A pair of studies of the brain activity and hand movements of children with Attention Deficit Hyperactivity Disorder may point to more accurate ways to measure the cause and severity of their problems.
The research, conducted at the Kennedy Krieger Institute In Baltimore and the Cincinnati Children's Hospital Medical Center and published in the current issue of Neurology, the journal of the American Academy of Neurology, found that children with ADHD show lower ability to control voluntary and involuntary muscle movements than children without ADHD.
The findings provide more evidence for the neurological cause of the disorder, and may also provide the first objective way to predict the severity of a child's symptoms, which could lead to more accurate identification and treatment for ADHD, the most commonly diagnosed child behavioral disorder. Children with ADHD show developmental delays in balance, motor control and behavior, and frequently show difficulty concentrating, sitting still in class and otherwise behaving in age-appropriate ways.
"The problem right now is ADHD is something diagnosed based on symptoms ... but it's not something that's diagnosed physiologically," said research team leader Dr. Stewart Mostofsky, a pediatric neurologist and the director of the Laboratory for Neurocognitive and Imaging Research at the Kennedy Krieger Institute. Most current assessments for ADHD rely on qualitative measures, such as behavior checklists filled out by parents or teachers. "It's subjective. In most medical diagnosis, ideally you are able to measure physiologically."
In the first study, the researchers asked 50 right-handed children between the ages of 8 and 14—half of whom were diagnosed with ADHD—to perform a finger-tapping sequence in which they touched each finger to the thumb in order, alternating hands across 10 repetitions of the exercise. During the exercise, researchers tracked muscle movements on the child's opposite hand.
The researchers were looking for "overflow" movements, unintended and unneeded movements that accompany voluntary movements. Among these is the "mirror overflow," a strong tendency for a person to move bilaterally. It's a natural developmental stage, Dr. Mostofsky said, and it's "why young children have difficulty patting their head and rubbing their stomach at the same time."
Typical children start to grow out of this mirroring in their tween years, but as the researchers found, children with ADHD still show twice as many mirroring movements. In boys with ADHD, mirror movements were four times as high as typically developing children, which Dr. Mostofsky said could point to faster motor-skill development in girls.
In a second, related study, researchers used transcranial magnetic stimulation to deliver electromagnetic pulses to the motor cortices of 49 right-handed children with and 49 without ADHD, all in the same age range as the first group. "If I was to stimulate motor cortex with a [strong] pulse, you will get a twitch in the hand," Dr. Mostofsky explained, but a weaker pulse would activate the brain's response to inhibit involuntary movements. So, the researchers paired a weak pulse with a stronger one a few moments later, and measured the inhibition of actual muscle activity in the child's hand. Then, they compared this to the activity caused simply by a single strong pulse. The difference between the two measures is the "short-interval cortical inhibition," or SICI, and it measures how well the child can stop unintended movements.
As it turns out, children with ADHD had 40 percent lower mean SICI measures than their peers, and the lower their neurological inhibition, the more severe their behavioral symptoms.
"That decrease [in SICI] was fairly robustly predictive" of ADHD, Dr. Mostofsky said. "It suggests the problems children with ADHD have with controlling their actions, their behavior and concentration show up even at the level of basic motor control.... We now have some kind of quantifiable measure of this disinhibition."
The link between physical and mental control makes sense, he said. "It's thought that ... children with ADHD have difficulty inhibiting themselves from paying attention to more interesting things in their environment," he said. "When you have to pay attention, your ability to control attention is very closely related to your ability to control your actions. We move toward things we're interested in engaging in, even moving our eyes toward something we are interested in."
The SICI measure, he said, may eventually help researchers determine ways to identify children with ADHD early and measure whether different interventions are more effective for different groups of children with ADHD.