Review The role of oxidative stress, inflammation and acetaminophen exposure from birth to early childhood in the induction of autism William Parker 1 , Chi Dang Hornik 2 , Staci Bilbo 3 , Zoie E. Holzknecht 1 , Lauren Gentry 1 , Rasika Rao 1 , Shu S. Lin 1 , Martha R. Herbert 4 and Cynthia D. Nevison 5 Abstract The wide range of factors associated with the induction of autism is invariably linked with either inflammation or oxidative stress, and sometimes both. The use of acetaminophen in babies and young children may be much more strongly associated with autism than its use during pregnancy, perhaps because of well-known deficiencies in the metabolic breakdown of pharmaceuticals during early development. Thus, one explanation for the increased prevalence of autism is that increased exposure to acetaminophen, exacerbated by inflammation and oxidative stress, is neurotoxic in babies and small children. This view mandates extreme urgency in probing the long-term effects of acetaminophen use in babies and the possibility that many cases of infantile autism may actually be induced by acetaminophen exposure shortly after birth. Keywords Autism, inflammation, oxidative stress, acetaminophen, paracetamol, paracetamolo Date received: 26 September 2016; accepted: 20 January 2017 Journal of International Medical Research 2017, Vol. 45(2) 407–438 ! The Author(s) 2017 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0300060517693423 journals.sagepub.com/home/imr 1 Departments of Surgery, Duke University Medical Center, Durham, NC USA 2 Departments of Pediatrics, Duke University Medical Center, Durham, NC USA 3 Departments of Pediatrics, Harvard Medical School, Charlestown, MA, USA 4 Departments of Neurology, Harvard Medical School, Charlestown, MA, USA 5 Institute for Arctic and Alpine Research, University of Colorado, Boulder, Boulder, CO, USA Corresponding author: William Parker, Duke University Medical Center, Box 2605, Department of Surgery, Durham, NC 27710, USA. Email: [email protected]Creative Commons CC-BY-NC: This article is distributed under the terms of the Creative Commons Attribution-NonCommercial 3.0 License (http://www.creativecommons.org/licenses/by-nc/3.0/) which permits non-commercial use, reproduction and distribution of the work without further permission provided the original work is attributed as specified on the SAGE and Open Access pages (https://us. sagepub.com/en-us/nam/open-access-at-sage).
Transcript
Review
The role of oxidativestress, inflammation andacetaminophen exposurefrom birth to early childhoodin the induction of autism
William Parker1, Chi Dang Hornik2,Staci Bilbo3, Zoie E. Holzknecht1,Lauren Gentry1, Rasika Rao1, Shu S. Lin1,Martha R. Herbert4 and Cynthia D. Nevison5
Abstract
The wide range of factors associated with the induction of autism is invariably linked with either
inflammation or oxidative stress, and sometimes both. The use of acetaminophen in babies and
young children may be much more strongly associated with autism than its use during pregnancy,
perhaps because of well-known deficiencies in the metabolic breakdown of pharmaceuticals during
early development. Thus, one explanation for the increased prevalence of autism is that increased
exposure to acetaminophen, exacerbated by inflammation and oxidative stress, is neurotoxic in
babies and small children. This view mandates extreme urgency in probing the long-term effects of
acetaminophen use in babies and the possibility that many cases of infantile autism may actually be
induced by acetaminophen exposure shortly after birth.
Creative Commons CC-BY-NC: This article is distributed under the terms of the Creative Commons Attribution-NonCommercial
3.0 License (http://www.creativecommons.org/licenses/by-nc/3.0/) which permits non-commercial use, reproduction and
distribution of the work without further permission provided the original work is attributed as specified on the SAGE and Open Access pages (https://us.
Autism is a complex disorder associatedwith a wide range of disparate and seeminglyunrelated factors such as (a) maternal expos-ure to various chemical substances, (b)maternal exposure to child abuse, (c) mater-nal evidence of diabetes or other auto-immune diseases, (d) age of either parentat conception, (e) exposure of the infant tovarious chemical substances, (f) vitamin Dlevels of the infant at birth, (g) gender of theinfant, and (h) a large number of geneticfactors. With this in mind, we believe it ishelpful to categorize the factors associatedwith autism in an effort to identify patternswhich may be informative. As describedbelow, these risk factors for autism fall intotwo primary categories: those associatedwith inflammation and those associatedwith oxidative stress.
A number of risk factors for autism canbe categorized as risk factors for inflamma-tion or indicators of inflammation (Table 1).Risk factors for inflammation associatedwith autism include maternal (odds ratio[OR]¼ 1.6, confidence interval [CI]95%¼ 1.1–2.2) and perhaps paternal(OR¼ 1.4, CI 95%¼ 1.0–2.0) autoimmunediseases such as diabetes, myasthenia gravis,idiopathic thrombocytopenic purpura, orrheumatic fever,1 maternal obesity(OR¼ 1.67, CI 95%¼ 1.1–2.56),2 and feb-rile episodes during the first two trimesters(OR¼ 1.6, CI 95%¼ 1.0–2.5).3 An increasein inflammation among individuals living inWestern societies is evidenced by the alarm-ing rise of allergy and autoimmune diseasein the United States and other Westernizedcountries over the past century and can beattributed to five major causes that resultdirectly from social and cultural changes: (a)an ‘‘inflammatory diet’’ high in fat andsimple sugars and low in fiber and nutrients,(b) lack of physical exercise, (c) chronic andunrequited psychological stress, (d) vitaminD deficiency, and (e) biome depletion.4–7
The recent rise in autism8,9 could be attrib-utable, at least in part, to this rise ininflammation.10,11
Aside from inflammation, oxidativestress is the second category in the landscapeof autism-associated factors. As pointed outby Chauhan and Chauhan, a wide range ofstudies ‘‘. . . suggest increased oxidativestress in autism that may contribute to thedevelopment of this disease.’’12 However, itis unknown whether factors causing oxida-tive stress have increased in Western socie-ties concomitantly with inflammation.Perhaps the weight of factors causing oxi-dative stress peaked during the height of theindustrial revolution and has since declinedfrom that peak,9 but this is unknown.Indeed, it may be difficult or even impossibleto determine whether, in terms of oxidativestress, modern factors such as pharmaceut-icals, pollutants from the combustion offossil fuels, and other factors from industrialprocesses outweigh more historical factorssuch as smoke from cooking fires and nat-urally occurring toxins from food obtainedby hunting and gathering. Nevertheless,oxidative stress is now associated withmodern diseases. Oxidative stress, likeinflammation, is associated with cancer,13
coronary artery disease,14 and a number ofpsychiatric disorders.15 It is widely thoughtthat inflammation and oxidative stress gohand in hand; as stated by Ghezzi andcolleagues, ‘‘The mechanism by which oxi-dative stress induces inflammation and viceversa is unclear but is of great import-ance.’’16 Therefore, either oxidative stresshas increased or other changes in modernsociety (e.g. increased inflammation) havemade oxidative stress more dangerous thanit was in past generations.
Thus, the list of diverse factors associatedwith the induction of autism can be viewedas interrelated when examined in the contextof two major categories, inflammation andoxidative stress (Figure 1). For the purposeof this discussion, factors involved in the
408 Journal of International Medical Research 45(2)
Table 1. Risk factors associated with autism, in very approximate descending order of risk.
Factor Classification
High Risk
Acetaminophen use in childrena Pharmaceutical: oxidative stressor, decreased capacity to
handle oxidative stress
Excessively high levels of vitamin B12
and folate in maternal blood
Linked to inflammation and possibly to oxidative stress
Down’s Syndrome Oxidative stress
Preterm delivery Risk factor for oxidative stress and indicator of
Male gender Risk factor for oxidative stress, susceptibility to oxidative
stress
Maternal exposure to childhood abuse Risk factor for inflammation later in life
Hepatitis B vaccine, first month
of life, pre-1999
Oxidative stressor, inflammatory stimulus
Polymorphic variants (various, involved in
methionine and glutathione pathways)
Decreased capacity to handle oxidative stress
Mother with diabetes Indicator of inflammation
Mother with lupus Indicator of inflammation
Moderate Risk
Father>50 years old Risk factor for inflammation
Parental autoimmune disorder Indicator of inflammation
Maternal obesity Risk factor for inflammation
Hyperbilirubinemia Risk factor for oxidative stress
Early childhood atopic disorders (dermatitis,
respiratory)
Indicator of inflammation
Maternal autoimmune disease Indicator of inflammation
Febrile episode >7 days Symptoms associated with oxidative stress and
inflammation
Use of acetaminophen during pregnancy Pharmaceutical, oxidative stressor
Infection during pregnancy Oxidative stressor, inflammatory stimulus
Low or Uncertain Risk
Mother smoking during pregnancy Oxidative stressor
Mother >40 years old Risk factor for inflammation
Environmental toxins: air pollution, including
vehicular emissions of heavy metals and
particulate matter
Oxidative stressors, inflammatory stimuli
Aspartame/other sources of methanol Oxidative stressor
Low vitamin D levels at birth Inflammatory mediator
Folate deficiency (anti-folate receptor
autoantibodies)
Risk factor for oxidative stress
Urbanization/Western society Risk factor for inflammation
Citations are in the text.aDetermined only in a single, survey-based study.
