HOMEResourcesArchive of Brown Newsletter
| Is there an autism epidemic? |
| Rowland P. Barrett, Ph.D. |
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A number of stories in the media recently have breathlessly discussed the "epidemic" of autism, raising concern about risks for autism at the same time the public is worried about avian flu and mad cow disease. Several recent epidemiological studies have reported a tenfold increase in the prevalence of autism in both England and the U.S. The indisputable fact that significantly more children are being diagnosed with autism than ever before, combined with ambiguity about the cause, has raised alarm and prompted the use of the term "epidemic" as a means of signaling serious concern. Autism, first identified by Kanner in 1943, is a neurodevelopmental disorder that profoundly impairs an individual's ability to form social relationships. It is believed that autism results from a neurological defect that impairs the brain's capacity to organize and interpret perceptual stimuli in a meaningful way. Consequently, the disorder seriously compromises the development of language and communication skills and, in the majority of cases, impairs intellectual growth. Approximately 75% of individuals diagnosed with autism also are diagnosed with varying degrees of mental retardation. Although autism is present from birth, it usually is not diagnosed until after the child has reached 15-18 months of age. The reason for the late diagnosis is the absence of a biological marker specific to the disorder. The diagnosis of autism depends entirely upon the recognition of behavior characteristic of the disorder. This has been the case for making a diagnosis of autism since it was first described 61 years ago. A diagnosis based on the recognition of a unique constellation of behavior patterns is inherently more difficult than a diagnosis arrived at through blood tests or imaging techniques. Although they are never perfect, laboratory studies can determine the presence or absence of a biological marker specific to the disorder. Achieving diagnostic accuracy via behavior is much more challenging, especially when behaviors of different disorders overlap, as is the case with autism and profound mental retardation. Simply put, the absence of a biological marker for autism lends itself to poor diagnostic reliability that, in turn, makes valid estimates of the prevalence of the disorder subject to question. Bearing these technical difficulties in mind, a number of possible causes and explanations have been offered to account for the current rise in the prevalence of autism. Genetic factors, it is agreed, play a central role in autism. Up to 10% of families having a child with autism have another member of the immediate or extended family, such as siblings, cousin, aunt or uncle with autism. Genetic factors alone, however, cannot adequately explain a ten-fold increase in prevalence. It has long been hypothesized that there are factors other than genetics - such as cytomegalovirus, toxoplasmosis and PKU (phenylketonuria) for example - that have a causal role in autism. Most recently, environmental toxins have been explored, especially thimerosal, the preservative in vaccines for measles, mumps, and rubella (MMR) and diptheria, pertussis, and tetanus (DPT). Studies of time trends in autism and MMR immunization in California, however, have demonstrated no correlation between increased prevalence rates of autism (373%) and increased rates of immunization (14%) for MMR. Another recent, large-scale study showed no increased risk for autism for children who had been vaccinated with a thimerosal-containing pertussis vaccine when compared to children who had been vaccinated with the same pertussis vaccine formulated without thimerosal. A less complicated explanation of the increased prevalence rates for autism involves a change in nosology. Broadening the definition of autism to include the so-called autistic-spectrum disorders, such as Asperger's Disorder and other pervasive developmental disorders, obviously has the effect of increasing the prevalence. A careful study in 2003 revealed a 300% increase in prevalence rate in the same population when criteria for autism were expanded to accommodate the full range of autistic spectrum disorders. Critics of this explanation agree that expanding the definition of autism to include autistic-spectrum disorders may account for some of the observed increase in prevalence, but not all of it. This may be true. It is highly likely that several other sources of influence exist and contribute to findings of increased prevalence rates. For example, advances in the training of child mental health professionals have led to improved recognition of disorders on the autistic-spectrum, leading to more frequent diagnoses. Additionally, heightened parent awareness of autistic-spectrum disorders results in more second opinions and fewer missed diagnoses. Parent advocacy also led to changes in federal funding in 1991 that added autism as a category eligible for special education services, thereby increasing parents' motivation to obtain a careful, accurate diagnosis. Thus, in the absence of evidence pointing to causal environmental pathogens, the prudent conclusion is the changes in nosology, better trained professionals, more informed parents and changes in public policy work together to improve the diagnosis of autism spectrum disorders. Under-diagnosis and missed diagnoses are now corrected, leaving us with the challenge of planning and funding more and proper treatment services, as well as dealing with the haunting realization of the vast number of individuals whose autistic disorder was unrecognized and untreated in years past. Rowland P. Barrett, Ph.D., is Associate Professor of Psychiatry at the Brown University Medical School. |