Neuropathies associated with cytomegalovirus infection
Nov. 15, 2022
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In this article, the author reviews the group of diseases known as prion diseases, also referred to as the subacute spongiform encephalopathies. These diseases have a subacute to chronic clinical course with a similar neuropathology. All the diseases are transmissible and induced by an abnormal misfolded form of the prion protein that is extremely resistant to physical and chemical inactivation. The unusual nature of the transmissible agent and the emergence of variant Creutzfeldt-Jakob disease (as a result of ingestion of contaminated beef) have had a significant impact on public health in addition to science and medicine. New diagnostic tests, such as protein misfolding cyclic amplification and real time quaking-induced conversion, and new ideas about treatment of the subacute spongiform encephalopathies are discussed.
• The transmissible subacute spongiform encephalopathies, or prion diseases, have a similar noninflammatory spongiform pathology and are caused by a similar transmissible agent -- an abnormal (“scrapie-like”) protease-resistant conformation of the prion protein (PrP), which is designated PrPSc.
• Sporadic human prion diseases include Creutzfeldt-Jakob disease and fatal sporadic insomnia. Prion diseases acquired by infection include kuru (a subacute cerebellar disease found in the Highlands of New Guinea that was spread by ritual endocannibalism), variant Creutzfeldt-Jakob disease, and iatrogenic Creutzfeldt-Jakob disease. Familial human prion diseases, which are 10% of the overall cases, include familial Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker syndrome, and fatal familial insomnia.
• Creutzfeldt-Jakob disease is a subacute fatal disease with a clinical triad of dementia, myoclonus, and EEG abnormalities that is usually associated with other neurologic abnormalities, along with neuropathological evidence of neuronal loss, spongiform changes, and astrocytosis. A new test that makes use of protein misfolding cyclic amplification, real-time quaking-induced conversion (RT-QulC), has high sensitivity and specificity and is helpful in making the diagnosis.
• Prion diseases are transmissible after a prolonged incubation period by inoculating the infected CNS into nonhuman primates and other species via multiple routes of inoculation; however, transmission is most efficient with an intracerebral inoculation into a species identical to the source of the infected CNS tissue.
• Some investigators consider Alzheimer disease, Parkinson disease, amyotrophic lateral sclerosis, and multiple system atrophy to be prion-like diseases in which a particular misfolded protein converts a normal form of the protein to the misfolded one that aggregates and allows it to propagate and spread throughout the CNS.
In March of 1957, D Carleton Gajdusek entered the kuru region in New Guinea where Vin Zigas was stationed as a medical officer (45). This led to the first clinical and neuropathological descriptions of kuru (28; 43). In 1959, Hadlow, a veterinarian neuropathologist, noted the similarity between kuru and scrapie, a transmissible disease of sheep that had an extremely prolonged incubation period (35). This prompted attempts to transmit kuru into nonhuman primates following intracerebral inoculation of affected CNS tissue and to the eventual successful transmission of this disease (27).
Kuru and a number of other human (eg, Creutzfeldt-Jakob disease) and animal (eg, scrapie, bovine spongiform encephalopathy, and chronic wasting disease) diseases are grouped together as prion diseases or transmissible subacute spongiform encephalopathies because of their similar clinical and pathologic features as well as their transmissibility by an unconventional agent (27; 73; 75). This review focuses on kuru and fatal familial insomnia, with some discussion of Creutzfeldt-Jakob disease. The transmission of these diseases is of special interest because of the noninflammatory nature of the clinical-pathologic syndrome, the long incubation period ("slow" virus infection), and the unusual nature of the transmissible agent, the prion. The prion agent consists of prion protein (PrP) that is misfolded and protease-resistant, PrPSc. The importance of these diseases was demonstrated by the receipt of the Nobel Prize in 1976 and 1997 by Gajdusek and Prusiner respectively.
Fatal familial insomnia was first described in 1986 (48). In 1992 fatal familial insomnia patients were demonstrated to carry a mutation in the PrP gene, PRNP, as is the case with individuals with other forms of familial prion diseases; for this reason, fatal familial insomnia was proposed to be a prion disease (61). The transmission of fatal familial insomnia to experimental animals has confirmed its inclusion in this group of diseases (85). A noninherited disease, called fatal sporadic insomnia, with a similar clinical and pathological phenotype to fatal familial insomnia has also been described (57; 01). The prion diseases became more visible because of an epidemic of bovine spongiform encephalopathy (“mad cow” disease), found primarily in the United Kingdom, and the subsequent emergence of variant Creutzfeldt-Jakob disease. Variant Creutzfeldt-Jakob disease is thought to result from oral transmission of the bovine spongiform encephalopathy agent to humans. In addition, the recognition of chronic wasting disease as a prion disease of deer and elk in the United States has focused attention on prion diseases.
Prion diseases present as multiple conditions (Table 1).
Human Prion Diseases
Cerebellar disease of New Guinea, now eradicated
Infection from endocannibalistic ritual
Dementia, myoclonus, and other neurologic signs. Positive CSF RT-QuIC test. MRI DWI can show cortical ribboning. EEG can show positive sharp wave complexes.
Autosomal dominant with clinical features similar to sporadic CJD
Mutation in PrP gene, PRNP
Clinical features similar to sporadic CJD except frequent cerebellar signs
Infection from corneal transplants, intracerebral electrodes, dura mater grafts, growth hormone administration
Early age of onset with psychiatric/behavioral symptoms. Declining incidence.
Infection from bovine spongiform encephalopathy tissue
Autosomal dominant usually with cerebellar dysfunction and slower tempo than CJD.
Mutation in PrP gene, PRNP
Fatal insomnia (either sporadic or familial)
Insomnia, autonomic abnormalities
Sporadic – unknown
Kuru is a progressive cerebellar syndrome found among the Fore Highlanders of Papua, New Guinea, primarily affecting adult females and children of both sexes (28; 45). The disease usually starts with subtle gait instability. Balance problems worsen until the patient can no longer walk or even sit. In addition to the ataxia, patients exhibit dysmetria, varied movement disorders (eg, intention tremor), and dysarthria. Emotional lability with inappropriate laughter can be seen. Dementia is unusual, and generally only apparent in advanced stages. Patients become incapacitated, usually dying within a year of onset. The pathology targets the cerebellum with neuronal loss, gliosis, mild status spongiosis involving the gray matter, and the frequent presence of amyloid-containing plaques.
