The previous post discussed a new study of autism prevalence. In The Politics of Autism, I write:
For all of our 21st century technology, autism is roughly where polio was a century ago. Before reliable lab tests were available, physicians could diagnose polio only by looking at symptoms. Because paralysis has many other causes (infections, injuries, tumors, toxins), doctors often mislabeled polio as something else, or vice versa.[i] Autism presents a similar muddle. “To diagnose autism reliably, we need to better understand what goes awry in people with the disorder,” writes Harvard biostatistician Nicholas Lange. “Until its solid biological basis is found, any attempt to use brain imaging to diagnose autism will be futile.”[ii] As the previous chapter noted, psychologists have to rely on questionnaires and observations of behavior. Therein lies a problem. Says Thomas Insel, director of the National Institute of Mental Health (NIMH):Unlike our definitions of ischemic heart disease, lymphoma, or AIDS, the DSM diagnoses are based on a consensus about clusters of clinical symptoms, not any objective laboratory measure. In the rest of medicine, this would be equivalent to creating diagnostic systems based on the nature of chest pain or the quality of fever. Indeed, symptom-based diagnosis, once common in other areas of medicine, has been largely replaced in the past half century as we have understood that symptoms alone rarely indicate the best choice of treatment.[iii]
[i] Gareth Williams, Paralysed with Fear: The Story of Polio (New York: Palgrave Macmillan, 2013), 36.[ii] Nicholas Lange, “Perspective: Imaging Autism,” Nature 491 (1 November 2012): S17. Online: http://www.nature.com/nature/journal/v491/n7422_supp/full/491S17a.html.[iii] Thomas Insel, “Transforming Diagnosis,” National Institute of Mental Health Director’s Blog, April 29, 20913. Online: http://www.nimh.nih.gov/about/director/2013/transforming-diagnosis.shtml.
The CDC has been tracking incidences of ASD for the past two decades in a number of ways. In addition to administering an annual survey to parents, government health officials also collect data through the National Survey of Children’s Health and the Autism and Developmental Disabilities Monitoring Network. The former is also a parent-reported survey conducted by telephone, while the latter—the largest source, involving a sample size of 360,000 families—is based on clinicians’ reviews of medical and educational records. However, the review by clinicians is only based on data from 14 communities across the U.S., while the two surveys look at a national sample.
Changing diagnostic criteria can also affect reported rates of ASD. Prior to 2013, a patient could be diagnosed with autistic disorder, Asperger’s syndrome, childhood disintegrative disorder or a pervasive developmental disorder not otherwise specified. But clinicians found criteria for various diagnoses were not used consistently across the country, so in 2013 the Diagnostic and Statistical Manual of Mental Disorders was updated; now a patient is diagnosed with ASD, along with a severity based on a given scale.
“It is difficult to interpret trends in prevalence over time because of the possibility of ‘diagnostic substitution,’” the researchers wrote, “whereby labeling practices might change and cause similar symptoms to be classified under different disabilities during different time periods.”