Hyponatremia

Hyponatremia is an abnormally low concentration of sodium in the blood, usually defined as serum sodium below 135 millimoles per liter.

Osmolality: A measure of the concentration of dissolved particles in a solution. Low serum sodium usually lowers blood osmolality, driving water into brain cells.
Serum sodium: The concentration of sodium measured in the liquid portion of blood. This laboratory value establishes the disorder and tracks its correction.
Cerebral edema: Swelling of brain tissue caused by excess fluid accumulation. A rapid sodium decline can draw water into brain cells and cause swelling.
Hypernatremia: An abnormally high concentration of sodium in the blood. It is the opposite sodium-concentration disorder and often reflects a water deficit.
Antidiuretic hormone: A hormone that promotes water reabsorption by the kidneys and helps regulate body-fluid concentration. Excess antidiuretic hormone retains water and can dilute blood sodium.
Urine osmolality: A measurement of how concentrated or dilute urine is. It helps distinguish impaired water excretion from appropriate production of dilute urine.
Seizure: A sudden episode of abnormal electrical activity in the brain. Severe acute hyponatremia can provoke seizures through brain swelling.
Hyperglycemia: An abnormally high concentration of glucose in the blood. High glucose shifts water into the bloodstream and can lower measured sodium by dilution.
Syndrome of inappropriate antidiuretic hormone secretion: A disorder in which antidiuretic hormone activity causes water retention despite low blood osmolality. It is a common cause of hypotonic hyponatremia in people with clinically normal fluid volume.
Urine sodium: The concentration of sodium in a urine sample. It helps assess kidney sodium handling and narrow the likely cause.

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