The most common human prion disease is Creutzfeldt-Jakob disease, a disease found throughout the world. Creutzfeldt-Jakob disease usually has a subacute course and is generally manifest with a "clinical triad" of dementia, myoclonus, and a characteristic EEG (periodic or pseudoperiodic paroxysms of sharp waves or spikes against a slow background) (82; 10). Although this triad is not always present in its entirety, the absence of all 3 features is distinctly unusual. A variety of other neurologic abnormalities are frequently seen, including behavioral abnormalities, sensory complaints, higher cortical dysfunction, and signs of cerebellar, pyramidal, extrapyramidal, and lower motor neuron dysfunction. Patients usually show a decline on virtually a weekly basis, leading to a mute akinetic state and eventually to death within 6 months. Less commonly, patients may have slow progression with late-onset dementia, myoclonus, or EEG abnormalities. Abnormalities on MRI and in the spinal fluid, including the very specific and ultrasensitive RT-QuIC test, are seen. Published criteria for the diagnosis of sporadic Creutzfeldt-Jakob disease are detailed in the diagnostic workup section of this chapter. The neuropathological hallmark of the disease includes neuronal loss with spongiform changes ("status spongiosis") and gliosis.
Creutzfeldt-Jakob disease can be acquired or familial. The acquired cases can result from iatrogenic transmission (11) and, in the case of variant Creutzfeldt-Jakob, from transmission of bovine spongiform encephalopathy to humans. For unclear reasons, iatrogenic cases, in contrast to sporadic Creutzfeldt-Jakob disease, have a far more frequent occurrence of cerebellar signs, with little early mental deterioration, myoclonus, or typical EEG abnormalities (10; 11). A typical case is described below as well in a Case Record of the Massachusetts General Hospital (41).
Over 225 cases of variant Creutzfeldt-Jakob disease have now been identified, most in the United Kingdom. Fortunately, the cases of variant Creutzfeldt-Jakob have become less frequent: for example, there was only 1 case in 2016 in the United Kingdom. These patients had a number of distinctive features compared to classical Creutzfeldt-Jakob disease. The average age of onset was less than 30 years, and disease duration was longer than classical Creutzfeldt-Jakob disease. Anxiety and depression were common early symptoms; other early signs were ataxia and sensory disturbances. Characteristic EEG abnormalities were frequently absent. Most cases had abnormal MRIs with a distinctive finding of increased signal in the posterior thalamus on T2-weighted images (the “pulvinar” sign). Postmortem examination of the brain revealed a much higher than usual occurrence of widespread protein clumps or "plaques." These plaques, known as “florid plaques,” are PrP-immunoreactive with an eosinophilic core and pale periphery surrounded by spongiform change. Almost all the patients have been homozygous for methionine at codon 129 of the prion protein gene (compared to the general population that includes a significant number of individuals who are valine homozygotes and heterozygotes at codon 129). The presence of homozygosity at codon 129 may allow for efficient oral entry of the bovine spongiform encephalopathy prion.
About 10% of prion disease cases are familial with an autosomal dominant inheritance pattern. Familial cases are classified as fatal familial insomnia, familial Creutzfeldt-Jakob disease, or Gerstmann-Sträussler-Scheinker syndrome, primarily on the basis of clinical and pathological features as well as the presence of a particular mutation in the prion protein gene. Familial cases that manifest a Creutzfeldt-Jakob disease clinical and a pathologic picture without features of Gerstmann-Sträussler-Scheinker syndrome or fatal familial insomnia are classified as familial Creutzfeldt-Jakob disease.
The onset of fatal familial insomnia is usually at about the age of 50. It is characterized by insomnia with a dramatic reduction or complete loss of the slow-wave and rapid-eye-movement phases of sleep (48; 52; 61; 80; 20). Accompanying abnormalities of the autonomic system include tachycardia, hyperthermia, hypertension, and hyperhidrosis. There are also endocrine disturbances with abnormalities of the circadian rhythm. Other neurologic findings such as ataxia, dysarthria, myoclonus, corticospinal tract signs, behavioral abnormalities, and dementia are seen with varying severity and varying times of presentation. The characteristic EEG changes of Creutzfeldt-Jakob disease with periodic or pseudoperiodic activity are not usually present. The disease selectively targets specific regions of the thalamus (especially the anterior ventral and mediodorsal nuclei) and, in contrast to Creutzfeldt-Jakob disease, demonstrates little or no spongiosis. A noninherited disease with a similar clinical and pathological phenotype has been described (57).
Patients with Gerstmann-Sträussler-Scheinker syndrome generally manifest a slower course than those with Creutzfeldt-Jakob disease, with prominent cerebellar findings, but less dementia and fewer EEG abnormalities; pathologic examination shows prominent amyloid plaques in the cerebellum (24). A report of a large kindred with Gerstmann-Sträussler-Scheinker syndrome highlights the phenotypic heterogeneity in this disease (91).
A novel inherited prion disease with remarkable involvement of peripheral organs and the peripheral nervous system in addition to the central nervous system was identified in a large kindred (59). The patients had chronic diarrhea, hereditary sensory and autonomic neuropathy, and late cognitive decline with seizures. This disease is associated with a specific truncation mutation of the PrP protein. PrP amyloid is deposited in peripheral organs, including the gut and peripheral nerves. The central nervous system shows frequent cortical amyloid plaques, cerebral amyloid angiopathy, and tauopathy. To date there has been no successful transmission of the disease into mice.
Thalamic degenerations or dementias can be classified into 3 types (54; 55; 69). Type 1 has severe pathology involving the thalamus with little extrathalamic pathology. It is probable that some of these patients with type 1 thalamic dementia (especially those with a family history) are actually cases of fatal familial insomnia (69). The second type is associated with multiple system atrophy, merging into spinocerebellar degeneration and spinal muscular atrophy; this type is probably distinct from the prion diseases and will not be discussed further here. The third type is similar to the first type, but demonstrates gliosis or spongiosis outside the confines of the thalamus. This third type probably represents a thalamic form of Creutzfeldt-Jakob disease. It is differentiated from fatal familial insomnia by the absence of prominent insomnia and by the presence of more widespread pathology.
