A dilemma of definition

In the 1980s, national estimates of autism--a disorder involving limited social and language skills--found the condition only occurred in about four in 10,000 children. But by 2002, the Centers for Disease Control and Prevention found that number to have more than doubled.

While there may be some true increase in the number of children born with autism, many researchers believe that the jump in prevalence is due, at least in part, to a broadened definition of the disorder. For example, when the authors of the Diagnostic and Statistical Manual of Mental Disorders third edition (DSM-III) first introduced autism as a diagnosis in 1980, anyone with normal intelligence was automatically disqualified, says Ami Klin, PhD, a psychology professor at the Yale Child Study Center. He and others, including Fred Volkmar, MD, professor of psychiatry at the Yale Child Study Center, retooled the criteria so that, by the fourth edition of the manual, autism also described individuals with high intelligence but significant impairments in communication, social interaction and behavioral flexibility.

But with the inclusion of high-functioning people with autism came some confusion among researchers and clinicians about an additional category within the autism spectrum, "Asperger's syndrome," which was first included as a disorder separate from autism in the DSM-IV, published in 1994.

"With two categories, we can better capture the state of knowledge [about these disorders]," says Klin. "To move ahead we need definitional consensus."

Some clinicians see this category shuffle as irrelevant to designing treatments and understanding the origins of autism and Asperger's syndrome, but Klin and his colleagues at the Yale Child Study Center make the case that the two disorders differ in their impact on early development as well as, potentially, the brain's biology. And these differences could potentially inform research on the origins of these disorders as well as their treatment, says Klin.

Early development

One aspect of Asperger's syndrome that differentiates it from autism appears early in affected children's language development, says Volkmar. Typical autistic children show language delays--pointing and grunting--while same-aged children without the disorder are stringing together simple sentences. In comparison, most children with Asperger's syndrome grasp language quickly, sometimes even more quickly than their cohorts.

"These are kids who talk before they walk," Volkmar says. "Language is their lifeline."

However, notes Volkmar, children with Asperger's syndrome generally do not master the usual give-and-take of conversation. More characteristically, they will drone on about their favorite topics, such as meteorology.

While many children with Asperger's syndrome acquire an impressive vocabulary, children with autism tend to find their strengths in motor skills and manipulating objects, according to ongoing research by Klin and Volkmar. In this study, they are comparing 21 children with Asperger's syndrome with 19 similarly aged children diagnosed as autistic--through all have normal IQs. Both groups took the Weschler Intelligence Scale for Children, including tests of their motor and language skills.

The participants with a diagnosis of autism performed well on tasks requiring visual-spatial perception--such as puzzle-solving--or motor skills. Those with Asperger's syndrome tended to show deficits in these areas, though they outperformed the participants with autism on vocabulary, auditory perception and verbal memory tasks.

"Though both of these groups have similar trouble with social functioning, they show different patterns of neuropsychological functioning," says Volkmar.

However, some researchers have found that differences in early language development even out by adulthood. In one such study, published in the Journal of Autism and Developmental Disorders (Vol. 33, No. 1), Patricia Howlin, PhD, a psychology professor at St. George's Hospital Medical School in London, compared the language abilities and social functioning of 32 adult participants who have autism with 42 adults who have Asperger's syndrome; all were diagnosed as children. Both groups, which were matched by IQ and age, scored similarly on tests of language ability.

Additionally, the two groups appeared to be equivalent in their social functioning: They had comparable numbers of relationships and reported about the same levels of satisfaction with them, as measured by the Autism Diagnostic Interview.

"Right now there is no good evidence to show Asperger's syndrome and autism are different disorders if groups are appropriately matched on age and IQ," says Howlin.

A preliminary separation

Though Klin agrees that the evidence so far is mixed, he believes that, at least for now, differentiating these two groups aids in their treatment.

"You want to build on [children with Asperger's] strengths, using language to teach social skills," he says, though he notes treatment for both disorders should be tailored to the child, rather than the diagnosis.

Adding additional evidence to the case for separate disorders, Volkmar notes that 20 percent of children diagnosed with autism also experience seizures, while those with Asperger's syndrome have seizures about as often as the general population. However, Tourette's syndrome and affective disorders more commonly co-occur with Asperger's syndrome than with autism, he adds.

"This makes us think that the two things are different disorders, operating differently in the brain, though we don't know for sure," says Volkmar.

Despite this uncertainty, placing the disorders into two categories may aid in the search for the genes that contribute to them, Volkmar says. In fact, if researchers can identify and compare the culpable genes of high-functioning autism versus Asperger's syndrome, they may be able to untangle the disorders' underpinnings, he notes.

"Looking at these two disorders makes for an interesting problem for academics," Volkmar says. "The better we can refine the phenotype [symptoms], the easier it will be to get at some of these genetic roots."

Shared symptoms

Both Asperger's syndrome and autism are characterized by profoundly disturbed social interactions and inflexible behavior and can be identified by:

• Deficits in nonverbal communication, such as facial expressions, eye contact and body language.

• A preference for interacting with objects rather than peers.

• Deep immersion in a single field of interest, such as meteorology.

• Strict adherence to a daily routine.

• Repetitive motions, such as hand flapping or finger twisting.

Source: Diagnostic and Statistical Manual of Mental Disorders (Fourth Edition, Revised.)