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Sodium - Hyponatraemia

Key points

  • A patient’s fluid status is key in determining the cause of hyponatraemia and dictating treatment
  • The rate of correction of hyponatraemia should not exceed 8 mmol/L in 24 hours as over rapid correction can cause osmotic demyelination syndrome
  • Hyponatraemic seizures and/or altered conscious state are a medical emergency and can cause irreversible neurological damage


  • Hyponatraemia is defined as serum sodium <135 mmol/L.
  • Most patients with Na >125 mmol/L are asymptomatic.
  • Hyponatraemia and rapid fluid shifts can result in cerebral oedema causing neurological symptoms.
  • If Na <125 mmol/L or if serum sodium has fallen rapidly vague symptoms such as nausea and malaise are more likely and may progress.
  • If severe hyponatraemia (Na <120 mmol/L): headache, lethargy, obtundation and seizures may occur.
  • Chronic hyponatraemia (developing >24 hours) may have more subtle features such as restlessness, weakness, fatigue or irritability (due to cerebral adaptation)
  • Rapid correction of hyponatraemia can result in osmotic demyelination syndrome which manifests as irreversible neurologic features (dysarthria, confusion, obtundation and coma) which often present days after sodium correction.

Causes of hyponatraemia:

Volume status

Cause hyponatraemia


Vomiting and/or diarrhoea


Diuretics (thiazides, indapamide, hydrochlorothiazide, chlortalidone)


Primary adrenal insufficiency



  • Drug (carbamazepine, SSRI, SNRI)
  • Cerebral or pulmonary pathology
  • Malignancy associated


Psychogenic polydipsia

Secondary adrenal insufficiency


Heart failure

Liver cirrhosis

Kidney failure

Nephritic syndrome



Severe hypertriglyceridaemia

Osmotic dilution


Mannitol administration

Mechanism uncertain


SIADH = syndrome inappropriate antidiuretic hormone secretion; SSRI = selective serotonin reuptake inhibitor; SNRI = serotonin and noradrenaline reuptake inhibitor


SIADH (Syndrome of inappropriate secretion of antidiuretic hormone) has several causes. These include cerebral or pulmonary pathology, drugs that can impair water excretion or malignancy.

If there is no diuretic or other obvious cause, SIADH is likely if the following results apply:

serum sodium concentration <130 mmol/L

serum osmolality < 275 mmol/kg

urine osmolality > 100 mmol/kg

urine sodium concentration > 30 mmol/L


History consistent with common causes for hyponatraemia

History of fluid intake/losses

Clinical assessment of the patient’s current hydration status

Neurological status

Red Flags:

Nausea and vomiting



Decreased conscious state



Investigations (recommended if Na <130 mmol/L)

  • UEC
  • Paired serum and urine osmolality
  • Urinary sodium
  • BSL (If hyperglycaemia present in addition to hyponatraemia see DKA/HHS)
  • Consider blood gas if sick
  • TFTs
  • Random serum cortisol levels or adrenocorticotropic hormone stimulation test - Considered in suspected adrenal suppression (e.g. recent steroid use).


  • Management is determined by presence of seizures/ altered conscious state and fluid status (see flow chart below).
  • The target rate of serum sodium correction is 4-8mmol/L in 24 hours (unless seizing- see flow chart below).
  • If additional risk factors (Na < 105 mmol/L, hypokalaemia, alcoholism, malnutrition, advanced liver disease), advise aim of rate of correction should be 4 – 6 mmol/L daily.
  • All patients should have a strict fluid balance including weight (minimum daily, but maybe 6-12 hourly for more unwell patients).
  • Remember to treat the underlying cause.

Hyponatraemia treatment flowchart:


Risk of morbidity from delayed treatment is greater than the risk of osmotic demyelination from overly rapid correction. Therefore, aggressive initial correction is indicated for the first 3-4 hours (or until symptoms resolve) at a rate not to exceed 2 mmol/L per hour

Hyponatraemic seizures may be refractory to anticonvulsants and sodium correction should not be delayed

Maximum 3 infusions of 3% Saline

3% Saline should preferably be given via a central line

Consider consultation with critical care team when

The patient has had CNS symptoms including seizures or altered conscious state


RCH Hyponatremia clinical practice guideline

ETG hyponatraemia

Diagnosis and Treatment: Emergency medicine

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