax skeletal muscle to facilitate surgery. Succinylcholine is ordinarily inactivated by plasma cholinesterase. In the absence of the enzyme its effects are very much prolonged and intensified. In anticholinesterase exposure the plasma enzyme is the first to be inhibited, so, as indicated in the story, it is a useful laboratory test for intensity of exposure and for following the progress of recovery.

A slightly different cholinesterase is found in the vicinity of cholinergic synapses, and it serves the vital function of terminating excessive cholinergic transmission. This enzyme is called "true" or acetylcholinesterase. Again for unknown reasons, this enzyme is also found on red blood cells.

The different chemical classes of cholinesterase inhibitors are distinguished primarily by their duration of action, and those that are the longest lasting are the most dangerous. Edrophonium (TensilonR) does not react chemically with cholinesterase, and it has a very brief duration of action. The carbamate class of cholinesterase inhibitors includes the drugs, physostigmine, as introduced in the preceding story for the management of atropine poisoning, and neostigmine. Neostigmine and physostigmine react chemically with cholinesterase, and leave a carbamyl group attached to the enzyme (carbamoylated enzyme). Over the course of several hours the carbamyl group spontaneously hydrolyzes off the enzyme, and it becomes reactivated. But, this makes the carbamates longer acting than edrophonium, namely, hours versus minutes, and certain derivatives (SevinR, BaygonR) are used as insecticides.

Even longer acting are the organophosphate cholinesterase inhibitors such as phosdrin, the culprit in this story, and its many chemical relatives. These inhibitors leave a phosphate group attached to the enzyme (phosphorylated enzyme) often with part of the organic moiety also attached. After variable periods of time the phosphate group undergoes spontaneous changes ("aging") and loses part of the organic moiety. At this point, the phosphorylation is essentially irreversible, and enzymatic activity can be restored only by the synthesis of new enzyme, a process that may require days. The toxic effects of this group have been exploited as insecticides and chemical warfare agents (nerve gases such as sarin and soman) and even useful drugs. Echothiophate is used in the management of glaucoma. Even within this group of organophosphate anticholinesterases there is a hierarchy of toxicity ranging from those thought safe enough to be used in household and garden insecticides or by physicians as drugs to the exquisitely poisonous nerve gases. So, it is very misleading to say as did Dr. Conrad, "The fatal skin dose is only about five drops". Not only are there wide differences in toxicity from agent to agent, but agents are often sold as concentrates with directions for large dilutions to actual use concentrations.

In all classes of anticholinesterases the resulting signs and symptoms can be ascribed to the effects of spontaneously released acetylcholine which accumulates in cholinergic synapses and intensifies or blocks the biological effects of normal cholinergic transmission. The accumulation of acetylcholine at muscarinic receptors results in a different series of molecular events than does the accumulation of acetylcholine at nicotinic synapses. At muscarinic receptors, the effect is an unrelenting intensification of muscarinic, cholinergic transmission. The pupil of the eye remains constricted almost to the point that it almost disappears. Salivation continues until the concentration of acetylcholine at the site of that receptor begins to fall.

At the nicotinic receptors in ganglia and at the myoneural junction the accumulation of acetylcholine at first leads to an intensification of cholinergic transmission, but with continued accumulation over time, the nicotinic receptors become desensitized. There follows a failure of nicotinic, cholinergic transmission, despite the continued presence of excess transmitter. This results in flaccid paralysis of skeletal muscle and a fall off of sympathetic autonomic function. Parasympathetic autonomic function, however, remains intensified even though the parasympathetic ganglia may have been desensitized. The cholinesterase inhibitor continues to act downstream from the ganglia at the muscarinic site. Thus, at nicotinic synapses, the effects of cholinesterase inhibitors resemble those of nicotine, whereas at muscarinic synapses their effects resemble those of muscarine. Thus, nicotine poisoning resembles anticholinesterase poisoning, but lacks the signs and symptoms referable to stimulation of muscarinic receptors. At both muscarinic and nicotinic sites the effects of cholinesterase inhibitors are, in fact, mediated through acetylcholine leading to the designation of inhibitors as indirectly acting, cholinergic agonists.

With the above in mind we can begin to dissect the signs and symptoms exhibited by Billy Cordoba. In general we can expect that some will be due to activation of muscarinic receptors, some will be due to activation and eventually to desensitization of sympathetic ganglia, some will be due to activation and then desentization of the motor end plate, and some will be due to effects on the central nervous system. Furthermore, the effects at muscarinic receptors will be the opposite of those exhibited by atropine in the last story. For example, where atropine produced mydriasis, phosdrin (or acetylcholine) will produce contraction of the pupil and contraction of the ciliary muscle (ciliospasm). The eye will be unable to accommodate for far vision. Instead of a dry mouth, eyes and skin as produced by atropine, there will copious salivation, tearing (lacrimation) and sweating. Instead of paralysis of the bowel, there will be diarrhea and abdominal cramps. Instead of urinary retention there will be incontinence. There may be wheezing from bronchospasm. It is common in the jargon to refer to these effects as "cholinergic", when, in fact, they are more properly called muscarinic. Similarly, many drugs are said to have anticholinergic side effects, when, again, what is really meant is antimuscarinic.

