Though fewer girls were diagnosed with attention deficit and hyperactivity disorder overall, girls born in December were about 70 per cent more likely to be diagnosed with ADHD than classmates born in January and 77 per cent more likely to be prescribed drugs for the condition.
The study compared children born in December and January because the two months sit on either side of the cut-off date for enrolling in Grade 1 in B.C. To start school, a child must turn six by Dec. 31. So children whose sixth birthday is in December and who just squeak in under the wire are nearly a year younger than their classmates born in January.
The sharply higher rates of ADHD diagnoses and prescriptions in children born in December begs the question of whether kids who are younger are being falsely labelled not because they have a neurological condition, but because they are immature in comparison to their classmates, suggests lead author Richard Morrow, a health research analyst with the Therapeutics Initiative at the University of British Columbia.
"What's clear from our study is that the younger children within a grade are much more likely to receive a diagnosis of ADHD and to receive a prescription drug for the disorder. And our interpretation of that is that it's the relative maturity of the children coming into play," said Morrow of the study, which appears in this week's issue of the Canadian Medical Association Journal.
"So in a sense the lack of maturity of some children is being misinterpreted in some cases as symptoms of a neurobehavioural disorder of ADHD. And that's what raises a concern about potential overdiagnosis and overprescribing."
The authors, from the Therapeutics Initiative, looked at medical records for children aged six to 12 who were born in B.C. from Dec. 1, 1997, to Nov. 30, 2008. There were 937,943 children in total.
They divided the children into groups by month of birth and looked at the percentage of kids in each month who were diagnosed as having ADHD and who were prescribed drugs for the condition. Children born in January had the lowest rates of diagnoses and prescriptions; from there on rates rose steadily through the year, peaking in October for boys and December for girls.
Morrow said the team suspects the fact that the rates in boys did not peak in December may relate to the fact that some parents will hold back a child they believe is too immature to start school.
The intent of the study was to test if something called "the relative age effect" is at play in Canadian schools. The idea is that the youngest children in any group may perform less well relative to the rest of the group.
U.S. studies have shown an association between being the youngest in a grade and being diagnosed with ADHD, but the Canadian researchers felt it was important to see if the effect exists here too.
Marie-Christine Brault, a PhD candidate at the University of Montreal who recently published a study on ADHD in young children, said she found the study results "really plausible."
Her study looked at rates of ADHD among preschool-aged children and children in primary school; it showed going to school appears to be associated with an increased risk of being diagnosed as having ADHD.
"It's hard for a child to adapt to the school setting," Brault said. "The younger the children the less mature they are. And it can make a big difference, especially with ADHD."
She and others noted a problem with ADHD is that there is no test for it. The diagnosis is a subjective one, based on observing a child and comparing his or her behaviour to others of his or her age.
"I think that there are kids with ADHD and these children need help. But maybe we identify too many of these children," Brault suggested.
Muhammad Mamdani, director of the Applied Health Research Centre at St. Michael's Hospital in Toronto, said the study cannot answer the question about whether children are being overdiagnosed. Because the researchers looked at medical records, not children, there's no way to tell if the children born late in the year who were diagnosed with ADHD actually had the condition or not.
He noted children who are diagnosed with ADHD are supposed to be reassessed from time to time, but he isn't sure how often that actually happens. That means children may acquire a label they cannot shake - and one that can have real consequences for them.
"When kids know that they're taking medication, when the parents know they're taking medication, when classmates, teachers know they're taking medication, that does impact not only their health but their perception of themselves," he said.
"I think it's really hard to get rid of that diagnosis when you have it," Brault said.
"We're raising this as an in issue of concern. But we would like it to contribute to a positive discussion of how ADHD could be approached differently in terms of assessment, or also in terms of making sure we try to meet children's needs within the classroom. And meet their needs for their developmental stage, which is sometimes somewhat independent of their grade," Morrow said.