Epidemiology of autism
Encyclopedia
The epidemiology of autism is the study of factors affecting autism spectrum disorders (ASD). Most recent reviews of epidemiology
estimate a prevalence
of one to two cases per 2,000 people for autism
, and about six per 1,000 for ASD; because of inadequate data, these numbers may underestimate ASD's true prevalence. ASD averages a 4.3:1 male-to-female ratio. The number of children known to have autism has increased dramatically since the 1980s, at least partly due to changes in diagnostic practice; it is unclear whether prevalence has actually increased; and as-yet-unidentified environmental risk factors cannot be ruled out. The risk of autism is associated with several prenatal factors, including advanced parental age and diabetes in the mother during pregnancy. ASD is associated with several genetic disorder
s and with epilepsy, and autism is associated with mental retardation
.
is a complex neurodevelopmental disorder. Many causes have been proposed, but its theory of causation
is still incomplete. Autism is largely inherited
, although the genetics of autism
are complex and it is generally unclear which genes are responsible. Little evidence exists to support associations with specific environmental exposures. In rare cases, autism is strongly associated with agents
that cause birth defects. Other proposed causes
, such as childhood vaccines, are controversial
and the vaccine hypotheses lack convincing scientific evidence
.
The doctor whose study linked Autism with childhood vaccines recently had his licence revoked in the United Kingdom.http://online.wsj.com/article/SB10001424052748704113504575263994195318772.html?mod=igoogle_wsj_gadgv1
defines several measures of the frequency of occurrence of a disease or condition:
When studying how diseases are caused, incidence rates are the most appropriate measure of disease frequency as they assess risk directly. However, incidence can be difficult to measure with rarer chronic diseases such as autism. In autism epidemiology, point or period prevalence is more useful than incidence, as the disorder starts long before it is diagnosed, and the gap between initiation and diagnosis is influenced by many factors unrelated to risk. Research focuses mostly on whether point or period prevalence is increasing with time; cumulative incidence is sometimes used in studies of birth cohorts
.
of autism vary widely depending on diagnostic criteria, age of children screened, and geographical location. Most recent review
s tend to estimate a prevalence of 1–2 per 1,000 for autism and close to 6 per 1,000 for ASD;
PDD-NOS is the vast majority of ASD, Asperger syndrome
is about 0.3 per 1,000 and the atypical forms childhood disintegrative disorder
and Rett syndrome
are much rarer. A 2006 study of nearly 57,000 British nine- and ten-year-olds reported a prevalence of 3.89 per 1,000 for autism and 11.61 per 1,000 for ASD; these higher figures could be associated with broadening diagnostic criteria. Studies based on more-detailed information, such as direct observation rather than examination of medical records, identify higher prevalence; this suggests that published figures may underestimate ASD's true prevalence. A 2009 study of the children in Cambridgeshire
, England used different methods to measure prevalence, and estimated that 40% of ASD cases go undiagnosed, with the two least-biased estimates of true prevalence being 11.3 and 15.7 per 1,000. A 2009 U.S. study based on 2006 data estimated the prevalence of ASD in 8-year-old children to be 9.0 per 1,000 (approximate range 8.6–9.3). A 2009 report based on the 2007 Adult Psychiatric Morbidity Survey by the National Health Service
determined that the prevalence of ASD in adults was approximately 1% of the population, with a higher prevalence in males and no significant variation between age groups; these results suggest that prevalence of ASD among adults is similar to that in children and rates of autism are not increasing.
The number of reported cases of autism increased dramatically in the 1990s and early 2000s, prompting investigations into several potential reasons:
The reported increase is largely attributable to changes in diagnostic practices, referral patterns, availability of services, age at diagnosis, and public awareness. A widely cited 2002 pilot study concluded that the observed increase in autism in California cannot be explained by changes in diagnostic criteria, but a 2006 analysis found that special education data poorly measured prevalence because so many cases were undiagnosed, and that the 1994–2003 U.S. increase was associated with declines in other diagnostic categories, indicating that diagnostic substitution had occurred. A 2007 study that modeled autism incidence found that broadened diagnostic criteria, diagnosis at a younger age, and improved efficiency of case ascertainment, can produce an increase in the frequency of autism ranging up to 29-fold depending on the frequency measure, suggesting that methodological factors may explain the observed increases in autism over time. A small 2008 study found that a significant number (40%) of people diagnosed with pragmatic language impairment as children in previous decades would now be given a diagnosis as autism. A study of all Danish children born in 1994–99 found that children born later were more likely to be diagnosed at a younger age, supporting the argument that apparent increases in autism prevalence were at least partly due to decreases in the age of diagnosis. A 2009 study of California data found that the reported incidence of autism rose 7- to 8-fold from the early 1990s to 2007, and that changes in diagnostic criteria, inclusion of milder cases, and earlier age of diagnosis probably explain only a 4.25-fold increase; the study did not quantify the effects of wider awareness of autism, increased funding, and expanding treatment options resulting in parents' greater motivation to seek services. Another 2009 California study found that the reported increases are unlikely to be explained by changes in how qualifying condition codes for autism were recorded.
Several environmental risk factors have been proposed to support the hypothesis that the actual frequency of autism has increased. These include certain foods, infectious disease, pesticides, MMR vaccine
, and vaccines containing the preservative thiomersal
, formerly used in several childhood vaccines in the U.S. Although there is overwhelming scientific evidence against the MMR hypothesis and no convincing evidence for the thiomersal hypothesis, other as-yet-unidentified environmental risk factors cannot be ruled out. Although it is unknown whether autism's frequency has increased, any such increase would suggest directing more attention and funding toward changing environmental factors instead of continuing to focus on genetics.