The effects on the heart and circulation are much more complex and difficult to predict because so many opposing factors are at work. Bradycardia would be the result of inhibition of cholinesterase at muscarinic receptors on the heart. This might be opposed by activation of sympathetic nerves, which also go to the heart. Thus, stimulation of sympathetic ganglia may result initially in tachycardia. The tachycardia could be compounded by stimulation of the adrenal medulla to release epinephrine. So, the heart might be under stimulation by both neural and hormonal influences. As the ganglionic receptors become desensitized, the effect could be reversed and become additive to the muscarinic action.

Similarly, effects on blood pressure are multifactorial and difficult to predict, although resistance blood vessels, which control blood pressure, are under the exclusive control of the sympathetic nervous system. Anticholinesterase poisoning is the only condition in which significant amounts of acetylcholine may be found in the systemic circulation. There are peculiar muscarinic receptors on blood vessels that are not innervated, but they respond to circulating acetylcholine by initiating vasodilatation. Thus, the blood pressure could fall. As in the case of the heart, however, stimulation of sympathetic ganglia and the adrenal medulla could lead to an initial rise in blood pressure followed by a fall when the sympathetic, ganglionic receptors become desensitized. Finally, changes in blood pressure might reflexly affect heart rate, and changes in heart rate can influence blood pressure. Thus, in this constantly changing milieu, there are ways of explaining tachycardia, bradycardia or no change in heart rate and a rise, fall or no change in blood pressure as observed at a single instant in time.

Since only somatic nerves go to skeletal muscle, the effects of cholinesterase inhibition at that site are less ambiguous. The muscle twitches exhibited by Billy Cordoba were a result of the initial stimulation of the motor end plate. As the poisoning progresses, the receptors become desensitized, and the muscles become unresponsive (flaccid paralysis). This is the effect that is sought for therapeutic purposes with the use of the drug, succinylcholine. This effect contributes to the eventual cause of death, respiratory paralysis, since the muscles of respiration such as the diaphragm and the intercostals are paralyzed. Thus, mechanical ventilation may be a part of the total program of management.

The finding of sugar in the urine might have been a result of initial sympathetic discharge which mobilizes sugar from glycogen in the liver. The increase in white cell count, however, might have been the result of a mild superimposed infection. The central nervous effects are less well characterized as to receptor type, but they are thought to include anxiety, restlessness, convulsions, coma and respiratory and circulatory depression.

It should not come as a surprise that in the last story a cholinesterase inhibitor was used in the treatment of atropine poisoning, whereas in this story atropine is used in the treatment of anticholinesterase poisoning. It is important, however, to recognize that atropine is not a "complete" antidote. Whereas one can reverse with adequate doses the effects mediated by muscarinic receptors, atropine does nothing for the effects at nicotinic receptors. Dr. Conrad was simply incorrect when he said, "Atropine would continue to counteract the potentially dangerous neuromuscular symptoms". The partial antidotal effect of atropine should not be minimized, however, since it provides considerable relief from distressing symptoms. Moreover, it halts the outpouring of respiratory tract secretions and bronchoconstriction which compound the course toward respiratory failure. Another important point is that patients poisoned with anticholinesterases are extremely refractory to atropine. At one point Dr. Conrad said, "I got Billy started on atropine....but he didn't respond as he had before". This was not a failure on the part of Billy, but on the part of Dr. Conrad. Had he the courage to act on his convictions, he would have increased the dose until Billy did respond, no matter how much it took. The safety of large doses of atropine was mentioned in the last story. Patients poisoned by cholinesterase inhibitors are extremely resistant to atropine, and heroic doses may be indicated. Instead of the usual single therapeutic dose of 0.5 to 1.0 mg, Billy was started on 1 mg every two hours, and that was still inadequate to control the signs and symptoms.

PAM (also 2-PAM or pralidoxime, to avoid confusion with the product used to prevent sticking to frying pan surfaces) was a rationally designed drug for the specific purpose of rapidly reactivating phosphorylated cholinesterase. If given soon after exposure, 2-PAM can react with the phosphorylated enzyme, and split off the phosphate group. Unfortunately, 2-PAM is much less effective in reactivating cholinesterase after the process of aging has occurred. Reactivators such as 2-PAM are not recommended for reactivation of the carbamate ester inhibitors because these are spontaneously reactivated much more rapidly, and because for poorly understood reasons, they make some members of that group more toxic.

The misdiagnoses of a brain tumor, bulbar polio, acute rheumatic fever and encephalitis must have been made by physicians who ignored the prominent parasympathetic signs, some of which are summarized in the mnemonic device, SLUDGE (Salivation, Lacrimation, Urination, Diarrhea, Gastrointestinal cramping and vomiting, Eye sign of miosis).

"The Fumigation Chamber" Addendum

These stories belong together because the chemicals involved are all cholinesterase inhibitors. They all produce essentially the same syndrome of intoxication even though two different chemical classes of cholinesterase inhibitors were involved, and even though there are some significant differences in the acute toxicities of the individual chemicals. On a scale of 1 to 6 where 1 is practically nontoxic, and 6 is super toxic, phosdrin, the organophosphate cholinesterase inhibitor in the first story is a 6. In the vacation cabin Betty Page was exposed to a mixture of chlorpyrifos (Dursban), an organophosphate with a toxicity rating of 4 or 5 and bendiocarb (Ficam), a carbamate also with a toxicity rating of 4 or 5. On the tennis court she was exposed to diazinon, another organophosphate with a toxicity rating of 4. The specific chemical exposures at the flower show could not be documented, but it reasonable to suppose that they might have included organophosphate and/or carbamate insecticides. The toxicity ratings are only part of the equation for the severity of the intoxication. The intensity of the exposure is the other determining factor. Given in a sufficiently large dose all of these chemicals are capable of producing severe intoxications or death.

