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Why autism diagnoses have become more frequent since the 1980s, whether autism has mainly a genetic or developmental cause, and the degree of coincidence between autism and mental retardation are all matters of current scientific controversy.
Scientific consensus holds that vaccines do not cause autism, but popular rumors and an article in a respected scientific journal, The Lancet, provoked concern among parents. The Lancet article was retracted for making false claims and because its author was found to be in the pay of litigants against vaccine manufacturers.
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 1,000 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 disorders and epilepsy. Autism is also associated with mental retardation.
There is evidence that autism has a genetic component, and ongoing research focuses on finding the biomarkers that determine autistic phenotypes. One yet unproven theory is that there may be genes which contribute to a vulnerability to environmental triggers or have another role in the etiology of autism.
Genetics is viewed as an underlying factor because there is a statistical pattern for a significantly high risk to have another autistic child in families with an already affected child. However, while some parents of those with autism are progressively also being diagnosed on the autism spectrum, at least some autistic children have apparently neurotypical parents. This suggests to some that genetics are either not a necessary cause or that they don't play a part in all cases of ASD, although it could also mean that the gene(s) responsible may be recessive or a spontaneous mutation. The spectrum of autistic disorders is notable for its significant gender disparity, with the incidence of autism in males greatly exceeding the incidence in females. Whilst this could be evidence for a genetic theory, it has also been argued that male brains may be more vulnerable during early development.
Several controversial claims have been made with regard to autism and vaccinations, leading notably to the MMR vaccine controversy, the thiomersal controversy, and the theory of vaccine overload. A 2011 journal article described the vaccine-autism connection as "the most damaging medical hoax of the last 100 years".
Vaccine overload is the notion that giving many vaccines at once may overwhelm or weaken a child's immune system and lead to adverse effects. The idea of vaccine overload is flawed, for several reasons. First, vaccines do not overwhelm the immune system; in fact, conservative estimates predict that the immune system can respond to thousands of viruses simultaneously. It is known that vaccines constitute only a tiny fraction of the pathogens already naturally encountered by a child in a typical year. Common fevers and middle ear infections pose a much greater challenge to the immune system than vaccines do. Second, studies have shown that vaccinations, and even multiple concurrent vaccinations, do not weaken the immune system or compromise overall immunity. Finally, autism is not an immune-mediated disease. The lack of evidence supporting the vaccine overload hypothesis, combined with these findings directly contradicting it, have led to the conclusion that currently recommended vaccine programs do not "overload" or weaken the immune system.
In 1999, the Centers for Disease Control (CDC) and the American Academy of Pediatrics (AAP) asked vaccine makers to remove the organomercury compound thiomersal (spelled "thimerosal" in the U.S.) from vaccines as quickly as possible, and thiomersal has been phased out of U.S. and European vaccines, except for some preparations of influenza vaccine. The CDC and the AAP followed the precautionary principle, which assumes that there is no harm in exercising caution even if it later turns out to be unwarranted, but their 1999 action sparked confusion and controversy that has diverted attention and resources away from efforts to determine the causes of autism. Since 2000, the thiomersal in child vaccines has been alleged to contribute to autism, and thousands of parents in the United States have pursued legal compensation from a federal fund. A 2004 Institute of Medicine (IOM) committee favored rejecting any causal relationship between thiomersal-containing vaccines and autism. Autism incidence rates increased steadily even after thiomersal was removed from childhood vaccines. Currently there is no accepted scientific evidence that exposure to thiomersal is a factor in causing autism.
In the UK, the MMR vaccine was the subject of controversy after publication in The Lancet of a 1998 paper by Andrew Wakefield, et al. This article reported a study of 12 children mostly with onset autism spectrum disorders soon after administration of the vaccine. During a 1998 press conference, Wakefield suggested that giving children the vaccines in three separate doses would be safer than a single vaccination. This suggestion was not supported by his paper, nor by several subsequent peer-reviewed studies that failed to show any association between the vaccine and autism. It later emerged that Wakefield had received funding from litigants against vaccine manufacturers and that Wakefield had not informed colleagues or medical authorities of his conflict of interest. Had this been known, the paper would not have been published in The Lancet the way that it was. Wakefield has been heavily criticized on scientific grounds and for triggering a decline in vaccination rates, as well as on ethical grounds for the way the research was conducted. In 2004 the MMR-and-autism interpretation of the paper was formally retracted by 10 of Wakefield's 12 co-authors, and in 2010 The Lancet's editors fully retracted the paper.
The CDC, the IOM of the National Academy of Sciences, and the UK National Health Service have all concluded that there is no evidence of a link between the MMR vaccine and autism. In 2009, The Sunday Times reported that Wakefield had manipulated patient data and misreported results in his 1998 paper, creating the appearance of a link with autism. A 2011 article in the British Medical Journal described how the data in the study had been falsified by Wakefield so it would arrive at a predetermined conclusion. An accompanying editorial in the same journal described Wakefield's work as an "elaborate fraud" which led to lower vaccination rates, putting hundreds of thousands of children at risk and diverting energy and money away from research into the true cause of autism.
The percentage 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 PDD-NOS the association with mental retardation is much weaker. The diagnosis of Asperger's excludes clinically significant delays in mental or cognitive skills.
A 2007 study suggested that Raven's Progressive Matrices (RPM), a test of abstract reasoning, may be a better indicator of intelligence for autistic children than the more commonly used Wechsler Intelligence Scale for Children (WISC). Researchers suspected that the WISC relied too heavily on language to be an accurate measure of intelligence for autistic individuals. Their study revealed that the neurotypical children scored similarly on both tests, but the autistic children fared far better on the RPM than on the WISC. The RPM measures abstract, general and fluid reasoning, an ability autistic individuals have been presumed to lack. A 2008 study found a similar effect, but to a much lesser degree and only for individuals with IQs less than 85 on the Wechsler scales.
|This section may be confusing or unclear to readers. (July 2012)|
Joint attention is the shared focus of two individuals on an object. It refers to a cluster of behaviors in one of two classes: a child's response to someone else pointing or shifting eye gaze, and a child seeking another's attention. Many joint-attention behaviors differ in children with autism: for example, eye contact is relatively absent or atypical. These joint attention skills seem to be prerequisites for functional language development. It has also been hypothesized that autistic children initiate joint attention perhaps even as often as their neurotypical peers, albeit in atypical ways, and that a parent should join an autistic child's focus of attention and try harder to notice the child's atypical requests for attention rather than insist on typical behavior from the child. The empirical data supporting the latter hypothesis has been questioned.