“The Mountain Model of Hyponatremia” is an educational project by Joel Topf and me.
Imagine this is the clinical scenario of severe hyponatremia:
- You are driving in a car at the top of a mountain. (The top of the mountain is the low sodium level.)
- There is a forest fire behind you. (The fire represents brain swelling and seizures if the sodium level drops much lower.)
- Your goal is to drive safely to the bottom of the mountain. (The bottom of the mountain is a normal sodium level.)
- But, to be safe, you should follow the “sodium speed limit”: a rise in sodium of 8 every 24 hours. (If you exceed this “sodium speed limit,” you increase the risk of a car crash: osmotic demyelination syndrome.)
Based on this model, there are five possible therapies for patients with hyponatremia:
- Press the accelerator (when the car is in “drive”): 3% (hypertonic) saline
- Press the accelerator (but the car may be in “drive,” “neutral,” or even “reverse”): 0.9% (normal) saline
- Cruise control: desmopressin/DDAVP and 3% (hypertonic) saline (“proactive strategy”)
- Press the brake: desmopressin/DDAVP (or large amounts of D5W) (“reactive strategy”)
- Reverse gear: desmopressin/DDAVP and D5W (“rescue strategy”)
Additional technical notes:
- The “sodium speed limit” might change depending on the clinical situation.
- The slope of the mountain represents rapid correction due to water diuresis that can occur in patients with a reversible cause of hyponatremia (hypovolemia, beer potomania/low solute hyponatremia, adrenal insufficiency, and other causes).
- If a patient has SIADH, the mountain might slope upward. (This is a separate figure.)
- If the patient has SIADH, giving 0.9% (normal) saline may not work or may cause the sodium to go down — when you press the accelerator, the car may be in “neutral” or even “reverse.” (See therapy #2.)
- Usually, fluid restriction is also important.
- In the right clinical circumstances, other therapies might include salt tablets, loop diuretics, urea, and vaptans. (These are not included in the model.)
The key is that in volume depletion and Tea and Toast and psychogenic polydipsia, the body "wants" to correct the sodium, so the car will roll down hill on its own. But with SIADH, the body "wants" the sodium to go down so the spotaneous rolling car makes the sodium go down.
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