Table of Contents
- 1 Alzheimer’s and Dentistry
Alzheimer’s and Dentistry
Recently, there have been news stories about the possible transmission of Alzheimer’s disease through dental instruments. To date, there is very little research to back this assertion up. Essentially, the question about the transmissibility of Alzheimer’s disease revolves around the similarity between the clinical and neuroanatomical properties of Alzheimer’s and certain other brain diseases such as Creutzfeldt-Jakob Disease (CJD), Fatal Familial Insomnia (FFI),Gerstmann-Straussler-Scheinker disease (GSS) and Kuru in humans,. There are similar diseases in other animals including Bovine Spongiform Encephalitis (BSE) in cattle, Chronic Wasting Disease (CWD) in Deer, and Scrapie in sheep and goats. All of these cause serious deterioration of the brain, and all eventually lead to the death of the person or animal. They all fall into the category of a class of illnesses called Transmissible Spongiform Encephalopathies (TSEs). Scientists have known about the similarities between Alzheimer’s disease and these wasting diseases for many years, but so far have not drawn the conclusion that Alzheimer’s is related to them. More research is needed, and almost certainly will come about over the next few years.
In the meantime, I have created this page to help interested patients to understand more about the TSEs. If Alzheimer’s turns out to be a member of this class of diseases, then you will already be armed with the information you need to make informed decisions. As more research becomes available, I’ll update this page. The bulk of the information here pertains to Creutzfeldt-Jakob disease (CJD), the most common form of TSE in humans.
Before I start, let me make a couple of things perfectly clear
1. The general medical consensus about Alzheimer’s disease is that with the exception of rare known genetic mutations, it is associated with multiple factors including advanced age, family history, cardiovascular disease, declining social and cognitive engagement, and educational attainment (People with fewer years of formal education are at higher risk for Alzheimer’s and other dementia than those with more years of formal education.) The association of Alzheimer’s with these factors is not just conjecture. There is solid statistical evidence that Alzheimer’s is likely to be a genetic predisposition exacerbated by the factors mentioned above.
In addition, the symptoms of Alzheimer’s are very different from those of other TSEs. Alzheimer’s causes a gradual progression of dementia, memory loss and personality changes. The symptoms of the other TSEs appear relatively suddenly and progress rapidly, generally killing the patient within one or a few years. Kuru causes, among other symptoms, uncontrollable laughter and tremors. Fatal Familial Insomnia kills by depriving its sufferers of sleep. None of these symptoms are seen in Alzheimer’s.
2. The link currently being drawn between Alzheimer’s and Creutzfeldt-Jakob Disease is based on the presence of a protein in the brains of patients with advanced Alzheimer’s. This protein is similar to one found in the brains of patients with CJD. Because of the biochemical and neurological similarities between Alzheimer’s disease and CJD, a few scientists believe that Alzheimer’s is actually a slow growing variant of CJD. For this reason, this paper emphasizes the etiology (cause) and epidemiology (how it spreads) of CJD, which, if these scientists are right, should be a good model for the etiology and epidemiology of Alzheimer’s and all the other TSEs (Transmissible Spongiform Encephalopathies) as well,—that is if Alzheimer’s turns out to be one of the TSEs
3. Very few people have actually been infected with CJD during surgical procedures, and none since 1976. No one has ever been infected through dental procedures.
About 85% of all CJD cases appear as “sporadic cases”. This means that the cases are the result of either random mutation of a gene or by a random accident causing a protein to unfold and refold in a pathological way. Another 10% of CJD cases are transmitted genetically from parents to their children.
Only about 5% of the people with this rare disease contract it through any form of contact with the environment. The vast majority of that 5% contract the disease through eating infected beef (mad cow disease–BSE).
Diseases and injuries caused by doctors or other medical personnel are labeled “iatrogenic“. Only a tiny fraction of the the iatrogenic cases of CJD reported worldwide were caused by contaminated surgical instruments. As of 2015 only 6 out of a total of 469 cases of iatrogenic CJD or its variant, vCJD (Varient of Creutzfeldt-Jakob Disease) were caused by surgical instruments or devices.
The bulk of the remainder were caused by contaminated allogenic dura mata grafts and growth hormone injections. (The dura mata is the membrane that covers the brain. An allogenic graft is the transfer of tissue from one patient to another–most frequently to close a traumatic injury.)
This very low rate of iatrogenic transmission of CJD implies that even if Alzheimer’s is transmissible through contaminated surgical instruments, its rate of transfer through such means would be very low as well.
More information about the nature of the iatrogenic cases of CJD and about modern sterilization techniques can be found at the end of this paper.