Parker et al. 409
induction of autism will be described as ‘‘lowrisk’’ (1%–20% increased risk of autism),‘‘moderate risk’’ (21%–100% increased riskof autism), or ‘‘high risk’’ (any risk of autismabove 100% or 2-fold).
Autism and inflammation
Several risk factors found for autismhave very straightforward connections toinflammation (Table 1). Episodes of fevergreater than 7 days in the first and secondtrimester carry a moderate risk factor for
autism (hazard ratio [HR]¼ 1.6, CI95%¼ 1.0–2.5).3 In addition, autoimmunediseases in the mother such as lupus, mul-tiple sclerosis, diabetes, and rheumatic feverare associated with moderate-to-high riskfor autism induction,17 as is maternal infec-tion during pregnancy.18 Further, atopicdermatitis and respiratory diseases in chil-dren under the age of 2 years are alsoassociated with a moderate risk for thedevelopment of autism.19
Another moderate risk factor for autismthat has a well-known association with
Figure 1. The role of oxidative stress (red), inflammation (blue), and possibly acetaminophen exposure
after birth in the induction of autism.
410 Journal of International Medical Research 45(2)
inflammation is obesity (OR¼ 1.67; CI95%¼ 1.1–2.56).2 Over one-third ofwomen in the United States of childbearingage are obese and this number is steadilyrising.20 Multiple studies link obesity tochronic, low-grade inflammation and thesubsequent secretion of pro-inflammatorycytokines and infiltration of immune cells.21
Although it may be tempting to blame therise in autism on the rise in obesity andautoimmune disease, these risk factorsalone, which are generally associated withlow-to-moderate increases in autism preva-lence, likely do not account for the 30-foldor greater increased prevalence of autismsince the 1970s.9,22
The prevalence of autism in children withcerebral palsy is approximately 7-fold greaterthan in the general population.23 Substantialevidence points toward the view that cerebralpalsy, the most common cause of severemotor disability in childhood, is associatedwith inflammation. For example, many stu-dies have linked fetal exposure to infection tothe development of cerebral palsy,24–26 andblood samples from neonates with cerebralpalsy contain higher concentrations of manypro-inflammatory cytokines and chemokinescompared with controls.27 Further, cerebralpalsy is associated with chorioamnioni-tis,26,28–30 which can stimulate fetal produc-tion of inflammatory cytokines including IL-6.31 Thus, cerebral palsy, a high risk factor forthe development of autism, is linked toinflammation.
Some risk factors for autism have lessobvious associations with inflammation. Forexample, increasing age of the mother or thefather at the time of birth is associated with alow or moderate risk of autism, respectively(OR¼ 1.15, >40 years for the mother andOR¼ 1.66, >50 years for the father).32,33
While age itself might not normally be con-sidered inflammatory, there have been directassociations found between increased age ofthe parents and inflammatory diseases in theiroffspring. For instance, maternal age �30
years at the time of birth is associated with anincreased incidence of one or more foodallergies in the child compared with controls(75% vs. 55%, p¼ 0.005).34 Further, the riskof having children diagnosed with multiplesclerosis steadily increases with paternal ageat birth from 21 to 55 years (adjusted OR for21–25-year-old fathers¼ 1.08 and 51–55-year-old fathers¼ 2.00).35 Thus, althoughparental age, a moderate risk factor forautism, is not intuitively associated withinflammation, the association is evidentbased on published studies.
Maternal exposure to severe emotionalstress during her childhood and/or adoles-cence is another risk factor (OR¼ 3.7, CI95%¼ 2.3–5.8) for autism36 that may not beintuitively associated with inflammation.However, the link between childhood andadolescent adversity and immune dysregula-tion and inflammation is widely appreciatedby the scientific community. Multiple stu-dies demonstrate that a wide range ofstressful events taking place in one’s child-hood can adversely affect health and inflam-matory responses even into the eighthdecade of life.37–40 Exposure of women totwo or more early childhood stressful events,including physical, emotional, or sexualabuse, is positively correlated with anincreased risk of Th1, Th2, and rheumaticautoimmune development later in life com-pared with women reporting no events inchildhood (p< 0.05).41 Markers of inflam-mation (IL-6 and C-reactive protein) havebeen found to be elevated in a systemicmanner in women who experienced sexualabuse in adolescence compared with womenwithout a history of abuse (p¼ 0.04 for IL-6and 0.03 for C-reactive protein).42 Further, arecent study of 28,456 African Americanwomen found a positive association betweenadult-onset asthma and physical or sexualabuse experienced during childhood or ado-lescence after adjusting for multiple con-founders such as parental history of asthma,body mass index, exercise, current smoking
Parker et al. 411
habits, and exposure to secondhandsmoke.43 Childhood stress also affects awoman’s response to current, daily stressors.For example, elevated levels (2.35-fold) ofIL-6 have been found within 24 hours inresponse to current daily stressful events insera from women who experienced child-hood abuse compared with women whoexperienced the same number of daily stres-sors without a history of child abuse.44
Other examples of the impact of early lifestress on inflammation abound.37,45–48
Thus, maternal exposure to stress duringearly life, a high risk factor for autism, isclearly associated with inflammation.
Autism and oxidative stress:Environmental toxins
Exposure to environmental toxins, a sourceof oxidative stress, has been associated withautism in a number of studies.49 Maternalexposure during gestation to agriculturalpesticides such as organophosphates,organochlorines, and pyrethroids has beenidentified as a moderate-to-high risk factorfor autism.50–52 The risk seems to be depend-ent at least in part on the proximity to theapplied chemical and the trimester duringwhich the exposure took place. However,the data are somewhat noisy and, as pointedout by the authors, possibly confounded byunavoidable misclassifications in the estima-tions of maternal exposure. In contrast toevidence of the effects of exposure in utero,evidence for exposure to pesticides afterbirth as a risk factor for autism is slim tonon-existent. Maternal exposure to traffic-related air pollutants also carries risk factorsfor autism. For example, exposure to par-ticulate matter during the third trimester is alow-to-moderate risk factor for autism,53–55
while maternal exposure to other traffic-related toxins (e.g. mercury, lead, arsenic,cadmium, manganese, styrene, trichloro-ethylene, and vinyl chloride) is a moderate-to-high risk factor for autism.56–58 However,
these findings were not replicated in a studyof four European countries.59 Furthermore,the range of variability in U.S. air pollution(e.g. interquartile ranges of only 4–5mcg/m3
in fine particulate matter [PM2.5]) overwhich associations with autism spectrumdisorder have been reported53–55 is quitesmall compared with the more severe airpollution encountered routinely in thedeveloping world (for example, >900mcg/m3 when indoor cooking stoves are used).60
Additionally, exposure to many environmen-tal toxins such as vehicular and air pollution,polychlorinated biphenyls (PCBs), lead, poly-cyclic aromatic hydrocarbons (PAHs), andorganochlorines and organophosphates hasdecreased over at least part, if notmost, of thetime frame in which autism prevalence hascontinued to climb.9 Further, factors such asexposures to phthalates, atmospheric mer-cury levels, and total blood mercury haveremained relatively stable since the 1990s,although exposure to polybrominated diphe-nyl ethers (PBDEs) and glyphosates hasincreased.9 However, to date, few studieshave associated exposure to phthalates,PCBs, PBDEs, PAHs, or glyphosates to thedevelopment of autism. With this in mind, itseems likely that the relative contribution ofany single environmental oxidative stressor tothe prevalence of autism is most likelydependent on the location or population inquestion.
Autism and oxidative stress:Jaundice
Hyperbilirubinemia (the symptoms of whichare known as jaundice) is associated withelevated oxidative stress61 and impedes theclearance of acetaminophen,62 a drug sus-pected of inducing autism (see discussionbelow). Jaundice is associated with autism,63
although the connection between jaundiceand autism had some early critics.64 The ideawas that early studies pointing toward aconnection between jaundice and autism
412 Journal of International Medical Research 45(2)
may have been confounded by some condi-tion or conditions associated with autism butunrelated to jaundice. These conditions mayhave been responsible for keeping the chil-dren in the hospital for longer periods of time,which could have resulted in an increasedchance of receiving a diagnosis of jaundice.However, a prospective study design using alarge sample size in Taiwan seems to haveaddressed major concerns with the previousstudies, and the connection between jaundiceand autism is apparently real.65
Autism and oxidative stress:Sex, Down syndrome, and
preterm birth
Perhaps the most widely known risk factorfor autism is beingmale. The increased risk ofautism for males was established very earlyduring work with patients having autism, andis approximately 4-fold that of females.Although being male is not widely known tobe associated with oxidative stress, maleinfants are more susceptible to oxidativestress than female infants,66–68 and havebeen found to have more oxidative stress.69
Further, male children seem to be moresusceptible to toxin-induced oxidative stressthan females.70 The occurrence of Downsyndrome is also a risk factor for autism,with more than 5%of individuals with Downsyndrome having autism.71 Down syndromeis also associated with a profound increase inoxidative stress and inflammation,72,73 point-ing again toward a connection between oxi-dative stress and the induction of autism.
