Plasma sodium levels are related to resting motor threshold in healthy humans

Electrolyte homeostasis is essential for normal neuronal function and clinically relevant imbalances can provoke neurological symptoms, including seizures. However, whether electrolyte variations within the normal physiological range affect cortical excitability remains unclear. In this exploratory secondary analysis, we examined baseline data from 42 healthy participants enrolled in a previously conducted clinical trial. We found a significant correlation between plasma sodium levels within the normal physiological range (136–143 mmol/L) and resting motor threshold (r = 0.47, p = 0.002), an indirect index of cortical excitability, suggesting that lower sodium concentrations may be associated with increased cortical excitability. No significant associations were observed for chloride, potassium, phosphate, or calcium. These preliminary findings raise the possibility that subtle interindividual differences in plasma sodium levels are related to variability in corticospinal excitability. Given that this was a non-prespecified, secondary analysis, further controlled studies are warranted to confirm the association and investigate the underlying mechanisms.