NEW YORK (Reuters Health) - Autism tends to go hand in hand with a variety of other mental andbehavioral conditions in kids, suggests a new study that highlights the fuzzy nature of autism diagnoses themselves.

Researchers said that other disorders that often go along with autism -- such as attention-deficit/hyperactivity disorder (ADHD) or learning disabilities -- may complicate the diagnosis, or slow down any improvement in kids who do get diagnosed and treated early.

"The most important message that (the study) underscores is that these children tend to have multiple disabilities, not just autism," said Johnny Matson, who studies autism spectrum disorders and intellectual disabilities at Louisiana State University in Baton Rouge but wasn't involved in the new research.

Dr. Andrew Zimmerman from Massachusetts General Hospital for Children in Lexington and his colleagues found that the conditions they typically saw occurring together with an autism spectrum disorder diagnosis varied depending on the age of the child.

Learning disabilities were more common in the youngest kids with an autism spectrum disorder, while anxiety, speech problems and seizures were more often seen in elementary school kids and teens.

The study also found that one-third of kids who had ever been diagnosed with an autism spectrum disorder no longer had the diagnosis at the time their parents were surveyed.

The researchers note in their report, published in Pediatrics, that past studies have also found that some kids who originally have an autism spectrum disorder eventually lose that diagnosis and are no longer considered autistic.

Whether that's due to a mistaken first diagnosis or actual changes in kids' brains and behaviors is controversial.

Autism spectrum disorders include autism, Asperger's syndrome and "pervasive developmental disorder-not otherwise specified," or PDD-NOS. The number of kids diagnosed with one of those conditions has spiked in recent years, with about one in every 110 kids in the United States now considered to have an autism spectrum disorder, according to estimates by the Centers for Disease Control and Prevention.

An upcoming revision of psychiatrists' Diagnostic and Statistical Manual of Mental Disorders is expected to narrow the definition of autism and related disorders, potentially shrinking the number of children and adults who qualify for the diagnosis.

The new study included data from a phone survey of about 92,000 parents of kids age 17 and younger in the U.S. in 2007 and 2008.

Parents were asked if a doctor had ever told them their child had an autism spectrum disorder and if the child had one of the disorders currently, as well as if kids had a range of other mental and behavioral conditions such as anxiety and developmental delays.

In total, 1,366 of the parents surveyed said their kid had a past or current diagnosis of an autism spectrum disorder. In 453 of those cases, kids had at one point been diagnosed as having one of the disorders but didn't anymore, parents said.

Compared to kids who had lost their diagnosis, the youngest kids that were still considered autistic were more likely to have a learning disability or delayed development, while elementary school kids had more anxiety disorders and teens more often had a speech problem or mild seizures and epilepsy.

In her own studies, "the kids that had more co-occurring diagnoses tended to be diagnosed (with autism) later," when an autism diagnosis may be more definitive and stable, said Dr. Susan Levy, an autism and child development researcher from The Children's Hospital of Philadelphia.

In addition, she told Reuters Health, "The kids that had more co-occurring diagnoses had a lot more difficulties and their progress may have been impeded," making it harder for their autism-related symptoms to improve with treatment.

Some of the kids who ended up not having an autism spectrum disorder may have originally been diagnosed incorrectly, especially if they got the diagnosis when they were very young, researchers said.

Kids and teens in the study that had once been considered autistic but weren't any longer were more likely to have hearing problems than those who still had an autism spectrum disorder -- suggesting that those problems might have been originally mistaken for autism.

But experts disagreed about whether it's possible for kids who are diagnosed correctly with one of the autism-spectrum disorders to improve to the point where they are no longer affected.

"When you're autistic, you're autistic. It's a very stable condition," Matson said, noting that even when symptoms improve, people with autism have to keep getting treatment and work to maintain that progress.

Zimmerman argued that recognizing autism early and starting treatment can increase the likelihood for real, lasting improvement.

"It's not unusual to see a child start out with more severe autism and then become more moderate and even mild as the years go by. A lot of the kids are improving, and we don't really know why, except we know there's a lot of moldability of the developing brain," he told Reuters Health.

"We think that earlier treatment is essential and there are reasons to think that we can improve the kids. I'm very optimistic."