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Researchers in British Columbia have discovered that children born in the latter months of the year are more likely to be diagnosed with attention deficit hyperactivity disorder than their peers born earlier in the year. The study, published Monday in the Canadian Medical Association Journal, found those children are also more likely to be placed on medication, some of which may be linked to side effects.

The finding should serve as a wake-up call for educators, parents and the public about the hazards of properly diagnosing ADHD.

It raises serious questions about whether certain behaviours, such as restlessness or inability to concentrate, are leaving younger, more immature classmates vulnerable to an inaccurate diagnosis of ADHD than their older, more developed peers.

"What's clear is that relative age is influencing who receives this diagnosis and who receives treatment for ADHD," said Richard Morrow, lead author of the study and health research analyst of the Therapeutics Initiative at the University of British Columbia. "It seems to be that sometimes a child's … lack of maturity can sometimes be mistaken for … a diagnosis.

The scale of the study also sets it apart – it is the largest of its kind. After looking at more than 937,000 children between ages 6 and 12 from 1997 to 2008, the researchers found those born in December were 39 per cent more likely to be diagnosed with ADHD and 48 per cent more likely to be given medication as treatment than their peers born in January of the same year.

A separate study, also released Monday, confirmed an upward trend in the last decade in ADHD diagnosis and medication among Canadian children. The researchers at the University of Montreal found that while preschoolers' rates of diagnosis have stabilized, diagnosis among school-aged children has climbed.

Schools are the frontlines of ADHD diagnosis, where teachers are often the first to identify when a student is struggling. Jack Kamrad, chief psychologist for the Peel District School Board in Ontario, said his staff receive a lot of referrals for students born late in the school year, especially in the youngest grades.

"Development is very fast in the early years and you can have a phenomenon where kids of equal ability but different stages of development are displaying different skills, and they just happen to be in the same grade," he said.

The B.C. study adds to a growing body of evidence that seems to suggest a similar pattern.

Mr. Morrow says this is a serious problem that needs to be addressed by the health-care system in order to protect vulnerable children. "It's important just to be aware of these relative age effects and the fact that a lack of maturity can sometimes lead to this diagnosis of neurobehavioural disorder," he said.

One of the risks of overdiagnosis of ADHD is putting children on medication unnecessarily, the study authors argue. Although many medications used to treat ADHD have been proven to be relatively safe in some clinical trials, others, such as second-generation anti-psychotics, which are increasingly being prescribed, may be more dangerous for some individuals. There's also issues of stigma attached to being singled out for having a disorder, the authors say.

But not everyone agrees with the conclusions of the study.

Declan Quinn, head of the division of child and adolescent psychiatry at the University of Saskatoon, argues the CMAJ study simplifies the complex issue of identifying and properly diagnosing ADHD in young people. Most people exhibit symptoms over a number of years; the real problem, Dr. Quinn suggests, is getting them the appropriate help and resources they need.

It's also quite possible that December babies aren't being overdiagnosed with ADHD; rather, the disorder may be going unrecognized in their older peers.

Lori King is a single mother in Victoria, and her oldest son, Shane, was diagnosed with ADHD in Grade 1. His birthday is in late December.

"I was told I should wait an extra to year to put my son in school, but he was smart enough; he just had trouble sitting still," she said.

Shane is now 19, and finishing high school. He struggled with ADHD throughout his schooling but was helped with medication. His mother ultimately feels that she made the right decision.

"I think [ADHD]has more to do with genetics than birthdays," she said.

Diagnosing ADHD can be a complex affair. There are no blood tests, and doctors must rely on determining whether a patient meets a set of often-arbitrary criteria before a diagnosis can be made. For this reason, it's possible that children born in early months of the year who have ADHD may go unrecognized because they don't display the reckless, disruptive or immature behaviour of their younger peers, says Alice Charach, head of the neuropsychiatry team and associate scientist at the research institute of Toronto's Hospital for Sick Children.

"Younger youngsters may be getting referred too frequently as opposed to the older ones not frequently enough," she said. "It's hard to know which side of that balance you want to fall on.

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