Canada
The rate of autism diagnoses in Canada was 1 in 450 in 2003. However, preliminary results of an epidemiological study conducted at Montreal Children’s Hospital in the 2003-2004 school year found a prevalence rate of 0.68% (or 1 per 147).
A 2001 review of the medical research conducted by the Public Health Agency of Canada concluded that there was no link between MMR vaccine and either inflammatory bowel disease or autism. The review noted, "An increase in cases of autism was noted by year of birth from 1979 to 1992; however, no incremental increase in cases was observed after the introduction of MMR vaccination." After the introduction of MMR, "A time trend analysis found no correlation between prevalence of MMR vaccination and the incidence of autism in each birth cohort from 1988 to 1993."
United States
The most recent estimate states that "up to 1 out of every 110 children born today has some form of ASD."
The number of diagnosed cases of autism grew dramatically in the U.S. in the 1990s and early 2000s. For the 2006 surveillance year, identified ASD cases were an estimated 9.0 per 1000 children aged 8 years (95% confidence interval
[CI] = 8.6–9.3). These numbers measure what is sometimes called "administrative prevalence", that is, the number of known cases per unit of population, as opposed to the true number of cases. This prevalence estimate rose 57% (95% CI 27%–95%) from 2002 to 2006. A further study in 2006 concluded that the apparent rise in administrative prevalence was the result of diagnostic substitution, mostly for findings of mental retardation and learning disabilities. "Many of the children now being counted in the autism category would probably have been counted in the mental retardation or learning disabilities categories if they were being labelled 10 years ago instead of today," said researcher Paul Shattuck of the Waisman Center at the University of Wisconsin at Madison, in a statement.
A population-based study of one Minnesota
county found that the cumulative incidence of autism grew eightfold from the 1980–83 period to the 1995–97 period. The increase occurred after the introduction of broader, more-precise diagnostic criteria, increased service availability, and increased awareness of autism. During the same period, the reported number of autism cases grew 22-fold in the same location, suggesting that counts reported by clinics or schools provide misleading estimates of the true incidence of autism.
Venezuela
A 2008 study reported a prevalence of 1.1 per 1000 for autism and 1.7 per 1000 for ASD.
Hong Kong
A 2008 Hong Kong study reported an ASD incidence rate similar to those reported in Australia and North America, and lower than Europeans. It also reported a prevalence of 1.68 per 1,000 for children under 15 years.
Israel
A 2009 study reported that the annual incidence rate of Israeli children with a diagnosis of ASD receiving disability benefits rose from zero in 1982–1984 to 190 per million in 2004. It was not known whether these figures reflected true increases or other factors such as changes in diagnostic measures.
Japan
A 2005 study of a part of Yokohama
with a stable population of about 300,000 reported a cumulative incidence to age 7 years of 48 cases of ASD per 10,000 children in 1989, and 86 in 1990. After the vaccination rate of MMR vaccine
dropped to near zero, the incidence rate grew to 97 and 161 cases per 10,000 children born in 1993 and 1994, respectively, indicating that MMR vaccine did not cause autism.
Saudi Arabia
Studies of autism frequency have been particularly rare in the Middle East. One rough estimate is that the prevalence of autism in Saudi Arabia is 18 per 10,000, slightly higher than the 13 per 10,000 reported in developed countries. Estimates for ASD prevalence in Saudi Arabia are not available.
Denmark
A 2003 study reported that the cumulative incidence of autism in Denmark began a steep increase starting around 1990, and continued to grow until 2000, despite the withdrawal of thiomersal-containing vaccines in 1992. For example, for children aged 2–4 years, the cumulative incidence was about 0.5 new cases per 10,000 children in 1990 and about 4.5 new cases per 10,000 children in 2000.
A subsequent critique of the Danish study by Mark Blaxill of SafeMinds pinpoints several discrepancies associated with the collection of autism data. Such discrepancies include changing the autism criteria in the middle of the study, 1994, from inpatients to outpatients, adding the numbers of a large Copenhagen clinic after 1992, etc.
France
Eric Fombonne
made some studies in the years 1992 and 1997. He found a prevalence of 16 per 10.000 for the global PDD
. The INSERM found a prevalence of 27 per 10,000 for the ASD
and a prevalence of 9 per 10,000 for the early infantile autism
in 2003. Those figures are considered as underrated as the WHO
gives figures between 30 and 60 per 10,000. The French Minister of Health gives a prevalence of 4.9 per 10.000 on its website but it counts only early infantile autism
.
Germany
A 2008 study found that inpatient admission rates for children with ASD increased 30% from 2000 to 2005, with the largest rise between 2000 and 2001 and a decline between 2001 and 2003. Inpatient rates for all mental disorders also rose for ages up to 15 years, so that the ratio of ASD to all admissions rose from 1.3% to 1.4%.
Norway
A 2009 study reported prevalence rates for ASD ranging from 0.21% to 0.87%, depending on assessment method and assumptions about non-response, suggesting that methodological factors explain large variances in prevalence rates in different studies.
United Kingdom
The incidence and changes in incidence with time are unclear in the UK.
