Association of Serum Sodium Levels and Delirium in Patients with Sepsis: A Retrospective Study

Background: This study aims to elucidate the relationship between serum sodium and the risk of sepsis-associated delirium (SAD), with particular emphasis on the critical threshold of 138.4 mmol/L. Methods: The retrospective study utilized data from the MIMIC-IV database. The analysis focused on serum sodium concentrations measured within the first 24 h of ICU admission. The association between sodium levels and the risk of delirium was assessed using restricted cubic spline (RCS) analysis and multivariable logistic regression. Subgroup analyses and propensity score matching (PSM) were used to mitigate potential confounding factors. Results: A total of 7356 septic patients were included, with 1861 (25.3%) developing delirium. RCS analysis revealed a significant non-linear relationship between sodium levels and delirium risk, with a threshold at 138.4 mmol/L. Sodium levels ≤ 138.4 mmol/L were associated with a reduced risk of delirium (OR 0.97, 95% CI: 0.95-0.99, p = 0.041), while levels > 138.4 mmol/L significantly increased the risk of delirium (OR 1.08, 95% CI: 1.06-1.11, p < 0.001). After PSM, hypernatremia was associated with a higher delirium incidence than hyponatremia (55.00% vs. 36.67%, p = 0.044) and remained an independent risk factor for delirium in logistic regression (OR 2.89, 95% CI 1.17-7.18, p = 0.022). Conclusions: This study identified a non-linear, threshold-dependent association between serum sodium and delirium susceptibility in septic patients, with 138.4 mmol/L as a critical tipping point. Hypernatremia emerged as a more potent risk factor for delirium compared to hyponatremia. These findings underscore the importance of sodium management in septic patients and suggest that serum sodium may serve as a potential biomarker for predicting neuropsychiatric outcomes in sepsis.