Updated July 2026 · 8 min read
Part of the ICU Emergencies Hub — browse every related guide in one place.
A low sodium in a brain-injured patient is a trap. Two different conditions — cerebral salt wasting and SIADH — produce nearly identical lab panels, yet their treatments point in opposite directions. Get the volume assessment wrong and you can restrict fluids in a patient who is already dry, worsening cerebral perfusion in exactly the population that cannot afford it. Telling them apart is one of the most clinically useful things a neuro ICU nurse can do.
Brain injury — subarachnoid hemorrhage, traumatic brain injury, tumors, meningitis — disrupts the systems that regulate water and sodium. SIADH is excess antidiuretic hormone, which makes the kidney retain free water and dilute the blood. Cerebral salt wasting is a primary renal loss of sodium (thought to be driven by natriuretic peptides), which drags water out with it. Both land the same low sodium on the morning labs, which is why the distinction is so easy to miss and so important to get right.
| Finding | SIADH | Cerebral salt wasting |
|---|---|---|
| Serum sodium | Low | Low |
| Urine sodium | High | High |
| Urine osmolality | Concentrated | Concentrated |
| Serum osmolality | Low | Low |
On paper they are nearly indistinguishable. This is exactly why you cannot resolve the question from the chemistry panel alone — you have to assess the patient.
| Clue | SIADH (retaining water) | Cerebral salt wasting (losing salt + water) |
|---|---|---|
| Volume status | Euvolemic / mildly up | Hypovolemic / dry |
| Urine output | Normal to low | High (polyuria) — often the tip-off |
| Fluid balance trend | Net even or positive | Net negative — ins < outs |
| Exam | Moist mucosa, no orthostatic signs | Dry mucosa, tachycardia, low CVP, weight loss |
Regardless of cause, the rate of correction matters as much as the direction. Chronic hyponatremia corrected too fast risks osmotic demyelination (central pontine myelinolysis), a devastating complication. So sodium is raised in a controlled way with frequent rechecks and a capped daily rise. The nursing role is to draw the serial sodiums on schedule, watch the trend against the ordered ceiling, and flag a correction that is moving too fast — not just too slow.
| Condition | Core treatment |
|---|---|
| SIADH | Fluid restriction; sometimes hypertonic saline for severe/symptomatic cases; address the trigger |
| Cerebral salt wasting | Replace salt and volume (isotonic or hypertonic saline, sodium supplementation); do not restrict fluids |
| Both | Correct at a controlled rate; frequent sodium checks; watch for over-rapid correction |
When a neuro patient's sodium drops, do not stop at the lab value — assess the volume. High urine output with a negative fluid balance and dry exam points to cerebral salt wasting, which needs salt and volume, not restriction. An even-or-positive balance in a euvolemic patient points to SIADH, which needs fluid restriction. Your intake-and-output flowsheet is what breaks the tie, and your serial sodiums keep the correction from moving too fast. In the SAH patient especially, getting this right protects the brain.
Related: Subarachnoid hemorrhage & vasospasm · ICP monitoring & EVD management · Status epilepticus · Sodium bicarbonate
Educational content for licensed clinicians. Always follow your facility's protocol and provider orders. Not medical advice.
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