The reported autism incidence in the UK rose starting before the first introduction of the MMR vaccine
in 1989. A 2004 study found that the reported incidence of pervasive developmental disorders in a general practice research database in England and Wales grew steadily during 1988–2001 from 0.11 to 2.98 per 10,000 person-years, and concluded that much of this increase may be due to changes in diagnostic practice.
studies now suggests that it is one of the most heritable of all psychiatric conditions. The first studies of twins estimated heritability
to be more than 90%; in other words, that genetics explains more than 90% of autism cases. When only one identical twin is autistic, the other often has learning or social disabilities. For adult siblings, the risk of having one or more features of the broader autism phenotype might be as high as 30%, much higher than the risk in controls. About 10–15% of autism cases have an identifiable Mendelian (single-gene) condition, chromosome abnormality, or other genetic syndrome, and ASD is associated with several genetic disorder
s.
Since heritability is less than 100% and symptoms vary markedly among identical twins with autism, environmental factors are most likely a significant cause as well. If some of the risk is due to gene-environment interaction the 90% heritability estimate may be too high; new twin data and models with structural genetic variation are needed.
Genetic linkage
analysis has been inconclusive; many association analyses
have had inadequate power. Studies have examined more than 100 candidate genes; many genes must be examined because more than a third of genes are expressed in the brain and there are few clues on which are relevant to autism.
Although the evidence does not implicate any single pregnancy-related risk factor as a cause of autism, the risk of autism is associated with several prenatal risk factors, including advanced age in either parent, and diabetes, bleeding, and use of psychiatric drugs in the mother during pregnancy. It is not known whether mutations that arise spontaneously in autism and other neuropsychiatric disorders come mainly from the mother or the father, or whether the mutations are associated with parental age. However, recent studies have identified advancing paternal age as a significant risk factor for ASD.
A large 2008 population study of Swedish parents of children with autism found that the parents were more likely to have been hospitalized for a mental disorder, that schizophrenia was more common among the mothers and fathers, and that depression and personality disorders were more common among the mothers.
It is not known how many siblings of autistic individuals are themselves autistic. Several studies based on clinical samples have given quite different estimates, and these clinical samples differ in important ways from samples taken from the general community.
Autism has also been shown to cluster in urban neighborhoods of high socioeconomic status. One study from California found a three to fourfold increased risk of autism in a small 30 by 40 km region centered on West Hollywood. Another study by a UC Davis group using a similar methodology,erroneous because of its stratification of space nevertheless found multiple clusters in urban high socioeconomic status neighborhoods of California.
Epidemiology
Epidemiology is the study of health-event, health-characteristic, or health-determinant patterns in a population. It is the cornerstone method of public health research, and helps inform policy decisions and evidence-based medicine by identifying risk factors for disease and targets for preventive...
estimate a prevalence
Prevalence
In epidemiology, the prevalence of a health-related state in a statistical population is defined as the total number of cases of the risk factor in the population at a given time, or the total number of cases in the population, divided by the number of individuals in the population...
of one to two cases per 2,000 people for autism
Autism
Autism is a disorder of neural development characterized by impaired social interaction and communication, and by restricted and repetitive behavior. These signs all begin before a child is three years old. Autism affects information processing in the brain by altering how nerve cells and their...
, and about six per 1,000 for ASD; because of inadequate data, these numbers may underestimate ASD's true prevalence. ASD averages a 4.3:1 male-to-female ratio. The number of children known to have autism has increased dramatically since the 1980s, at least partly due to changes in diagnostic practice; it is unclear whether prevalence has actually increased; and as-yet-unidentified environmental risk factors cannot be ruled out. The risk of autism is associated with several prenatal factors, including advanced parental age and diabetes in the mother during pregnancy. ASD is associated with several genetic disorder
Genetic disorder
A genetic disorder is an illness caused by abnormalities in genes or chromosomes, especially a condition that is present from before birth. Most genetic disorders are quite rare and affect one person in every several thousands or millions....
s and with epilepsy, and autism is associated with mental retardation
Mental retardation
Mental retardation is a generalized disorder appearing before adulthood, characterized by significantly impaired cognitive functioning and deficits in two or more adaptive behaviors...
.
Autism and its causes
AutismAutism
Autism is a disorder of neural development characterized by impaired social interaction and communication, and by restricted and repetitive behavior. These signs all begin before a child is three years old. Autism affects information processing in the brain by altering how nerve cells and their...
is a complex neurodevelopmental disorder. Many causes have been proposed, but its theory of causation
Etiology
Etiology is the study of causation, or origination. The word is derived from the Greek , aitiologia, "giving a reason for" ....
is still incomplete. Autism is largely inherited
Heritability
The Heritability of a population is the proportion of observable differences between individuals that is due to genetic differences. Factors including genetics, environment and random chance can all contribute to the variation between individuals in their observable characteristics...
, although the genetics of autism
Heritability of autism
The heritability of autism is the proportion of autism that can be explained by genetic variation; if the heritability of a condition is high, then the condition is considered to be primarily genetic...
are complex and it is generally unclear which genes are responsible. Little evidence exists to support associations with specific environmental exposures. In rare cases, autism is strongly associated with agents
Teratology
Teratology is the study of abnormalities of physiological development. It is often thought of as the study of human birth defects, but it is much broader than that, taking in other non-birth developmental stages, including puberty; and other non-human life forms, including plants.- Etymology :The...
that cause birth defects. Other proposed causes
Causes of autism
Many causes of autism have been proposed, but understanding of the theory of causation of autism and the other autism spectrum disorders is incomplete. Heritability contributes about 90% of the risk of a child developing autism, but the heritability of autism is complex and typically it is...
