Longitudinal Sodium Dynamics After Aneurysmal Subarachnoid Hemorrhage: A Single-Center Cohort Study with a Contemporary Literature Review.

Background and Objectives: Disorders of sodium homeostasis are common after aneurysmal subarachnoid hemorrhage (aSAH), but the prognostic value of longitudinal sodium dynamics remains unclear. We evaluated whether dynamic sodium parameters provide prognostic information beyond admission sodium concentration. Methods: This retrospective cohort study included 232 consecutive patients with angiographically confirmed aSAH admitted between January 2014 and December 2025. Serial serum sodium measurements were used to assess admission sodium, mean sodium, interday sodium variability, and hypernatremia burden. Associations with clinical outcomes were evaluated using multivariable regression, survival analysis, receiver operating characteristic analysis, and linear mixed-effects models. A landmark analysis was performed to assess the prognostic value of sodium variability during the first 5 days after admission. Results: Admission sodium concentration showed limited discrimination for 30-day mortality (area under the curve [AUC] 0.606, 95% confidence interval [CI] 0.509-0.703). Greater interday sodium variability was independently associated with mortality (odds ratio [OR] 1.55, 95% CI 1.29-1.86; p < 0.001) and remained significant after adjustment for severe hypernatremia (OR 1.32, 95% CI 1.09-1.60; p = 0.005). Adding interday variability to the baseline clinical model improved its discriminative performance (AUC 0.845 vs. 0.899; ΔAUC 0.054, 95% CI 0.010-0.099; p = 0.017). The association remained significant in the adjusted landmark analysis (hazard ratio [HR] 1.21, 95% CI 1.13-1.29; p < 0.001). Greater sodium variability was also associated with poorer neurological outcomes. Conclusions: Longitudinal sodium dynamics provide prognostic information beyond admission sodium concentration in patients with aSAH. Greater interday sodium variability was independently associated with 30-day mortality and remained robust after accounting for potential immortal-time bias. Prospective multicenter studies are warranted to validate its prognostic value and the cohort-derived variability threshold.