Hyponatremia

Causes and classifications

Clinicians classify hyponatremia by the patient’s volume status and by serum osmolality. Common categories include:

• Hypovolemic hyponatremia: loss of both sodium and water (for example from vomiting, diarrhea, or diuretics) with greater sodium loss.
• Euvolemic hyponatremia: normal total body sodium but increased free water (syndrome of inappropriate antidiuretic hormone secretion, or SIADH; psychogenic polydipsia).
• Hypervolemic hyponatremia: increased total body sodium and water with relatively more water (heart failure, cirrhosis, advanced kidney disease).

There are also laboratory distinctions: pseudohyponatremia can occur with very high blood lipids or proteins, and hypertonic hyponatremia results from osmotic agents such as markedly elevated blood glucose.

Symptoms and complications

Symptoms depend on severity and how quickly sodium falls. Mild hyponatremia may cause nausea, headache, or malaise. More severe or rapidly developing hyponatremia can cause confusion, drowsiness, seizures, respiratory arrest, or coma due to cerebral edema. A major treatment hazard is overly rapid correction of chronic hyponatremia, which can cause osmotic demyelination (central pontine myelinolysis) and permanent neurological damage.

Diagnosis and management

Evaluation typically includes measurement of serum sodium and osmolality plus urine osmolality and urine sodium to identify the underlying mechanism. Management targets the cause and the patient’s symptoms:

• Asymptomatic or mild cases: often managed by fluid restriction and addressing contributing drugs or conditions.
• Symptomatic or life‑threatening cases: managed in hospital; short‑term use of hypertonic saline and close monitoring may be required.
• Chronic cases: gradual correction and treatment of the root disorder to avoid neurological injury.

Prevention and notable facts

Prevention focuses on treating underlying disease, avoiding inappropriate fluid intake, and careful use of medications that affect sodium balance. Athletes and endurance competitors have developed hyponatremia by drinking excessive hypotonic fluids during events such as a marathon, illustrating that overhydration as well as salt loss can be harmful. Hyponatremia is a common laboratory abnormality and often signals an underlying medical problem that merits investigation and tailored treatment.