Study links subtle motor signs in children to white matter structure, but not uniquely to ADHD

A new study published in Human Brain Mapping has found that subtle motor difficulties in children, such as problems with coordination and involuntary movements, are linked to differences in the brain’s white matter structure. However, these differences were not specific to children with attention-deficit/hyperactivity disorder (ADHD), suggesting that white matter organization plays a role in motor development across all children. The researchers observed that smaller fiber bundles in key brain pathways were associated with greater motor difficulties, but these white matter differences were present in children with and without ADHD alike.

ADHD is a neurodevelopmental condition characterized by persistent difficulties with attention, impulse control, and hyperactivity. It affects children and adults, often leading to challenges in academic, social, and daily life activities. While ADHD is primarily known for its effects on behavior and executive functioning, research has increasingly recognized that it also involves motor difficulties.

Many children with ADHD display subtle motor signs, such as trouble with coordination, involuntary movements, and difficulty performing precise motor tasks. These difficulties are not as severe as those seen in full-blown movement disorders but can still impact a child’s ability to complete everyday tasks smoothly. Despite their prevalence, the underlying brain mechanisms of these motor difficulties in ADHD are not well understood.

White matter plays a crucial role in brain function by serving as the brain’s communication network. It consists of nerve fibers, or axons, that connect different brain regions, allowing signals to travel efficiently between them. Unlike gray matter, which primarily contains nerve cell bodies, white matter is coated in a fatty substance called myelin that helps speed up electrical signals.

White matter pathways are essential for coordinating movement, integrating sensory information, and regulating higher-order cognitive functions. In typically developing children, white matter undergoes significant changes as the brain matures, improving motor control and cognitive abilities over time. Previous research has suggested that abnormalities in white matter may be linked to the motor difficulties seen in ADHD, but this connection had not been directly tested.

The rationale for this study was to investigate whether subtle motor difficulties in children with ADHD are associated with differences in white matter structure. Given that white matter pathways play a key role in motor control, the researchers aimed to determine whether variations in white matter organization could explain the increased presence of subtle motor signs in ADHD. They also wanted to test whether these differences were specific to ADHD or whether they were part of a broader pattern of motor development in children.

To examine this, the researchers recruited 92 children with ADHD and 185 typically developing children, all between the ages of 8 and 12. Each child underwent a standardized motor assessment called the Physical and Neurological Examination for Soft Signs (PANESS), which evaluates subtle motor difficulties through tasks like timed hand and foot movements, coordination tests, and assessments of involuntary movements.

In addition to these motor tests, the children completed brain imaging using diffusion MRI, a technique that maps the brain’s white matter pathways. The researchers applied a sophisticated analysis called fixel-based analysis, which allowed them to examine the size of fiber bundles in specific motor-related tracts. These included the corpus callosum, which connects the brain’s two hemispheres; the corticospinal tract, which controls voluntary movement; the superior longitudinal fasciculus, which is involved in coordination; and the fronto-pontine tract, which connects the motor cortex to deeper brain regions involved in movement planning.

The researchers found that children with ADHD had significantly more motor difficulties than typically developing children, as indicated by higher PANESS scores. However, when looking at white matter structure, they discovered that differences in fiber bundle size were associated with greater motor difficulties in both groups, rather than being specific to children with ADHD.

Lower fiber cross-section in the corticospinal tract was linked to worse overall motor performance. Similar patterns were seen in the corpus callosum and fronto-pontine tract when looking at specific timed movement tasks, such as hand and foot coordination exercises. Interestingly, these effects were consistent across both children with ADHD and typically developing children, meaning that white matter structure appeared to influence motor ability regardless of whether a child had ADHD or not.

No significant differences in white matter structure were found between children with ADHD and those without in the brain regions that were linked to motor difficulties. This suggests that while white matter plays an important role in motor function, it does not explain why children with ADHD have more pronounced motor difficulties compared to their peers.

These findings provide important insights into the neurological basis of motor development in childhood. They suggest that white matter structure contributes to subtle motor signs in children but is not uniquely altered in ADHD. This challenges previous assumptions that motor difficulties in ADHD are driven by distinct white matter abnormalities and instead highlights the possibility that broader developmental factors influence these differences.

However, the study had some limitations. Because it was cross-sectional, meaning it only measured children at one point in time, it could not determine whether these white matter differences cause motor difficulties or simply reflect normal variation in development. A longitudinal study following children over time would be needed to see how these white matter pathways change with age and whether they influence long-term motor outcomes.

Additionally, while the study used advanced imaging techniques to assess white matter structure, it focused on macroscopic differences in fiber bundle size rather than microstructural properties like fiber density or myelination. Future research using higher-resolution imaging might provide more insight into the finer details of white matter organization and its relationship to motor function.

The study, “Subtle motor signs in children with ADHD and their white matter correlates,” was authored by C. Hyde, I. Fuelscher, K. S. Rosch, K. E. Seymour, D. Crocetti, T. Silk, M. Singh, and S. H. Mostofsky.