What is Hyponatremia?
Hyponatremia is a common electrolyte disorder1
Hyponatremia is a common electrolyte disorder that occurs in up to 30% of all hospitalized patients.1 The disorder is
important to recognize because it can have serious clinical consequences.2
Hyponatremia can be defined as:
- Serum sodium concentration <135 mEq/L3
- Disorder of water balance
- Body water is in excess relative to total body sodium4
Types of Hyponatremia
Hyponatremia can be classified by several clinical parameters, and treatment strategies are often based on a combination of these parameters.4 The following parameters are used to classify hyponatremia:
Hyponatremia is classified into mild, moderate, or severe based on the concentration of sodium in the serum.2,5
|Hyponatremia Severity||Serum [Na+] (mEq/L)|
|Mild6||<135 and ≥130|
|Moderate6||<130 and ≥120|
Hyponatremia can be classified by the effective osmolality or plasma tonicity. Because sodium is an effective solute resulting in osmotic movement across cell membranes, the plasma osmolality in hyponatremia should be hypotonic; however, there are instances where hyponatremia is isotonic or hypertonic.5 These cases of hyponatremia occur
when there are increases in effective solutes other than sodium—such as glucose, glycine, or mannitol—that cause
an osmotic movement of water from the intracellular or extracellular compartment. In some cases, increases in
proteins or lipids can cause a type of isotonic hyponatremia called pseudohyponatremia, which is an artifactual
decrease in serum sodium caused by the excess proteins or lipid.5,8
Hyponatremia Classification by Plasma Osmolality5,8,9
|Hypotonic||<280||Heart failure, cirrhosis, SIADH|
|Hypertonic||>295||Severe hyperglycemia with dehydration, mannitol|
|SIADH=Syndrome of Inappropriate Antidiuretic Hormone.|
Hypotonic hyponatremia can further be classified into 3 categories based on the extracellular fluid (ECF) volume of the patient. This classification can be seen on the chart below.4,8,9
Hyponatremia Classification by Volume Status4,8,9
|ECF Volume||Edema||Total Body Water||Total Body Sodium|
The rate of onset of hyponatremia can be either acute or chronic, and is an important factor to be considered when determining a treatment strategy.4,8
< 48 hours4
Hyponatremia of a duration
≥ 48 hours4
Classification of hyponatremia based on the status of symptoms is also an important factor to be considered when determining a treatment strategy.4,8 Patients can be symptomatic or asymptomatic,4,8 although some experts believe that patients are never fully asymptomatic.5,10
Incidence of hyponatremia is high in hospitalized patients11,12
Reported incidences of hyponatremia vary. Below are 2 large studies that look at the incidence of hyponatremia present at admission and the incidence of hospital-acquired hyponatremia.
In a retrospective analysis of over 100,000 patients in an acute care hospital in Singapore over a 2-year period11:
- 28% of patients admitted to the hospital had hyponatremia defined as serum sodium concentration <136 mEq/L
- 22% of patients admitted to the hospital had hyponatremia defined as serum sodium <135 mEq/L11
- More severe hyponatremia (serum [Na+] <126 mEq/L) at admission occurred much less frequently at 2.6%
In a retrospective analysis of 7 years of discharge data (53,236 hospitalizations)12:
- 38.2% (N=10,662) out of 27,897 hospitalized patients with normonatremia (serum [Na+]=138-142 mEq/L) on admission, developed hyponatremia (serum [Na+] <138 mEq/L) during their hospital stay of more than 1 day
- Upadhyay A, Jaber BL, Madias NE. Incidence and prevalence of hyponatremia. Am J Med. 2006;119(Suppl 1):S30-S35.
- Adrogué HJ, Madias NE. The challenge of hyponatremia. J Am Soc Nephrol. 2012;23(7):1140-1148.
- Vaidya C, Ho W, Freda BJ. Management of hyponatremia: providing treatment and avoiding harm. Cleve Clin J Med. 2010;77(10):715-726. doi:10.3949/ccjm.77a.08051.
- Douglas I. Hyponatremia: why it matters, how it presents, how we can manage it. Cleve Clin J Med. 2006;73(Suppl 3):S4-S12.
- Verbalis JG, Goldsmith SR, Greenberg A, et al. Diagnosis, evaluation, and treatment of hyponatremia: expert panel recommendations. Am J Med. 2013;126(10 Suppl 1):S1-S42. doi:10.1016/j.amjmed.2013.07.006.
- Reynolds RM, Seckl JR. Hyponatremia for the clinical endocrinologist. Clin Endocrinol (Oxf). 2005;63(4):366-374.
- Sterns RH, Cappuccio JD, Silver SM, Cohen EP. Neurologic sequelae after treatment of severe hyponatremia: a multicenter perspective. J Am Soc Nephrol. 1994;4(8):1522-1530.
- Assadi F. Hyponatremia: a problem-solving approach to clinical cases. J Nephrol. 2012;25(4):473-380. doi:10.5301/jn.5000060.
- Sra J, Repp AB. Hyponatremia. Hosp Med Clin. 2012;1:e199-e208. doi:10.1016/j.ehmc.2012.03.007.
- Renneboog B, Musch W, Vandemergel X, Manto MU, Decaux G. Mild chronic hyponatremia is associated with falls, unsteadiness, and attention deficits. Am J Med. 2006;119(1):71.e1-71.e8. doi:10.1016/j.amjmed.2005.09.026.
- Hawkins RC. Age and gender as risk factors for hyponatremia and hypernatremia. Clin Chim Acta. 2003;337(1-2):169-172.
- Wald R, Jaber BL, Price LL, Upadhyay A, Madias NE. Impact of hospital-associated hyponatremia on selected outcomes. Arch Intern Med. 2010;170(3):294-302. doi:10.1001/archintemmed.2009.513.