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KelRN215, BSN, RN

1 Article; 7,349 Posts

Specializes in Pedi.
The short way to think about this is that serum sodium is not telling you about sodium; it really is telling you about water balance. Low water balance (de-hydrat-ion) makes the sodium more concentrated-> higher serum sodium. Water excess dilutes serum sodium -> lower serum sodium.

You change these not by adding or withholding sodium, but by regulating the water.

So someone with hyponatremia is usually on a free water restriction to prevent further dilution.

Except, of course, in the case of cerebral salt wasting when the patient is actually hyponatremic because of excessive sodium loss. Fluid restriction doesn't correct the hyponatremia with this underlying process- though restricting fluid is usually the first step to distinguish between CSW and SIADH. I had a patient develop acute CSW once and I don't think the Resident had ever seen it before- it was in a child with rapidly progressing brain cancer. He continued with polyuria and persistent hyponatremia despite fluid restriction and was losing weight. I remember having to actually explain to the Resident how his clinical picture was not consistent with SIADH as he obviously wasn't retaining water. Poor kid, we spent days torturing him with Na+ checks q 4hrs (he had no permanent access at that point), finally diagnosed CSW and started him on Na+ supplements, he went to the OR and had a port placed for some reason or another (either because of the frequency of lab draws or because they were going to try some experimental chemo) and then he died a few days later (from the cancer, not the endocrine issues).

nurseprnRN, BSN, RN

1 Article; 5,115 Posts

Altogether true, and another reason why med students, when they hear hoofbeats, think "Zebra!" CSW is not a common condition by any stretch, but you see garden-variety water excess all the time. :)

(I know my colleague KelRN215 knows that!)

Dollars to doughnuts nobody will get a question on CSW in NCEX, but you will see one or three on low serum sodium.

Low serum sodium, water excess, means that water will be on the move from the area where there's lower sodium (the vascular space, which is where you got the serum to measure its low-ness) to an area of higher solute concentration (the cells). This is why you get cellular edema in garden-variety hyponatremia-- the cells swell up with the extra water entering them. This is particularly dangerous inside the rigid box where the brain lives, because cerebral edema can kill by making it too crowded in there to get decent blood flow to the most demanding and least adaptable organ the body has.

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