Syndrome of inappropriate antidiuretic hormone and cerebral salt wasting in critically ill patients.

S sodium levels in critically ill patients can be altered by many factors. The human body is 60% to 70% water, with approximately 30% of that water as extracellular fluid and sodium chloride as the major electrolyte (135-145 mEq/L). Hypo natremia occurs when a person’s serum sodium level is less than 135 mEq/L; it is the most common electrolyte abnormality among hospitalized patients, occurring in up to 30% of patients in the intensive care unit (ICU).1 A sodium level less than 125 mEq/L is an independent predictor of mortality, especially among critically ill patients, and should be avoided or corrected when it occurs.2 Syndrome of inappropriate antidiuretic hormone (SIADH) and cerebral salt wasting (CSW) represent a particularly challenging subset of hyponatremias. These conditions are exceedingly common in patients with an intracranial disorder and neurosurgical patients but also may be seen in other critically ill populations. In the neurosurgical population, 62% of hyponatremias are caused by SIADH, and 4.8% to 31.5% are caused by CSW.3 These 2 conditions are very similar and may be hard to differentiate in critically ill patients. This column outlines the unique characteristics of SIADH and CSW and provides guidance about management strategies for optimal outcomes.