Preterm birth, a complex and often idio-pathic complication of pregnancy, is a highrisk factor for the development of autism.Limperopoulos and colleagues found that26% of preterm, low birth weight infantshad a positive result on the ModifiedChecklist for Autism in Toddlers (M-CHAT) autism screening tool74 comparedwith 5.7% of non-preterm infants(OR¼4.56).75 This risk is corroborated by
several additional studies with odds ratios of6.3 (CI 95%¼ 2.2–18.3) for children bornpreterm in the United Kingdom andIreland75 and 3.2 (CI 95%¼ 2.6–4.0) forchildren who were born preterm in Swedenusing a sibling-comparison approach.76
Preterm birth has long been associated withinflammation, with approximately 50% ofpreterm births being associated with chor-ioamnionitis.77,78 In addition, oxidativestress has recently been hypothesized as aco-mechanism for the initiation of pretermbirth.77,79,80 Further, oxidative stress is asso-ciated with preterm birth.81–83 Thus, pretermbirth, a risk factor for autism, is associatedwith both inflammation and oxidative stress.
Autism and oxidative stress:Vitamin B
Excessively high levels of maternal vitaminB12 and vitamin B9 (folate) are additionalrisk factors linked to autism.84 In a study ofthe association between vitamin B levels andrisk of autism, excessively high prenatalvitamin B12 levels in mothers (HR¼ 3.01,CI 95%¼ 1.64–5.52; P value: 0.001) andexcessively high prenatal folate levels(HR¼2.27, CI 95%¼ 1.26–4.09; P value:0.007) were each found to be associated witha significantly increased risk of infantautism. Together, excessively high vitaminB12 and folate levels in maternal blood serashow the highest increased risk of infantileautism (HR: 17.59; P value: <0.001).84
Vitamin B9 and B12 are antioxidants, andthus, intuitively, high levels of these vitaminsshould be associated with a decreased riskfor oxidative stress. However, excessivelyhigh levels of vitamin B12 in the blood arecommon and are often paradoxically indi-cative of a clinical deficiency of vitaminB12.85 In one case, for example, excessivelyhigh levels of vitamin B12 were caused by thepresence of anti-vitamin B12 antibodies thatled to the formation of functionally inactiveIgG-IgM-B12 immune complexes.86 In
Parker et al. 413
general, excessively high levels of vitaminB12 and the associated functional deficiencyof that antioxidant could lead to difficultiesin the ability to cope with oxidative stress,and indeed have been consistently linked toinflammatory diseases such as neoplasms,hematological malignancies, and liver andkidney diseases.85
Not only are exceedingly high levels offolate associated with autism,84 the presenceof anti-folate receptor autoantibodies hasbeen shown to be highly prevalent (75.3%)in children with autism.87 Such autoantibodieswould create a functional deficiency of folatein the brain regardless of the concentration offolate in the serum. Folate deficiency is highlydeleterious and is associated with reducedactivity of antioxidant enzymes, as well asoverall increased oxidative stress.88,89
Hypothetically, excessive levels of folatein the serum could be an indication of aclinical insufficiency of folate, just as exces-sively high levels of vitamin B12 are linkedto a clinical insufficiency of that vitamin.Regardless of the reasons for excessivelyhigh levels of folate, they have been asso-ciated with inflammatory bowel disease,90
indicating that the high levels are related insome way to inflammation.
Autism and oxidative stress:
Genetic variation
Polymorphic variants related to the metab-olism of methionine transmethylation andtranssulfuration, which increase susceptibil-ity to endogenous and environmental oxi-dative stress, are significantly different inchildren with autism.91–93 Studies show adecreased ability in children with these gen-etic variants to handle oxidative stress asmeasured by several metabolic biomarkersincluding S-adenosylmethionine (SAM), S-adenosylhomocysteine (SAH), adenosine,homocysteine, cystathionine, cysteine, oxi-dized and reduced glutathione, endogenoussecretory receptor for advanced glycation
end-products (RAGE), and the pro-inflam-matory ligand S100A9. In addition, poly-morphisms in glutathione pathways, whichmodulate the response to oxidative stress,strongly affect risk for autism. For example,the homozygous GSTM1 deletion genotypeimposes a near 2-fold increased risk forautism.94,95 Further, polymorphisms of theglutathione S-transferase P1 gene (GSTP1)in the mother, which could affect the fetusduring pregnancy, are high risk factors forthe induction of autism (OR¼ 2.67, CI95%¼ 1.39–5.13).96 Thus, several geneticvariants that affect pathways involved inoxidative stress are risk factors for autism.
Synergism between oxidativestress and inflammation:Evidence from animal models
Inflammation can be caused by a diverse setof factors associated with Western culture,including chronic psychological stress andbiome depletion (Figure 1), which at firstglance might seem unrelated to inflamma-tion. However, it is now recognized thatthese non-chemical inflammatory mediators(e.g. limited resources or social support forthe mother, leading to chronic psychologicalstress) can increase vulnerability of the fetusto chemical stressor exposures (e.g. oxida-tive stressors such as pollution or toxins).97
This view potentially explains why a singleexposure or risk factor in isolation is amodest predictor of autism risk. Given thecomplex nature of environmental and socialexposures, attempts at deciphering themechanisms that contribute to autismsuffer from fatal oversimplification if thosemodels involve only single agents. In con-trast, useful and relevant models mustinclude multiple factors. For example, anexperimental animal model has been devel-oped that employs the combined effects ofan ethologically relevant maternal stressorand an environmentally relevant pollutant,diesel exhaust, both of which have been
414 Journal of International Medical Research 45(2)
implicated in autism.55,57,98–102 Using thismodel, it was demonstrated that maternalexposure to diesel exhaust particles com-bined with maternal stress, but neither inisolation, produced long-term cognitive def-icits and strikingly increased anxiety in malebut not female offspring.97
Evidence for a mediator otherthan inflammation andoxidative stress
The likely role of oxidative stressors andinflammation in the pathogenesis of autismis apparent based on the nature of riskfactors associated with autism. This conclu-sion is corroborated by numerous studies,described above and reviewed else-where,10,103–109 that have identified immuneactivation and inflammation in patients withautism. Further, treatment with antioxi-dants or anti-inflammatory agents such assulforaphane110 and helminth therapy,111
respectively, can help some patients withautism. However, it seems unlikely thatoxidative stress and inflammation aloneaccount for the dramatic rise in the inci-dence of autism since 1980. Inflammation ingeneral has increased steadily since the turnof the twentieth century, as indicated by aslow and steady rise in a wide range ofallergic disorders, autoimmune conditions,and other inflammation-associated dis-eases.112 The very rapid rise in autism sincethe early 1980s might suggest that one ormore specific environmental factors are atplay. As we and others have pointedout,9,10,113 it is possible that the rapid‘‘increase’’ in autism is due simply toincreased awareness and changing diagnos-tic criteria, but substantial evidence whichcontradicts this conclusion is available and aworking hypothesis that autism has pro-foundly increased over the last 40 yearsholds the most promise for a rapid reso-lution of the problem.10 That is to say, ifautism is indeed an ancient and natural
consequence of ‘‘being human,’’ then autismmay be much more difficult to prevent thanif it is induced by factors present in modernsociety.10
We have suggested10 that arguments forand against autism as an epidemic (orpandemic) can only be resolved (a) if aspecific trigger for autism is found, facilitat-ing the elimination of autism; or (b) ifcomplete normalization of the immunesystem (leading to a pre-industrial conditionof essentially no allergies) succeeds in elim-inating autism. Given that complete nor-malization of the immune system may takeyears or even generations to accomplish, andgiven the recent apparent rise in autismdescribed above, the search for very specificand potent triggers for the induction ofautism seems worthwhile.