Of great interest to the neurologic and neuroscience community is the possibility that other degenerative diseases such as Alzheimer disease, Parkinson disease, and amyotrophic lateral sclerosis are prion diseases that have low or no infectivity. Some investigators consider these diseases to be proteinopathies, like Creutzfeldt-Jakob disease, in which a particular misfolded protein is able to convert a normal form of the protein to the misfolded one that aggregates and propagates throughout the CNS (74; 89; 14). Relevant to this topic is a report of transmission of multiple system atrophy to transgenic mice that express a mutant α–synuclein transgene (76), as well as presumed transmission of amyloid-beta pathology and cerebral amyloid angiopathy to recipients of pituitary-derived growth hormone who died of Creutzfeldt-Jakob disease (40). In some cases, it appears that misfolded proteins associated with one neurodegenerative disease promote the spreading of misfolded proteins associated with another neurodegenerative disease (03).
Kuru patients generally develop increasing walking disturbances, become bedridden, and die within a year of onset. The actual cause of death may be pneumonia and sepsis.
Fatal familial insomnia patients usually die within a year (although longer durations are described), with increasing insomnia and worsening dysautonomia, hyperthermia, tachycardia, and blood pressure instability. As in Creutzfeldt-Jakob disease, patients usually develop a mute, bedridden, stuporous state. Survival of Creutzfeldt-Jakob disease patients is rarely prolonged over a year or 2 except in atypical and some familial cases.
A 41-year-old, right-handed, white male was admitted in October of 1992 because of bilateral blurred vision and left upper-extremity weakness and numbness (80). The patient’s father died in his 50s of alcoholism and cirrhosis. It was not until the patient died that it was discovered that his paternal great-grandmother was the sister of the paternal great-grandfather of a patient who had died with a diagnosis of fatal familial insomnia. Beginning approximately 1 year previous to his admission, the patient developed insidious onset of episodic paresthesias in the left side of his neck radiating into the first and second fingers of his left hand. These symptoms accompanied weakness and spasms of this same extremity and resolved within a week. After that, he complained of episodic blurred vision, diplopia, paresthesias of the left side of his face and body, generalized weakness, and fatigue. For the first time, in April of 1992, he developed impotence and difficulty initiating sleep that resulted in progressive daytime fatigue. One month prior to entry into the hospital, the patient experienced episodic dizziness and dysequilibrium. Beginning 2 weeks before admission he experienced bilateral blurred vision, and 1 day prior to admission he had diplopia on lateral gaze. On the day of admission, he developed paresthesias along the dorsal aspect of his left arm. There was no associated dysarthria, dysphagia, or urinary or bowel incontinence. On admission, his temperature was 37.3° C. His blood pressure was 145/95 with no orthostasis, his pulse was 67 and regular, and his respirations were 18 and regular. The red-glass test showed a mild right medical rectus weakness with resultant diplopia on left lateral gaze.
The patient was admitted with a presumed acute exacerbation of what was thought to be multiple sclerosis, based on both history and clinical exam. A subsequent ear, nose, and throat exam was normal. He was given a 3-day course of IV adrenocorticotropic hormone with mild objective improvement in his strength.
From his discharge on October 28, 1992 to another admission on December 29, 1992, he experienced a progression of daytime sleepiness, insomnia, fatigue, and numerous brief episodes wherein his head would loll forwards after which he would correct himself. He was profoundly depressed and was initially admitted to the psychiatry service. He became increasingly despondent, and within 2 weeks after admission, became near mute, with decreased appetite, followed by increasingly bizarre behavior such as confabulating about previous wives and past personal events.
He began psychotropic medications, but 3 days after starting these medications he developed fever to 38.3°C, tachycardia, and mild hypertension. He also had nuchal rigidity, axial tremors, and rigidity. The serum creatinine phosphokinase was found to be 578 IU per liter (normal less than 150). He was presumed to have mild neuroleptic malignant syndrome and the psychotropic medications were stopped. The rigidity and tremor abated and the creatinine phosphokinase returned to normal, but his temperature, blood pressure, and heart rate remained elevated. A head CT without contrast revealed mild atrophy out of proportion to his age, but otherwise was unremarkable. A lumbar puncture showed that his red blood cell count was 25, white blood count 1, protein 60 mg/dl, and glucose 88 mg/dl.
This patient was subsequently transferred to the neurology service for further evaluation. At the time of his transfer his temperature was 37.7°C, blood pressure was 166/114, heart rate was 130, respiration rate was 22, and pulse oximeter reading was greater than 95%.
He appeared disheveled and made grunting noises on inspiration. Although he had his eyes open and appeared to follow simple commands, he spoke little and responded to questions with appropriate nods of his head. He exhibited purposeful tracking movements with his eyes and continued to exhibit a mild weakness of the right medial rectus on extraocular movements to the left. He had bilateral horizontal nystagmus. He moved all extremities equally well, with bilateral extensor plantar reflexes and sustained clonus at the left ankle. He resisted all passive movements, making it difficult to assess tone. The cerebellar and gait exams could not be evaluated secondary to noncompliance. He responded to deep pressure and pinprick sensation by grimacing.
One day after transfer to the neurology service he became progressively tachycardic, tachypneic to 35, diaphoretic, and hypertensive. Over the course of 2 hours he became lethargic and bradypneic. A laryngoscopic exam revealed paradoxical movements of both vocal cords. On inspiration, his vocal cords were almost completely closed. He was intubated, with subsequent improvement in his condition. The paradoxical movements of his vocal cords diminished, and he appeared more alert. Following extubation he was easy to arouse and responded appropriately to simple commands but kept his eyes closed, did not initiate conversation, and had few spontaneous movements. He remained tachycardic and hypertensive, although he was afebrile. He became incontinent of urine and stool and was unable to dress, bathe, or feed himself. Myoclonic jerks involved his trunk and extremities, especially with tactile or auditory startle.
An EEG at the end of January 1993 showed mild gray matter encephalopathy, without periodic sharp waves or epileptiform activity. Several polysomnograms revealed marked diminution of total sleep time and the percentage of total sleep time spent in REM sleep. Furthermore, he had an abnormal sleep pattern wherein REM sleep preceded slow wave sleep. The patient carried a mutation and a polymorphism of the PrP gene, both of which have been seen in most cases of fatal familial insomnia.
The patient was subsequently transferred to a nursing home. Soon after, he began experiencing marked episodic hyperthermia to 42°C; no obvious source of infection could be found. He became increasingly restless and unresponsive, and died on March 22, 1993.
At autopsy, typical pathological findings of fatal familial insomnia were found.
