Changes with time Epidemiology of autism

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.