By Lisa Rapaport

(Reuters Health) - Attention deficit hyperactivity disorder (ADHD), usually diagnosed in children, may show up for the first time in adulthood, two recent studies suggest.

And not only can ADHD appear for the first time after childhood, but the symptoms for adult-onset ADHD may be different from symptoms experienced by kids, the researchers found.

“Although the nature of symptoms differs somewhat between children and adults, all age groups show impairments in multiple domains – school, family and friendships for kids and school, occupation, marriage and driving for adults,” said Stephen Faraone, a psychiatry researcher at SUNY Upstate Medical University in Syracuse, New York and author of an editorial accompanying the two studies in JAMA Psychiatry.

Faraone cautions, however, that some newly diagnosed adults might have had undetected ADHD as children. Support from parents and teachers or high intelligence, for example, might prevent ADHD symptoms from emerging earlier in life.

It’s not clear whether study participants “were completely free of psychopathology prior to adulthood,” Faraone said in an email.

One of the studies, from Brazil, tracked more than 5,200 people born in 1993 until they were 18 or 19 years old.

At age 11, 393 kids, or 8.9 percent, had childhood ADHD. By the end of the study, 492 participants, or 12.2 percent, met all the criteria for young adult ADHD except the age of diagnosis.

Childhood ADHD was more prevalent among males, while adult ADHD was more prevalent among females, the study also found.

Just 60 of the nearly 400 kids with ADHD still had symptoms at the end of the study, and only 60 of the nearly 500 adults with ADHD had been diagnosed as children.

“The main take-home message is that adult patients experiencing significant and lasting symptoms of inattention, hyperactivity or impulsivity that cause impairment should seek evaluation, even if they began recently by their perception or if family members deny their existence in childhood,” senior study author Dr. Luis Augusto Rohde, a psychiatry researcher at Federal University of Rio Grande Do Sul in Brazil said by email.

The second study focused on 2,040 twins born in England and Wales in 1994 and 1995. During childhood, 247 of them met the diagnosis criteria for ADHD. Of those, 54 still met the diagnosis criteria for the disease at age 18.

Among 166 individuals with adult ADHD, roughly one third didn’t meet the criteria for ADHD at any of four evaluations during childhood, the study also found.

It’s possible some of these adults had undiagnosed ADHD as kids, but symptoms may also look different in older people than they do in children, said senior study author Louise Arseneault of King’s College in London.

People with adult ADHD may have more inattentive symptoms like being forgetful or having difficulty concentrating, whereas children with ADHD may have more hyperactive symptoms, Arseneault said by email. “And if adults do experience hyperactive symptoms, these symptoms may manifest more as feelings of internal restlessness rather than obvious hyperactive behavior like running or climbing around in inappropriate situations,” she said.