The results of this study indicated that screen exposure (television, mobile phones) had a statistically significant causal effect on the onset of ADHD. The conclusion of this study contributed to a further understanding of the etiology of ADHD, providing important references for developing prevention strategies for ADHD and identifying potential intervention measures.
Previous studies have shown that screen exposure may have an impact on the cognition, language, and social emotions of children and adolescents. The study shared that the time of watching television and using mobile phones had a risk of ADHD. A survey study from a longitudinal and cross-sectional in children (n = 596) [22] and a cross-sectional in young adults (n = 408) also confirmed this conclusion [23] that there was an association between early screen exposure and neurodevelopmental disorders. The previous study showed that individuals with ADHD began to be exposed to electronic screens earlier and for more extended periods, which easily affected their cognition development. Specifically, the ADHD population typically begins to be exposed to electronic screens during adolescence; they spend an average of 2 hours per day using electronic devices [24, 25]. Through a retrospective questionnaire survey, it was found that prolonged screen exposure increases the risk of ADHD [26]. Therefore, in order to promote the healthy development of cognitive and social abilities in children and young adults, the use of electronic devices by infants and young children should be limited.
An individual's genotype and phenotype had a causal relationship. However, there is a risk bias in the commonly used methods of estimating genotype-phenotype associations based on unrelated individuals. Factors such as population stratification and selection of sex can correlate between genotype and phenotype, which in turn affects the evaluation of causal relationships [27]. There was heterogeneity in the instrumental variables of TV viewing time and mobile phone usage, which may come from different analysis platforms, different experiments, or different populations. Therefore, future research should consider re-validating causal relationships through further single-sample MR methods in prospective ADHD cohorts.
Although the exact pathological mechanism between screen exposure and ADHD is not yet precise, some theories supply possible explanations. The "substitution hypothesis" suggests that screen exposure can attract children's attention, thereby replacing developmental learning opportunities. The present study found that the OR value of the incidence of attention deficit symptoms increases with the increased time of watching TV and using mobile phones. The results of a series of studies by Mortaza SM et al. that found a significant correlation between mobile phone use and attention deficit were consistent [8, 28]. Attention deficit is related to the abnormal structure and function of the prefrontal cortex circuit in the brain [29, 30]. Aalto et al. found that when mobile phone use was close to the human head, local blood flow near the prefrontal cortex of the brain was affected, which may be due to an increase in the incidence of attention deficit symptoms caused by mobile phone use [31]. The other potential reason was that the rapid parallel processing of visual information in screen exposure may hinder children's ability to think independently, leaving them with insufficient time to understand and reflect on the social context they encounter [32]. These theories were consistent with the results of this study, which suggested that early screen exposure might increase the risk of ADHD.
At the molecular level, more and more genes have been identified to be associated with ADHD. The forkhead box p2 (FOXP2) locus encodes a transcription factor expressed in the brain, closely related to the human ability to communicate through complex speech [33]. The FOXP2 gene discovered by GWAS was the only gene that overlaps with ADHD and telephone use [34]. The FOXP2 expression is related to sex hormone levels, which are related to various aspects of human speech. Due to the difference in FOXP2 expression, the incidence rate of male and female patients is different. Dopamine is considered the primary neurotransmitter involved in the pathophysiology of ADHD [35]. Strong evidence suggests that patients with ADHD have reduced dopamine metabolism, particularly related to impulsivity. However, playing video and computer games was observed to increase the release of dopamine in the striatum [36]. The random effects IVW method showed no correlation between time of using computers and ADHD in the study. The above results indicated that although some overlapping genes were found between screen exposure and ADHD, the role of genes in causing symptoms of ADHD was still controversial, which needs to be explored in the future.
Although this study has a large sample size and sufficient statistical power, the following shortcomings need to be considered: 1.The samples for this study were from European countries, so these conclusions might not be applicable to the people from other countries. 2. IVs were based on adult screen exposure choices, which could not be necessarily suitable for adolescents or children. 3. The impact of heterogeneity cannot be completely eliminated, which may be attributed to the complex and unclear biological functions of many genetic variations. 4. GWAS can provide new insights into ADHD related genes; further research needs to explore the molecular biology of ADHD.