The youngest boys and girls in a classroom are dramatically more likely to be diagnosed with — and prescribed drugs for — attention deficit hyperactivity disorder, a new study based on nearly a million B.C. children shows.
Boys who were born in December were 30 per cent more likely to receive a diagnosis of ADHD than boys born in January. The youngest girls, meanwhile, were 70 per cent more likely to be diagnosed with the brain disorder than the oldest, the analysis, published in this week’s edition of the Canadian Medical Association Journal, finds.
“Our study suggests younger, less mature children are inappropriately being labelled and treated,” lead author Richard Morrow, of the University of British Columbia, said in a statement released with the study.
“It is important not to expose children to potential harms from unnecessary diagnosis and use of medications.”
Children within the same grade can be almost one year apart in age. Yet only a handful of studies — and none of them outside of the U.S. — have examined what is known as the “relative-age” effect on the diagnosis of ADHD.
The UBC team wanted to see whether the same phenomenon exists in Canada.
A total of 937,943 children were included in the study. The team included children who were aged six to 12 at any time during the study period from 1997 to 2008.
In B.C., the cut-off for entry into kindergarten or Grade 1 is Dec. 31. Children with birthdays in mid- to late-December were compared to children born in early January.
The researchers found that the youngest and least mature children within the grade were at higher risk for treatment and diagnosis of ADHD at all ages, from six to 12 — a pattern that held for the duration of the 11-year study.
The researchers note in the CMAJ that “even children born during the three days before the end of the calendar year had a higher risk than children born during the first three days of the following year” — even though they were all born in the same season and within a six-day period.
Overall, girls were 77 per cent more likely, and boys were 41 per cent more likely, to be given a prescription for a medication to treat ADHD if they were born in December than if they were born in January.
“The potential harms of over-diagnosis and over-prescribing and the lack of an objective test for ADHD strongly suggest caution be taken in assessing children for this disorder and providing treatment,” the authors write.
Drugs used to treat ADHD can cause sleep problems, decreased appetite, anxiety and irritability. Concerns have also been raised over whether stimulant medications such as methylphenidate (the active ingredient in Ritalin) and amphetamines increase the risk of heart attack or stroke.
In 2006, Health Canada advised Canadians not to use ADHD drugs if they have high blood pressure, heart disease or abnormalities, hardening of the arteries or an overactive thyroid gland. The Canadian Paediatric Society has recommended that all children undergo a careful history and physical examination before being started on stimulant medications to check for risk factors for sudden death.
However, a study published last month involving more than 171,000 youth aged six to 21 found no link between stimulant use and an increased risk of heart-related events in otherwise healthy young people.
ADHD is the most commonly diagnosed behavioural disorder of childhood.
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