Proposed triggers of the autismepidemic: Aspartame
Aspartame is an artificial sweetener that hasbeen in use since 1981, about the time thatthe autism epidemic started (Figure 2).Aspartame breaks down in the body andreleases methanol, an oxidative stressor andtoxin. Its use has been attributed to the rise inautism.114 However, the use of other sweet-eners has replaced aspartame in the past 10years, except in diet soft drinks, and theconsumption of diet soft drinks has been indecline. Thus, there is no association overtime between autism and aspartame. Further,the presence of methanol in canned vege-tables and in cigarette smoke is not new, andthus the relatively low incidence of autismprior to the mid-1900s cannot be accountedfor if methanol, by itself, is a major trigger.Further, Pepsi Cola� has at least temporarilyremoved aspartame from its drinks, whicheliminates at least for a time one of the majorremaining sources of aspartame consumptionin the United States. If indeed aspartame isthe major trigger for autism, then the ‘‘experi-ment’’ has been accomplished already and
Parker et al. 415
the rate of autism should be in the process ofdecreasing. This does not appear to be thecase, apparently disproving the hypothesis.
Proposed triggers of the autismepidemic: Ethyl mercury andvaccines
The view that vaccines and ethyl mercury inparticular, a component of the preservativethimerosal used in some vaccines, can
induce the development of autism is wide-spread among non-scientists115,116 and hasbeen discussed widely in the literature.117,118
Proposed mechanisms of induction dependon the difference between methyl and ethylmercury119 to account for the relativelyrecent rise of autism compared with thecenturies-old use of methyl mercury.However, a study addressing this issuefound no association between levels ofmercury exposure during vaccination and
Figure 2. Apparent changes in the quality and quantity of autism extending over a decade, starting in the
early 1980s. In the top diagram, data are from Rimland’s summary168 of the number of surveys (the ‘‘E-1
Diagnostic Checklist’’ and the ‘‘E-2 Diagnostic Checklist’’) that were collected in a given time period through
grass-roots efforts of the Autism Research Institute and the Autism Society of America, the only two national
autism organizations in the United States at the time of the data collection. The Y-axis describes the actual
number of surveys received, and changes in the number of reports received were attributed by Rimland to
increases in the number of children with autism. ‘‘Shortly after birth’’ in this case refers to parents’ reports
that symptoms of autism were evident within weeks of birth. In the lower diagram, the prevalence of autism
in California as compiled by Nevison9 is shown. Data are a composite of ‘‘snapshot’’ data (information
collected at one point in time) from the California Department of Developmental Services (collected in 2002
and covering birth years 1970–1997) and tracking data evaluating 5-year-olds collected under the US
Individuals with Disabilities Education Act (covering birth years 1995–2005).9
416 Journal of International Medical Research 45(2)
the incidence of autism 120. Further, asubstantial reduction in exposure to ethylmercury as a result of elimination of thimer-osal from vaccines has not reduced the rateof autism. It has been counter-argued that‘‘no level is safe,’’ suggesting that anyamount of ethyl mercury may be dangerous.However, since the induction of autism (bywhatever agent) is apparently not near thesaturation point (presumably yielding a rateof 100% autism in the population, at whichpoint the rate of autism would be independ-ent of increasing or decreasing levels ofinducing agent), then it is expected that asubstantial reduction in the amount ofinducing agent would lead to at least somereduction in the level of autism. It has beenargued that perhaps aluminum adjuvantsnow replace mercury preservative as thevaccine-associated agent, but then ethylmercury and aluminum must be differentfrom methyl mercury in some regard, strain-ing the original hypothesis regarding theuniqueness of ethyl mercury. It could becounter-argued that the route of exposure(injection vs. dietary intake) of the metal isimportant, but in the one widely acceptedinstance in which a vaccine caused theinduction of a neuropsychiatric disorder(narcolepsy with cataplexy), no metal pre-servatives or metal adjuvants wereinvolved.120 Thus it could be argued that itis the vaccination in general that is criticallyimportant, not the metal per se. Yet vaccinesare much older in their origins than autism,and it is not intuitive that a vaccine would beworse than an actual life-threatening infec-tion, the origins of which are ancient andpre-date the human race. This view iscorroborated by a cohort of parents whodid not vaccinate younger siblings of chil-dren with autism; failure to vaccinate withone or even all vaccines did not preventautism.122,123 Most importantly, acetamino-phen, the analgesic most commonly admin-istered in conjunction with vaccination andthe only analgesic administered to children
under the age of 6 months following vaccin-ation, has been identified as a likely inducerof autism.124 Initial studies described belowsuggest that it is the co-administration ofthis analgesic with vaccines that may havegiven many parents the false impression thattheir child’s autism was induced by avaccine.
Proposed mediators of the autismepidemic: Acetaminophen
Acetaminophen has been widely used inadults for more than half a century as apain reliever and anti-pyretic, and is by farthe most commonly used medication forpain and fever during pregnancy125 and inthe pediatric population.126,127 By the early1980s, acetaminophen had effectivelyreplaced the analgesic phenacetin, which ismetabolized to acetaminophen by thebody128,129 and which had been widelyused since the late 1800s despite its carcino-genic and toxic nature. The therapeuticaction of acetaminophen involves inhibitionof prostaglandin synthesis, a surprisinglyimportant biochemical process involved indevelopment and neurological function, asshown in Figure 3. Acetaminophen elimin-ation from the body typically involves bio-chemical modification in the liver by phaseII metabolism, which entails the addition ofsulfate (or glucuronide more often inadults130) to the molecule, facilitating itselimination (Figure 3). However, the drugcan also be modified via phase I metabolism,producing NAPQI, a highly toxic metabolitewhich is then processed via phase II metab-olism to a non-toxic product by the additionof cysteine in a manner dependent on gluta-thione (Figure 3). Acetaminophen is knownto be safer in children than in adults,presumably because the drug cannot berapidly converted by the child’s relativelyundeveloped liver into toxic metabolites.131
However, this view of acetaminophen’ssafety is based strictly on the low rate of
Parker et al. 417
acute adverse events such as liver and kidneyfailure and intestinal bleeding.131,132
Unfortunately, the long-term effects of acet-aminophen exposure on neural developmenthave never been evaluated in humans.However, Margaret McCarthy’s lab hasshown that drug-mediated inhibition ofprostaglandin synthesis (as is accomplishedby acetaminophen) in laboratory rats during‘‘a time sensitive window in early postnatallife’’ not only results in significant long-termmodifications to brain development andmorphology but also leads to decreasedsocial interactions and reduced sensoryfunction in male but not female animals.133
Further, in vitro studies using human celllines have shown that acetaminophen can
cause ‘‘an immediate, reversible, dose-dependent loss of oxygen uptake followedby a slow, irreversible, dose-independentdeath’’ and have suggested mechanisms bywhich acetaminophen may cause toxicity intissues other than the liver.134
Given its suppression of the febrileresponse, one might expect that the generaleffects of exposure to acetaminophenwould be anti-inflammatory in nature.Unfortunately, this expectation is false.Studies in adult humans demonstrate thateven low-dose acetaminophen triggersimmune system activation and oxidativestress responses.135 Further, medical profes-sionals evaluating immune responses from abiologists’ perspective have warned that the
Figure 3. Action and metabolism of acetaminophen in babies and children. Phase II metabolism involving
glucorinadation, like sulfation, leads to detoxification of acetaminophen, but sulfation is the primary
mechanism active in infants and children.
418 Journal of International Medical Research 45(2)
febrile response is both adaptive (beneficialto survival) and ancient in its origins,136 withbehaviors which induce fever-like tempera-tures evident in cold-blooded animals.137
Thus, biologists argue that inhibition ofthe febrile response is likely not withoutcosts.137 It has even been postulated that theelimination of fevers may be responsible inpart for the induction of autism.138
However, the health costs of eliminatinglow-grade fevers in general are not known.On the other hand, some data regarding thehealth costs of using acetaminophen aredescribed in the literature. A multinationalstudy with more than 200,000 childrenfound a dose-dependent association betweenuse of acetaminophen in the first year of lifeand the occurrence of inflammatory diseasessuch as asthma, rhinoconjunctivitis, andeczema later in life.139 The causal relation-ship between use of acetaminophen and theoccurrence of asthma has been ques-tioned,140 but the consensus is that a rela-tionship exists141,142 and a population-wideincrease in asthma of more than 40% maybe caused by the use of acetaminophen.143
Perhaps even more concerning than stu-dies demonstrating acetaminophen-inducedsocial impairment in animal models, acet-aminophen-induced injury to cultured cells,and acetaminophen-induced inflammationin adults, are studies pointing toward aconnection between acetaminophen andneurological problems in children. A sum-mary of such studies is shown in Table 2. Asibling-controlled study with over 48,000children in Norway showed that the use ofacetaminophen but not ibuprofen bymothers during pregnancy was associatedwith problems in the psychomotor, behav-ioral, and temperamental development ofchildren at 3 years of age.144 Further, a studyin Bristol, United Kingdom, of more than7000 children showed that maternal use ofacetaminophen during pregnancy was asso-ciated with hyperactivity and ‘‘emotionalsymptoms’’ at age 7.145 In addition, a study
at UCLA in collaboration with scientists inDenmark and Taiwan found that childrenwhose mothers used acetaminophen duringpregnancy were at higher risk of beingdiagnosed with hyperkinetic disorder(HR¼ 1.37, CI 95%¼ 1.19–1.59). Toquote the authors, ‘‘Results did not appearto be confounded by maternal inflamma-tion, infection during pregnancy, themother’s mental health problems, or other[variables they examined].’’146 Amore recentDanish study came to the same conclu-sions.147 Further, a study from NewZealand found associations between acet-aminophen use during pregnancy and atten-tion-deficit/hyperactivity disorder (ADHD)at 7 and 11 years of age,148 with the authorsconcluding that their work supports ‘‘earlierclaims that findings [of increased ADHDwith acetaminophen use] are specific toacetaminophen.’’ The authors further statethat ‘‘The finding that even low doses ofacetaminophen (indicated by the number ofweeks of drug exposure) can affect behavior7 years later is alarming because acetamino-phen (paracetamol) is the most commonlyused antenatal drug.’’ Indeed, new studiessupporting the view that prenatal acet-aminophen use is associated with long-term negative effects on brain function arecurrently being published on a monthlybasis.145,149
Acetaminophen rapidly enters the cere-brospinal fluid to exert its effects.150 Inaddition to reducing fever and physicalpain, acetaminophen has a profound effecton adult brain function, blunting theresponse to both negative and positivestimuli, including threatening stimuli,151,152
and reducing behavioral responses to socialrejection.153 Further, the drug impairs theability of adults to identify errors madeduring the performance of simple tasks.154
In animal models, the use of acetaminophenduring development has been shown tocause permanent alterations in cognitivefunction.155 Despite these effects on
Parker et al. 419
Tab
le2.
