SIADH vs Diabetes Insipidus and Cerebral Salt Wasting
How do DI, SIADH and cerebral salt wasting differ?
DI concerns impaired vasopressin-mediated water conservation. SIADH involves inappropriate antidiuresis in a hypotonic setting after other causes are excluded. Cerebral salt wasting describes renal sodium loss with volume depletion in intracranial disease. Similar serum or urine concentrations do not establish the same water balance, sodium loss or diagnosis.
Key takeaways
- Serum sodium depends on water intake and losses as well as hormone action.
- Urine sodium concentration is different from the amount of sodium lost.
- SIADH and salt wasting cannot be separated by one concentrated urine sample alone.
In this video
- DI, SIADH and cerebral salt wasting
- Sodium is a concentration
- How ADH saves water
- DI: AVP deficiency and resistance
- Why DI can have a normal sodium
- Other reasons for high urine output
- SIADH and diagnosis by exclusion
- Salt wasting and volume depletion
- Urine sodium concentration versus amount
- Neuro patient: reassess the new evidence
- Why volume is hard to assess
- CRNA interview reasoning
- Three practice questions
- Recap and close
Can DI occur with normal sodium?
Yes. A person with impaired water conservation may maintain sodium through sufficient intake. Vasopressin deficiency and resistance can produce dilute urine, but the serum response depends on thirst, access to water and ongoing intake and losses.
Current terminology also uses arginine vasopressin deficiency and resistance. The familiar DI term remains useful for recognition, provided the underlying water-handling problem is explained.
Why can SIADH and salt wasting look similar in a sample?
Both can be associated with low serum sodium and concentrated urine. The reason for antidiuresis differs: volume depletion can appropriately stimulate vasopressin in salt wasting, while SIADH is inappropriate antidiuresis in its diagnostic context. SIADH requires exclusion of other explanations, including relevant adrenal, renal and medication effects.
Cerebral salt wasting additionally requires renal sodium loss and volume depletion in the neurological context. Volume assessment is imperfect and the diagnosis can be uncertain, so a label should not be inferred from a single urine sodium concentration.
Why does amount lost differ from concentration?
The amount of sodium excreted depends on urine concentration multiplied by urine volume over time. Two patients with the same spot concentration can lose very different amounts when their urine outputs differ.
Paired serum and urine findings, intake and output, weight, medications and the clinical course help explain the balance. This comparison does not prescribe fluid restriction, replacement, desmopressin or a sodium-correction rate.
