Hand movements may give clues to ADHD severity

WASHINGTON - Two studies have found markers for measuring the ability of children with Attention Deficit Hyperactivity Disorder (ADHD) to control impulsive movements, which may reveal insights into the neurobiology of ADHD, inform prognosis and guide treatments.

“Despite its prevalence, there is a lack of understanding about the neurobiological basis of ADHD,” said Dr. Stewart Mostofsky, the study’s senior author and Director of the Laboratory for Neurocognitive and Imaging Research at the Kennedy Krieger Institute in Baltimore.

In the first study, researchers looked at 50 right-handed children - 25 with ADHD and 25 typically developing, ages 8-12 years. Each subject completed five tasks of sequential finger-tapping on each hand. In this exercise, the children tapped each finger to the thumb of the same hand, in sequence. The tapping hand alternated between left-handed finger sequencing and right-handed finger sequencing.

Excessive mirror overflow, defined as unintentional and unnecessary movements occurring in the same muscles on the opposite side of the body, were measured using video and a device that recorded finger position.

These methods provided precise quantification of the amount of overflow movement, a major advance over prior studies that relied on qualitiative scales. During left-handed finger tapping, children with ADHD showed more than twice as much mirror overflow than typically developing children.

The differences were particularly prominent for boys with ADHD who showed nearly four times as much mirror overflow than typically developing boys on one of the two measures used in the study.

“This study used quantitative measures to support past qualitative findings that motor overflow persists to a greater degree in children with ADHD than in typically developing peers,” said Dr. Mostofsky.

“The findings reveal that even at an unconscious level, these children are struggling with controlling and inhibiting unwanted actions and behavior. Studying motor control weakness gives us a window to understanding the similar challenges that children with ADHD face in controlling more complex behavior, which can lead to improved diagnosis and treatment.”

In a second study, the researchers investigated motor control in children with ADHD further by measuring activity within the motor cortex, the part of the brain that controls voluntary movement.

Researchers used Transcranial Magnetic Stimulation (TMS) to apply mild magnetic pulses for brief durations to trigger muscle activity in the hand, causing hand twitches. Researchers performed 60 trials, with single or paired pulses to measure the level of muscle activity and monitored the resulting brain activity, called short interval cortical inhibition (SICI).

Overall, children with ADHD showed a substantial decrease in SICI, with significantly less inhibition of motor activity during the paired pulse stimulation compared to typically developing children. The degree of inhibition in children with ADHD, measured by SICI, was 40 percent less than typically developing children.

Furthermore, within the ADHD group, less motor inhibition (decreased SICI) correlated with more severe symptoms. The measure of SICI not only predicted motor impairment in ADHD children but also robustly predicted their behavioral symptoms as reported by parents. The findings suggest that reduced SICI may be a critical biomarker of ADHD.

The study has been published in Neurology, the medical journal of the American Academy of Neurology. (ANI)