Although the intensisty of the exposure of Betty Page cannot now be determined, there are a number of hints to suggest that she was unusually sensitive to the chemicals in her environment. Admittedly, Lewis Page spent far less time in the vacation cabin than she did, but he did spend some time there without exhibiting any of the signs or symptoms which distressed his wife. Only Betty Page seems to have become ill on the sprayed tennis court, and the exposures in the flower show were not reported to have affected anyone else. Unusual sensitivity to chemicals often suggests an allergic basis, but this could not have been the case with Betty Page. The signs and symptoms she experienced and exhibited were consistent with anticholinesterase poisoning and not with any known form of allergy. Moreover, the signs and symptoms responded to atropine proving that they were due to excessive stimulation of nuscarinic receptors. Therefore, one must seek another explanation for her sensitivity.

We now enter the realm of pure speculation, but it is possible that Betty Page was one of those rare individuals with a congenital deficiency of plasma cholinesterase. (This condition is also mentined in the addendum to "The Hoofbeats of a Zebra".) Plasma cholinesterase as opposed to the essential true or acetylcholinesterase found at cholinergic junctions seems to have no physiological function. Rare individuals with virtually zero plasma cholinesterase activity, appear perfectly normal unless they are challenged with a particular drug (see "Hoofbeats of a Zebra"). When a normal individual is exposed to a cholinesterase inhibitor, the inhibitor first acts on the plasma enzyme, because that enzyme is the first encountered. The inhibitor must block all of the plasma enzyme before it can gain access via the plasma to the essential junctional enzyme. Thus, the plasma enzyme acts as a harmless sink for cholinesterase inhibitors, and it has to be fully inhibited before the enzyme can get to the synaptic sites to inhibit the true acetylcholinesterase and produce signs and symptoms of poisoning. If Betty Page lacked plasma cholinesterase, she might have been much more sensitive to cholinesterase inhibitors that a normal individual. A simple laboratory test for plasma cholinesterase activity could have provided the proof for this hypothesis. What a pity that it was not done.

This may be the only Roueché story where he published a postscript in a subsequent issue of The New Yorker, and a strange postscript it was that really adds nothing to the original story. Roueché received many letters in response to the original article. Some described similar experiences, some feared that they might be suffereing from a similar poisoning and some expressed indignation at the seeming indifference of the federal government to the dangers posed by such chemicals. The incident that Roueché selected for elaboration, however, concerned a professor of medicine and his docile, male cat. The professor has treated the cat liberally with a tick powder containing 5% carbaryl. In the process, he accidentally managed to expose himself. Within ten days the cat had become an aggressive hunter of birds and mice, an activity he had not previously engaged in. When he remarked on this change to his housemate, he was told that he, too, had undergone a markedly more aggressive and hostile personality change. Only at this point, did the professor explicitly acknowledge his extreme and inappropriate behavior. He terminated the use of the tick powder and within a week both cat and professor were their old selves. The scientific literature both before and after this incident, however does not substantiate such an effect of carbaryl.

Additional reading:

Taylor, P. Chapter 8, Anticholinesterase agents. In Goodman and Gilman's The Pharmacological Basis of Therapeutics, 9th ed. J. G. Hardman and L. E. Limbird, Editors-in-Chief, McGraw-Hill, New York, 1996.

Holladay, M. W., M. J. Dart and J. K. Lynch. Neuronal nicotinic acetylcholine receptors as targets for drug discovery. J. Med. Chem. 40:4169-4194, 1997.


 

The story, "The Hoofbeats of a Zebra" can be found in Roueché, B., The Man Who Grew Two Breasts, Truman Talley Books/Plume, New York, New York, 1995.

 

"Hoofbeats of a Zebra" Addendum

Myasthenia gravis is not a poisoning, but the similarity of this condition to the effects of the South American arrow or dart poison, curare, was pointed out in the story. The effects of curare are almost immediate, whereas Sheila Allen's signs and symptoms developed slowly over four years. Curare, or tubocurarine as the purified active ingredient is called in medicine, is a competitive, reversible, and non-desensitizing blocker of the myoneural junction. Curare-like drugs are used to produce muscular relaxation to facilitate abdominal surgery. These drugs have little or no effect on nicotinic receptors in ganglia.

Although the end result of curare poisoning is similar to that of excess acetylcholine at the motor end plate, as described in the last story, there is an important and fundamental difference. The paralysis by curare is competitive and reversible. By making more acetylcholine available in the synapse through the judicious use of an anticholinesterase, curare can be dislodged from the receptor, and the course of the poisoning can be reversed. The desensitization blockade resulting from excess acetylcholine, as in anticholinesterase poisoning, cannot be reversed except by removal of the excess transmitter through the activity of cholinesterases. The muscular paralysis of excess acetylcholine was caused by cholinesterase inhibitors, whereas cholinesterase inhibitors are actually used to treat myasthenia gravis and curare overdose.

It is interesting that although curare is quite toxic when it gets into the blood stream, the flesh of animals hunted down with curare-tipped darts can be eaten with impunity. Curare is large, highly charged molecule that is not absorbed from the gastrointestinal tract.