4. If it turns out that Alzheimer’s can be transmitted between patients through contaminated surgical instruments, the risk of transmission would not be limited to medical or dental instruments. The risk would be the same for tatoo needles, body piercing instruments, blood transfusions and shared drug needles.
5. Alzheimer’s disease is NOT the only human disease to resemble CJD in their neurological effects. Parkinson’s disease and Amyotrophic Lateral Sclerosis (ALS) also fall into this category.
CJD also occurs sporadically (spontaneously) in cattle, and is spread to other animals through feed manufactured using infected bovine brain and spinal chord tissue. The agent that causes CJD in humans is nearly identical with the one that causes Bovine Spongiform Encephalitis (BSE). The agent that causes BSE is slightly different than the one that causes CJD. This variant is called vCJD, and when it is found to be the cause of human illness, it marks a case of CJD that was acquired from eating contaminated beef. The common name for BSE in cattle is “mad cow disease”. vCJD is not transmitted through steaks or other muscle meats unless they were contaminated with brain tissue during processing. A small number of cases (both CJD and vCJD) have been transmitted through blood transfusions from infected persons.
Kuru, the fourth TSE mentioned in the first paragraph, is a degenerative brain disease identified in the Fore people, an obscure and rather well isolated tribe in Papua New Guinea. It is transmitted through cannibalism. However with the modern suppression of cannibalism, it is now nearly extinct.
FFI and GSS are both exceedingly rare and are genetically transmitted diseases. Familial CJD, FFI and GSS are all caused by different point mutations on the same chromosome (chromosome 20).
None of these diseases is transmissible through the air or through touching, kissing or most other forms of casual contact.
What causes TSEs?
TSEs are known to be transmissible through an infective agent called a “Prion” (rhymes with neon). A prion is not a bacteria, a yeast, a parasite or a virus. All of those agents contain DNA (or RNA) which is the molecule that is responsible for the the replication of the infective agent. Prions have NO nucleic acids to transmit their genetic information. They are a sort of “vampire” variation of a simple protein that has the remarkable ability to transform neighboring proteins of the same type into identical vampire proteins.
The image above shows what appears to be two different proteins, but in reality are the both the same, each one with identical amino acid chains. The difference is ion the way they fold themselves together. The function of any given protein is dependent on its three dimensional shape, and it is apparent that the two forms pictured above would not serve the same function. The two forms are called isomers, and it is the ability of the misfolded isomer to stimulate the same transformation in its normally shaped sibling.
If a prion enters a healthy organism and finds its way to the brain, it will induce existing, properly folded proteins there to transform into the misfolded prion form. In this way, the prion acts as a template to guide the misfolding of more proteins into prion form. These prions join together in string-like strands which grow longer, and replicate by breaking into two strands, then four, then eight, then sixteen…etc. in a geometric fashion. These strands continue growing and breaking into more and more strands until they tangle together to form sheets, and the sheets form into clumps. This cascading transformation happens rather quickly within the brain turning functional proteins into a scar-like mass of non functional tissue called amyloid. As this stuff spreads, the brain and spinal tissue come to resemble (microscopically) a sponge, with countless holes connected by isolated nervous and connective tissues, along with clumps and strings of amyloid.
It should be noted that in Alzheimer’s disease, a similar form of amyloid forms and crowds out normal brain tissue which is what causes dementia. The main differences between the clinical appearance of CJD and Alzheimer’s are the speed of their respective progress, the order of the appearance of symptoms and the degree of sponginess seen in histological sections of the brain. Alzheimer’s progresses much more slowly than CJD, and the slower progress of Alzheimer’s may account for the differences seen in the microscopic appearance of their histological sections.
Tissue histology involves taking thin slices of tissue, processing them using stains, and looking at them under a microscope. The three images above are presented to compare the differences in the microscopic appearance of tissues from normal, Alzheimer’s and CJD brains. The most stark difference is the appearance of the holes in the CJD specimen when compared with the normal brain section. This gives the specimen the appearance of a sponge. Hence the name “Spongiform Encephalitis” when one refers to diseases of this nature. The Alzheimer’s brain also shows the formation of vacuoles (holes), although of a different kind than the ones seen in the CJD slide. The arrows point to amyloid plaques in the Alzheimer’s slide. The amyloid plaques in the CJD slide are not obvious, but are scattered all over the slide as solid pink blobs.
