Mortality and serum sodium: do patients die from or with hyponatremia?

BACKGROUND AND OBJECTIVES: Severe hyponatremia (<120 mEq/L) in hospitalized patients has a high mortality rate. We hypothesized that underlying diseases causing hyponatremia attribute to mortality rather than hyponatremia itself.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS: The relationship between mortality and serum sodium (sNa) was examined in 45,693 patients admitted to a single community teaching hospital between January 1996 and December 2007. We conducted a comprehensive retrospective review of the medical records of 53 patients who died after developing sNa <120 mEq/L before or after admission and of 32 patients who survived after developing sNa <110 mEq/L.

RESULTS: Mortality rates tended to increase as the sNa fell from 134 to 120 mEq/L, rising above 10% for patients with sNa of 120 to 124 mEq/L. However, below sNa of 120 mEq/L, the trend reversed, such that the mortality rate progressively decreased as sNa fell. More than two thirds of patients who died after sNa <120 mEq/L had at least two additional acute severe progressive illnesses, most commonly sepsis and multiorgan failure. Three deaths (5.6%) in 12 years could plausibly be related to adverse consequences of hyponatremia, and one (1.8% of the fatal cases and 0.15% of all patients with sNa <120 mEq/L) was from cerebral edema. Most patients who survived with sNa <110 mEq/L had medication-induced hyponatremia. Severe underlying illnesses were uncommon in this group.

CONCLUSIONS: The nature of underlying illness rather than the severity of hyponatremia best explains mortality associated with hyponatremia. Neurologic complications from hyponatremia are uncommon among patients who die with hyponatremia.