The prion diseases or subacute spongiform encephalopathies are generally transmissible via multiple routes of inoculation into nonhuman primates or other species, although the intracerebral route is the most efficient. The transmissible agent is an isoform of the PrP that is protease-resistant, PrPSc, possibly because of a change in conformation from the normal cellular form of PrP, PrPC (73; 75). The abnormal PrP isoform is generally considered to be all or part of a proteinaceous infectious agent, the prion, which apparently lacks nucleic acid. Data indicate that interactions of PrPSc with PrPC lead to the conversion of PrPC to PrPSc and are critical for disease pathogenesis. PrPSc aggregates to produce amyloid. Fatal familial insomnia tissue was more difficult to transmit than the other subacute spongiform encephalopathies (85), possibly because the amount of protease-resistant PrP is relatively less in this disease than in cases of Creutzfeldt-Jakob disease (61; 80).
Amino acid sequence variation with either a methionine or valine at codon position 129 of PrP (ie, a polymorphism that is found in the normal population) affects the infectivity and phenotype of prion disease. Interestingly, variant Creutzfeldt-Jakob disease seems to be generally confined to patients that are homozygous for methionine at codon 129.
Different PrPSc “strains” can be identified or typed on the basis of Western blots of affected tissues that show varying glycosylation patterns and PrPSc electrophoretic mobilities after proteinase K digestion, presumably because of their different conformation (67; 78). The Creutzfeldt-Jakob disease agent can be classified into 6 subtypes (MM1 or 2, VV 1 or 2, and MV or 2) on the basis of the PrP gene haplotype (methionine [M], or valine [V], homozygosity or heterozygosity [MV]) and the electrophoretic mobility and degree of glycosylation of proteinase K-digested fragments (67). Clinical phenotypes of prion diseases are thought to correlate with these molecular subtypes. For example, most patients with MM1 or MV1 subtype have a rapid disease with progressive dementia along with myoclonus and visual disturbance, whereas patients with MV2 or VV2 have an atypical disease with a longer duration and late dementia (70). Variant Creutzfeldt-Jakob disease manifests a distinctive electrophoretic subtype that is also seen in bovine spongiform encephalopathy.
In some forms of human prion disease resembling Creutzfeldt-Jakob disease, PrP accumulates and is not sensitive to protease treatment (29). These cases of variably protease-sensitive proteinopathies can have atypical courses without the usual EEG and MRI findings seen in Creutzfeldt-Jakob disease.
An understanding of the pathogenesis of the subacute spongiform encephalopathies remains incomplete. It is known that these diseases are generally transmissible to nonhuman primates and other animals following filtration of the inoculum, indicating that the agent is small and "replicating" (33). The intracerebral route of inoculation is more efficient than the peripheral route, with an incubation period ranging, for example, from 14 to 39 months following intracerebral inoculation of kuru into chimpanzees. The transmissible agent is unusually resistant to chemical and physical agents (eg, heat inactivation, ultraviolet irradiation, x-irradiation), which originally led to suggestions that it does not contain nucleic acid.
As noted above, a distinctive protein that corresponds to an abnormal scrapie-like isoform of PrP, PrPSc (73; 75), is present in the CNS tissue of patients with prion disease. This protein differs from its normal cellular PrP counterpart, PrPC, because it is protease-resistant, presumably due to a change in conformation of the protein (although it has an identical amino acid sequence). It has been hypothesized that PrPSc deposits in the CNS, causing dysfunction, and that in the presence of PrPSc, PrPC is converted to PrPSc. In the case of experimentally transmitted prion disease, the inoculated PrPSc is believed to enter the CNS and thereby start the conversion of PrPC to PrPSc, with the eventual induction of disease. In the case of kuru, ritual endocannibalistic ingestion of brain tissue by females and children in the Fore tribe leads to entry of large amounts of PrPSc into the body; PrPSc makes its way into the CNS and leads to a change in conformation of the normal PrP into PrPSc and its subsequent deposition. The pathogenesis of sporadic Creutzfeldt-Jakob disease and sporadic fatal insomnia remains unclear. It has been hypothesized that a spontaneous change in conformation of PrP in the CNS of patients with these disorders might initiate the disease.
In the case of fatal familial insomnia, as well as Gerstmann-Sträussler-Scheinker syndrome and familial Creutzfeldt-Jakob disease (37), a mutated form of the PrP gene appears to lead to PrP deposition. All cases of fatal familial insomnia have been reported to have a mutation in codon 178, resulting in the substitution of asparagine (Asn178) for aspartic acid (61); interestingly, all cases of fatal familial insomnia also have methionine at codon 129 on the same allele as the Asn178 (34). In contrast, the presence of the Asn178 mutation on the same allele as valine at codon 129 is associated with a familial syndrome that is typical of Creutzfeldt-Jakob disease rather than the more distinctive clinical and pathologic fatal familial insomnia disease. In other words, 2 familial prion diseases are both associated with the same mutation, but the specific syndrome that results depends on an associated polymorphism at codon 129. An additional genotypic abnormality, consisting of a 24-base pair deletion from the normal octapeptide repeat of the PrP gene, has been found in some patients with fatal familial insomnia (08; 80).
The same genotypic abnormalities found in fatal familial insomnia (Asn178 and Met129) are also seen in cases previously classified as thalamic degenerations (or familial Creutzfeldt-Jakob disease) (47; 42; 08). It is now apparent that patients thought in the past to have thalamic dementia have clinical, pathological, and genotypic features that are more appropriately classified as fatal familial insomnia. These cases demonstrate that sequencing of the PRNP gene, which encodes PrP, may have a critical role in diagnosis (69).
In a landmark experiment, a mutated PrP gene (from a patient with Gerstmann-Sträussler-Scheinker syndrome) that was engineered as a transgene in mice was found to induce a spongiform neuropathology (38). This latter result suggests that this mutant PrPSc is (at least partly) sufficient to produce disease; ie, a mutant form of PrP replicates and induces disease. The importance of interactions between PrPSc and PrPC in disease was further supported by experiments using a PrP knockout mouse (12). This knockout mouse does not develop subacute spongiform encephalopathy following inoculation of (mouse-adapted) PrPSc; reintroduction of PrPC as a transgene into the knockout mouse leads to a return in susceptibility of these mice.
The pathogenesis of sporadic Creutzfeldt-Jakob disease remains unclear. It has been hypothesized that a spontaneous somatic change in conformation of prion protein in the CNS initiates the disease. However, it is likely that some of the presumed sporadic cases of Creutzfeldt-Jakob disease may represent an unrecognized transmission from infectious material, either as result of iatrogenic exposure or an environmental focus.