An overview of the biochemistry of prions
Normally, the only molecular components in a living cell that can replicate themselves are long molecules of DNA (or in the case of retroviruses, RNA). The process is enormously complex using an incomprehensible combination of nucleic acid components (the building blocks of DNA and RNA) and large numbers of equally complex protein “helpers” called enzymes. The replication of prions does not require any of this complexity. It only requires that a single rogue variant of a protein comes into contact with another normal protein of the same type. Contact between the rogue protein and the normal neighboring protein causes the neighboring brother to become a second rogue protein. Now there are two rogues which go off and transform their neighbors…..and so on, until the entire brain becomes filled with masses of this scar-like tissue.
In order to understand what a prion is, you have to understand what a protein is. In short, different proteins constitute almost all of the building blocks in your body, and do all of the biochemical work that your body needs to do to keep you alive. Collagen is the protein that makes up most of the connective tissue in your body. Hemoglobin is the protein that carries oxygen in your red blood cells. Fibrin is a complex protein that makes your blood clot. An unimaginable number of proteins operate as individual microscopic machines to carry out complex metabolic and other cellular functions. The normal protein that can be transformed into the rogue protein is called the “Prion Protein” (PrP). In its normal form it is called PrPc–(the c stands for Common). It is found on cell membranes throughout the body, but mostly on the membranes of neurons in the brain and spinal chord. The abnormal form is called PrPscr or PrPsc–(The scr and the sc stand for scrapie. Scrapie is the TSE that infects sheep).
Proteins are very long chains of small molecules called amino acids. There are twenty amino acids all together, and any amino acid can occupy any position along the protein chain. The exact order of the amino acids along the chain determines the final function of the protein.
The exact order of the amino acids in the protein chain is specified by the order of nucleotides in chains of DNA within the cells’ chromosomes. The order of the nucleotides in the string of DNA acts as a sort of code, and the translation of this code is the beginning of a remarkably reliable and complex manufacturing process that produces a specific protein. The string of DNA that contains the code for a specific protein is called a gene. Any given gene always specifies exactly the same protein, and without fail, all the proteins that a given gene produces are identical. The Gene acts as a sort of permanent blueprint to make a constant supply of the same protein chain.
Protein chain folding
The basic three dimensional form that proteins naturally take is a helix. They are called alpha helices. These alpha helices tend to fold over to form secondary three dimensional shapes including beta sheets as well as amorphous forms. The folding of an alpha helix into its secondary forms is dependent on the exact order of the amino acids in the chain. Some amino acids have hydrophilic (water loving) side chains, and some have hydrophobic (water repelling) side chains. Some side chains have positive charges and some negative charges. The exact shape of the side chains enters into the equation as well. The interplay of all these forces, as well as the order in which the amino acids are added to the chain determine the exact way a protein folds as it forms. The way it folds determines its biological function. Folding is everything when it comes to protein function.
Sometimes however, a random mutation causes a gene to suddenly start producing an aberrant protein, and sometimes, in a rare occurrence, a protein might just fold itself in a new shape. These are the two ways that the rogue prion might come about. Obviously, if something goes wrong with the blueprint, the final product will be adversely affected. However, an unfortunate folding error takes some additional explaining.
Even though most proteins fold themselves into stable shapes, some shapes are more stable than others. Some proteins have several stable states. In this case, each stable shape is called an isomer. As a rule, during protein formation, the chain tends to fold into its normal isomeric state, but upon rare occasion, something interferes with the normal folding process and allows a protein to fold into one of its other stable isomer states. In most cases, the misfolded protein has lost its normal function and eventually is simply recycled or excreted from the cell. But if the new state happens to be thermodynamically more stable than the original state, then, also in very rare instances, the new isomer can become a prion. Bad things can happen when this new prion comes into close contact with a normally folded isomer. The normal protein chain may be stimulated to “flip” into the more stable state creating a second prion identical to original prion that caused it to flip. For people who remember their high school chemistry, it happens a bit like the crystallization of a solution when a single crystal is introduced into a saturated solution. All the dissolved molecules are stimulated to crystallize, fanning almost instantly out from the added crystal.
There are three ways in which the original prion particle may be acquired by a host:
In Familial transmission, something has gone wrong with the DNA that created the protein in the first place. In the case of fatal familial insomnia (FFI) and Gerstmann-Straussler-Scheinker disease (GSS), this is exactly what has happened. In those cases, the gene that codes for the original protein mutated sometime during the genetic history of the family, and was passed on through succeeding generations.
In sporadic Transmission, a random mutation in the DNA of the gene that codes for PrPc causes the gene to produce PrPscr instead. This can happen at any time in a person’s life. In this case, the disease is in the “sporadic” category, and is not genetically passed on to that person’s children.