, such as childhood vaccines, are controversial
Controversies in autism
Controversies in autism encompass the disagreement over the exact nature of autism, its causes and manifestations. Autism is considered to be a neurodevelopmental condition which manifests itself in markedly abnormal social interaction, communication ability, and patterns of interests.The cause of...
and the vaccine hypotheses lack convincing scientific evidence
Evidence-based medicine
Evidence-based medicine or evidence-based practice aims to apply the best available evidence gained from the scientific method to clinical decision making. It seeks to assess the strength of evidence of the risks and benefits of treatments and diagnostic tests...
.
The doctor whose study linked Autism with childhood vaccines recently had his licence revoked in the United Kingdom.http://online.wsj.com/article/SB10001424052748704113504575263994195318772.html?mod=igoogle_wsj_gadgv1
Frequency
Although incidence rates measure autism risk directly, most epidemiological studies report other frequency measures, typically point or period prevalence, or sometimes cumulative incidence. Attention is focused mostly on whether prevalence is increasing with time.Incidence and prevalence
EpidemiologyEpidemiology
Epidemiology is the study of health-event, health-characteristic, or health-determinant patterns in a population. It is the cornerstone method of public health research, and helps inform policy decisions and evidence-based medicine by identifying risk factors for disease and targets for preventive...
defines several measures of the frequency of occurrence of a disease or condition:
- The incidence rateIncidence (epidemiology)Incidence is a measure of the risk of developing some new condition within a specified period of time. Although sometimes loosely expressed simply as the number of new cases during some time period, it is better expressed as a proportion or a rate with a denominator.Incidence proportion is the...
of a condition is the rate at which new cases occurred per person-year, for example, "2 new cases per 1,000 person-years". - The cumulative incidenceCumulative incidenceCumulative incidence or incidence proportion is a measure of frequency, as in epidemiology, where it is a measure of disease frequency during a period of time...
is the proportion of a population that became new cases within a specified time period, for example, "1.5 per 1,000 people became new cases during 2006". - The point prevalencePoint prevalenceIn epidemiology, point prevalence is a measure of the proportion of people in a population who have a disease or condition at a particular time, such as a particular date. It is like a snap shot of the disease in time. It can be used for statistics on the occurrence of chronic diseases...
of a condition is the proportion of a population that had the condition at a single point in time, for example, "10 cases per 1,000 people at the start of 2006". - The period prevalencePeriod prevalenceIn epidemiology, Period prevalence is the proportion of the population with a given disease or condition over a specific period of time. It could describe how many people in a population had a cold over the cold season in 2006, for example....
is the proportion that had the condition at any time within a stated period, for example, "15 per 1,000 people had cases during 2006".
When studying how diseases are caused, incidence rates are the most appropriate measure of disease frequency as they assess risk directly. However, incidence can be difficult to measure with rarer chronic diseases such as autism. In autism epidemiology, point or period prevalence is more useful than incidence, as the disorder starts long before it is diagnosed, and the gap between initiation and diagnosis is influenced by many factors unrelated to risk. Research focuses mostly on whether point or period prevalence is increasing with time; cumulative incidence is sometimes used in studies of birth cohorts
Cohort (statistics)
In statistics and demography, a cohort is a group of subjects who have shared a particular time together during a particular time span . Cohorts may be tracked over extended periods in a cohort study. The cohort can be modified by censoring, i.e...
.
Estimation methods
The three basic approaches used to estimate prevalence differ in cost and in quality of results. The simplest and cheapest method is to count known autism cases from sources such as schools and clinics, and divide by the population. This approach is likely to underestimate prevalence because it does not count children who have not been diagnosed yet, and it is likely to generate skewed statistics because some children have better access to treatment. The second method improves on the first by having investigators examine student or patient records looking for probable cases, to catch cases that have not been identified yet. The third method, which is arguably the best, screens a large sample of an entire community to identify possible cases, and then evaluates each possible case in more detail with standard diagnostic procedures. This last method typically produces the most reliable, and the highest, prevalence estimates.Frequency estimates
Estimates of the prevalencePrevalence
In epidemiology, the prevalence of a health-related state in a statistical population is defined as the total number of cases of the risk factor in the population at a given time, or the total number of cases in the population, divided by the number of individuals in the population...
of autism vary widely depending on diagnostic criteria, age of children screened, and geographical location. Most recent review
Review
A review is an evaluation of a publication, a product or a service, such as a movie , video game, musical composition , book ; a piece of hardware like a car, home appliance, or computer; or an event or performance, such as a live music concert, a play, musical theater show or dance show...
s tend to estimate a prevalence of 1–2 per 1,000 for autism and close to 6 per 1,000 for ASD;
PDD-NOS is the vast majority of ASD, Asperger syndrome
Asperger syndrome
Asperger's syndrome that is characterized by significant difficulties in social interaction, alongside restricted and repetitive patterns of behavior and interests. It differs from other autism spectrum disorders by its relative preservation of linguistic and cognitive development...
is about 0.3 per 1,000 and the atypical forms childhood disintegrative disorder
Childhood disintegrative disorder
Childhood disintegrative disorder , also known as Heller's syndrome and disintegrative psychosis, is a rare condition characterized by late onset of developmental delays in language, social function, and motor skills...
and Rett syndrome
Rett syndrome
Rett syndrome is a neurodevelopmental disorder of the grey matter of the brain that almost exclusively affects females. The clinical features include small hands and feet and a deceleration of the rate of head growth . Repetitive hand movements, such as wringing and/or repeatedly putting hands into...
are much rarer. A 2006 study of nearly 57,000 British nine- and ten-year-olds reported a prevalence of 3.89 per 1,000 for autism and 11.61 per 1,000 for ASD; these higher figures could be associated with broadening diagnostic criteria. Studies based on more-detailed information, such as direct observation rather than examination of medical records, identify higher prevalence; this suggests that published figures may underestimate ASD's true prevalence. A 2009 study of the children in Cambridgeshire
Cambridgeshire
Cambridgeshire is a county in England, bordering Lincolnshire to the north, Norfolk to the northeast, Suffolk to the east, Essex and Hertfordshire to the south, and Bedfordshire and Northamptonshire to the west...