An alternative drug to curare for skeletal muscle relaxation during surgery is succinylcholine. Succinylcholine is simply two molecules of acetylcholine joined together. It acts on the motor end plate in the same way as acetylcholine. Thus, succinylcholine also results in a desensitization blockade of the receptor. Instead of being antagonized by cholinesterase inhibitors, as in the case of curare, succinylcholine blockade is actually intensified by cholinesterase inhibitors for two reasons. Since succinylcholine and acetylcholine act in the same way on the motor endplate, they would be expected to have additive effects. More acetylcholine translates into an intensified effect in the presence of succinylcholine. Secondly, the cholinesterase inhibitor, by definition, blocks the effects of cholinesterases, particularly in this case, plasma cholinesterase. Plasma cholinesterase is the major means for the inactivation of succinylcholine, just as it would inactivate any circulating acetylcholine. Indeed, it is only because most of us have functional plasma cholinesterase, that succinylcholine can be used safely in humans. In those rare patients in the last addendum, born with a congenital deficiency of plasma cholinesterase, the effects of succinylcholine last for many hours instead of the few minutes it lasts in normal people. Thus, the danger of using a drug like succinylcholine for which there is no chemical antagonist, is offset by the rapidity with which our cholinesterase can inactivate it.

A trained and careful observer could distinguish between curare and succinylcholine during the period of administration of the two drugs. Succinylcholine would produce some fine tremors before flaccid paralysis due to an initial activation of the motor end plate, whereas curare would not. Once paralysis has been achieved, however, the curarized patient would be indistinguishable from the one given succinylcholine. However, the mechanisms by which paralysis is achieved by the two drugs is quite different. The cause of death in myasthenia, in poisonings by curare or anticholinesterases and in succinylcholine overdose is respiratory failure secondary to paralysis of respiratory muscles.

Myasthenia is an auto-immune disease, but the antibodies are directed against the motor end plate, not acetylcholine as stated in the story. It is not rare; the prevalence is about 1:7,500. Acetylcholine production and release are normal, but there are less and less post-synaptic receptors that can be activated as the disease progresses. There are so-called myoid cells in the thymus that have receptors similar to or identical with motor end plates. As pointed out in the story, the thymus is necessary for the early development of immunological function, but in myasthenia gravis something goes awry. The thymus then plays a role in the production of abnormal antibodies because of the myoid cell receptors. When these antibodies escape to the circulation, they get to motor endplates to first block them, then eventually to destroy them. Hence the value of thymectory as an adjunct to drug treatment of myasthenia. Also, the presence of serum antibodies directed against the motor end plate can be used to confirm the diagnosis.

The amount of acetylcholine stored in the nerve terminal is limited so that in normal repeated activity of the nerve, the amount of acetylcholine released per nerve action potential tends to decrease ("presynaptic rundown"). In myasthenia, this normal phenomenon of presynaptic rundown is coupled with an abnormally low number of postsynaptic receptors. Muscle activity may start out normally, but there is an abnormally rapid onset of what is called myasthenic fatigue. A rapid decremental response to repetitive nerve stimulation can be used in electrodiagnostic testing. This is the reason why Sheila Allen could pick up a tray of drinks, but could not hold it for the usual amount of time. In the early years of the disease the course is irregularly downhill, and there is waxing and waning of the signs and symptoms (exacerbations and incomplete remissions) so there were times when Sheila felt much better than other periods. For example, an intercurrent infection sometimes brings on a myasthenic crisis.

Sheila Allen had so-called generalized myasthenia gravis of which there are at least three varieties with differing genetic susceptibilities. There is also a form of the disease in which only to the extraocular muscles are involved, and a form of the disease in newborns in which there was transplacental passage of the antibodies to motor end plate. Finally, another form of the disease occurs in patients several months after treatment with D-penicillamine, a heavy metal chelator (see "Live and Let Live").

The ultra-short acting anticholinesterase, edrophonium, introduced in the last addendum was used here as a diagnostic test for myasthenia. By temporarily making more acetylcholine available in the synapse, cholinergic transmission could be at least partially restored despite the decrement in postsynaptic receptors. Edrophonium can be used in another diagnostic way. The definitive drug treatment in this story was with MestinonR, also known as pyridostigmine. This drug is closely related to neostigmine as introduced previously, which is also useful in myasthenia. Both drugs are carbamate type cholinesterase inhibitors, which have the advantage over edrophonium of a longer duration of action. Sometimes a patient who has been stabilized for a long period of time on pyridostigmine will come to the emergency room with muscle weakness that cannot be distinguished from that of myasthenia. However, there are two possible mechanisms for the weakness. If the dose of pyridostigmine has become inadequate for any reason, the weakness is indeed a myasthenic crisis, but if the dose of pyridostigmine is excessive, the crisis is due to the accumulation of acetylcholine in the synapse and desensitization blockade (cholinergic crisis). The usual dose of edrophonium will improve the patient's condition in a myasthenic crisis, but it will make things worse in a cholinergic crisis. Indeed, it may make things so much worse that the patient may have to have temporary respiratory support. Again, the short duration of action of edrophonium is a great advantage in performing this test. Common side effects in the use of cholinesterase inhibitors in the management of myasthenia are often related to undesired activation of muscarinic receptors.

In diagnostic testing for myasthenia, the edrophonium test is usually given first. If a second test is required, it should be for the presence of acetylcholine receptor antibodies in plasma. Finally, if either of the first two tests give equivocal results, one can resort to electromyographic testing to look for an abnormally rapid decremental reponse to repetitive nerve stimulation.

