(8 March 2005-HealthDay)The apparent rise in the proportion of children in the United States with autism appears to be real, a new analysis suggests.


Autism prevalence is increasing with successively younger children, particularly those born between 1987 and 1992, epidemiologists report in the March issue of Pediatrics.


It's a worrisome trend, experts said, magnifying questions on the causes for the increasing rates while placing a tremendous strain on the nation's special education resources.


"The financial burden that this will place on our society is going to be just stunning, and that is really the wake-up call here," said Andy Shih, director of research and programs at the National Alliance for Autism Research in Princeton, N.J.


Autism is a complex disorder affecting the ability to communicate and socialize with others; many children engage in repetitive behaviors, such as rocking, self-abuse or obsessive behavior.


Most experts agree that autism actually is a variety of separate but related disorders, Shih said.


The disability usually strikes by age 3, and lasts a lifetime. Boys are three-to-four times more likely than girls to be affected, according to the National Institute of Child Health and Human Development.


What causes autism isn't known, but some research points to a genetic origin. Scientists also are exploring neurological, infectious, metabolic, immunologic and environmental factors.


To assess prevalence trends in autism, researchers from Johns Hopkins Bloomberg School of Public Health and the University of Minnesota used data reported to U.S. Department of Education's Office of Special Education Programs. The study included children who were aged 6 to 17 between 1992 and 2001.


Prevalence refers to the proportion of people in a given population affected by a disease at a point in time, yielding a snapshot of the impact of the disease.


Researchers compared the results for autism with trends for other disabilities, including traumatic brain injury, mental retardation and speech/language impairment.


"The analysis shows that the number of children receiving a special ed classification of autism has increased drastically over the past decade," said Craig J. Newschaffer, director of Johns Hopkins' Center for Autism and Developmental Disabilities Epidemiology.


While the exact number of autism cases in the United States in not known, estimates range from one in 500 to one in 1,000 diagnosed cases each year, according to government statistics.


Difficulty in pinning down the actual number of causes stems from changes in how autism is diagnosed, what is considered autism and how cases are reported.


Some people have attributed the rising rate of autism to "diagnosis shifting," meaning children who in past years might have been classified as having mental retardation or speech/language difficulties are now being diagnosed as having autism.


This study refutes that theory.

"By looking at trends in other classifications, we see that this increase is not seen across the board in all [special] ed classifications," Newschaffer said. "This is not a rising tide lifting all boats."


Increases in autism prevalence were greatest for kids born from 1987 to 1992. And while prevalence continued to increase among kids born after 1992, the increases were not as great.

"This may represent a slowing of the rate of increase -- not a decrease," Newschaffer reasoned. It may also be the result of a 1997 change in federal law allowing special ed agencies to classify kids over age 5 as having a "developmental delay," he added. It could mean more children will be classified as being autistic at a later age.

All of this presents a major challenge to the nation's special education system. And to parents of kids with autism, it means either qualifying for financial assistance or paying for special therapists themselves.

"It's not uncommon for me to hear parents spending $30,000 to 40,000 a year," Shih said.

He added, "If this trend continues, it could be possible that we would no longer be able to care for everyone who deserved the care."

Meantime, controversy continues to swirl over a hypothetical link between the childhood vaccine given to prevent measles, mumps and rubella (MMR) and autism.

That possible connection, first raised in a widely reported 1998 study in The Lancet, led by British gastroenterologist Dr. Andrew Wakefield, has been discredited in several prominent studies since then.

A May 2004 report from the Institute of Medicine ) concluded that neither the mercury-based vaccine preservative thimerosal nor the MMR are associated with autism. And last week, Japanese and British researchers again disproved the connection in a study of 30,000 Japanese kids that was published in the Journal of Child Psychology and Psychiatry.

Yet, many parents and patient advocates remain suspicious. The National Autism Association, for one, insists that vaccines have not been cleared as a cause of autism.

"There is a great deal of evidence supporting a link between vaccines and autism," the association said in statement reacting to a report on NBC Nightly News. "Even [Centers for Disease Control and Prevention ] CDC director Julie Gerberding has not ruled out that possibility, as she indicated in her statement on the Today show, 'Right now, the scientific evidence doesn't provide any framework for concluding that thimerosal or immunizations in any way affect autism, but we have to have an open mind about that.'"

Journalist David Kirby explored the issue in his new book Evidence of Harm. "No one can say with certainty that thimerosal, the vaccine preservative made with 49.6 percent mercury, helped fuel the explosion in cases of autism, attention-deficit disorder, speech delay and other disorders over the past decade," he wrote. "But no one can say for certain that it did not."