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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 and 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.
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.
A study by Giordano et al found a significant increase in the prevalence of hyponatremia in elderly patients visiting a university hospital emergency department during the summer. Prevalence during the summer was 12.5% (zenith) in the elderly, compared with a mean monthly prevalence of 10.3% in these patients. The investigators suggested that factors such as reduced renal function, salt loss, a decline in salt intake, and increased water ingestion may play a role in the increased prevalence of hyponatremia in the elderly during hot-weather months.
Similarly, a study by Huwyler et al found an increased incidence of adult patients with profound hyponatremia in a university hospital emergency department during the summer (1.29%, compared with 0.54% in the winter). Based on multivariate analysis, the investigators reported that the rise in incidence was related to patient age, the presence of psychiatric disorders, and the use of diuretics (either potassium-sparing or thiazide).