SIADH Treatment

[dyspareunia]

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Why treat with hypertonic saline? Isn't normal saline more hypertonic than the TBW of someone with SIADH? So wouldn't normal saline work to increase serum Na+ and also carry a lower risk of CPM?

 

[Transposony]

You treat SIADH with hypertonic saline only in patients with symptoms.
Otherwise, you do treat them with N saline.
Think of it this way-You want to give as little water to someone who is already overloaded with water.
Remember, SIADH patients retain H2O but they do not have problem excreting Na.

P.S. FA is a great book for review but not for learning.

 

[PB n J]

You treat SIADH with hypertonic saline only in patients with symptoms.
Otherwise, you do treat them with N saline.
Think of it this way-You want to give as little water to someone who is already overloaded with water.
Remember, SIADH patients retain H2O but they do not have problem excreting Na.

P.S. FA is a great book for review but not for learning.

SIADH actually causes a euvolemic hyponatremia. This is very important to know about this clinical condition. Many sources say that it may evolve into a slight hypervolemic hyponatremia, but for boards, remember euvolemia.

Free water restriction is first line. NS is also given if Na+ is considerably low; demeclocycline may also be used. 3% NS is given with symptoms (vomiting, lethargy, deliurium, confusion, coma) or Na+ <120, but sodium again is corrected no more than 8-12 mEq/L/24hr in an attempt to lessen the propensity for fluid shifts (the aforementioned dreaded CPM causing locked in syndrome; I've seen it and it is HORRIFIC), not because the patient is volume overloaded.

 

[Transposony]

SIADH actually causes a euvolemic hyponatremia.

Yes. That was discussed here.
The point I was trying to make was that SIADH patients are already overloaded with water relative to Na and you want to give them as little water as possible especially when they are symptomatic. That's the reasoning behind giving hypertonic saline. Otherwise in asymptomatic patients NS works fine.
The treatment also depends on whether the hyponatremia is relatively acute or chronic.

 

[Donald Juan]

I was taught you don't give NS because the patient will excrete the sodium but keep the water since they are concentrating their urine, therefore NS can act to exacerbate the condition. Just for example, the salt you give them in 1 L they can urinate out in .5 L, leaving them a gain of .5 L free water.

 

[Modeselektor]

It actually depends on the presence of symptoms or not:

Asymptomatic: Restrict fluids
Minimal confusion: Normal saline + Diuretic
Lethargy, Seizure or Coma: Hypertonic saline, conivaptan/tolvapatan (ADH antagonists)

For the purposes of step 1 they will most likely give you the symptoms, labs, and then ask you to pick one of those options amidst wrong options like amiloride, hypotonic saline to ensure you recognize/understand the correct pathophysiology and mechanism-based treatment. (Choice of treatment depending on severity is a step 2 objective and it is very unlikely you will see two correct treatments in a question unless there is a contraindication for one like g6pd def in the stem). They may also have lithium and demeclocycline listed as viable options.

 

[Transposony]

I was taught you don't give NS because the patient will excrete the sodium but keep the water since they are concentrating their urine, therefore NS can act to exacerbate the condition. Just for example, the salt you give them in 1 L they can urinate out in .5 L, leaving them a gain of .5 L free water.

You are absolutely right.
Giving NS without a Loop diuretic will make it worse.
However, that's encroaching into CK territory.
For Step 1:
Fluid restriction is the first step.
Severe hyponatremia (Na <115 meq/L) or mental status changes (seizure etc) require treatment with hypertonic (3%) saline infusion.
For milder mental status changes you give normal saline infusion together with a loop diuretic to enhance free water clearance.

Demeclocycline is used for chronic SIADH without symptoms since it takes 1-2 weeks to take effect.