Causes of Hyponatremia: Diet Coke and The Magic Water Faucet

I once cared for a patient with cellulitis who had a sodium of 113.  Kind of a strange presentation for an otherwise pretty healthy young female.  You'll never guess why.  What are the causes of hyponatremia?  On an academic basis the causes of hyponatremia can be broken down into hypovolemic, euvolemic and hypervolemic hyponatremia.   Us doctors will often order special urine electrolytes (spot urine osmolality and spot urine sodium) to understand exactly how the kidneys are managing a patient's sodium excretion. I rarely find serum osmolality helpful clinically and I never order it. 

When I'm asked to evaluate a patient in consultation for hyponatremia, one of the first things I'll do is check a urine osmolality and look at their medication profile.  In the absence of hypervolemia, the most common causes of hyponatremia are volume depletion,  medications or the reset osmotat of SIADH.  

I was always taught that making the diagnosis of syndrome of inappropriate antidiuretic hormone required one to exclude the presence of hypothryoidism or adrenal insufficiency.  So most people I see with hyponatremia get a TSH and occasionally a random cortisol, although I've never seen a patient with adrenal insufficiency present with isolated hyponatremia.  If I feel like it I may order a random cortisol.  I don't think there is any evidence  to this practice. 

What medications are classic causes of hyponatremia?  Of course, we have the thiazide diuretics.  After that it's a crap shoot. Occasionally, some of the seizure medications  and antipsychotics can do it as well. If in doubt I either look it up or have a pharmacist assist with a medication side effect profile.  If the diuretic is the culprit a little normal saline usually does the trick.

In SIADH, the body is inappropriately secreting anti diuretic hormone.  That means the body is holding on to more free water than they should (anti diuretic) and therefore inappropriately concentrating the urine. So SIADH is present when you have a low serum sodium and an inappropriately high urine osmolality.  If that's the case you can
  • Order a free water restriction (most common)
  • Order 3% hypertonic saline if the serum sodium is critically low and symptoms are present
  • Conivaptan (not a fan yet)
  • Demeclocylcine (a really expensive anitbiotic)
  • Sometimes a combination of normal saline (or 3%) with Lasix
What causes SIADH?  Lot's of things.  In hospitalist medicine the causes are usually tweaked out with a little good history talking.  Often the patient has a primary pulmonary process (lung mass, bronchiectasis, pneumonia etc) or a central nervous system process (brain tumor/stroke etc) or cancers of many kinds. 

There you have it.  The most common clinical causes of hyponatremia I see as a hospitalist are hypovolemic hyponatremia that readily resolves with normal saline or SIADH from medications, primary pulmonary disease, primary CNS disease, or neoplasm of many kinds.  If I can't get the problem fixed  or at least trending toward improvement after two or three days,  I'm calling in God  the nephrologist for their assistance.

So what happened?  I once saw a healthy woman with cellulitis and a sodium of 113.  I know she didn't have beer potomania, but she did tell me she drinks ten diet cokes a day.  I diagnosed her with psychogenic polydypsia and told her to cut it out.  Her sodium recovered by doing nothing at all.  It's that magic force field around a hospital known as the compliance vector.  Most things, if you just get the patient out of their home environment will fix themselves.  Too bad Medicare doesn't consider the magic hospital force field to be an inpatient qualifier for intensity of therapy.

This leads me to one of my other dramatic causes of hyponatremia:  My young bipolar patient I saw as a medical student  on my internal medicine rotation.  We were asked to see the patient in the inpatient psychiatry ward for a sodium of 102.  Yes folks.  102.  And he was walking and talking and fully functional. After watching this man use the water fountain, unrestricted, at least five times in a ten minute period, I asked him what he was doing.
"I've got this water faucet memorized.  I can start drinking and stop exactly before the cooling motor comes on.  Check me out."
Psychogenic polydypsia at its finest.  We asked that they shut the water faucet off and they said they couldn't do that.  We asked that he be monitored closely to keep from drinking water.  They said they couldn't do that.  We signed off with a recommendation for free water restriction.
Thank you for allowing us to care for your patient. Please call back if you have any other questions or if you need CPR.
If I was the attending I would have ordered Duct tape. And it would have worked.  These cases of hyponatremia are pretty straight forward. Most are too difficult, even for the doctors to figure out, as this original Happy crude medical E-card humor depicts:

"Symptomatic hyponatremia.  It's confusing, I know."

Symptomatic hyponatremia ecard humor photo Medical Humor Store Banner

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