Defined as a serum sodium concentration of <135 mmol/L.
Most common electrolyte disorder encountered in clinical practice.
Can occur in settings of volume depletion, volume overload, or euvolaemia.
Serum osmolality, urine osmolality, and urine sodium concentration help to determine the underlying cause.
Cerebral oedema is a medical emergency and occurs more frequently when hyponatraemia develops over <48 hours. Patients should be treated promptly with hypertonic 3% saline.
Other treatment depends on the underlying cause and may include fluid restriction and stopping causative medications.
Overly rapid correction of serum sodium concentration can lead to the life-threatening complication of myelinolysis and should be avoided.
Hyponatraemia is defined as a serum sodium concentration of <135 mmol/L. Normal serum sodium concentration is in the range of 135-145 mmol/L. It is a disorder of water balance reflected by an excess of total body water relative to electrolytes (total body sodium and potassium) leading to low plasma osmolality (i.e., <275 mmol/kg).  Hyponatraemia is generally caused by an increase in renal water reabsorption due to release of vasopressin (arginine vasopressin also known as antidiuretic hormone) along with water intake, and can occur in situations of volume depletion, volume overload, or normal volume. (It should be noted that loss of sodium is minor compared with gains in water in most types of hyponatraemia.)
This topic principally focuses on hypotonic (hypo-osmolar) hyponatraemia, the most common type of hyponatraemia.
MedStar Washington Hospital Center
JHV declares that she has no competing interests.
Department of Internal & Emergency Medicine
Hirslanden Klinik Im Park
GL is an author of a number of references cited in this topic.
Professor of Medicine
Division of Nephrology
Department of Medicine
Medical University of South Carolina
MEU declares that he has no competing interests.
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