What causes hyponatremia?

Updated: Aug 24, 2020
  • Author: Kartik Shah, MD; Chief Editor: Romesh Khardori, MD, PhD, FACP  more...
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Hypovolemic hyponatremia develops as sodium and free water are lost and replaced by inappropriately hypotonic fluids, such as tap water, half-normal saline, or dextrose in water. Sodium can be lost through renal or nonrenal routes. Nonrenal routes include GI losses, excessive sweating, third spacing of fluids (eg, ascites, peritonitis, pancreatitis, burns), and cerebral salt-wasting syndrome.

  • Excess fluid losses (eg, vomiting, diarrhea, excessive sweating, GI fistulas or drainage tubes, pancreatitis, burns) that have been replaced primarily by hypotonic fluids
  • Salt-wasting nephropathy

Cerebral salt-wasting syndrome seen in patients with traumatic brain injury, aneurysmal subarachnoid hemorrhage, and intracranial surgery. Cerebral salt-wasting must be distinguished from SIADH because both conditions can cause hyponatremia in neurosurgical patients, and yet the pathophysiology and treatment are different. [11]

Prolonged exercise in a hot environment, especially in patients who hydrate aggressively with hyposmolar fluids during exertion, is another cause of hyponatremia. Severe symptomatic hyponatremia has been reported in marathon runners and in recreational hikers in the Grand Canyon.

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