How To Manage Hyponatraemia

by

Dr Zaid Ismail

Hyponatraemia, defined as a serum sodium concentration of <135 mmol/L, is one of the most common electrolyte disturbances encountered in primary care. It can be associated with significant morbidity, particularly in elderly and frail patients.

Causes of Hyponatraemia

The causes of hyponatraemia can be broadly classified based on volume status:

Hypovolaemic Hyponatraemia

  • Gastrointestinal losses – vomiting, diarrhoea
  • Renal losses – diuretics, Addison’s disease, salt-wasting nephropathy
  • Third-spacing – pancreatitis, burns

Euvolaemic Hyponatraemia

  • Syndrome of inappropriate antidiuretic hormone (SIADH) – malignancy (e.g., small cell lung cancer), CNS disorders, pulmonary infections, certain medications (SSRIs, carbamazepine)
  • Hypothyroidism
  • Glucocorticoid deficiency

Hypervolaemic Hyponatraemia

  • Congestive heart failure
  • Cirrhosis
  • Nephrotic syndrome
  • Chronic kidney disease

Clinical Presentation

Symptoms depend on the severity and rapidity of onset:

  • Mild (<135 mmol/L): Often asymptomatic
  • Moderate (120-130 mmol/L): Lethargy, confusion, nausea, headache
  • Severe (<120 mmol/L): Seizures, coma, respiratory distress

Investigations

Key investigations include:

  • Serum osmolality – to distinguish true hyponatraemia from pseudohyponatraemia
  • Urine osmolality and sodium – to differentiate SIADH from hypovolaemic causes
  • Thyroid function tests – to exclude hypothyroidism
  • Cortisol levels – to check for adrenal insufficiency

Management in Primary Care

Management depends on severity and underlying cause:

  • Mild, asymptomatic cases – Address reversible causes (e.g., medication review, fluid restriction in SIADH)
  • Symptomatic or severe cases – Requires urgent secondary care referral to prevent complications such as cerebral oedema
  • Chronic hyponatraemia – Gradual correction to avoid osmotic demyelination syndrome

When to Refer to Secondary Care

  • Severe hyponatraemia (<120 mmol/L) or symptomatic patients
  • Suspected SIADH requiring specialist input
  • Adrenal insufficiency requiring endocrinology referral
  • Hyponatraemia unresponsive to initial management

Conclusion

Hyponatraemia is a common yet potentially serious electrolyte disturbance. Primary care clinicians should assess volume status, identify underlying causes, and initiate appropriate management. Severe or symptomatic cases require urgent secondary care input.

References

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