, England used different methods to measure prevalence, and estimated that 40% of ASD cases go undiagnosed, with the two least-biased estimates of true prevalence being 11.3 and 15.7 per 1,000. A 2009 U.S. study based on 2006 data estimated the prevalence of ASD in 8-year-old children to be 9.0 per 1,000 (approximate range 8.6–9.3). A 2009 report based on the 2007 Adult Psychiatric Morbidity Survey by the National Health Service
National Health Service (England)
The National Health Service or NHS is the publicly funded healthcare system in England. It is both the largest and oldest single-payer healthcare system in the world. It is able to function in the way that it does because it is primarily funded through the general taxation system, similar to how...
determined that the prevalence of ASD in adults was approximately 1% of the population, with a higher prevalence in males and no significant variation between age groups; these results suggest that prevalence of ASD among adults is similar to that in children and rates of autism are not increasing.
Changes with time
Attention has been focused on whether the prevalence of autism is increasing with time. Earlier prevalence estimates were lower, centering at about 0.5 per 1,000 for autism during the 1960s and 1970s and about 1 per 1,000 in the 1980s, as opposed to today's 1–2 per 1,000.- More children may have autism; that is, the true frequency of autism may have increased.
- There may be more complete pickup of autism (case finding), as a result of increased awareness and funding. For example, attempts to sue vaccine companies may have increased case-reporting.
- The diagnosis may be applied more broadly than before, as a result of the changing definition of the disorder, particularly changes in DSM-III-R and DSM-IV.
- Successively earlier diagnosis in each succeeding cohort of children, including recognition in nursery (preschool), may have affected apparent prevalence but not incidence.
- A review of the "rising autism" figures compared to other disabilities in schools shows a corresponding drop in findings of mental retardation.
The reported increase is largely attributable to changes in diagnostic practices, referral patterns, availability of services, age at diagnosis, and public awareness. A widely cited 2002 pilot study concluded that the observed increase in autism in California cannot be explained by changes in diagnostic criteria, but a 2006 analysis found that special education data poorly measured prevalence because so many cases were undiagnosed, and that the 1994–2003 U.S. increase was associated with declines in other diagnostic categories, indicating that diagnostic substitution had occurred. A 2007 study that modeled autism incidence found that broadened diagnostic criteria, diagnosis at a younger age, and improved efficiency of case ascertainment, can produce an increase in the frequency of autism ranging up to 29-fold depending on the frequency measure, suggesting that methodological factors may explain the observed increases in autism over time. A small 2008 study found that a significant number (40%) of people diagnosed with pragmatic language impairment as children in previous decades would now be given a diagnosis as autism. A study of all Danish children born in 1994–99 found that children born later were more likely to be diagnosed at a younger age, supporting the argument that apparent increases in autism prevalence were at least partly due to decreases in the age of diagnosis. A 2009 study of California data found that the reported incidence of autism rose 7- to 8-fold from the early 1990s to 2007, and that changes in diagnostic criteria, inclusion of milder cases, and earlier age of diagnosis probably explain only a 4.25-fold increase; the study did not quantify the effects of wider awareness of autism, increased funding, and expanding treatment options resulting in parents' greater motivation to seek services. Another 2009 California study found that the reported increases are unlikely to be explained by changes in how qualifying condition codes for autism were recorded.
Several environmental risk factors have been proposed to support the hypothesis that the actual frequency of autism has increased. These include certain foods, infectious disease, pesticides, MMR vaccine
MMR vaccine controversy
The MMR vaccine controversy was a case of scientific misconduct which triggered a health scare. It followed the publication in 1998 of a paper in the medical journal The Lancet which presented apparent evidence that autism spectrum disorders could be caused by the MMR vaccine, an immunization...
, and vaccines containing the preservative thiomersal
Thiomersal controversy
The thiomersal controversy describes claims that vaccines containing the mercury-based preservative thiomersal contribute to the development of autism and other brain development disorders...
, formerly used in several childhood vaccines in the U.S. Although there is overwhelming scientific evidence against the MMR hypothesis and no convincing evidence for the thiomersal hypothesis, other as-yet-unidentified environmental risk factors cannot be ruled out. Although it is unknown whether autism's frequency has increased, any such increase would suggest directing more attention and funding toward changing environmental factors instead of continuing to focus on genetics.
CanadaCanadaCanada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
The rate of autism diagnoses in Canada was 1 in 450 in 2003. However, preliminary results of an epidemiological study conducted at Montreal Children’s Hospital in the 2003-2004 school year found a prevalence rate of 0.68% (or 1 per 147).
A 2001 review of the medical research conducted by the Public Health Agency of Canada concluded that there was no link between MMR vaccine and either inflammatory bowel disease or autism. The review noted, "An increase in cases of autism was noted by year of birth from 1979 to 1992; however, no incremental increase in cases was observed after the introduction of MMR vaccination." After the introduction of MMR, "A time trend analysis found no correlation between prevalence of MMR vaccination and the incidence of autism in each birth cohort from 1988 to 1993."
United StatesUnited StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
The most recent estimate states that "up to 1 out of every 110 children born today has some form of ASD."