Transgenic mice that overproduce wild-type rodent PrPC or part of PrPC develop neurodegeneration, suggesting that prion disease can be induced by excessive amounts of PrPC or particular peptides of PrPC, presumably because the chance of misfolding of the overexpressed PrPC into an abnormal PrPSc conformation is more likely than if there is a normal amount of PrPC expressed (92; 25).
Several observations demonstrate the potential of a broad spectrum of abnormalities that can be caused by PrP. One study found that overexpression of PrPC in muscle leads to a primary myopathy in transgenic mice (39). In addition, cardiac disease can be induced by overexpression of PrPSc in the heart (87).
Interestingly, a number of reports have been published concerning prion-like elements in yeast that lead to amyloidogenic aggregation of a protein and involve the inheritance of an altered protein rather than nucleic acid (46; 94). These studies have demonstrated that prions (ie, abnormally folded proteins) are more prevalent than previously realized and have led to the hypothesis that prion-like mechanisms are important in the pathogenesis of a number of neurodegenerative diseases (74; 89; 14). Although these neurodegenerative diseases may not be transmissible in the same way that the subacute spongiform encephalopathies are, the pathogenic protein specific for these diseases is misfolded and is thought to spread throughout the CNS by means of a prion-like mechanism in which the misfolded protein leads to misfolding of the wild-type protein. In fact, a CSF RT-QuIC assay has been used for the detection of misfolded proteins associated with nonprion neurodegenerative diseases (13). As noted above, transmission of multiple system atrophy to transgenic mice that express a mutant α-synuclein transgene has been reported (76), as well as transmission of amyloid-beta pathology and cerebral amyloid angiopathy to recipients of pituitary-derived growth hormone who died of Creutzfeldt-Jakob disease (40).
Kuru is confined to the highlands of Papua New Guinea among the Fore tribesmen and individuals with whom they intermarry. The disease primarily involved adult females and children of both sexes. Knowledge of ritual endocannibalism among the Fore tribesmen in New Guinea served to clarify the issue of how kuru was spread. In this region, cannibalistic eating of tribesmen constituted a form of mourning. Brain tissue that had large amounts of the transmissible agent (PrPSc) was eaten by females and children of both sexes, providing an explanation for the remarkable sex and age distribution of the disease. Kuru has disappeared with the cessation of cannibalism. A study of kuru recorded only 11 patients from July 1996 to June 2004, with all patients born before the cessation of cannibalism (17). Presumed incubation periods ranged from 39 to 56 years and could have been longer than 70 years. Most of the patients with long incubation periods were heterozygous at codon 129. The amino acid sequence of PrP varies in the normal population, with either a methionine or valine at codon position 129 of PrP. This polymorphism affects the infectivity and phenotype of prion disease. Interestingly, variant Creutzfeldt-Jakob disease seems to be almost exclusively confined to patients that are homozygous for methionine at codon 129. As noted above, the amino acid at codon 129 on the same allele as the Asn178 mutation determines whether the patient will manifest fatal familial insomnia or familial Creutzfeldt-Jakob disease. The polymorphism also affects the susceptibility and phenotype of scrapie (60).
There have been a number of reports of iatrogenically transmitted Creutzfeldt-Jakob disease. Transmission has occurred as a result of improperly sterilized intracerebral electrodes, transplants of cornea or dura mater from an affected individual, and purified preparations of human growth hormone (11; 02; 65). Infrequent cases have been reported among members of the medical profession.
The occurrence of an epidemic of bovine spongiform encephalopathy ("mad-cow disease") in the United Kingdom focused attention on prions as significant agents of transmissible human disease. Compelling data indicate that variant Creutzfeldt-Jakob disease is a result of transmission of bovine spongiform encephalopathy to humans (95). Fortunately, there has been a decline in variant Creutzfeldt-Jakob cases, with only 1 case occurring in 2016 in the United Kingdom, presumably because of the decline in bovine spongiform encephalopathy:https://www.cjd.ed.ac.uk/. An atypical sporadic strain of bovine spongiform encephalopathy found in a number of countries is under observation. More information is available at the following site:https://www.aphis.usda.gov.
Although there has been a remarkable decline in variant Creutzfeldt-Jakob disease, there is significant concern related to a 2013 report from the United Kingdom that found 16 of 32,441 (approximately 1 in 2000) appendices that were surgically removed from 2000 to 2012 stained positive for PrP accumulation. Although all but 1 patient with clinical signs of variant Creutzfeldt-Jakob disease have been homozygous for methionine at codon 129 of the prion protein gene (compared to the United Kingdom population, which includes a significant number of individuals who are valine homozygotes and heterozygotes at codon 129), the codon 129 genotype of the individuals with PrP positive-stained appendices was: 8 with methionine homozygosity, 4 with methionine-valine, and 4 homozygous for valine. The results from this study of appendices suggest that in the case of bovine spongiform encephalopathy, the barrier for infection of human lymphoreticular infection is lower than for clinical disease, as also suggested from animal experiments (06). The data also suggest that clinical disease seems to selectively target patients with a homozygous methionine genotype, but this is not the case with lymphoreticular infection. It remains unclear whether the appendix-positive individuals with valine and methionine-valine genotypes will eventually develop clinical disease. If these individuals do develop Creutzfeldt-Jakob disease, it is unclear whether the clinical disease will have a different phenotype from variant Creutzfeldt-Jakob disease in individuals who are homozygous for methionine.
A subsequent study of appendices collected before 1980 (prior to the epidemic of bovine spongiform encephalopathy) and after 1996 (a year when measures to remove bovine spongiform encephalopathy from the food chain was fully in place) was carried out in order to clarify the effect of PrPSc dietary exposure (77). Surprisingly, 2 out of 14,692 (about 1 in 7000) of appendices collected prior to 1980 were positive, and 5 out of 14,824 (about 1 in 3000) appendices were positive from patients born after 1996. Despite the difference in age and dietary exposure among the 3 groups, there was no statistical difference in the frequency of positive appendices collected prior to 1980, from 2000 to 2012, or from individuals born after 1996. These results are difficult to interpret with certainty. One suggestion is that human exposure to bovine spongiform encephalopathy occurred prior to 1980 and after 1996. Another interpretation is that there is a background of variant Creutzfeldt-Jakob disease infection that does not progress to disease, ie, there is a disconnection between a positive appendix result and variant Creutzfeldt-Jakob disease (77). In other words, prion infectivity can be separated from toxicity (05).