We are protected against some poisons, because they do not get into the brain. The so-called blood-brain-barrier tends to exclude molecules that are water-soluble, that are large and/or that are charged or ionized. Small, lipid-soluble, neutral compounds penetrate the blood-brain-barrier readily. Physostigmine gets into the brain, and this property is an advantage in the treatment of atropine poisoning, because it can antagonize the central effects of atropine on muscarinic receptors. That same property of physostigmine is a disadvantage in the management of myasthenia. Anticholinesterases have potentially lethal effects in the brain as we saw in the last story. Nicotinic receptors in the brain of myasthenic patients are not affected by the disease, because the antibodies against the peripheral receptors do not penetrate the blood-brain-barrier. Neostigmine and pyridostigmine are "designer" synthetic drugs based on the structure of the naturally occurring, physostigmine. They were deliberately modified to make them highly charged, which keeps them out of the brain and in the periphery, where they can act at the myoneural junction to increase transmission.

Physostigmine is found in the seed of a plant native to West Africa. It is also called the calabar bean or ordeal bean. In a primitive system of native justice, people accused of witchcraft were forced to consume calabar beans. Innocence was thought to be rewarded by survival; guilt by death.

Other, more dangerous or invasive, treatments for refractory myasthenia include immunosuppressive drugs such as glucocorticoids or cyclosporine in an attempt to suppress the production of the abnormal antibodies. Plasmophophoresis can be done to remove the antibodies with plasma, which is replaced with normal plasma.

This is truly a pathetic story in which the diagnosis was missed time and time again even by specialists who should have been able to recognize the condition. If only the young physician had not taken so literally the textbook description of initial signs and symptoms of drooping of the eyelids (ptosis) and difficulty in swallowing, the diagnosis might have been made two years earlier. Instead Sheila was given anti-anxiety and anti-depressant medication that did no good, and may have had undesirable side effects. She thought that the ElavilR (amitryptyline) made things worse. She may have been right, but the anticholinergic side effects of amitryptyline are usually directed against muscarinic receptors. At the same time, there are a host of other drugs and toxins that are deleterious to the myoneural junction and may aggravate muscle weakness in myasthenic patients.

Additional reading:

Verma, P. and J. Oger. Treatment of acquired autoimmune myasthenia gravis: A topic review. Canad. J. Neurolog. Sci. 19: 360-375 (1992).


 

The story "Family Reunion" can be found in Rouché, B.,
Eleven Blue Men and Other Narratives of Medical Detection, Little Brown and Company, Boston, 1953. (Out of print)

Also found in Rouché, B., Annals of Epidemiology, Little Brown and Company, Boston, 1967. (Out of Print)


 

"Family Reunion" Addendum

The gram-positive, spore-forming anaerobic bacteria that make up the Clostridium family include some nasty critters. Besides C. botulinum, there is C. tetani, the cause of tetanus and C. perfringens and C. septicum, causes of gas gangrene. Botulism was first recognized in the late 1700s and early 1800s in the kingdom of Würtemburg as a result of the consumption of spoiled sausages (botulus is Latin for sausage). It is a coincidence that the organism itself is a sausage-shaped rod. It is found in soil world wide. Different strains of C. botulinum produce eight antigenically distinct exotoxins, meaning that the toxin is secreted by the bacteria into the surrounding media. The toxins are designated as types A through G (there are two types of C toxin), but only types A, B, E and F are well established causes of human disease. An antibody raised against one toxin type fails to neutralize other toxin types. Thus, the therapeutic antitoxin obtained from horses is polyvalent, meaning that it contains more than one antibody, usually A, B and E. The toxins are large globular proteins, and they include some of the most toxic materials known to man. A dangerous dose of the A type toxin in man might be on the order of 1/1,000 of a milligram. It is estimated that a little over an ounce of botulinum toxin could be enough to kill every human on earth. The first investigators to purify and crystallize A toxin described it with a macabre sense of humor as a "white odorless protein of........unknown taste".

To say that the last outbreak of botulism in the United States involving a commercial packer was in 1925 is a public admission of an inadequate literature search. It is easy to locate reports of sporadic outbreaks up to the present time. Since more than half of all cases terminate fatally, it is indeed fortunate that such occurrences are rare. Except for isolated cases, as pointed out in the story, botulism is largely a result of human activity, and its sudden appearance in larger numbers of victims around the turn of the eighteenth century coincident with the spread of food preservation practices must have been baffling to early investigators.

In addition to adequate heat treatment, a more controversial measure to prevent botulism has been adopted by the meat packing industry, namely pickling in a brine containing, among other ingredients, potassium nitrate or salt-peter. The food industry claims three beneficial effects in these "cured" or "processed" meat products such as hams, bacon, balogna, etc. It imparts a characteristic taste; it is responsible for the appealing pink color of the meat; and, the food industry maintains it prevents the growth of C. botulinum. The Food and Drug Administration remains unconvinced about the efficacy of the latter, and they have been trying in vain for years to make the use of nitrates/nitrites in food illegal. The problem is that nitrate, which is biologically inert, is converted in part to nitrite during the process. Nitrite, as we will see, has toxic effects, and it can be converted to other toxic chemicals, such as nitric oxide and carcinogenic nitrosamines, in the food product or in the body. In the meantime, the amounts of nitrate and nitrite in processed meats are carefully regulated.

