研究：ADHD儿童脑铁水平或与开始服药年龄有关

研究关注的问题

ADHD与多巴胺信号减弱有关，而脑铁是多巴胺合成酶酪氨酸羟化酶发挥作用的必要成分，也参与髓鞘生成。此前研究显示，从未服药的ADHD儿童脑铁往往低于同龄人，而服用精神兴奋剂的儿童铁水平接近神经典型儿童，这引出了药物是否随时间影响脑铁的疑问。

研究设计与主要发现

澳大利亚珀斯儿童与青少年健康服务中心的Hugo A.E. Morandini团队利用俄勒冈ADHD-1000公开数据，选取29名持续服用精神兴奋剂的ADHD儿童和38名无精神科诊断的神经典型儿童，在基线和两年后分别进行磁共振扫描，并用专门技术估算脑组织铁浓度，重点分析苍白球、壳核、尾状核、丘脑和黑质等富含多巴胺与铁的区域。

两组比较中，黑质、丘脑和大部分基底节区域的铁水平差异不具统计显著性，但ADHD儿童苍白球铁含量更低。作者认为，本研究中用药暴露的平均时长比过去研究短，可能解释这一差异。

用药年龄与时长

在29名ADHD儿童中，用药总时长与脑铁几乎无统计关系，仅丘脑出现极小的负向关联，作者判断其不太可能有临床意义。按开始服药年龄分组后，八岁及以前开始用药的儿童在丘脑、尾状核、壳核和苍白球的铁含量高于九岁及以后开始者。研究者提出，儿童期脑铁与多巴胺系统快速发育，较早用药可能与此类发育窗口相交，使多巴胺系统对外部影响更敏感。

需要保留的限制

该分析样本量较小，观察到的模式需要进一步验证；研究为观察性关联，不能据此推断用药年龄导致脑铁差异。

Children with attention deficit hyperactivity disorder who begin taking stimulant medications at a younger age may develop different brain iron levels than those who start treatment later. A new analysis of brain imaging data suggests that starting medication by age eight is associated with higher iron concentrations in dopamine-rich brain regions compared to starting at age nine or later. The research was published in the journal Brain Research.

Attention deficit hyperactivity disorder, commonly known as ADHD, is a neurodevelopmental condition characterized by inattention, impulsivity, and hyperactivity. These symptoms often create challenges with everyday tasks and social interactions. The condition is closely tied to reduced signaling of dopamine, a chemical messenger in the brain that helps regulate movement, attention, and reward. To manage symptoms, doctors routinely prescribe psychostimulants, which work by increasing the amount of dopamine available in the central nervous system.

Brain iron plays an essential role in this process because it is a necessary ingredient for the brain to manufacture dopamine. Tyrosine hydroxylase, the enzyme responsible for creating dopamine, relies entirely on iron to function properly. Iron also aids in the production of myelin, the protective sheath that insulates nerve fibers and speeds up communication between brain cells.

Because of this relationship, iron tends to concentrate heavily in dopamine-rich regions, such as the basal ganglia and the substantia nigra. These deep brain structures are heavily involved in motor control and learning. As a child grows into adolescence, iron levels in these areas typically increase alongside the maturation of the dopamine system.

Prior research has shown that children with ADHD who have never taken medication often have lower levels of brain iron compared to their peers without the condition. At the same time, some brain imaging research suggests that children treated with psychostimulants have iron levels similar to those of neurotypical children. This pattern raised the question of whether stimulant medications directly influence brain iron levels over time.

Hugo A.E. Morandini, a researcher at the Child and Adolescent Health Service in Perth, Australia, along with a team of colleagues, wanted to better understand how psychostimulant treatment relates to brain iron. They designed a study to track iron levels over a two-year period. They specifically looked at how both the duration of treatment and the age a child started taking medication might relate to iron concentrations.

To conduct the research, Morandini and his team used brain imaging data from the Oregon ADHD-1000 dataset, a public repository of clinical information. They identified 29 children with ADHD who were consistently taking psychostimulant medications and 38 neurotypical children with no psychiatric diagnoses. The children underwent magnetic resonance imaging scans at the beginning of the study and again two years later.

The researchers used a specific type of scan called functional magnetic resonance imaging. While this technology is often used to map blood flow, the team analyzed the data using a specialized technique that estimates the concentration of iron in brain tissue based on its magnetic properties. They focused their analysis on several brain regions rich in dopamine and iron, including the globus pallidus, putamen, caudate nucleus, thalamus, and substantia nigra.

First, the team compared the iron levels of the children with ADHD to the neurotypical children across the two-year period. The researchers adjusted their statistical models to account for variables like age, sex, and intelligence quotient. They found that differences in iron levels in the substantia nigra, thalamus, and most of the basal ganglia were not statistically significant between the two groups.

However, the children with ADHD had lower iron content in the globus pallidus. This structure is a key part of the basal ganglia that helps regulate dopamine circuitry and reward behaviors. The authors noted that a shorter average duration of medication exposure in their study compared to past research might explain why globus pallidus iron had not reached the levels seen in neurotypical children.

Next, the researchers looked exclusively at the 29 children with ADHD to see how their medication histories related to their brain iron. They calculated the total time each child had been taking psychostimulants and noted the age at which they first started the treatment. The researchers then tested whether these factors predicted iron levels across the different brain regions.

The length of time a child had been taking stimulants had almost no statistical relationship with brain iron. The researchers found a very small negative association between treatment duration and iron in the thalamus, but they noted this effect was so minor it is unlikely to have any clinical meaning. In all other regions, the models for treatment duration were not statistically significant.

The age at which a child began treatment showed a different pattern. The team split the children with ADHD into two subgroups based on when they started medication. One group began treatment at age eight or younger, while the other began at age nine or older.

The analysis revealed that children who started taking psychostimulants at an earlier age had higher iron content in the thalamus, caudate nucleus, putamen, and globus pallidus. The researchers noted that the brain undergoes rapid developmental changes in iron and dopamine during childhood. They proposed that starting medication earlier in childhood might intersect with these developmental windows, making the dopamine system more sensitive to external influences.

The findings come with several limitations that affect how the results should be interpreted. The analysis relied on a relatively small number of participants, meaning the patterns observed need to be confirmed in larger populations. A larger study could provide more precise estimates of how medication timing interacts with brain development.

The researchers also had to rely on parent reports to determine when the children began taking medication and how long they had been treated. Parent recall can sometimes be imprecise, which could introduce errors into the timelines. The study also lacked information on the exact dosages the children were taking and how strictly they adhered to their daily medication schedules.

Because the researchers observed existing medication habits rather than assigning treatments, the results establish an association rather than a direct cause and effect. It is possible that children prescribed medication at a younger age have more severe symptoms, which could be tied to underlying differences in brain biology. The researchers could not measure the children’s brain iron before treatment started, making it difficult to isolate the exact impact of the drugs.

Future research will need to follow patients from before they start medication into adulthood to fully chart the effects of early treatment. Tracking these changes over a longer lifespan could help researchers understand whether the differences in brain iron persist as patients grow older.

The study, “Neurochemical impact of psychostimulants: Longitudinal MRI analysis of brain iron content in children with ADHD,” was authored by Hugo A.E. Morandini, Sjoerd B. Vos, Paola Chivers, Tim J. Silk, and Pradeep Rao.