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About Hyponatremia

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:

Serum Sodium Concentration

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
Severe2,6,7 <120
Plasma Osmolality or Plasma Tonicity

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

  Plasma Osmolality
(mOsm/kg H2O)
Possible Causes
Hypotonic <280 Heart failure, cirrhosis, SIADH
Isotonic 280-295 Pseudohyponatremia, hyperglycemia
Hypertonic >295 Severe hyperglycemia with dehydration, mannitol
SIADH=Syndrome of Inappropriate Antidiuretic Hormone.
Volume Status

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
Hypovolemic Absent  
Euvolemic Absent  
Hypovolemic   Present
Duration and Rate of Onset

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

Acute hyponatremia:

Hyponatremia develops

< 48 hours4

Chronic hyponatremia:

Hyponatremia of a duration

≥ 48 hours4

Symptom status

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

The 3 Types of Hypotonic Hyponatremia

Watch videos discussing the definitions, etiologies, and pathogenesis 
of hypovolemic, euvolemic, and hypervolemic hyponatremia.