Great morning report case at the VA by both Rich Bloomingdale and Mathur Karan. Our discussion was about hyponatremia; euovolemic hyonatremia to be exact. The question is:
Q: Can hyponatremia due to SIADH be treated with isotonic saline?
A: The control of water and sodium excretion at the level of the kidney is independent (water by ADH and sodium by aldosterone / ANP). In SIADH, sodium excretion mechanisms are not affected. There is high ADH which causes antidiuresis, i.e. less water in urine, hence urine in SIADH has high osmolality. Isotonic saline has an osmolality of 308 mOsmol/Kg. According to this wildly cited 1986 article by Dr. Burton Rose:
To effectively raise the plasma sodium concentration, sodium can be given, but the osmolality of the administered fluid must exceed that in the urine. Since the urine osmolality usually is greater than 300 mOsmol/kg in the syndrome of inappropriate antidiuretic hormone secretion, there is essentially no role for the use of isotonic saline in this disorder. However, hypertonic saline (3 or 5 percent) can be given for severe (plasma sodium concentration below 115 meq/liter) or symptomatic hyponatremia.
There is a great example on UpToDate (linked).
So what happens if a SIADH patient gets started on 0.9% NaCl? It worsens the hyponatremia. Sodium will continue to be dumped out in urine while water is held back because of inappropriately high ADH. Also called “desalination phenomenon”. Yes, someone did try that before.
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