The number of diagnosed cases of autism grew dramatically in the U.S. in the 1990s and early 2000s. For the 2006 surveillance year, identified ASD cases were an estimated 9.0 per 1000 children aged 8 years (95% confidence interval
Confidence interval
In statistics, a confidence interval is a particular kind of interval estimate of a population parameter and is used to indicate the reliability of an estimate. It is an observed interval , in principle different from sample to sample, that frequently includes the parameter of interest, if the...
[CI] = 8.6–9.3). These numbers measure what is sometimes called "administrative prevalence", that is, the number of known cases per unit of population, as opposed to the true number of cases. This prevalence estimate rose 57% (95% CI 27%–95%) from 2002 to 2006. A further study in 2006 concluded that the apparent rise in administrative prevalence was the result of diagnostic substitution, mostly for findings of mental retardation and learning disabilities. "Many of the children now being counted in the autism category would probably have been counted in the mental retardation or learning disabilities categories if they were being labelled 10 years ago instead of today," said researcher Paul Shattuck of the Waisman Center at the University of Wisconsin at Madison, in a statement.
A population-based study of one Minnesota
Minnesota
Minnesota is a U.S. state located in the Midwestern United States. The twelfth largest state of the U.S., it is the twenty-first most populous, with 5.3 million residents. Minnesota was carved out of the eastern half of the Minnesota Territory and admitted to the Union as the thirty-second state...
county found that the cumulative incidence of autism grew eightfold from the 1980–83 period to the 1995–97 period. The increase occurred after the introduction of broader, more-precise diagnostic criteria, increased service availability, and increased awareness of autism. During the same period, the reported number of autism cases grew 22-fold in the same location, suggesting that counts reported by clinics or schools provide misleading estimates of the true incidence of autism.
VenezuelaVenezuelaVenezuela , officially called the Bolivarian Republic of Venezuela , is a tropical country on the northern coast of South America. It borders Colombia to the west, Guyana to the east, and Brazil to the south...
A 2008 study reported a prevalence of 1.1 per 1000 for autism and 1.7 per 1000 for ASD.
Hong KongHong KongHong Kong is one of two Special Administrative Regions of the People's Republic of China , the other being Macau. A city-state situated on China's south coast and enclosed by the Pearl River Delta and South China Sea, it is renowned for its expansive skyline and deep natural harbour...
A 2008 Hong Kong study reported an ASD incidence rate similar to those reported in Australia and North America, and lower than Europeans. It also reported a prevalence of 1.68 per 1,000 for children under 15 years.
IsraelIsraelThe State of Israel is a parliamentary republic located in the Middle East, along the eastern shore of the Mediterranean Sea...
A 2009 study reported that the annual incidence rate of Israeli children with a diagnosis of ASD receiving disability benefits rose from zero in 1982–1984 to 190 per million in 2004. It was not known whether these figures reflected true increases or other factors such as changes in diagnostic measures.
JapanJapanJapan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
A 2005 study of a part of Yokohama
Yokohama
is the capital city of Kanagawa Prefecture and the second largest city in Japan by population after Tokyo and most populous municipality of Japan. It lies on Tokyo Bay, south of Tokyo, in the Kantō region of the main island of Honshu...
with a stable population of about 300,000 reported a cumulative incidence to age 7 years of 48 cases of ASD per 10,000 children in 1989, and 86 in 1990. After the vaccination rate of MMR vaccine
MMR vaccine
The MMR vaccine is an immunization shot against measles, mumps, and rubella . It was first developed by Maurice Hilleman while at Merck in the late 1960s....
dropped to near zero, the incidence rate grew to 97 and 161 cases per 10,000 children born in 1993 and 1994, respectively, indicating that MMR vaccine did not cause autism.
Saudi ArabiaSaudi ArabiaThe Kingdom of Saudi Arabia , commonly known in British English as Saudi Arabia and in Arabic as as-Sa‘ūdiyyah , is the largest state in Western Asia by land area, constituting the bulk of the Arabian Peninsula, and the second-largest in the Arab World...
Studies of autism frequency have been particularly rare in the Middle East. One rough estimate is that the prevalence of autism in Saudi Arabia is 18 per 10,000, slightly higher than the 13 per 10,000 reported in developed countries. Estimates for ASD prevalence in Saudi Arabia are not available.
DenmarkDenmarkDenmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...
A 2003 study reported that the cumulative incidence of autism in Denmark began a steep increase starting around 1990, and continued to grow until 2000, despite the withdrawal of thiomersal-containing vaccines in 1992. For example, for children aged 2–4 years, the cumulative incidence was about 0.5 new cases per 10,000 children in 1990 and about 4.5 new cases per 10,000 children in 2000.
A subsequent critique of the Danish study by Mark Blaxill of SafeMinds pinpoints several discrepancies associated with the collection of autism data. Such discrepancies include changing the autism criteria in the middle of the study, 1994, from inpatients to outpatients, adding the numbers of a large Copenhagen clinic after 1992, etc.
FranceFranceThe French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
Eric Fombonne
Eric Fombonne
Eric Fombonne, MD, FRCP, is a professor of psychiatry and an epidemiologist. He directs the child psychiatry division at McGill University in Canada and the psychiatry department at the Montreal Children's Hospital, where he played a key role in the launch of its autism clinic. Fombonne is also...
made some studies in the years 1992 and 1997. He found a prevalence of 16 per 10.000 for the global PDD
Pervasive developmental disorder
Pervasive developmental disorders is a diagnostic category refers to a group of disorders characterized by delays or impairments in communication, social behaviors, and cognitive development.Pervasive developmental disorders include Autism, Asperger's syndrome, Rett's syndrome, Childhood...