The occurrence of chronic wasting disease, a prion disease of deer and farmed elk, in the United States and Canada provides another environmental risk (04). Chronic wasting disease has also been detected in 3 reindeer in Norway, triggering extermination of a herd of 2000 (84). Chronic wasting disease has spread and is documented in 26 states in the United States as well as internationally (72). Although presently no evidence suggests that this disease can be transmitted to humans, experimental transmission into squirrel monkeys has been carried out following intracerebral inoculation (53). Chronic wasting disease is efficiently spread and very contagious among cervids in the field. Studies have demonstrated that this prion disease can be spread by blood and saliva from preclinical infected deer. In addition, transmission occurs from exposure to feed buckets, water, and bedding (as a result of fecal, urine, and saliva contamination) from the housing areas of infected deer—without any direct animal-to-animal contact (58). Of note, even earthworms are capable of taking up and then excreting infectious prions (71). The amount of PrPsc excreted in the feces into the environment can be very substantial, even comparable to the amount in the brain at terminal disease (71). Of concern is the finding that prions can persist and retain infectivity in the environment for at least 16 years (31). These observations underline the important role of the environment in prion transmission.
Although Creutzfeldt-Jakob disease is worldwide, foci have been identified in a number of regions including Slovakia and a region inhabited by Libyan Jews (56). These foci are generally considered to result from clusters of unrecognized familial cases or exposure to a common infectious source, as in the cases of variant Creutzfeldt-Jakob disease in the United Kingdom and kuru in New Guinea. The apparent overrepresentation of individuals of Italian ancestry with fatal familial insomnia probably reflects the pioneering work of Italian investigators in identifying large Italian fatal familial insomnia pedigrees.
The pathogenesis of sporadic Creutzfeldt-Jakob disease remains unclear. It has been hypothesized that a spontaneous somatic change in the conformation of PrP in the CNS leads to its aggregation and the subsequent induction of disease. It is likely that some of these cases may actually represent an unrecognized transmission from infectious material, either as result of iatrogenic exposure or an environmental focus; surgical transmission from prion-contaminated instruments has been suggested by some epidemiological studies (49). Epidemiological studies have found little evidence of horizontal spread from person to person, and only rare reports of conjugal Creutzfeldt-Jakob disease. Furthermore, a number of epidemiologic studies have failed to find a relationship between the incidence of scrapie and the presence of Creutzfeldt-Jakob disease (10); however, there is less knowledge concerning the potential of human transmission of atypical scrapie strains.
Data have shown that in some of the prion diseases a variety of body fluids are infectious, including spinal fluid, blood, saliva, urine, and feces (07). Several studies have found infectious material in the blood of patients with variant and sporadic Creutzfeldt-Jakob patients (23). An ongoing United States surveillance study found no evidence of transmission of Creutzfeldt-Jakob disease in recipients of blood transfusions from donors who developed Creutzfeldt-Jakob disease (21); however, the latter result must be considered tentative because one would anticipate an extremely prolonged incubation period if transmission from this source occurred, considering the low infectivity of blood and the peripheral route of entry.
There are more concerns about blood transfusion as a potential route of infection in countries where variant Creutzfeldt-Jakob disease exists because of the unusual tropism of this prion for lymphoid tissue. Secondary transmission of variant Creutzfeldt-Jakob disease, which is more lymphotrophic than classical Creutzfeldt-Jakob disease, is thought to have occurred through transfusion of blood from patients with variant Creutzfeldt-Jakob disease (88). There is an additional case of a hemophiliac patient with preclinical variant Creutzfeldt-Jakob (on the basis of detection of the PrP antigen in the spleen) following treatment with Factor VIII that was prepared using plasma from a donor who developed variant Creutzfeldt-Jakob 6 months after donating blood (68).
A variety of tissues of prion disease patients are infectious. Although the skin of Creutzfeldt-Jakob patients was demonstrated to have PrPSc and to be infectious, there is no evidence that skin transmission of PrPSc leads to disease in humans (66). Another study demonstrated PrPSc in the cornea, ocular fluid, retina, choroid, sclera, and optic nerve of some patients with sporadic Creutzfeldt-Jakob disease; in some cases, PrPSc was demonstrated by immunohistochemical staining of retinal tissue (65). It is likely that there is variable infectivity of peripheral tissues from both variant and sporadic Creutzfeld-Jakob patients, depending on when the tissue is obtained during the disease course and how long the assay is monitored (22).
The identification of particular mutations of the PRNP gene among cases of familial prion diseases signifies that in utero identification of this disease and an informed decision regarding elective therapeutic abortion are possible. In addition, PRNP sequencing is important to make the diagnosis of inherited prion disease. Prevention of disease among individuals who are known to carry the mutation but are not yet affected is unfortunately not possible at present.
The transmissible agent of Creutzfeldt-Jakob disease is highly resistant to chemical and physical agents, prompting the publication of guidelines concerning inactivation of the agent's infectivity and safety precautions at the time of autopsy. These can be found on the following website:Centers for Disease Control and Prevention website. Patients should generally be kept on "stool and needle" precautions in the hospital and body fluids labeled as potentially dangerous. Invasive procedures should be carried out with precautions similar to those used with AIDS patients. Attempts to decrease the incidence of iatrogenic disease have also been implemented. Intracerebral electrodes are no longer sterilized by paraformaldehyde vapor. Guidelines regarding transplantation of tissues, administration of blood-derived products, and transfusion of blood from demented individuals are under continuing consideration and refinement. Genetically engineered material, rather than purified human pituitary gland extracts, is presently available and recommended for growth hormone treatment. Generally, there is an effort to try to avoid transplantation of tissues or delivery of biologic products that were collected from a pool of donors. If the pool of donors is unavoidable as a source, then it is important to try to prevent inclusion of prions and to screen for prions in the final product.
The sequence of PRNP has a significant effect on disease. Most well known is amino acid sequence variation with either a methionine or valine at codon position 129 of PrP, which affects susceptibility and phenotype of prion disease. Also, studies of kuru have demonstrated that the amino acid at this codon affects the length of the incubation period (60). These insights may have a future impact on preventing prion disease in animals and even perhaps in humans.
