Salt wasting syndrome in brain trauma patients: a pathophysiologic approach using sodium balance and urinary biochemical analysis

Abstract

Background: To explore the underlying mechanisms leading to the occurrence of hyponatremia and enhanced urinary sodium excretion in brain trauma patients using sodium balance and urinary biochemical analysis.

Methods: We conducted a retrospective analysis of a local database prospectively collected in 60 brain trauma patients without chronic renal dysfunction. Metabolic and hemodynamic parameters were averaged over three consecutive periods over the first seven days after admission. The main outcome investigated in this study was the occurrence of at least one episode of hyponatremia.

Results: Over the study period, there was a prompt decrease in sodium balance (163 ± 193 vs. -12 ± 154 mmol/day, p < 0.0001) and free water clearance (- 0.7 ± 0.7 vs. -1.8 ± 2.3 ml/min, p < 0.0001). The area under the ROC curves for sodium balance in predicting the occurrence of hyponatremia during the next period was 0.81 [95% CI: 0.64-0.97]. Variables associated with averaged urinary sodium excretion were sodium intake (R² = 0.26, p < 0.0001) and fractional excretion of urate (R² = 0.15, p = 0.009). Urinary sodium excretion was also higher in patients with sustained augmented renal clearance over the study period (318 ± 106 vs. 255 ± 135 mmol/day, p = 0.034).

Conclusion: The decreased vascular volume resulting from a negative sodium balance is a major precipitating factor of hyponatremia in brain trauma patients. Predisposing factors for enhanced urinary sodium excretion were high sodium intake, high fractional excretion of urate and augmented renal clearance over the first seven days after ICU admission.

Keywords: Augmented renal clearance; Brain trauma; Hyponatremia; Intensive care; Salt wasting syndrome.

Fig. 1

Comparison of sodium balances in patients who presented or not a first episode of hyponatremia (a) during the intermediate period [day 3–4] or (b) during the late period [day 5–7]. Dark grey: patients with hyponatremia; light grey: control group

Fig. 2

Ability of sodium balance during one period to predict the occurrence of hyponatremia the following period: ROC curve andTwo-curve graph showing the sensitivity and specificity of the different values of sodium balance to predict the occurrence of hyponatremia the following period; the inconclusive grey zone is displayed as a grey rectangle for a sodium balance between − 130 and + 100 mmol/day