Publis
hed
studie
spro
bin
gth
eeffect
sofac
eta
min
ophen
on
neuro
psy
chia
tric
funct
ion.
Study
Gro
up
Contr
ols
or
Var
iable
sC
onsi
dere
d
(oth
er
than
aceta
min
ophen
use
)Sy
mpto
ms
Odds
Rat
io
1.163
US
child
ren,
aged
5ye
ars
or
less
aIb
upro
fen
use
use
das
aco
ntr
ol.
Confo
unders
incl
uded
mat
ern
al
and
pat
ern
aldem
ogr
aphic
s,an
dch
ild’s
heal
th,ag
e,an
dse
x.
Autism
6.1
155
2.48,6
31
Norw
egi
an
child
ren,
aged
3ye
ars
Ibupro
fen
use
use
das
aco
ntr
ol.
Confo
unders
incl
uded
mat
ern
al
dem
ogr
aphic
s,heal
th(infe
ctio
ns,
feve
rs,pai
n),
use
ofoth
er
medic
atio
ns,
alco
holuse
,to
bac
couse
,an
dpsy
cholo
gica
l
stre
ss.
Pro
ble
ms
inpsy
chom
oto
r,
behav
iora
l,an
dte
mpera
-
menta
ldev
elo
pm
ent
1.5
1to
1.6
9143
3.64,3
22
Dan
ish
chil-
dre
n,ag
ed
up
to
15.6
year
s
Confo
unders
incl
uded
mat
ern
aldem
ogr
aphic
s,heal
th(f
ever,
infe
ctio
n,j
oin
tan
dm
usc
ledis
eas
es)
,use
ofo
ther
medic
atio
ns,
alco
holuse
,to
bac
couse
,psy
cholo
gica
lco
nditio
ns,
and
child
’s
age
and
sex.
Autism
acco
mpan
ied
by
hyp
erk
inetic
sym
pto
ms
1.5
1123
4.1491
Dan
ish
chil-
dre
n,
aged
5ye
ars
Confo
unders
incl
uded
mat
ern
aldem
ogr
aphic
s,heal
th(infe
c-
tions,
feve
rs,pai
n),
ibupro
fen
use
,as
pir
inuse
,al
coholuse
,
tobac
couse
,psy
cholo
gica
lco
nditio
ns,
and
child
’sse
x.
Impai
red
atte
ntion
and
execu
tive
funct
ion
1.5
146
5.7796
Bri
tish
child
ren,ag
ed
7ye
ars
Mat
ern
alpost
nat
alan
dpar
tner
aceta
min
ophen
use
use
das
contr
ols
.C
onfo
unders
incl
uded
mat
ern
aldem
ogr
aphic
s,
heal
th(infe
ctio
ns,
feve
rs,pai
n),
genetic
risk
fact
ors
,al
cohol
use
,to
bac
couse
,psy
cholo
gica
lst
ress
,and
child
’sbir
thw
eig
ht
and
gest
atio
nal
age.
Hyp
era
ctiv
ity
and
‘‘em
otional
sym
pto
ms’
’
1.2
9to
1.4
6144
6.64,3
22
Dan
ish
chil-
dre
n,
aged
7ye
ars
Confo
unders
incl
uded
mat
ern
aldem
ogr
aphic
s,heal
th(infe
c-
tions,
feve
rs,pai
n),
alco
holuse
,to
bac
couse
,psy
cholo
gica
l
stre
ss,an
dch
ild’s
bir
thw
eig
ht
and
sex.
Hyp
erk
inetic
dis
ord
er
and
AD
HD
b1.1
3to
1.3
7145
7.871
New
Zeal
and
child
ren,
aged
7an
d
11
year
s
Anti-infla
mm
atori
es,
aspir
in-b
ased
pai
nkill
ers
,an
taci
ds,
and
anti-
bio
tic
use
use
das
contr
ols
.C
onfo
unders
incl
uded
mat
ern
al
dem
ogr
aphic
s,heal
th(f
ever
and
infla
mm
atio
n),
alco
holuse
,
psy
cholo
gica
lst
ress
,an
dm
edic
atio
ns
for
psy
cholo
gica
l
conditio
ns.
AD
HD
Odds
ratio
not
calc
ula
ted:
appro
xim
ately
10%
dif-
fere
nce
inA
DH
D
asse
ssm
ent1
47
8.2644
Span
ish
chil-
dre
n,ag
ed
5ye
ars
Confo
unders
incl
uded
mat
ern
aldem
ogr
aphic
san
dheal
th(f
ever,
infe
ctio
n,ch
ronic
illness
),par
enta
lm
enta
lheal
th,an
dch
ild’s
heal
th,ag
ean
dse
x.
Autism
Odds
ratio
not
calc
ula
ted:
diff
ere
nce
inC
AST
c
Pva
lue¼
0.0
06
161
aT
his
study
was
the
only
study
that
eval
uat
ed
aceta
min
ophen
exposu
reduri
ng
child
hood.
The
rest
of
the
studie
sev
aluat
ed
the
effect
sof
aceta
min
ophen
exposu
reduri
ng
pre
gnan
cy.
bA
DH
D;A
ttention-d
efic
it/h
ypera
ctiv
ity
dis
ord
er
cC
AST
;C
hild
hood
Autism
Spect
rum
Test
420 Journal of International Medical Research 45(2)
neurological function in adults, the widelyappreciated connections between acet-aminophen use during pregnancy and neu-rodevelopmental problems, and studies inanimal models pointing toward potentialproblems with acetaminophen for braindevelopment,133,155 acetaminophen is nowthe most widely used medicine in the pedi-atric population, readily available forinfants in an over-the-counter form labeledas ‘‘safe, gentle, and effective,’’ with nowarnings of side effects other than allergicreactions.
A connection between acetaminophenand autism was first identified in 2008 bySchultz et al.,156 who found that acetamino-phen use by children was significantlyassociated with autism in children aged 5years or less (OR¼6.11, CI 95%¼ 1.42–26.3). Subsequently, several investigatorsnoted that the marked increase in autism,asthma, and ADHD in the early 1980scorresponded with the replacement ofaspirin with acetaminophen.157,158 In add-ition, Schultz noted that the long-term,steady increase in the prevalence of autismwas punctuated by short-term decreasescoinciding with widely publicized cases ofacetaminophen poisoning that temporarilydeterred the public from using the drug.159
Further, evidence has surfaced indicatingthat neural pathways affected by acetamino-phen may be ‘‘different’’ in some regards inpeople with autism.159,160 This observationis potentially a ‘‘smoking gun,’’ suggestive ofthe role of acetaminophen in the pathogen-esis of autism. Interestingly, Bauer andcolleagues noted that acetaminophen usewith circumcision may be associated with anincreased prevalence of autism in somelocations.157 A second and more recentstudy looking at the connection betweencircumcision and autism, this one by Frischand Simonsen, found a 2-fold increased riskof autism identified before age 5 in circum-cised boys compared with uncircumcisedboys.161 But of course, the degree to which
acetaminophen use during circumcision isassociated with autism will depend not onlyon the ability of acetaminophen to induceautism but also on other factors such as therelative amount of acetaminophen used fol-lowing circumcision compared with otheruses in the pediatric population.
