Attention deficit hyperactivity disorder (ADHD) is widely considered a condition that affects children, but some people may not develop it until they become adults, international researchers said Wednesday.
Two separate studies from Britain and Brazil, published in the Journal of the American Medical Association (JAMA) Psychiatry, suggest that late-onset ADHD may be its own distinct disorder, since many young adults diagnosed with ADHD did not have the disorder as children.
And when adults were diagnosed with ADHD, often the symptoms of inattentiveness, hyperactivity and impulsive behaviors were more severe than those seen in children, and tended to be accompanied by more traffic accidents, incarceration and criminal behavior, researchers said.
“Our research sheds new light on the development and onset of ADHD, but it also brings up many questions about ADHD that arises after childhood,” said co-author Louise Arseneault, a professor at Kings College London.
“How similar or different is ‘late-onset’ ADHD compared with ADHD that begins in childhood? How and why does late-onset ADHD arise? What treatments are most effective for late-onset ADHD? These are the questions we should now be seeking to answer.”
ADHD is believed to occur in about four percent of adults.
It is defined in clinical terms when a child under 12 shows at least six inattentive or impulsive behaviors that interfere with functioning and development for six months straight.
In the British study of more than 2,000 twins, a total of 166 individuals were found to have adult ADHD, and 68% of those “not meet criteria for ADHD at any assessment in childhood.”
The study measured ADHD in children based on mother and teacher reports collected at ages 5, 7, 10 and 12.
For adults, who were between 18 and 19 when studied, a diagnosis was derived following an interview in which subjects discussed their own symptoms and behaviors.
Researchers at King’s College London found among adults “a smaller group with persistent ADHD.” that endured from childhood.
Perhaps childhood-onset and late-onset adult ADHD have different causes, which “has implications for genetic studies and treatment of ADHD,” said the study.
Analyzing the twins’ data, researchers also found that adult ADHD was less likely to run in the family than childhood ADHD, and was almost as common in men as in women.
Typically, childhood ADHD is far more common in boys.
“We found that those with late-onset ADHD exhibit elevated rates of anxiety, depression, and marijuana and alcohol dependence,” added the study.
The Brazil study, which followed more than 5,000 people beginning in 1993, found very few adults (12%) with ADHD had been diagnosed as children, and very few children diagnosed with ADHD (17%) continued to have the syndrome as adults.
This suggests “the existence of two syndromes that have distinct developmental trajectories,” said the study.
Those who were diagnosed with late-onset ADHD showed “high levels of symptoms, impairment and other mental health disorders,” said the study.
More researchers is needed to uncover other factors that may contribute to adult ADHD, and whether it should be considered a distinct disorder, separate from childhood ADHD.
An accompanying editorial by Stephen Faraone of SUNY Upstate Medical University in Syracuse, New York and Joseph Biederman of Harvard Medical School in Boston, Massachusetts called the conclusions “provocative,” but “premature.”
The ADHD diagnoses in young adults were derived from self-reported symptoms, which are “less reliable” than reports by parents and teachers, they said.
“The adults in the Brazil and UK studies were aged 18 to 19 years. That is too small a slice of adulthood to draw firm conclusions.”
Instead, researchers should consider the data a “call to arms” to pursue more rigorous study of ADHD, and doctors who treat patients should be aware that adult-onset ADHD exists and deserves treatment -- even if the medical literature says it must be diagnosed by age 12.
“The current age-at-onset criterion for ADHD, although based on the best data available, may not be correct,” they wrote.
“We hope that future research will determine whether and how it should be modified.”