Hyponatremia Etiology/Pathophysiology

Hyponatremia's etiology and pathophysiology are very interesting.  First of all, I will talk about the etiology of hyponatremia (how it's caused).  Before that though, I must reiterate from my previous blog post:  there are two types of hyponatremia; dilutional and depletional hyponatremia.  Dilutional hyponatremia is just what it sounds like.  There is too much fluid in your blood, which decreases the concentration of sodium in the blood to less than 135 mEq/L.  This is much like the effect of pouring distilled water into salt water, making the water less "salty".  Depletional hyponatremia is when the body loses too much sodium via fluid loss.  Prime examples include urine and sweat.  People who are most at risk of this are people who exercise too much, losing too much sodium through sweat.  Most people with hyponatremia have resided/reside in a healthcare setting.  "Surely, it must be some kind of disease that causes the sodium level to go down" you may think.  That is not the case at all.  One of the biggest causes of hyponatremia is iatragenic (caused by the healthcare professional).  Many times, there are co-morbidities associated with hyponatremia.  Hyponatremia serves as a marker for heart and liver diseases.  It is also associated with kidney injury, brain tumors, and brain hemorrhaging (1). In a study, the hospital mortality rate for patients with hyponatremia was 6.1% as compared to 2.3% with non-hyponatremic patients (1).

Mortality vs. Serum Sodium

With the chart shown above, we can see that mortality percentage peaks at patients with serum sodium at 115 to 124 mEq/L.  Several points of evidence suggests that hyponatremia is rarely a cause of death, but rather a marker for  the severity of the underlying disease (1).  "(1) once sNa falls below 120 mEq/L, mortality rate does not seem to increase as the severity of hyponatremia worsens; in fact the trend is in the opposite direction; (2) deaths in patients with sNa <120 mEq/L were associated with serious comorbidities and were mostly attributable to conditions other than hyponatremia; (3) neurologic symptoms attributable to hyponatremia were uncommon among fatal cases, and deaths attributable to neurologic complications were rare. (1)"

Fatal cases with uncorrected hospital-acquired or hospital-aggravated hyponatremia

Age/Gender	Admission SNa (mEqL)	Lowest SNa (mEq/L)	SNa at Death (mEq/L)	Day of Death	Clinical Course	Comfort Care?	Charlson Score
66/M	145	114	114	27	Metastatic carcinoma, cardiac arrest, respiratory failure, anoxic brain damage, acute kidney injury, refractory shock. First SNa <120 mEq/L 4 days before death	Yes	11
67/M	139	115	115	163	Sepsis with several months in hospital with irreversible multiorgan failure; first sNa <120 mEq/L on day 155, 8 days before death	Yes	5
63/M	138	117	117	16	Metastatic carcinoma, acute kidney injury, acute respiratory failure. First SNa <120 mEq/L 1 day before death	Yes	11
87/M	136	118	123	24	ESRD. Comfort care on admission.	Yes	4
65/M	132	118	118	11	End-stage liver disease, sepsis, multiorgan failure. First sNa <120 mEq/L on day of death	Yes	5
65/M	131	119	119	11	Cardiogenic shock. First sNa <120 mEq/L 1 day before death	Yes	4
55/M	124	119	122	12	End-stage heart disease, acute kidney injury	No	7
54/F	124	119	122	4	End-stage lung disease	Yes	3
83/M	120	118	123	7	End stage heart disease	No	8
63/M	121	119	122	2	Advanced lung cancer	No	7

As you can see from this table, almost all patients who had hyponatremia died of an underlying progressive illness (1).  We can conclude that many cases of hyponatremia can be directly linked to or tied to another disease.  Sure, it's possible to die from hyponatremia, but most of the time, it's more complicated than that. 

Works Cited:

1) Chawla, A., Sterns, R., Nigwekar, S., & Cappuccio, J. (2011). Mortality and Serum Sodium: Do Patients Die from or with Hyponatremia? Clinical Journal of the American Society of Nephrology, 960-965. Retrieved April 17, 2015, from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3087791/#B18