A Danish National Birth Cohort studyrecently found that prenatal use of acet-aminophen is associated with an increasedrisk of autism accompanied by hyperkineticsymptoms (HR¼ 1.51; CI 95%¼ 1.19–1.92).124 More recently, a Spanish studysupported the connection between acet-aminophen exposure during pregnancy andautism in the offspring.162 Although theincrease from prenatal exposure is statistic-ally significant and indeed concerning, therisk was more than 10-fold less than thatoriginally identified by Schultz when evalu-ating the use of acetaminophen in children.As discussed below, post-partum (infancyand early childhood) exposure to acet-aminophen may result in a much higherrisk of developing autism than prenatalexposure.
Schultz, whose 2008 study was the first toidentify acetaminophen use in small childrenas a potential cause of autism, very recentlyfound that older children with autism actu-ally use less (not more) acetaminophen thanneurotypical controls.163 Thus, small chil-dren who eventually develop autism havemore often been exposed to acetaminophenthan controls, but older children with autismare less likely to use the drug. Schultzsuggested that this observation may behypothetically explained if parents noticedthat their child with autism did not respondto acetaminophen. Studies in animal modelshave shown that early life exposure toacetaminophen causes a lack of response tothe same drug later in life,155 and Schultzpostulated that the same may occur inhumans.163 Another explanation, not mutu-ally exclusive, is that parents may havelearned that acetaminophen is associated
Parker et al. 421
with autism, and thus discontinued admin-istration of the drug to their child. Theconnection between acetaminophen andautism is common knowledge among par-ents who have children with autism, as it hasbeen promoted by grass-roots organizationssuch as Reset.Me and SafeMinds.
Plausible mechanisms for the induction ofautism by acetaminophen have been formu-lated,113,158,159 adding further support to thepotential role of acetaminophen in the patho-genesis of autism. McCarthy, for example,has provided a plausible explanation for theidea that inhibition of prostaglandin synthe-sis (by acetaminophen, for example) wouldhave a much different effect on the male brainthan on the female brain.164 However, theexistence of a plausible mechanism alonemaybe considered inadequate by itself to supportstudies regarding the acetaminophen–autismconnection, since detailed mechanisms havebeen proposed for the induction of autism byseveral factors, including ethyl mercury fromvaccine preservatives165 and methanol fromdiet soft drinks.114 The removal of ethylmercury from vaccines and the reduction ofaspartame consumption have removed theseenvironmental factors from prime suspicionfor inducers of autism (see discussion above),despite the publication of hypotheticalmechanistic underpinnings. Thus, theavailability of a hypothetical mechanism forthe induction of autism by a particular agentis not a good indicator for its actual role inthe pathogenesis of disease. However, giventhe weight of the burden of autism on society,it seems reasonable to test all plausibleenvironmental triggers for the induction ofautism.
Albeit indirectly, yet another factor pointstoward a role of acetaminophen in the devel-opment of autism: the lack of any publishedassociation between cystic fibrosis (CF) andautism stands out as the exception to the ruleof association between inflammatory condi-tionsandautism.CFisclearlyassociatedwithmucosal inflammation166 but no connection
between autism and CF has been published,suggesting that if anyassociationdoes exist, itis not as conspicuous as that seen with otherinflammation-associated conditions (e.g.cerebral palsy, preterm birth, or Down syn-drome). Indeed, physicians at two independ-ent clinics, each having treated hundreds ofCF patients, recalled no patients with autismwhatsoever in discussions with one of theauthors (WP). An explanation for this poten-tial exception to the rule of associationbetween inflammation and autism lies in thefact that adults and children with CF tend tometabolize acetaminophen through phase IIpathways much more so than do healthycontrols.167,168 This feature of metabolism inCF patients may be a ‘‘consequence ofdisease-specificchanges inbothenzymeactiv-ity and/or drug transportwithin the liver.’’168
In this regard, CF patients are very differentfrom patients with autism, who tend to haveimpaired phase II metabolism (Figure 3).Although speculative at present, the ideathat CF may be protective from autismmerits further study and may provide insightinto the pathogenesis of the latter.
Why evaluate the connectionbetween post-partumacetaminophen exposure
and autism?
The possible role of acetaminophen exposurein neonates and young children in the patho-genesis of autism demands further study for anumber of reasons. First, as pointed outabove, the odds ratio for autism associatedwith acetaminophen exposure in children isone of the highest ever reported, exceedingthe odds ratios consistently reported forexposure in utero. The initial study basedon surveys156 was conducted almost 10 yearsago and has yet to be confirmed or refuted bymore rigorous studies.
Perhaps even more compelling than thereason above, a second line of evidence
422 Journal of International Medical Research 45(2)
points toward the need to evaluate theconnection between post-partum use ofacetaminophen and autism. This evidenceis based in part on two important epidemio-logical observations made by BernardRimland, founder of the Autism ResearchInstitute in 1967 and the individual respon-sible for undermining the horrendous‘‘refrigerator mother’’ hypothesis that pre-vailed during the first 20 years of research onautism. As shown in Figure 2, Rimlandnoted a rapid rise in the rate of autismbeginning in the early 1980s.169 Although hedid not connect this rise with the use ofacetaminophen in neonates and children, thewidespread use of acetaminophen in infants,neonates, toddlers, and small children beganin the early 1980s as a result of the discoverythat aspirin may be associated with Reyesyndrome.170 This apparent rise in theprevalence of autism starting in the early1980s has been corroborated by data col-lected in the state of California (Figure 2,lower diagram). In addition to an increase inprevalence, Rimland also noted a simultan-eous increase in the relative ratio of regres-sive to infantile autism (Figure 2), as mightbe expected if a new and extremely potenttrigger for the disease was introduced intothe population which affected newborns andsmall children but not necessarily fetuses.
Rimland’s study was not the last tosuggest the emergence of a preponderanceof regressive autism. In 2010, Ozonoff andcolleagues published a prospective evalu-ation of behavior in children who wouldeventually be diagnosed with autism.171
Ozonoff collected data starting at 6 monthsof age and focused on at-risk children,mostly those with siblings that had autism.Surprisingly, more than 85% of the childreneventually diagnosed with autism wereindistinguishable from neurotypicallydeveloping children at 6 months of age,but showed declines in social communica-tion between 6 and 18 months. Thus, shortlyprior to 2010, when Ozonoff’s data were
collected, most children with autism appar-ently presented with a regressive phenotype.
Intuitively, a preponderance of theregressive phenotype weakens any assump-tion that prenatal exposure is centrally orexclusively important. Indeed, there is no‘‘proof’’ that autism is based on architec-tural changes laid down before birth.Associations between imaging and neuro-pathic findings and phenotype have beenfound,172,173 but no one has systematicallyassessed the extent to which documentedbrain architectural differences in individualsdiagnosed with autism (a diagnosis thatcannot presently be made prenatally or inthe first year and a half of life) derive fromaltered prenatal neurodevelopmental pro-cesses as compared with tissue changesacquired over time from disturbances inkey phenomena such as excitation/inhib-ition balance, bioenergetics, immune func-tion, or other metabolic processes. Indeed, itis widely appreciated that pathophysio-logical changes can be set off not just byearly genetic or environmental influencesbut also in many ways at many times of life.
Another compelling reason to probe thepost-partum acetaminophen–autism con-nection is that, if indeed acetaminophenexposure during early childhood is foundto be an important player in the pathogen-esis of autism, then reduction of the inci-dence of autism is readily achievable, withthe primary concern being establishing bestpractices for the treatment of fevers and painin children under 6 months of age. Giventhat acetaminophen exposure in pregnantwomen is associated with an increase in therisk of autism,124 it would be quite surpris-ing and perhaps very informative from amechanistic perspective if indeed a dramaticreduction in acetaminophen exposureduring early childhood did not cause adramatic reduction in the incidence ofautism.
Yet another reason to investigate the roleof early childhood exposure to
Parker et al. 423
acetaminophen in the pathogenesis ofautism is that eliminating acetaminophenexposure early in life is unlikely to cause netharm, regardless of the effect on autism.Acetaminophen has never been shown tosave lives in any controlled study and, asdescribed above, is known to be associatedwith the induction of asthma and the pres-ence of a variety of developmental delays.Going even further, Ohlsson and Shah statein their recent Cochrane report of acet-aminophen use during a major thoracicsurgery frequently performed on infants(surgical closure of the patent ductusarteriosus):
In view of a recent report in mice of adverse
effects on the developing brain from para-cetamol [acetaminophen], and anotherreport of an association between prenatal
paracetamol and the development of autismor autism spectrum disorder in childhood,long-term follow-up to at least 18 to 24
months postnatal age must be incorporatedin any studies of paracetamol in the newbornpopulation. Such trials are required before
any recommendations for the use of para-cetamol in the newborn population can bemade.174
In other words, Ohlsson and Shahargue that acetaminophen should not berecommended, even for major and neces-sary surgical procedures, until it is knownwhether or not it does in fact causeautism.
