What are the mortality and morbidity associated with hyponatremia?

Updated: Aug 24, 2020
  • Author: Kartik Shah, MD; Chief Editor: Romesh Khardori, MD, PhD, FACP  more...
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Pathophysiologic differences between patients with acute and chronic hyponatremia engender important differences in their morbidity and mortality.

Patients with acute hyponatremia (developing over 48 h or less) are subject to more severe degrees of cerebral edema for a given serum sodium level. The primary cause of morbidity and death is brainstem herniation and mechanical compression of vital midbrain structures. Rapid identification and correction of serum sodium level is necessary in patients with severe acute hyponatremia to avert brainstem herniation and death.

Patients with chronic hyponatremia (developing over more than 48 h) experience milder degrees of cerebral edema for a given serum sodium level. Brainstem herniation has not been observed in patients with chronic hyponatremia. The principal direct causes of morbidity and death are status epilepticus (when chronic hyponatremia reaches levels of 110 mEq/L or less) and cerebral pontine myelinolysis (an unusual demyelination syndrome that occurs in association with chronic hyponatremia and its rapid correction).

The distinction between acute hyponatremia and chronic hyponatremia has critical implications in terms of morbidity and mortality and in terms of proper corrective therapy.

A study of 98,411 hospitalized patients found that even mild degrees of hyponatremia were associated with increased in-hospital, 1-year and 5-year mortality rates. Mortality was particularly increased in those with cardiovascular disease, metastatic cancer, and those undergoing orthopedic procedures. [6]

Similarly, a study by McCarthy et al found that patients with lower sodium levels at emergency admission tended to have a longer hospital stay than did those with normal sodium concentrations (6.8 vs 4.9 days, respectively), with the hyponatremic patients also having a higher 30-day inhospital mortality rate (6.4% vs 4.4%, respectively). [7]

A study in Copenhagen concluded that hyponatremia in the range of 130-137 mEq/L is also associated with increased mortality rates in the general population. [8]

In the aforementioned Swiss-Austrian study, hyponatremia and severe hypernatremia independently increased mortality risk in emergency patients with AKI. [3]

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