. The INSERM found a prevalence of 27 per 10,000 for the ASD
Autism spectrum
The term "autism spectrum" is often used to describe disorders that are currently classified as pervasive developmental disorders. Pervasive developmental disorders include autism, Asperger syndrome, Childhood disintegrative disorder, Rett syndrome and Pervasive Developmental Disorder Not Otherwise...
and a prevalence of 9 per 10,000 for the early infantile autism
Autism
Autism is a disorder of neural development characterized by impaired social interaction and communication, and by restricted and repetitive behavior. These signs all begin before a child is three years old. Autism affects information processing in the brain by altering how nerve cells and their...
in 2003. Those figures are considered as underrated as the WHO
Who
Who may refer to:* Who , an English-language pronoun* who , a Unix command* Who?, one of the Five Ws in journalism- Art and entertainment :* Who? , a 1958 novel by Algis Budrys...
gives figures between 30 and 60 per 10,000. The French Minister of Health gives a prevalence of 4.9 per 10.000 on its website but it counts only early infantile autism
Autism
Autism is a disorder of neural development characterized by impaired social interaction and communication, and by restricted and repetitive behavior. These signs all begin before a child is three years old. Autism affects information processing in the brain by altering how nerve cells and their...
.
GermanyGermanyGermany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
A 2008 study found that inpatient admission rates for children with ASD increased 30% from 2000 to 2005, with the largest rise between 2000 and 2001 and a decline between 2001 and 2003. Inpatient rates for all mental disorders also rose for ages up to 15 years, so that the ratio of ASD to all admissions rose from 1.3% to 1.4%.
NorwayNorwayNorway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
A 2009 study reported prevalence rates for ASD ranging from 0.21% to 0.87%, depending on assessment method and assumptions about non-response, suggesting that methodological factors explain large variances in prevalence rates in different studies.
United KingdomUnited KingdomThe United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
The incidence and changes in incidence with time are unclear in the UK.
The reported autism incidence in the UK rose starting before the first introduction of the MMR vaccine
MMR vaccine
The MMR vaccine is an immunization shot against measles, mumps, and rubella . It was first developed by Maurice Hilleman while at Merck in the late 1960s....
in 1989. A 2004 study found that the reported incidence of pervasive developmental disorders in a general practice research database in England and Wales grew steadily during 1988–2001 from 0.11 to 2.98 per 10,000 person-years, and concluded that much of this increase may be due to changes in diagnostic practice.
Genetics
As late as the mid-1970s there was little evidence of a genetic role in autism; evidence from genetic epidemiologyGenetic epidemiology
Genetic epidemiology is the study of the role of genetic factors in determining health and disease in families and in populations, and the interplay of such genetic factors with environmental factors...
studies now suggests that it is one of the most heritable of all psychiatric conditions. The first studies of twins estimated heritability
Heritability
The Heritability of a population is the proportion of observable differences between individuals that is due to genetic differences. Factors including genetics, environment and random chance can all contribute to the variation between individuals in their observable characteristics...
to be more than 90%; in other words, that genetics explains more than 90% of autism cases. When only one identical twin is autistic, the other often has learning or social disabilities. For adult siblings, the risk of having one or more features of the broader autism phenotype might be as high as 30%, much higher than the risk in controls. About 10–15% of autism cases have an identifiable Mendelian (single-gene) condition, chromosome abnormality, or other genetic syndrome, and ASD is associated with several genetic disorder
Genetic disorder
A genetic disorder is an illness caused by abnormalities in genes or chromosomes, especially a condition that is present from before birth. Most genetic disorders are quite rare and affect one person in every several thousands or millions....
s.
Since heritability is less than 100% and symptoms vary markedly among identical twins with autism, environmental factors are most likely a significant cause as well. If some of the risk is due to gene-environment interaction the 90% heritability estimate may be too high; new twin data and models with structural genetic variation are needed.
Genetic linkage
Genetic linkage
Genetic linkage is the tendency of certain loci or alleles to be inherited together. Genetic loci that are physically close to one another on the same chromosome tend to stay together during meiosis, and are thus genetically linked.-Background:...
analysis has been inconclusive; many association analyses
Genetic association
Genetic association is the occurrence, more often than can be readily explained by chance, of two or more traits in a population of individuals, of which at least one trait is known to be genetic....
have had inadequate power. Studies have examined more than 100 candidate genes; many genes must be examined because more than a third of genes are expressed in the brain and there are few clues on which are relevant to autism.
Risk factors
Boys are at higher risk for autism than girls. The ASD sex ratio averages 4.3:1 and is greatly modified by cognitive impairment: it may be close to 2:1 with mental retardation and more than 5.5:1 without. Recent studies have found no association with socioeconomic status, and have reported inconsistent results about associations with race or ethnicity.Although the evidence does not implicate any single pregnancy-related risk factor as a cause of autism, the risk of autism is associated with several prenatal risk factors, including advanced age in either parent, and diabetes, bleeding, and use of psychiatric drugs in the mother during pregnancy. It is not known whether mutations that arise spontaneously in autism and other neuropsychiatric disorders come mainly from the mother or the father, or whether the mutations are associated with parental age. However, recent studies have identified advancing paternal age as a significant risk factor for ASD.
A large 2008 population study of Swedish parents of children with autism found that the parents were more likely to have been hospitalized for a mental disorder, that schizophrenia was more common among the mothers and fathers, and that depression and personality disorders were more common among the mothers.
