Frequency Epidemiology of autism

1 frequency

1.1 incidence , prevalence
1.2 estimation methods
1.3 frequency estimates
1.4 changes time
1.5 geographical frequency

1.5.1 africa
1.5.2 americas

1.5.2.1 canada
1.5.2.2 united states
1.5.2.3 venezuela


1.5.3 asia

1.5.3.1 hong kong
1.5.3.2 japan


1.5.4 middle east

1.5.4.1 israel
1.5.4.2 saudi arabia


1.5.5 europe

1.5.5.1 denmark
1.5.5.2 france
1.5.5.3 germany
1.5.5.4 norway
1.5.5.5 united kingdom

frequency

although incidence rates measure autism risk directly, epidemiological studies report other frequency measures, typically point or period prevalence, or cumulative incidence. attention focused on whether prevalence increasing time.

incidence , prevalence

epidemiology defines several measures of frequency of occurrence of disease or condition:

the incidence rate of condition rate @ new cases occurred per person-year, example, 2 new cases per 1,000 person-years .
the cumulative incidence proportion of population became new cases within specified time period, example, 1.5 per 1,000 people became new cases during 2006 .
the point prevalence of condition proportion of population had condition @ single point in time, example, 10 cases per 1,000 people @ start of 2006 .
the period prevalence proportion had condition @ time within stated period, example, 15 per 1,000 people had cases during 2006 .

when studying how diseases caused, incidence rates appropriate measure of disease frequency assess risk directly. however, incidence can difficult measure rarer chronic diseases such autism. in autism epidemiology, point or period prevalence more useful incidence, disorder starts long before diagnosed, , gap between initiation , diagnosis influenced many factors unrelated risk. research focuses on whether point or period prevalence increasing time; cumulative incidence used in studies of birth cohorts.

estimation methods

the 3 basic approaches used estimate prevalence differ in cost , in quality of results. simplest , cheapest method count known autism cases sources such schools , clinics, , divide population. approach underestimate prevalence because not count children have not been diagnosed yet, , generate skewed statistics because children have better access treatment.

the second method improves on first having investigators examine student or patient records looking probable cases, catch cases have not been identified yet. third method, arguably best, screens large sample of entire community identify possible cases, , evaluates each possible case in more detail standard diagnostic procedures. last method typically produces reliable, , highest, prevalence estimates.

frequency estimates

estimates of prevalence of autism vary depending on diagnostic criteria, age of children screened, , geographical location. recent reviews tend estimate prevalence of 1–2 per 1,000 autism , close 6 per 1,000 asd; pdd-nos vast majority of asd, asperger syndrome 0.3 per 1,000 , atypical forms childhood disintegrative disorder , rett syndrome rarer.

a 2006 study of 57,000 british nine- , ten-year-olds reported prevalence of 3.89 per 1,000 autism , 11.61 per 1,000 asd; these higher figures associated broadening diagnostic criteria. studies based on more detailed information, such direct observation rather examination of medical records, identify higher prevalence; suggests published figures may underestimate asd s true prevalence. 2009 study of children in cambridgeshire, england used different methods measure prevalence, , estimated 40% of asd cases go undiagnosed, 2 least-biased estimates of true prevalence being 11.3 , 15.7 per 1,000.

a 2009 u.s. study based on 2006 data estimated prevalence of asd in 8-year-old children 9.0 per 1,000 (approximate range 8.6–9.3). 2009 report based on 2007 adult psychiatric morbidity survey national health service determined prevalence of asd in adults approximately 1% of population, higher prevalence in males , no significant variation between age groups; these results suggest prevalence of asd among adults similar in children , rates of autism not increasing.

changes time

attention has been focused on whether prevalence of autism increasing time. earlier prevalence estimates lower, centering @ 0.5 per 1,000 autism during 1960s , 1970s , 1 per 1,000 in 1980s, opposed today s 1–2 per 1000.

reports of autism cases per 1,000 children grew dramatically in u.s. 1996 2007. unknown how much, if any, growth came changes in autism s prevalence.