A promising route for protection and treatment of Creutzfeld-Jakob disease is knocking down PrP. This plan has led to the identification of antisense oligonucleotides and other means of PrP knockdown (15). Unfortunately, cases of Creutzfeldt-Jakob disease can have very severe impairment by the time they are diagnosed. Therefore, PrP knockdown may need to be utilized for patients who carry PrPSc but are not yet affected with disease. The methods of PrP knockdown are discussed under the management section.
The diagnosis of kuru posed no problem in diagnosis in the past because few other conditions were considered as a cause of cerebellar disease in a Fore tribesman from New Guinea; in fact, it was common for the affected native to be the first to diagnose this condition.
There are, similarly, few diagnostic dilemmas when insomnia appears in a member of a fatal familial insomnia pedigree. A polysomnogram is clearly important for confirming the sleep disorder and differentiating the condition from other causes of sleep disturbances. In addition, PRNP gene sequencing is critical for the diagnosis. However, some cases appear without a clear family history and, in some cases, insomnia only appears with disease progression. In these cases, one must differentiate the condition from other diseases that might mimic prion disease, especially atypical cases.
The usual manifestations of Creutzfeldt-Jakob disease are fairly distinctive, and other diseases rarely produce a similar picture; however, Creutzfeldt-Jakob disease can, at times, have atypical clinical features. In addition, early in the disease course, Creutzfeldt-Jakob disease may be confused with other disorders. The availability of a new laboratory CSF test, RT-QuIC, and the identification of MRI abnormalities in Creutzfeldt-Jakob disease are important tools with excellent sensitivity and specificity to clarify the diagnosis.
Many of the diseases in the differential diagnosis of Creutzfeldt-Jakob disease fall into a group of rapidly progressive dementias (32). Paraneoplastic syndromes (including, for example, limbic encephalitis or cerebellar degeneration) and autoimmune encephalitis should be included in the differential diagnosis. Infectious or granulomatous processes (eg, neurosyphilis, CNS fungal disease, sarcoid, HIV-1-related diseases, Lyme disease), tumors, and vasculitis (possibly confined to the CNS) may present a clinical picture similar to that seen with Creutzfeldt-Jakob disease; however, imaging studies and blood and CSF investigations usually identify neoplastic or inflammatory processes. More confusing may be disease processes related to toxins, especially inorganic mercury, a primary central nervous system vasculitis with little or no laboratory evidence of systemic inflammation, Hashimoto thyroiditis, and "mixed" neurodegenerative processes (eg, Lewy body dementia, frontotemporal dementia, Parkinson disease with dementia, and amyotrophic lateral sclerosis with frontotemporal dementia).
A study from the U.S. National Prion Disease Pathology Surveillance Center noted that 352 (32%) of 1106 brain autopsies from cases that were suspicious for prion disease had another diagnosis, most frequently Alzheimer disease (154 cases) or vascular dementia (36 cases) (16). Of note, these cases were diagnosed prior to the use of RT-QuIC. Other diagnoses of “incurable” neurologic disease included: unspecified degenerative brain disease (10), frontotemporal dementia (9), mesial temporal lobe sclerosis (5), diffuse Lewy body disease (4), tauopathy (4), hereditary diffuse leukoencephalopathy with spheroids (3), progressive supranuclear palsy (3), corticobasal ganglionic degeneration (1), adult polyglucosan body disease (1), Huntington disease (1), Marchiafava-Bignami disease (1), and superficial siderosis (1). Of special concern was the finding that 71 individuals (23%) had a potentially treatable neurologic disease that included: immune mediated disorders (26) (primary CNS angiitis, acute disseminated encephalomyelitis, limbic encephalitis, neurosarcoidosis, paraneoplastic cerebellar degeneration, and Wegener granulomatosis), neoplasms, infections (fungal, viral, and parasitic), and toxic encephalopathies (6).
Some subacute spongiform encephalopathies result from an environmental source. For example, kuru is caused by endocannabalism involving ingestion of prion-infected tissue by individuals in the New Guinea Highlands. Variant Creutzfeldt-Jakob disease is caused by ingestion of bovine spongiform encephalopathy-contaminated beef. In some cases, there is infection as a result of an iatrogenic exposure, such as via corneal transplants.
Patients or animals with familial prion diseases have a mutation in PRNP. At times, the mutation is specific for a particular prion disease. It is likely that non-PRNP host genetics influence the age of onset and duration of the prion disease; however, these genes are still being investigated.
Patients suspected of having fatal familial insomnia clearly need a polysomnogram evaluation to confirm the sleeping disorder. Most important for the diagnosis is identifying the pathogenic mutated PRNP gene by sequencing. The latter study should be performed on any patient with a family history suspicious for a prion disease or even patients without a family history who have features of familial insomnia.
The most important assay in making the diagnosis of Creutzfeldt-Jakob disease are a protein misfolding cyclic amplification test called RT-QuIC. The MRI can also be helpful, as well as the EEG (30).
The results of RT-QuIC on CSF from 126 patients who were suspected to have Creutzfeldt-Jakob disease (111 sporadic disease and 15 inherited disease) and had prion disease confirmed at autopsy and from 67 patients who had other diseases at autopsy had a specificity of 98.5% and sensitivity of 92.1% (26). A subsequently published review notes that the sensitivity of RT-QuIC for CSF in Creutzfeldt-Jakob disease CSF is 96%, and the specificity is 100% (98). Modifications of the amplification test have been used to detect variant Creutzfeldt-Jakob prions in blood (19) and urine (62).
MRI is also frequently helpful in making the diagnosis of Creutzfeldt-Jakob disease. A consensus review of 48 cases of Creutzfeldt-Jakob disease by 2 neuroradiologists blinded to the diagnosis found that MRI had a sensitivity of 96% and 93%, respectively and that the pattern of FLAIR/DWI can effectively differentiate Creutzfeldt-Jakob disease from other rapidly progressive dementias (90). There were gray matter hyperintensities in DWI sequences of the MRI in all the 48 cases, with certain regions preferentially involved, as found in this study and others, especially the cortex (“cortical ribboning”), basal ganglia, and thalamus. The hyperintensity was always greater on DWI than FLAIR, and the ADC in these regions was hypointense, indicating restricted diffusion. Patients with variant Creutzfeldt-Jakob disease have a distinctive finding with increased signal in the posterior thalamus on T2-weighted images (the “pulvinar sign”). MRI criteria for the diagnosis of sporadic Creutzfeldt-Jakob disease are available (86). MRI abnormalities have also been found preclinically in at-risk patients who carry a mutation that is highly penetrant for familial Creutzfeldt-Jakob disease; diffusion was significantly reduced in thalamic-striatal structures, including the putamen and mediodorsal, ventrolateral, and pulvinar thalamic nuclei (44).
