Supplementary Material for: The Role of Heavy Metals as a Biomarker in the Pathogenesis of Dyslexia and Specific Learning Disabilities: A Systematic Review

Introduction: Among the possible risk factors for dyslexia and specific learning disabilities (SLD) in children, being exposed to heavy metals in the environment has been considered a significant one, serving as a biological marker. The precise mechanisms by which heavy metals affect cognitive functions were discussed. However, their shared affinity for NMDA receptors or the Na+/K+ ATPase pump can disrupt the balance between reactive oxygen species (ROS) and antioxidants. Several studies have scrutinized the effect of heavy and toxic metals on dyslexia. Consequently, this research aimed to systematically assess the most recent evidence of the effects of heavy metals on SLD and dyslexia.
Methods: Four databases (i.e., Scopus, Web of Science, Google Scholar, and PubMed/Medline) were searched; 450 studies published up to 2024 were found. After applying inclusion/exclusion criteria, five studies were selected, and data regarding the participants’ age, measurement instruments, the year of the studies, and the outcomes were extracted and analyzed.
Results: Generally, the total sample size in the five studies was 1909 individuals. These studies examined 25 heavy metals, with selenium (Se), copper (Cu), lead (Pb), zinc (Zn), and nickel (Ni) showing consistent associations with dyslexia and SLD. Metals such as aluminum (Al), arsenic (As), antimony (Sb), cadmium (Cd), cobalt (Co), mercury (Hg), iron (Fe), and manganese (Mn) were cited only once. Notably, heavy metals such as Pb, Al, Sb, and Cd had a definitive role in the neuropathogenesis of SLD.
Conclusion: These findings highlighted the importance of implementing environmental control measures to minimize children’s exposure to toxic heavy metals.