the number of reported cases of autism increased dramatically in 1990s , 2000s, prompting investigations several potential reasons:

more children may have autism; is, true frequency of autism may have increased.
there may more complete pickup of autism (case finding), result of increased awareness , funding. example, attempts sue vaccine companies may have increased case-reporting.
the diagnosis may applied more broadly before, result of changing definition of disorder, particularly changes in dsm-iii-r , dsm-iv.
an editorial error in description of pdd-nos category of autism spectrum disorders in dsm-iv, in 1994, inappropriately broadened pdd-nos construct. error corrected in dsm-iv-tr, in 2000, reversing pdd-nos construct more restrictive diagnostic criteria requirements dsm-iii-r.
successively earlier diagnosis in each succeeding cohort of children, including recognition in nursery (preschool), may have affected apparent prevalence not incidence.
a review of rising autism figures compared other disabilities in schools shows corresponding drop in findings of mental retardation.

the reported increase largely attributable changes in diagnostic practices, referral patterns, availability of services, age @ diagnosis, , public awareness. cited 2002 pilot study concluded observed increase in autism in california cannot explained changes in diagnostic criteria, 2006 analysis found special education data poorly measured prevalence because many cases undiagnosed, , 1994–2003 u.s. increase associated declines in other diagnostic categories, indicating diagnostic substitution had occurred.

a 2007 study modeled autism incidence found broadened diagnostic criteria, diagnosis @ younger age, , improved efficiency of case ascertainment, can produce increase in frequency of autism ranging 29-fold depending on frequency measure, suggesting methodological factors may explain observed increases in autism on time. small 2008 study found significant number (40%) of people diagnosed pragmatic language impairment children in previous decades given diagnosis autism. study of danish children born in 1994–99 found children born later more diagnosed @ younger age, supporting argument apparent increases in autism prevalence @ least partly due decreases in age of diagnosis.

a 2009 study of california data found reported incidence of autism rose 7- 8-fold 1990s 2007, , changes in diagnostic criteria, inclusion of milder cases, , earlier age of diagnosis explain 4.25-fold increase; study did not quantify effects of wider awareness of autism, increased funding, , expanding treatment options resulting in parents greater motivation seek services. 2009 california study found reported increases unlikely explained changes in how qualifying condition codes autism recorded.

several environmental risk factors have been proposed support hypothesis actual frequency of autism has increased. these include foods, infectious disease, pesticides. there overwhelming scientific evidence against mmr hypothesis , no convincing evidence thiomersal (or thimerosal) hypothesis, these types of risk factors have ruled out. although unknown whether autism s frequency has increased, such increase suggest directing more attention , funding toward changing environmental factors instead of continuing focus on genetics.

geographical frequency
africa

the prevalence of autism in africa unknown.

americas
canada

the rate of autism diagnoses in canada 1 in 450 in 2003. however, preliminary results of epidemiological study conducted @ montreal children s hospital in 2003-2004 school year found prevalence rate of 0.68% (or 1 per 147).

a 2001 review of medical research conducted public health agency of canada concluded there no link between mmr vaccine , either inflammatory bowel disease or autism. review noted, increase in cases of autism noted year of birth 1979 1992; however, no incremental increase in cases observed after introduction of mmr vaccination. after introduction of mmr, time trend analysis found no correlation between prevalence of mmr vaccination , incidence of autism in each birth cohort 1988 1993.

united states

cdc s recent estimate 1 out of every 68 children, or 14.7 per 1,000, have form of asd of 2010. number of diagnosed cases of autism grew dramatically in u.s. in 1990s , 2000s. 2006 surveillance year, identified asd cases estimated 9.0 per 1000 children aged 8 years (95% confidence interval [ci] = 8.6–9.3). these numbers measure called administrative prevalence , is, number of known cases per unit of population, opposed true number of cases. prevalence estimate rose 57% (95% ci 27%–95%) 2002 2006.

the number of new cases of austism spectrum disorder (asd) in caucasian boys 50% higher found in hispanic children, , approximately 30% more occur in non-hispanic white children in united states.