Few people today would agree that the practice of eating fruit sprayed with lead arsenate or other conventional insecticides is an innocuous practice. Aluminum hydroxide is safe in normal adults, but in victims of renal failure it has produced "dialysis dementia". At one time tin cans in the United States had seams that were soldered with materials containing lead. The inside of the can was shellacked to minimize the leaching of lead, but minuscle amounts often got into the contents. This practice is now banned, but it might still be true in other countries.

It is surprising that the botulinum toxins survive digestion in the stomach, the fate of normal protein, and also surprising, because of their size, that they are absorbed into the blood stream. Indeed, some evidence exists that the toxins are actually activated by proteases in the gastrointestinal tract. In the blood stream the toxin finds its way to cholinergic nerve terminals, where the larger of the two protein chains attaches itself irreversibly to the nerve membrane, and the smaller of the two chains is transported into the nerve. Once in the nerve, all seven serotypes of the toxin block the release of acetylcholine in response to a normal nerve action potential. The toxin is denied access to the central nervous system by the blood-brain-barrier. The signs and symptoms are primarily a result of transmission failure at muscarinic junctions, myoneural junctions and to a lesser extent, at autonomic ganglia. Thus, botulism is a sort of combination of atropine and curare poisonings, or atropine and nicotine poisonings without the initial excitatory phase.

In cholinergic nerve terminals, the acetylcholine is stored in tiny vesicles each containing millions of molecules of the neurotransmitter. When an action potential reaches the nerve terminal, calcium enters the nerve and promotes the fusion of vesicles in close proximity to the inside of the nerve membrane with that membrane. Both the vesicle membrane and the nerve membrane open, and the vesicular contents are dumped into the synaptic cleft (exocytosis). The vesicles are "guided" to specific sites on the interior of the nerve membrane by a so-called docking complex on the exterior of the vesicles. The docking complex has three distinct protein components: synaptobrevin, SNAP 25 and syntaxin. Different toxin types enzymatically cleave different proteins in the docking complex. Each docking complex protein is cleaved by at least one toxin. When the docking complex proteins are damaged, the vesicle can no longer dock at the appropriate site on the interior of the nerve cell membrane to release its content of neurotransmitter, and transmission is blocked. Thus, botulinum toxins have become exquisitely sensitive and powerful tools for dissecting the intricate mechanisms involved in not only cholinergic transmission, but other types of neurotransmission as well, which use the same docking complex mechanism.

Botulism is acquired after the consumption of food contaminated with the toxin. However, the statement in the story that "the organism is incapable of establishing itself in man" is not correct. Wound botulism can occur when traumatic wounds are contaminated with soil, in chronic drug abusers and after cesarean delivery. Under such circumstances, the spores may germinate into viable organisms that secrete the toxin. The syndrome is very much like the food-borne illness, but it is an exceptionally rare disease. In infant botulism, the most common form of the disease, ingested spores or inhaled and subsequently ingested spores germinate in the large intestine and produce toxin that is absorbed. Cases usually occur in infants under six months of age, the same pattern as in the sudden infant death syndrome. One identified source of infant botulism is honey containing the spores

The failure of cholinergic transmission begins with the cranial nerves innervating the head, face, mouth and throat (the bulbar musculature), and progresses gradually to the extremities. As in the more typical cases of myasthenia, the first signs are visual and difficulty in swallowing and speaking. In Mr. Pappone's case his vision was blurry and double. Often the eyelids droop (ptosis) as in myasthenia. The headache may have been coincidental as also was the heart attack of Mr. Gagliono. The weakness can progress rapidly from the head to involve the neck, arms, chest and legs. The absence of fever points away from infectious diseases such as polio or meningitis. A descending paralysis in a nonfebrile, alert patient with no sensory impairment is suggestive of botulism. It is difficult to see how this syndrome could have been confused with atropine poisoning as in the story "Something a Little Unusual". The definitive diagnosis is somewhat complicated unless the toxin can be identified in the food consumed, and treatment is often instituted on the basis of clinical findings. Complete recovery involves the sprouting of new nerve terminals. When these sprouts locate skeletal muscle, they re-innervate them after inducing the synthesis of new motor end plates, a process that may take a month.

The antitoxin is effective only against free toxin circulating in blood. It has no effect against toxin that has already entered nerve terminals. Antibiotic therapy is usually not warranted except in wound botulism and secondary infections. Good nursing care is essential.

Oddly, botulinum toxin has found use in clinical medicine in the treatment of certain muscular disorders such nonparallel visual axes of the eyes and spasmodic winking. With these modest beginnings, the list of conditions in which the toxin might be of benefit has expanded rapidly to a variety of skeletal muscle disorders such as cerebral palsey and other spastic states. There may be applications to gastrointestinal smooth muscle as well. More recently, it was found effective in the management of chronic anal fissures. Local infiltration into the internal anal sphincter produces relaxation that allows the fissure to heal. Needless-to-say, the doses in these applications are very carefully controlled.

Additional reading:

Arnon, S. S. Chapter 7, Human tetanus and human botulism. In The Clostridia: Molecular Biology and Pathogenesis, J. I. Rood, B. A. McClane, J. G. Songer and R. W. Titball, Eds., Academic Press, San Diego, 1997.