Another reason to conduct a study of theearly childhood acetaminophen–autism con-nection is the observations of parents.During the 70-year history of investigatingthe pathogenesis of autism, scientists andpediatricians have made two well-documen-ted and costly errors, both associated withdismissing the observations of parents. Thefirst error was the acceptance for 20 years ofthe ‘‘refrigerator mother’’ hypothesis.175
The second error was a firm belief, helduntil 2005, that autism could not be
regressive and associated with a decline inpreviously existing neuropsychiatric func-tion. At present, half of all parents ofchildren with autism suspect vaccines as anunderlying cause of their child’s condi-tion.176 One study found that 29% of allmothers, with or without a child withautism, believe that autism can be inducedby vaccines.177 Other studies,178 as well assurveys by various polling organizationssuch as Harris, Thompson Reuters, andthe National League of Consumers, havereached similar conclusions. For reasonsdiscussed above, it seems apparent thatvaccines are not the underlying cause ofthe epidemic of autism, but at the same time,if history is any indication, ignoring theperspectives of parents is a grave error. Theidea that vaccines induce autism has beenwidely blamed179 on a single article pub-lished by Wakefield,180 but articles pub-lished in scientific journals are unlikely tosway public opinion. Even repeated publi-cations regarding the dangers of acetamino-phen for neural development, for example,have had little impact on the use of acet-aminophen in the pediatric population.Rather, it is social networks that are morelikely to be persuasive. Acceptance of socialnetworks is deeply ingrained in the humanpsyche,181 most likely because of theirancient and critical importance for humansurvival.182,183 Thus, parents of autistic chil-dren are likely to believe that vaccines areresponsible for the induction of autism notbecause of a peer-reviewed article but ratherbecause of their observations and the obser-vations of individuals within their trustedsocial networks. With this in mind, the storyof Steve Schultz is informative. He wasconvinced that his son’s autism was inducedby a vaccine because of what was obvious tohim as an observer. Thus, Schultz’s workboth accounts for parents’ observations andfor the science surrounding the pathogenesisof autism. Whatever the factor inducing thedevelopment of autism, it is critical that the
424 Journal of International Medical Research 45(2)
scientific community learns from its pasterrors in ignoring the observations ofparents.
It is estimated that the lifetime cost ofsupporting an individual with autism iscurrently US$2.4 million for an individualthat also has an intellectual disability, orUS$1.4 million for an individual with autismbut without an intellectual disability.184 It isalso estimated that 38% of individuals withautism in the United States also have anintellectual disability.185 This leads to anoverall average cost estimation of approxi-mately US$1.8 million to support one childwith autism for his or her lifetime. Weestimate that the total cost of running adefinitive clinical study to test the connec-tion between postnatal acetaminophenexposure and the induction of autism isless than the cost of caring for three to fiveindividuals who have already developedautism.
Conducting an ‘‘acetaminophenwithdrawal study’’: Practicalconsiderations
Given the already established adverse effectsof acetaminophen on the developing brain,described above, and the known risk ofasthma following acetaminophen exposure,it seems inappropriate to intentionallyexpose anyone under the age of 5 years toacetaminophen for the purpose of a medicalstudy. Rather, other approaches must beconsidered. One approach involves large-scale experiments in which acetaminophenexposure is essentially eliminated until theage at which a patient can be considered tobe at zero risk for regression into autism.Such an ‘‘acetaminophen withdrawal study’’would not be trivial. The feasibility of anacetaminophen withdrawal study waspreviously examined for the purpose ofevaluating the connection between acet-aminophen and asthma.186 However, thatfeasibility study involved only 120 infants
admitted to hospital after birth, at a timewhen some acetaminophen exposure mayhave already happened (e.g. during circum-cision). Further, the study allowed some useof acetaminophen in all patients, and onlyinvolved 3 months of restricted use in theexperimental population versus liberal use ina control population. Complete withdrawalup until the age of 5 years, on the otherhand, would require all forms (injectable,oral, and suppository) of acetaminophen tobe prospectively identified and reduced to an‘‘essential minimum,’’ if not eliminated. Afew thousand patients may need to beenrolled, depending of course on the mag-nitude of the anticipated effect. However,fewer patients may be needed if at-riskpopulations (e.g. preterm delivery, Downsyndrome) are selected for the study. Thepractices associated with circumcision willneed to be reconsidered, and the use of acet-aminophen during vaccinations, a commonbut already highly questioned practice,187,188
would need to be eliminated. The possibilityof passing acetaminophen to nursing infantsand children through breast milk would alsoneed to be eliminated. Extensive educationof parents and a wide range of health-careworkers (pharmacists and all physicians andnurses associated with obstetrics and pedi-atrics) regarding the avoidance of acet-aminophen would be necessary. Further,comparable control groups with liberal use(current standard) of acetaminophen wouldneed to be monitored simultaneously inorder to obtain unequivocal results.Unfortunately, follow-up would requiresome time, and it would take years beforea result is obtained. However, it is hopedthat the weight of the current evidence willbe considered sufficient for a dramaticreduction in acetaminophen use until inci-sive studies are completed.
A primary concern with an acetamino-phen-elimination experiment would be thetreatment of fever and pain. Alternativepharmacological methods of treating pain
Parker et al. 425
are available. Although all pharmacologicalmethods have drawbacks, acetaminophen isnot considered to be highly effective for painrelief.189 Indeed, as stated by McCullough,‘‘The few clinical studies of (acetaminophen)and ibuprofen in children have struggled tofind objective measures of pain capable ofreliably distinguishing between active treat-ment and placebo.’’189 Thus it seemsunwise to risk potentially permanent neuro-logical injury for apparently ineffective painrelief. Alternative pharmacological meansare also available for fever reduction, butthese have drawbacks. On the other hand,traditional methods of fever reduction(physical, non-pharmacological) look pro-mising and have few side effects. However,these may be less convenient for cliniciansand parents alike, and may prove ineffectivein some cases. At the same time, the specterof inducing autism while treating pain orfever with acetaminophen should beweighed when considering the benefits ofusing acetaminophen, even in direcircumstances.
Given the current value attributed to thetherapeutic effects of acetaminophen, it isworthwhile to consider possible ways toutilize the drug in the pediatric population,even if it is shown to be responsible for theepidemic of autism. One possibility is thatindividuals who are susceptible to acetami-nophen-induced autism (e.g. perhaps thosewhose mothers have excessively high levelsof vitamin B) might be identified, and useof the drug in these individuals could beavoided while others could benefit fromuse of the drug. Another possibility is thatco-administration of N-acetyl cysteine(NAC) with acetaminophen could be usedto attenuate the toxic effects of acetamino-phen in cases where the use of acetamino-phen is considered highly desirable. Itmight be hypothesized that any insufficien-cies in phase I or phase II metabolism(Figure 3), which may lead to acetamino-phen-induced autism, would be prevented
by co-administration of NAC. NAC worksboth orally and parenterally, which may beimportant since acetaminophen is availablefor oral, intravenous, or rectal administra-tion (Table 3). In patients with normalliver and renal function, the half-life ofNAC is slightly longer than the half-life ofacetaminophen, so a single dose of NACmay be sufficient to cover one dose ofacetaminophen. However, the respectivehalf-lives of the two compounds wouldneed to be evaluated in the treatmentpopulation, and the idea that NAC maybe protective against acetaminophen-induced autism is speculative.
Education of medical professionals isimperative for any acetaminophen with-drawal study, given the current medicalenvironment. Acetaminophen use is cur-rently ubiquitous and thought to be theonly humane approach to pain and feverreduction for children from the time of birthto 6 months. The drug is available in a verywide range of formats for both prescriptionand over-the-counter use (Table 3). Almostone-quarter of all infants are given acet-aminophen in any given week when in thehospital, making it the number one medica-tion used in infants.190 Given that mostmedical professionals currently practicingbegan their careers after 1982, it is notsurprising that acetaminophen use isdeeply engrained in practice and that alter-natives may seem foreign and even primi-tive. Further, the idea that acetaminophenexposure in early childhood could be dan-gerous is extremely novel and even disturb-ing to most medical practitioners (personalobservation by co-author WP). Even thebest-trained pediatricians are generally una-ware that the long-term effects of acetamino-phen were never tested in children incontrolled trials.191 Further, with the excep-tion of toxic metabolite production (Figure3) that primarily affects liver function inadults but not babies, most physicians arelargely unaware of the varied and complex
426 Journal of International Medical Research 45(2)
Tab
le3.
Ava
ilable
US
aceta
min
ophen
pro
duct
sfr
om
bir
thto
ear
lych
ildhood.
