Hyponatraemia
Low sodium — causes, classification and safe management
Hyponatraemia (serum sodium <135 mmol/L) is the most common electrolyte disturbance in hospitalised patients, affecting up to 30% of inpatients. It ranges from asymptomatic incidental findings to life-threatening cerebral oedema. Safe management requires accurate classification of the underlying cause — rapid correction can be more dangerous than the imbalance itself.
What is Hyponatraemia?
Hyponatraemia is defined as a serum sodium concentration below 135 mmol/L. It is classified by severity: mild (130–134 mmol/L), moderate (125–129 mmol/L), and severe (<125 mmol/L). It can also be classified by onset: acute (<48 hours) or chronic (≥48 hours or unknown duration). Sodium is the principal extracellular cation and the primary determinant of plasma osmolality. When sodium falls, water moves into cells by osmosis — in the brain, this causes cerebral oedema, which is the basis of most serious neurological symptoms.
Causes & Classification
Hyponatraemia is classified by the patient's volume status into three types. Hypovolaemic hyponatraemia (low sodium + low fluid volume): causes include vomiting, diarrhoea, diuretic use, Addison's disease, and burns. The body retains water to restore volume, diluting sodium. Euvolaemic hyponatraemia (low sodium + normal fluid): most commonly SIADH (Syndrome of Inappropriate ADH) — caused by lung disease (pneumonia, malignancy), CNS disease (stroke, meningitis), drugs (SSRIs, carbamazepine, cyclophosphamide, NSAIDs, PPIs), surgery, pain, and nausea. Hypothyroidism and adrenal insufficiency also cause euvolaemic hyponatraemia. Hypervolaemic hyponatraemia (low sodium + excess fluid): seen in heart failure, nephrotic syndrome, and cirrhosis — sodium is diluted by retained water.
Signs & Symptoms
Symptoms correlate with both the severity and rate of decline of sodium. Mild hyponatraemia is often asymptomatic. Moderate symptoms include nausea, headache, confusion, fatigue, and muscle cramps. Severe or rapidly developing hyponatraemia causes progressive neurological deterioration: seizures, respiratory arrest, and herniation from cerebral oedema — all potentially fatal. Chronic hyponatraemia (develops slowly over >48h) allows the brain to adapt, so patients may have surprisingly few symptoms even with sodium as low as 115 mmol/L. Acute hyponatraemia is far more dangerous at the same sodium level because adaptation has not occurred.
Diagnosis & Safe Management
A thorough evaluation of volume status, urine sodium and osmolality, and drug history is essential before treatment. Serum and urine osmolality distinguish true hyponatraemia from pseudohyponatraemia (normal osmolality — e.g. hyperproteinaemia, hyperlipidaemia). Urine sodium >20 mmol/L with normal/high urine osmolality points to SIADH. The rate of sodium correction must never exceed 8–10 mmol/L per 24 hours — correcting too fast causes osmotic demyelination syndrome (ODS), a devastating and irreversible brainstem injury (previously called central pontine myelinolysis). Acute symptomatic hyponatraemia (seizures, obtundation) is the only indication for hypertonic saline — and only under specialist supervision.