The EEG characteristically shows positive sharp wave complexes. In a large well-controlled study, 58% of sporadic Creutzfeldt-Jakob disease patients (1261 of 2083 cases) showed this typical EEG, although the chances of finding it were more likely when a patient’s age was over 50 years and the disease duration under 6 months (18); in addition, the typical EEG was more likely seen with patients with the MM1 subtype and less likely with the MV1, MV2, and VV2 subtypes.
The chart that follows includes the European criteria for the diagnosis of sporadic Creutzfeldt-Jakob disease (86). It is very likely that the availability of RT-QuIC will play a major role in future diagnostic criteria.
Rapidly progressive cognitive impairment with 2 of the following clinical features
In combination with either:
(a) periodic EEG discharges or
Other criteria for the diagnosis of sporadic Creutzfeldt-Jakob are also available (86), as well as criteria for the diagnosis of iatrogenic and genetic transmissible spongiform encephalopathy and variant Creutzfeldt-Jakob disease. They can be found on the following website:http://www.cjd.ed.ac.uk/sites/default/files/criteria.pdf.
As investigators await results of the RT-QuIC and in order to rule out inflammatory processes, the following studies may be performed: complete blood count, comprehensive metabolic profile, sedimentation rate, collagen vascular screen, paraneoplastic antibody panel, autoimmune encephalitis panel, HIV-1 antibody, antithyroid antibody, and a consideration for CSF analysis with culture of the fluid and tests for paraneoplastic antibody and autoimmune encephalitis. In addition, an MRI with gadolinium infusion should be performed to look for hyperintensities in the striatum, thalamus, or cortex and to rule out the possibility of a mass lesion or inflammatory focus. A toxic screen may also be indicated. An EEG is important in patients suspected of having Creutzfeldt-Jakob disease in order to identify evidence of the characteristic abnormality seen in Creutzfeldt-Jakob disease and to rule out seizures. If the characteristic EEG findings are not seen, it may be helpful to perform additional EEGs because these abnormalities frequently appear as the clinical syndrome evolves; however, fatal familial insomnia patients rarely if ever manifest these EEG abnormalities. If a vasculitis is strongly suspected, one should consider performing an arteriogram.
CNS tissue usually shows the typical neuronal loss with spongiform changes as well as evidence of protease-resistant PrPSc. Affected tissue usually transmits disease in experimental animals. An issue with carrying out an autopsy, however, is the possibility of contamination from prion-infected tissue.
The fear of an epidemic of variant Creutzfeldt-Jakob disease, the poor prognosis of classical Creutzfeldt-Jakob disease, and the presence of individuals who carry a PrP pathogenic mutation but are still asymptomatic have challenged the scientific community to identify potential treatments. A number of drugs have been tested in mice that carry PrPSc; however, there remains no confirmed effective treatment for any prion disease (97). Of interest is a publication that showed that treatment of a scrapie-infected mouse with a drug that inhibits a key kinase in the unfolded protein response (triggered by the aggregation of PrPSc) abrogates the development of clinical disease and decreases the neuropathological abnormalities (63). A number of subsequent studies that used different strategies to target this pathway again found an amelioration of disease (36; 83).
A promising path for treatment of prion diseases is related to knocking down or knocking out PrP. As noted above, PrP knock-out mice do not develop prion disease following inoculation of (mouse-adapted) PrPSc, and reintroduction of PrPC as a transgene into the knock-out mouse leads to a return in susceptibility of these mice to prion disease (12). Furthermore, turning off expression of PrPSc in the mice rescues them from disease (50) and leads to recovery from cognitive and behavioral symptoms (51), which is encouraging and suggests that the disease process is not inexorably fatal after it starts. Other experiments showed that knockdown of PrPC following lentiviral delivery of RNAi into the hippocampus of scrapie-infected mice led to a prolongation in survival, with a decrease in behavioral defects and neuropathology (93). Another promising avenue for treatment is intracerebroventricular injection of antisense oligonucleotides directed against the prion protein. This intervention led to a significant extension in survival of scrapie-infected mice (79). One could also deliver shRNA that knocks down PrP via virus vector delivery. A potential issue in these knock-out or knock-down experiments is that PrP knockdown or knockout may cause toxicity.
There have also been attempts to knockdown PrP by means of immunotherapy, primarily with antibody directed to all or part of PrP (81). These antibodies have been shown to increase lifespan, and in some cases, prolong the incubation period of scrapie mice.
Of interest is a publication that showed that treatment of a scrapie-infected mouse with a drug that inhibits a key kinase in the unfolded protein response (triggered by the aggregation of PrPSc) abrogates the development of clinical disease and decreases the neuropathological abnormalities (63). A number of subsequent studies that used different strategies to target this pathway again found an amelioration of disease (36; 83).
Unfortunately, as of this date, there is still no treatment recommended to slow the course of Creutzfeldt-Jakob disease, and there is no way to prevent the disease from developing in a clinically asymptomatic individual who carries a mutant PrP gene associated with familial Creutzfeldt-Jakob disease. Furthermore, it is clear that there is significant damage to the brain when most Creutzfeldt-Jakob disease patients are diagnosed, so that treatments may sadly have little benefit in symptomatic individuals.
No information is available regarding the effect of pregnancy on the course of prion diseases. To date, there is no evidence that children born to pregnant women with Creutzfeldt-Jakob disease develop the disease (96). An investigation of a child born from a pregnant woman with Creutzfeldt-Jakob disease found no evidence of proteinase K-resistant protein in gestational tissues, including the placenta and amniotic fluid (96); however, transmission studies or amplification-based detection assays were not performed. Of concern is the detection of chronic wasting disease prions in fetal tissues of white-tailed deer using an amplification method, demonstrating vertical transmission of prions (09; 64).
All contributors‘ financial relationships have been reviewed and mitigated to ensure that this and every other article is free from commercial bias.
All contributors‘ financial relationships have been reviewed and mitigated to ensure that this and every other article is free from commercial bias.
All contributors‘ financial relationships have been reviewed and mitigated to ensure that this and every other article is free from commercial bias.
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