Ingr
edie
nt
Dosa
geFo
rmSt
rengt
hIn
dic
atio
nSp
eci
alC
onsi
dera
tions
Inje
ctab
le
Ace
tam
inophen
Solu
tion
1000
mg/
100m
LA
rthra
lgia
;denta
lpai
n;fe
ver;
head
ache;
mild
/modera
te/s
eve
repai
n;
musc
ulo
skele
talpai
n;
mya
lgia
;pat
ent
duct
us
arte
riosu
s
treat
menta
;
post
-opera
tive
/pro
cedura
lpai
na
Notst
udie
din
infa
nts<
28
weeks
gest
atio
nal
age
Ora
l
Ace
tam
inophen
(nat
ura
lberr
y,gr
ape,
white
grap
e,ch
err
y,an
d
bubble
gum
flavo
rs)
Solu
tion
120
mg/
5m
L
160
mg/
5m
L
Art
hra
lgia
;denta
lpai
n;fe
ver;
head
ache;
mild
/modera
te/s
evere
pai
n;
musc
ulo
skele
talpai
n;m
yalg
ia;
post
-opera
tive
/pro
cedura
lpai
n
All
dosa
gefo
rms
avai
lable
OT
C
Liq
uid
160
mg/
5m
L
500
mg/
15
mL
Susp
ensi
on
160
mg/
5m
L
325
mg/
10.1
5m
L
80
mg/
2.5
mL
80
mg/
0.8
mL
Dro
ps
80
mg/
0.8
mL
Chew
able
table
t80
mg
160
mg
Rap
idta
ble
t80
mg
160
mg
Ace
tam
inophen/
Codein
eb
phosp
hat
e
Solu
tion
120
mg/
5m
LM
ild/m
odera
tepai
n;A
rthra
lgia
a;
bone
pai
na;co
ugh
a;head
ache
a;
mya
lgia
a
Not
reco
mm
ended:fo
ruse
in
neonat
es,
for
post
-opera
tive
tonsi
llect
om
yan
d/o
rad
enoi-
dect
om
ypai
nm
anag
em
ent;
contr
aindic
ated
inpat
ients
who
are
CY
P2D
6ultra
-rap
id
meta
boliz
ers
(continued)
Parker et al. 427
Tab
le3.
Continued.
Ingr
edie
nt
Dosa
geFo
rmSt
rengt
hIn
dic
atio
nSp
eci
alC
onsi
dera
tions
Hyd
roco
done
bitar
trat
eb/
aceta
min
ophen
Solu
tion
108
mg/
5m
L
167
mg/
5m
L
163
mg/
7.5
mL
217
mg/
10
mL
Cough
a;m
odera
te/s
evere
pai
n;p
ost
-
opera
tive
/pro
cedura
lpai
na
Not
reco
mm
ended
inch
ildre
n
<2
year
sofag
e
Oxyc
odone
hydro
chlo
ride
b/
aceta
min
ophen
Solu
tion
325
mg/
5m
LM
odera
te/s
evere
pai
n;
post
-opera
tive
/pro
cedura
lpai
na
Ace
tam
inophen/
chlo
rphenir
amin
eb/
dextr
om
eth
orp
han
b
Susp
ensi
on
OT
C
7.5
mg/
5m
LC
om
mon
cold
;co
ugh
Not
reco
mm
ended
inch
ildre
n
<4
year
sofag
e
Ace
tam
inophen/
dextr
om
eth
orp
han
bSu
spensi
on
160
mg/
5m
LC
ough
;so
reth
roat
Not
reco
mm
ended
inch
ildre
n
<4
year
sofag
e
Ace
tam
inophen/
chlo
rphenir
amin
eb/
dextr
om
eth
orp
han
b/
Phenyl
ephri
ne
b
Efferv
esc
ent
table
t250
mg
Com
mon
cold
;co
ugh
;fe
ver;
head
ache;m
ildpai
n;
nas
alco
nge
stio
n;phar
yngi
tis;
rhin
orr
hea;
sneezi
ng
Both
dosa
gefo
rms
avai
lable
OT
C;
not
reco
mm
ended
in
child
ren<
4ye
ars
ofag
e
Susp
ensi
on
160
mg
Supposi
tory
Ace
tam
inophen
Rect
alsu
pposi
tory
OT
C
80
mg
120
mg
325
mg
Art
hra
lgia
;denta
lpai
n;fe
ver;
head
ache;
mild
/modera
te/s
eve
repai
n;
musc
ulo
skele
talpai
n;m
yalg
ia;
post
-opera
tive
/pro
cedura
lpai
n
Notst
udie
din
infa
nts<
28
weeks
gest
atio
nal
age
‘‘Ove
r-th
e-c
ounte
r’’is
abbre
viat
ed
OT
C.
aO
ff-a
bel
bC
om
bin
atio
npro
duct
sco
nta
inva
ryin
gam
ounts
ofoth
er
active
ingr
edie
nts
.T
he
stre
ngt
hlis
ted
isfo
rth
eac
eta
min
ophen
com
ponent
only
.
428 Journal of International Medical Research 45(2)
effects of acetaminophen on the nervoussystem and on metabolism. Few practi-tioners are aware of the well-establishedconnection between asthma and acetamino-phen use, and fewer still are aware of theincreased possibility of cryptorchidism(undescended testis) known to be associatedwith acetaminophen use.192
The exceedingly common and widespreaduse of acetaminophen both in the hospitaland at home may have led to some degree ofcomplacency in proper dose administration.For example, in a study of outpatient careprescription errors, 15% of children whowere prescribed acetaminophen were given adosage by medical professionals that washigher than recommended.193 Similarly,another study found that 12% of childrenwho were prescribed acetaminophen at apediatric emergency department were givena prescription for acetaminophen higherthan the recommended dosage.194
Interestingly, Li et al.195 found that par-ents administered acetaminophen in amanner comparable to physicians; dosagesadministered by parents exceeded recom-mended amounts about 15% of the time. Liet al.195 also noted that infants were morelikely than older children to be given aninaccurate dosage of acetaminophen(RR¼ 1.40, P< 0.04, CI 95%¼ 1.06–1.86)by their parents. This latter observation isparticularly concerning since it is duringinfancy that the brain may be most sensitiveto damage by acetaminophen from oxida-tive stress. However, it is unknown to whatextent acetaminophen-induced neuropath-ology can be induced by the recommendeddose of the drug, and to what extent exces-sive doses are responsible for the inductionof neurological disorders in the pediatricpopulation.
The bottom line is that hundreds of studiesdescribing the epidemiology of autism andthe numerous and varied risk factors forautism have a straightforward explanation:autism could be an acetaminophen-induced
brain injury facilitated by oxidative stress andinflammation in newborns and young chil-dren. This is certainly an attractive view froman intellectual perspective, as it satisfiesOccam’s razor. Most importantly, thismodel merits urgent testing because (a) it isintuitive and accounts for the observations,(b) the experiment to test the hypothesis isvery feasible, and (c) if proven correct, theprevalence of autism in future generationswill be dramatically reduced. The urgency ofthe issue cannot be underestimated, as pre-cious time and resources that could beallocated much more constructively and use-fully in this time of serious need are beingpoured into approaches that offer little to nohope of prevention.
Acknowledgements
The authors are very grateful to Stephen M.
Edelson, current director of the Autism Research
Institute, for sharing his knowledge of the work
with autism that he and Bernard Rimland
performed during the 1960s through the 1990s.
The study team is also very grateful to Susanne
Meza-Keuthen for her generous financial support
of this study. The authors wish to thank Michael
R. Knowles, Jeffrey P. Baker, Margaret Leigh
and Carolyn Durham for very helpful
discussions.
Declaration of conflicting interest
The Authors declare that there is no conflict of
interest.
Funding
This review received no specific grant from any
funding agency in the public, commercial, or not-
for-profit sectors.
Ethical approval
No studies using animal models and no research
involving human subjects were performed for this
study.
Parker et al. 429
References
1. Keil A, Daniels JL, Forssen U, et al. Parental
autoimmune diseases associated with autism
spectrum disorders in offspring.
Epidemiology 2010; 21: 805–808.2. Tanne JH. Maternal obesity and diabetes are
et al. Social networks and cooperation inhunter-gatherers. Nature 2012; 481:497–501.
184. Buescher AV, Cidav Z, Knapp M, et al.Costs of autism spectrum disorders in theUnited Kingdom and the United States.JAMA Pediatr 2014; 168: 721–728.
185. Autism and Developmental DisabilitiesMonitoring Network Surveillance Year2008 Principal Investigators; Centers for
Disease Control and Prevention.Prevalence of Autism spectrum disorders —autism and developmental disabilities
186. Riley J, Braithwaite I, Shirtcliffe P, et al.Randomized controlled trial of asthma riskwith paracetamol use in infancy–a feasibil-ity study. Clin Exp Allergy 2015; 45:
448–456.187. Prymula R, Siegrist CA, Chlibek R, et al.
Effect of prophylactic paracetamol admin-
istration at time of vaccination on febrilereactions and antibody responses in chil-dren: two open-label, randomised con-