a further study in 2006 concluded apparent rise in administrative prevalence result of diagnostic substitution, findings of mental retardation , learning disabilities. many of children being counted in autism category have been counted in mental retardation or learning disabilities categories if being labelled 10 years ago instead of today, said researcher paul shattuck of waisman center @ university of wisconsin–madison, in statement.

a population-based study in olmsted county, minnesota county found cumulative incidence of autism grew eightfold 1980–83 period 1995–97 period. increase occurred after introduction of broader, more-precise diagnostic criteria, increased service availability, , increased awareness of autism. during same period, reported number of autism cases grew 22-fold in same location, suggesting counts reported clinics or schools provide misleading estimates of true incidence of autism.

venezuela

a 2008 study in venezuela reported prevalence of 1.1 per 1000 autism , 1.7 per 1000 asd.

asia

a journal reports median prevalence of asd among 2–6-year-old children reported in china 2000 upwards 10.3/10,000.

hong kong

a 2008 hong kong study reported asd incidence rate similar reported in australia , north america, , lower europeans. reported prevalence of 1.68 per 1,000 children under 15 years.

japan

a 2005 study of part of yokohama stable population of 300,000 reported cumulative incidence age 7 years of 48 cases of asd per 10,000 children in 1989, , 86 in 1990. after vaccination rate of triple mmr vaccine dropped near 0 , replaced mr , m vaccine, incidence rate grew 97 , 161 cases per 10,000 children born in 1993 , 1994, respectively, indicating combined mmr vaccine did not cause autism. 2004 japanese autism association reported 360.000 people have typical kanner type autism.

middle east
israel

a 2009 study reported annual incidence rate of israeli children diagnosis of asd receiving disability benefits rose 0 in 1982–1984 190 per million in 2004. not known whether these figures reflected true increases or other factors such changes in diagnostic measures.

saudi arabia

studies of autism frequency have been particularly rare in middle east. 1 rough estimate prevalence of autism in saudi arabia 18 per 10,000, higher 13 per 10,000 reported in developed countries. estimates asd prevalence in saudi arabia not available.

europe
denmark

in 1992, thiomersal-containing vaccines removed in denmark. study @ aarhus university indicated rates of autism had stabilized , may in decline after removal of thiomersal vaccines. 1 co-author of study stated incidence , prevalence still decreasing in 2001 . in 2003 pediatrics magazine claimed same study indicated cumulative incidence of autism in denmark stable until 1990, began increase after removal of thiomersal containing vaccines in 1992. other doctor (dr. poul thorsen) involved in study omitted 2001 data indicating dropping rates of autism. thorsen later charged fraud , embezzlement while working aarhus university.

france

france made autism national focus year 2012 , health ministry evaluates rate of autism 67 per 10000 (1 out of 150).

eric fombonne made studies in years 1992 , 1997. found prevalence of 16 per 10.000 global pervasive developmental disorder (pdd). inserm found prevalence of 27 per 10,000 asd , prevalence of 9 per 10,000 infantile autism in 2003. figures considered underrated gives figures between 30 , 60 per 10,000. french minister of health gives prevalence of 4.9 per 10,000 on website counts infantile autism.

germany

a 2008 study in germany found inpatient admission rates children asd increased 30% 2000 2005, largest rise between 2000 , 2001 , decline between 2001 , 2003. inpatient rates mental disorders rose ages 15 years, ratio of asd admissions rose 1.3% 1.4%.

norway

a 2009 study in norway reported prevalence rates asd ranging 0.21% 0.87%, depending on assessment method , assumptions non-response, suggesting methodological factors explain large variances in prevalence rates in different studies.

united kingdom

the incidence , changes in incidence time unclear in united kingdom.

the reported autism incidence in uk rose starting before first introduction of mmr vaccine in 1989. however, perceived link between 2 arising results of fraudulent scientific study, has caused considerable controversy, despite being subsequently disproved. 2004 study found reported incidence of pervasive developmental disorders in general practice research database in england , wales grew steadily during 1988–2001 0.11 2.98 per 10,000 person-years, , concluded of increase may due changes in diagnostic practice.