Preoperative hyponatremia is associated with increased postoperative complications and short- to long-term mortality in geriatric hip fracture patients: a retrospective cohort study

BACKGROUND: Preoperative hyponatremia is common in elderly hip fracture patients, but its severity-dependent effects on postoperative complications and short-/long-term mortality, as well as the mediating role of postoperative factors, remain incompletely understood. METHODS: This retrospective cohort study included 1,901 consecutive patients aged ≥ 60 years who underwent hip fracture surgery between 2015 and 2021. Preoperative hyponatremia was categorized as mild (130-134 mmol/L) or moderate-to-severe (< 130 mmol/L). Multivariable logistic and Cox regression models were used to assess associations with postoperative complications, 30-day and 1-year mortality, with sequential adjustment for confounders. Mediation analyses quantified the contributions of postoperative hyponatremia and postoperative complications. RESULTS: Preoperative hyponatremia was present in 243 patients (12.8%): 191 (10.0%) had mild hyponatremia, and 52 (2.7%) had moderate-to-severe hyponatremia. After full adjustment, mild hyponatremia was associated with increased complication risk (OR 1.97, 95% CI 1.18-3.20) but not with 30-day mortality (HR 1.10, 95% CI 0.54-2.26). Moderate-to-severe hyponatremia was associated with both complications (OR 4.43, 95% CI 2.03-9.65) and 30-day mortality (HR 3.75, 95% CI 1.74-8.08). Mediation analysis revealed that postoperative complications and postoperative hyponatremia together accounted for 40% of the 30-day mortality risk; after further adjustment for these postoperative factors, the hazard ratio was attenuated (HR 2.25, 95% CI 1.10-4.98). For 1-year mortality, moderate-to-severe hyponatremia remained a significant predictor in the baseline-adjusted model (HR 2.07, 95% CI 1.21-3.53), but the association lost statistical significance after additional adjustment for postoperative factors (HR 1.47, 95% CI 0.86-2.54). CONCLUSIONS: Preoperative hyponatremia is a severity-dependent risk factor for postoperative complications and short-term mortality, with moderate-to-severe hyponatremia also associated with 1-year mortality through mediation by postoperative factors. Prevention of complications and correction of electrolyte disturbances may represent potential targets to improve early outcomes. The potential impact on long term survival, however, would likely be indirect and requires further prospective validation.