In this study, we explored changes in brain WM microstructural properties of drug-naïve children with ADHD. We found that, compared to HC children, drug-naive ADHD children showed significantly reduced FA in the right CC splenium, left IFOF, and the intersection of the anterior and posterior internal capsule, as well as increased RD in the right CC splenium, SLF, and PCR; however, there were no differences in AD and MD. These data supported the hypothesis that the function of long-distance connectivity in WM, particularly the connections among the occipital, temporal, and frontal lobes and the cingulum, are essential for the development of ADHD. We also confirmed that the higher the hyperactivity/impulsivity symptom score, the lower the FA value of long-range fibers in drug-naïve patients.
In the current study, we found reduced FA and increased RD in the right splenium of the CC. The CC is the largest WM bundle in the brain and is responsible for interhemispheric communication4. This tract connects the bilateral cerebral hemispheres, transferring excitatory and inhibitory signals14. The splenium of the CC connects interhemispheric somatosensory, auditory, occipital, and motor areas, which are important for visual object recognition and discrimination15. Moreover, the splenium of the CC influences the speed of visual information transmission and dynamic distribution of processing resources16–17. Thus, the deficits along the splenium pathway might be related to the inattention, distractibility, and visual dysfunctions observed among patients with ADHD. Decreased FA in the splenium of the CC is likely to result from axonal damage, or can be found in areas with decreased or delayed myelination. Moreover, RD has been suggested to indicate increased freedom of cross-fiber diffusion and reflect variations in myelination18. The impaired WM microstructure in the CC splenium might impede interhemispheric communication and activity in the posterior brain circuitries, particularly the occipital- and temporal-lobe-related pathways, and it might be a pathological component of attention dysfunction in ADHD.
In addition to the CC splenium, we found reduced FA in the left intersection of the anterior and posterior internal capsule; these observed differences in the posterior regions of the corona radiata are consistent with the findings of previous ADHD studies in childhood19. The posterior regions of the corona radiata are continuations of the posterior limb of the internal capsule to the sensorimotor cortex, and contain axons primarily involved in low-level motor functions. The internal capsule includes connections to cortical areas and subcortical areas, such as the brain stem, thalamus, and basal ganglia, in addition to connections with the corticospinal tract, which conducts nerve impulses from the motor cortex to the spinal cord20. A recent study has found that decreased FA in the right intersection of anterior and posterior internal capsule might contribute to sensorimotor deficits in adult ADHD21.
Our finding of reduced FA in the left intersection of the anterior and posterior internal capsule further underscores the possible role of dysfunctions within the visual system in ADHD. Changes in the corona radiata and internal capsule may explain the attention, visual processing, and motor function in ADHD. Higher RD is thought to reflect decreased myelination, while lower AD reflects axonal damage or degeneration. Our findings of decreased FA in the left intersection of the anterior and posterior internal capsule, as well as increased RD in the left PCR connections point to involvement of the basal ganglia in the pathophysiology of ADHD14, which is consistent with previous TBSS studies involving ADHD children and adolescents18, 21.
Furthermore, we found that the left part of the IFOF, which is a part of the SS, presented significantly lower FA in children with ADHD than in controls. The SS is a large WM tract that is oriented in a sagittal plane along the temporal occipital lobes and cingulum, and it has been associated with altered FA in patients with ADHD, according to voxel-based analyses and ROI studies14, 16. This tract contains fibers of the IFOF and ILF. The IFOF is a large WM tract connecting the frontal, temporal, and occipital lobes. The IFOF also constitutes one of the major efferent and afferent neuronal projections to the frontal lobes. Additionally, IFOF is implicated in attention-shifting abilities, which are deficient in individuals with ADHD17, 22–23. We found markedly decreased FA in the left IFOF in ADHD children; these changes in the IFOF may explain the attention, visual processing, and language problems involved in ADHD.
Moreover, as in previous studies24, we identified a large area of markedly higher RD in the right SLF in our study. The SLF is a WM bundle spanning the posterior and anterior regions of the cerebrum and contains connections to the frontal, temporal, parietal, and occipital lobes25. The association between the SLF and verbal and spatial working memory has already been reported in healthy children, adolescents, and young adults26–27. In addition, the left SLF also connects Broca’s and Wernicke’s areas, which are responsible for language comprehension and production. Increased RD is associated with dysmyelination of the nerve sheath in the left SLF, which might also be related to the visual and auditory deficits of ADHD children.
We found that the hyperactivity/impulsivity symptom severity was negatively correlated with FA values in the right CC splenium, left IFOF, and the intersection of the anterior and posterior internal capsule. This result was consistent with previous studies that have shown that WM differences in ADHD are related to disease severity26–27. Given that previous studies in young children with ADHD suggested reduced or delayed myelination, we surmise that our results may reflect alterations of myelination in ADHD. The regions in which there were significant between-group differences in our study were similar to those reported in the previous diffusion studies in ADHD25.
Our study had several limitations. Our sample size was relatively small, resulting in reduced power to examine other potential confounding effects on the DTI metrics. This was because of the strict selection criteria we had adopted. A further study of drug-naive children, without a history of other psychiatric disorders, could provide more reliable results. Additionally, further longitudinal studies involving the same patient cohort would be required to establish the generalizability of our findings. In addition, TBSS has limitations in analyzing small fiber tracts, regions of crossing fibers, and tract junctions. In later studies, we hope to be able to combine the data of structural and functional data with the help of deep learning to further explore the changes in the brain networks of ADHD children.