A second form of sporadic transmission may be as a result of a spontaneous unfolding of a normal PrPc protein and subsequent refolding into its pathological PrPscr form.
Sporadic transmission is by far the most frequent mode of transmission of PrPscr.
Only about 5% of all cases of CJD are acquired. There are several modes of acquiring CJD or vCJD that are not sporadic or genetic.
1. Orally by eating BSE infected beef (especially brain or spinal chord). this type of infection in humans is called vCJD.
2. Iatrogenic transmission is transmission through medical procedures. This involves mostly neurological grafts and contaminated growth hormone injections. As you can see by the table below, out of 469 cases of iatrogenic CJD worldwide, only four cases were attributed to surgical transmission, and those were all neurosurgical procedures. Note that none have ever been reported from dental instruments.
The following is a list of case types that have been known to transmit CJD. Each case type is followed by the number of cases reportee:
- Dura mater graft–228
- Neurosurgical instruments–4
- Stereotactic EEG needles–2
- Corneal transplant–2
- Growth hormone–222
- Blood transfusions–3
Below is a table recounting the number of cases in the UK from 1990 to 2015. Note that the bulk of the cases were caused by sporadic (spontaneous) infection and were not acquired from any external source. Note also the jump in the number of cases (vCJD) corresponding with the BSE epidemic among cattle between 1995 and 2011.
What about the 469 reported cases of iatrogenic transmission of CJD?
According to this table from the National Institute of Health (NIH), all but six of the iatrogenic (doctor caused) transmission cases were due to transplantation of tissues from cadavers to patients during surgery. In other words, tissues were removed from the bodies of recently deceased people who had CJD and transplanted into patients as tissue grafts. 288 of them were dura mata grafts. The dura mata is the covering of the brain and the grafts were performed to repair traumatic injuries to living patients’ skulls. Another 230 involved injections of growth hormone or gonadotropin obtained from the pituitary glands of similar cadavers. (Since 1985, human growth hormone has been manufactured by the use of recombinant DNA technology, which eliminated this risk.) Two additional cases involved corneal transplants, also from infected cadavers.
Of the remaining six cases, only two are proven to have been infected by way of a medical device. Both cases involved the implantation of stereotactic electrodes used to find the location of the focal points where seizures begin within the brain of epileptics. This happened at a time when normal means of sterilization were not available for this type of instrument. They were cleaned with benzine and alcohol and sterilized using formaldehyde vapor. This type of sterilization is no longer done today.
The other four cases turned out to be somewhat equivocal in that all of the cases of CJD associated with neurosurgical instruments occurred in Europe during the period from 1952 through 1976, and details of the methods used to reprocess the instruments are incomplete. Prior to 1982, no one had ever heard of a prion or had any idea that an infection could be transfered by anything other than an organism that did not reproduce with DNA or RNA.
Prions and the sterilization of surgical equipment
Hospital sterilization utilizes steam under pressure to heat surgical instruments, handpieces and other implements used to kill all living organisms that might cause infection. No living thing can survive direct exposure to saturated steam at 250F (121C) longer than 15 minutes. As temperature is increased, the same results can be achieved in less time. Instruments must first be thoroughly cleaned prior to heat sterilization. Most medical practices use an ultrasonic unit to clean the instruments prior to placing them in the autoclave. Modern dental offices and and medical practices follow these guidelines in order to sterilize their instruments. Hospitals often take additional precautions, especially when dealing with instruments used in neurosurgery. This includes a presoak in solutions that denature proteins.
In order to fully remove the very slight risk of exposure to prions, medical and dental practices would need only to add a single step to their existing sterilization procedures. That step would be an hour long presoak in sodium hydroxide solution prior to normal sterilization in a steam autoclave. The cost of the solution would be minimal, but the cost in time and the frequent need to buy new instruments due to the caustic nature of the presoak would add substantially to the cost of dental and all in-office surgical services. Considering that there have been no documented cases of surgical instrument transnsference of any known prion mediated disease except the six reported prior to 1976, (all of them involving direct surgery on the brain), it seems unlikely that there will be any federal or international recommendations for a change in sterilization procedures in small offices unless future research demonstrates a real risk of iatrogenicly introduced infection by newly discovered prion diseases.
Baring more research showing that additional diseases are caused by prions (including Alzheimer’s disease), and that they are more prone to transmission through surgical instruments than CJD, (including root canal files and other dental instruments), it seems unlikely that the general public has any reason to fear having dental or any other form of surgery.
It seems, in the light of the information above, very unlikely that the transmission of Alzheimer’s disease, if indeed Alzheimer’s turns out to be caused by a prion, would happen in any modern medical or dental practice today.