Hyponatremia is defined as a lower than normal serum sodium concentration using routine laboratory testing of serum electrolytes. Although chronic hyponatremia below 120 mEq almost universally prompts medical investigation, milder forms are usually overlooked or disregarded as being of little clinical significance. Recently, mild-moderate hyponatremia (i.e., a serum sodium concentration >125-135 mEq/L) has been associated with distinct symptoms as well as with increased mortality after hospitalization.
What causes hyponatremia? Many conditions are associated with chronic hyponatremia, the single most common electrolyte abnormality. These include inflammation in the brain or lung, cancers, thyroid disease, kidney disease, heart failure, and several categories of medications such as oral hypoglycemics (used in patients with diabetes mellitus), narcotics, diuretics, and antidepressants, particularly those agents referred to as selective serotonin receptor inhibitors (SSRI agents).
What are the consequences of mild hyponatremia? Recent epidemiologic evidence associates mild hyponatremia (average 131 mEq/L) with impairment of short-term memory as well as an abnormal gait. While both deficits are concerning, the risk of fall is significant (more than double compared to patients with a normal serum sodium), especially in the elder population at greatest risk for hip and femoral fractures.
Standard of Care: Serum sodium should be evaluated in all patients receiving routine laboratory testing for serum electrolytes, glucose and kidney function. The finding of a serum sodium concentration below 135 mEq/L should prompt inquiry for associated changes in short-term memory, as well as an evaluation of gait. If either is implied, a search for the cause of hyponatremia is important in normalizing these parameters, thereby potentially reducing the risk of short-term memory loss, the likelihood of falling due to an unsteady gait, and decreasing the risk of mortality, particularly in patients with underlying chronic disease conditions.
- Hufschmidt A, Shabarin V, Zimmer T. Drug-induced confusional states: the usual suspects? Acta Neurol Scand. 2009;120(6):436-8.
- Sandhu HS, Gilles E, DeVita MV, Panagopoulos G, Michelis MF. Hyponatremia associated with large-bone fracture in elderly patients. Int Urol Nephrol. 2009;41(3):733-7.
- Waikar SS, Mount DB, Curhan GC. Mortality after hospitalization with mild, moderate, and severe hyponatremia. Am J Med. 2009;122(9):857-65.