 

The story "A Pinch of Dust" can be found in Rouché, B., Eleven Blue Men and Other Narratives of Medical Detection, Little Brown and Company, Boston, 1953. (Out of Print)

Also found in Rouché, B., Annals of Epidemiology, Little Brown and Company, Boston, 1967. (Out of Print)

The story "The Case of Mrs. Carter" can be found in Roueché, B., The Orange Man and Other Narratives of Medical Detection, LittleBrown and Company, Boston, 1971. (Out of Print)


 

"A Pinch of Dust" Addendum

The contrast between the clinical presentations of botulism and tetanus could hardly be more dramatic. To further confuse the picture, tetanus was recognized as a distinct disease entity since the time of Hippocrates, whereas botulism seemed to suddenly appear in the late 1700's to early 1800's. Physicians of that era would have no reason to think that there was any relationship between the two diseases, until it was shown that they were caused by two species of the same genera of bacteria. There is even considerable overlap in the chemical makeup of the two types of toxins, and in their mechanisms of action. What then accounts for the paralysis of botulism vis-a-vis the spastic convulsions of generalized tetanus?

As we saw in the last addendum, botulism is occasionally the result of a wound, but that is the usual way in which tetanus is acquired. Many recreational drugs are injected intravenously, but at least one (Charles Williams), and possibly all, of the addicts in this story used subcutaneous injections, or "skin popping". This provides an ideal environment for the spores to germinate into viable, toxin-secreting bacteria. In the blood stream, the spores are apt to be destroyed by white blood cells or other body defense mechanisms. From the subcutaneous sites, the toxin finds its way to the blood stream, and then to myoneural junctions. To a lesser extent it seeks out other cholinergic junctions as well. Like botulinum toxin, tetanus toxin enters the nerve terminal, but unlike botulinum toxin, it keeps going, traveling backwards by axoplasmic flow all the way up the nerve axon to the cell bodies in the spinal cord and brain stem. Nor, does the journey stop there, because it leaves the cell bodies and travels in a retrograde fashion across the synapse to enter the terminals of nerves that impinge on or innervate the neuron it had just departed. These interneuron final targets are not cholinergic nerves; they secrete either glycine or gamma-amino butyric acid (GABA).

Acetylcholine is usually an excitatory neurotransmitter. When it activates a cholinergic receptor, muscles contract and glands secrete. Glycine and GABA are examples of inhibitory neurotransmitters. When these transmitters activate their own receptors on a somatic nerve, they tend to inhibit muscular activity. These transmitters are also stored in vesicles that have docking complexes like those in cholinergic nerves, and tetanus toxin again interferes with the docking mechanism by cleaving the protein, synaptobrevin, which is also the target for four of the botulinum toxins. Interrupting inhibitory neurotransmission is like releasing the brakes on an automobile while the accelerator is fully depressed. All of the skeletal muscles of the body are eventually contracted simultaneously, and the body assumes a position dictated by the strongest muscle groups. Hence, the bizarre posture of opisthotonus in which the back is arched, and the body rests on the heels and back of the head. Contractures may be so violent that vertebrae are fractured, and muscles assume a board-like rigidity. High fevers may result because of the excess expenditure of metabolic energy.

Patient in opisthotonus from Taylor, D. A., Chapt. 31. Central Nervous System Stimulants. In Craig. C. R. and Stitizel, R. E. Modern Pharmacology with Clinical Applications, 5th ed. Little Brown and Company, Boston, 1997

Trismus or lockjaw is due to spasmodic contraction of the muscles of chewing, the masseters. The story did not mention another distinctive feature of generalized tetanus. Contraction of the facial and buccal muscles results in a grotesque sneer, called risus sardonicus or risus caninus. In the 1960's the Harvard mathematician turned novelty song writer, Tom Lehrer, wrote these lyrics, "Her mother she could never stand, and so a cyanide soup she planned. Her mother died with her spoon in her hand, and her face in a horrible grin." He should have said a strychnine soup. Strychnine, although much less potent, also produces opisthotonus and risus sardonicus. Strychnine blocks glycine receptors in the spinal cord, which is another way of shutting down inhibitory neurotransmission. Occasionally, tetanus toxin (formerly known as tetanospasmin) may result in autonomic hyperactivity with tachycardia, hypertension, sweating and fever of non-metabolic origin. More rarely, tetanospasmin may act like botulinum toxin to produce weakness or paralysis. With either toxin recovery from this effect requires the sprouting of new nerve terminals. Tetanus toxin is about one tenth as toxic as botulinum toxin, but that is still almost beyond belief.

Strychnos nux vomica, a small tree native to India, the Malay peninsula and Australia from which strychnine is isolated, with orange-sized fruit and fruit cut open.

Drawing in ink and watercolor by Louise Glass, Piermont, NH. The original is the property of Dartmouth College and hangs in Dana Biomedical Library.

 

Like botulinum toxin, tetanus toxin does not gain access to the central nervous system, but the implications of that exclusion in the case of tetanus are truly horrifying. Without medical intervention, the victim is conscious and in extreme pain awaiting the next tetanic seizure. Seizures can be triggered by sensory input, and the smallest or slightest of sounds, touches or visual stimuli may trigger a spasm. Patients should be isolated and protected from all unnecessary sensory stimuli.