It is not known how many siblings of autistic individuals are themselves autistic. Several studies based on clinical samples have given quite different estimates, and these clinical samples differ in important ways from samples taken from the general community.
Autism has also been shown to cluster in urban neighborhoods of high socioeconomic status. One study from California found a three to fourfold increased risk of autism in a small 30 by 40 km region centered on West Hollywood. Another study by a UC Davis group using a similar methodology,erroneous because of its stratification of space nevertheless found multiple clusters in urban high socioeconomic status neighborhoods of California.
Comorbid conditions
Autism is associated with several other conditions:- Genetic disorderGenetic disorderA genetic disorder is an illness caused by abnormalities in genes or chromosomes, especially a condition that is present from before birth. Most genetic disorders are quite rare and affect one person in every several thousands or millions....
s. About 10–15% of autism cases have an identifiable Mendelian (single-gene) condition, chromosome abnormality, or other genetic syndrome, and ASD is associated with several genetic disorders. - Mental retardationMental retardationMental retardation is a generalized disorder appearing before adulthood, characterized by significantly impaired cognitive functioning and deficits in two or more adaptive behaviors...
. The fraction of autistic individuals who also meet criteria for mental retardation has been reported as anywhere from 25% to 70%, a wide variation illustrating the difficulty of assessing autistic intelligence. For example, a 2001 British study of 26 autistic children found about 30% with intelligence in the normal range (IQ above 70), 50% with mild to moderate retardation, and about 20% with severe to profound retardation (IQ below 35). For ASD other than autism the association is much weaker: the same study reported about 94% of 65 children with PDD-NOS or Asperger syndrome had normal intelligence. - Anxiety disorderAnxiety disorderAnxiety disorder is a blanket term covering several different forms of abnormal and pathological fear and anxiety. Conditions now considered anxiety disorders only came under the aegis of psychiatry at the end of the 19th century. Gelder, Mayou & Geddes explains that anxiety disorders are...
s are common among children with ASD, although there are no firm data. Symptoms include generalized anxiety and separation anxietySeparation anxietySeparation anxiety may refer to:*Separation anxiety disorder or Separation anxiety test*Spider-Man & Venom: Separation Anxiety, a 1995 SNES video game*Separation Anxieties, a 2000 album by 12 Rods...
, and are likely affected by age, level of cognitive functioning, degree of social impairment, and ASD-specific difficulties. Many anxiety disorders, such as social phobiaSocial anxiety disorderSocial anxiety disorder , also known as social phobia, is an anxiety disorder characterized by intense fear in social situations causing considerable distress and impaired ability to function in at least some parts of daily life...
, are not commonly diagnosed in people with ASD because such symptoms are better explained by ASD itself, and it is often difficult to tell whether symptoms such as compulsive checking are part of ASD or a co-occurring anxiety problem. The prevalence of anxiety disorders in children with ASD has been reported to be anywhere between 11% and 84%. - EpilepsyEpilepsyEpilepsy is a common chronic neurological disorder characterized by seizures. These seizures are transient signs and/or symptoms of abnormal, excessive or hypersynchronous neuronal activity in the brain.About 50 million people worldwide have epilepsy, and nearly two out of every three new cases...
, with variations in risk of epilepsy due to age, cognitive level, and type of language disorder; 5–38% of children with autism have comorbid epilepsy, and only 16% of these have remission in adulthood. - Several metabolic defects, such as phenylketonuriaPhenylketonuriaPhenylketonuria is an autosomal recessive metabolic genetic disorder characterized by a mutation in the gene for the hepatic enzyme phenylalanine hydroxylase , rendering it nonfunctional. This enzyme is necessary to metabolize the amino acid phenylalanine to the amino acid tyrosine...
, are associated with autistic symptoms. - Minor physical anomaliesMinor physical anomaliesMinor physical anomalies are relatively minor congenital physical abnormalities consisting of features such as low-seated ears, single transverse palmar crease, telecanthus, micrognathism a furrowed tongue...
are significantly increased in the autistic population. - Preempted diagnoses. Although the DSM-IV rules out concurrent diagnosis of many other conditions along with autism, the full criteria for ADHD, Tourette syndromeTourette syndromeTourette syndrome is an inherited neuropsychiatric disorder with onset in childhood, characterized by multiple physical tics and at least one vocal tic; these tics characteristically wax and wane...
, and other of these conditions are often present and these comorbid diagnoses are increasingly accepted. A 2008 study found that nearly 70% of children with ASD had at least one psychiatric disorder, including nearly 30% with social anxiety disorderSocial anxiety disorderSocial anxiety disorder , also known as social phobia, is an anxiety disorder characterized by intense fear in social situations causing considerable distress and impaired ability to function in at least some parts of daily life...
and similar proportions with ADHD and oppositional defiant disorderOppositional defiant disorderOppositional defiant disorder is a diagnosis described by the Diagnostic and Statistical Manual of Mental Disorders as an ongoing pattern of disobedient, hostile and defiant behavior toward authority figures which goes beyond the bounds of normal childhood behavior...
. Childhood-onsetPediatric schizophreniaPediatric schizophrenia is a type of mental disorder that is characterized by degeneration of thinking, motor, and emotional processes in children and young adults under the age of 18...
schizophreniaSchizophreniaSchizophrenia is a mental disorder characterized by a disintegration of thought processes and of emotional responsiveness. It most commonly manifests itself as auditory hallucinations, paranoid or bizarre delusions, or disorganized speech and thinking, and it is accompanied by significant social...
, a rare and severe form, is another preempted diagnosis whose symptoms are often present along with the symptoms of autism.