The rare cases of tetanus in the United States are usually among the indigent, who were never immunized, senior citizens who neglected to stay up to date with booster shots, or among recent immigrants, from third world countries without immunization programs. In such countries, infantile tetanus takes a very high toll; hundreds of thousands of neonates die each year simply because their mothers were not immunized. Packing umbilical stumps with materials such as dung or soil is responsible for many infections. In addition, tens of thousands of women also die yearly from maternal tetanus after live births or abortions. The diagnosis is made on the basis of the clinical findings. Treatment includes antibiotics to eradicate any viable organisms, human tetanus immune globulin or bovine-derived tetanus antitoxin to neutralize any remaining free tetanus toxin, diazepam (ValiumR) and related drugs usually relieve the muscle spasms, but curare-like drugs may be required in severe cases. Remarkably, botulinum toxin has been used successfully in a few cases. Any such muscle relaxant in effective doses is apt to result in paralysis of the respiratory musculature requiring mechanical support. The usual cause of death in tetanus is respiratory failure. The victims cannot ventilate so mechanical support is again essential.

There is a bit of a mystery about strychnine poisoning and tetanus when compared with anticholinesterase poisoning or the therapeutic effects of succinylcholine. All of these are due to excessive cholinergic transmission at the myoneural junction, so why is it that strychnine and tetanus do not cause depolarization-desensitization blockade and flaccid paralysis as do anticholinesterases and succinylcholine? I can only guess at the answer, but I suspect that the phenomenon of depolarization-desensitization blockade is critically dependent on the concentration of acetylcholine in the vicinity of the motor end plate. In that case the excessive cholinergic transmission induced by strychnine and tetanus must not reach the critical concentration of agonist at the motor end plate that can be achieved by cholinesterase inhibition or succinylcholine.

"The Case of Mrs. Carter" Addendum

The resemblance of the stiff-man syndrome to tetanus and strychnine poisoning was recognized by many of the people who examined Mrs. Carter, but they were unable to extrapolate to etiology and treatment until she went to the Mayo Clinic. She must have some sort of failure of GABAergic transmission. At one point she was given phenobarbital, which is now known to be a GABA agonist that is effective in the major motor seizures of epilepsy. It was prescribed, however, for nervousness, and, indeed, it does have mild antianxiety effects. Despite the fact that phenobarbital was a step in the right direction from two standpoints, it did not produce dramatic relief of signs and symptoms. Perhaps the dose was not large enough, but larger doses are apt to produce daytime sedation.

She was also given mephenesin and chlorzoxazone, drugs that are closely related to meprobamate. None of these is used today, but all have muscle relaxant activity, and meprobamate in particular had marked "tranquilizing" effects. It became very widely used in the treatment of anxiety. Unfortunately, like phenobarbital, it was capable of producing dangerous depression in overdose. Although structurally unrelated, meprobamate had many properties in common with the newer class of drugs that are now known as the benzodiazepines, the premier example of which is diazepam (Valium). Diazepam, of course, was ultimately shown to be highly effective in the stiff-man syndrome. As a GABA agonist, diazepam was much more effective in restoring synaptic inhibition than the earlier drugs, and, equally important, it did not produce dangerous central nervous depression even in large overdoses.

However, diazepam does share with the earlier drugs one dangerous property, in high doses for prolonged periods of time it is capable of producing physical dependence with an abstinence (withdrawal) syndrome. In Mrs. Carter's case the abstinence syndrome took the form of status epilepticus, a series of unremitting convulsions that is extremely dangerous. There is the distinct possibility of death in hypoxia because it is impossible to breathe during major motor seizures. Dr. Cohen was able to recognize immediately that this was a withdrawal reaction, and not a recurrence of the stiff-man syndrome. The latter, however, could equally well have been anticipated under the circumstances. She was treated with pentobarbital, which should have been given intravenously to be most rapidly efficacious, although the narrator does not tell us that. Today, the treatment of choice would be intravenous diazepam.

Despite the many similarities between styrchnine poisoning, tetanus and the stiff-man syndrome, there is one major difference, namely, the time course. The syndrome of strychnine poisoning rarely lasts more than a day. Tetanus is longer lasting because full recovery involves the regeneration of new nerves, but recovery should be complete in a few weeks. Without intervention, there seems to be no end to the stiff-man syndrome. The thinking today is that the stiff-man syndrome is an autoimmune disease, as also is myasthenia gravis, because (1) antibodies are found in the plasma of victims directed against glutamic acid decarboxylase (GAD), the rate-limiting enzyme for the synthesis of GABA, (2) the association of the disease with other autoimmune conditions, (3) the presence of various autoantibodies, and (4) a strong immunogenetic association. With that recognition, in addition to drugs that enhance GABAergic transmission, the therapy can be expanded to immunomodulatory measures such as steriods, plasmapheresis and intravenous immunoglobulin. These measures are also effective in myasthenia gravis.

Additional reading:

Schiavo, G. and C. Montecucco. Chapter 18, The structure and mode of action of botulinum and tetanus toxins. In The Clostridia: Molecular Biology and Pathogenesis, J. I. Rood, B. A. McClane, J. G. Songer and R. W. Titball, Eds., Academic Press, San Diego, 1997.

Scully, R. E. et al., Case records of the Massachusetts General Hospital. N. Engl. J. Med. 344:1232-39 (2001).

Levy, L. M., M. C. Dalakas and M. K. Floeter. The stiff-person syndrome: An autoimmune disorder affecting neurotransmission of gamma-aminobutyric acid. Ann. Intern. Med. 131:522-30 (1999).