Hyponatraemia, defined as a serum sodium concentration of less than 135 mmol/L, is a common electrolyte disturbance in children. The UK’s paediatric guidelines, including those from the Royal College of Paediatrics and Child Health (RCPCH) and NICE, have provided clear guidance on management, with a strong emphasis on understanding fluid status and preventing hyponatraemia in hospitalised patients.
Clinical Presentation and Assessment
The clinical presentation depends on the severity and rate of onset.
Severity: Hyponatraemia is generally asymptomatic unless the serum sodium falls below 125 mmol/L. Severe symptoms typically occur below 120 mmol/L.
Symptoms: Due to fluid shifts into brain cells (cerebral oedema), neurological symptoms are most prominent. These can range from subtle signs like headache, irritability, and lethargy to severe manifestations such as reduced consciousness, seizures, and coma.
Assessment: A thorough assessment of the child’s fluid status is crucial. This helps to determine the underlying cause and guide management.
Hypovolaemic: The child is dehydrated, with signs like poor skin turgor, sunken eyes, tachycardia, and hypotension.
Hypervolaemic: The child has fluid overload, with signs like oedema, ascites, or pulmonary oedema.
Euvolaemic (Normovolaemic): The child appears to have a normal fluid volume.
Aetiology and Classification
The classification of hyponatraemia is based on the child’s fluid status and helps to narrow down the cause.
Hypovolaemic Hyponatraemia: This occurs due to a loss of both sodium and water, with a relative deficiency of sodium.
Renal Losses: The kidneys inappropriately excrete sodium (urine Na >20 mmol/L). Causes include diuretic use (e.g., thiazides), adrenal insufficiency (e.g., congenital adrenal hyperplasia), or salt-wasting nephropathies.
Extra-renal Losses: Sodium and water are lost from other sources (urine Na <20 mmol/L). Common causes are severe vomiting and diarrhoea, burns, or third-spacing of fluid (e.g., in pancreatitis).
Euvolaemic Hyponatraemia: This is the most common cause of hyponatraemia in hospitalised children and is typically due to excess free water.
Syndrome of Inappropriate Antidiuretic Hormone (SIADH): This is the most common cause, where excess ADH leads to water retention and a dilutional hyponatraemia. Causes include central nervous system infections (meningitis, encephalitis), lung diseases (pneumonia, bronchiolitis), certain drugs (e.g., carbamazepine), or post-operative stress.
Other Causes: Primary polydipsia (excessive fluid intake) or giving inappropriate hypotonic fluids (e.g., very dilute milk formula).
Hypervolaemic Hyponatraemia: This occurs due to an excess of both sodium and water, with a relative excess of water.
Renal Failure: Both acute and chronic kidney failure can impair the excretion of water.
Fluid Overload: Inappropriate administration of hypotonic intravenous fluids.
Other Causes: Hypoalbuminaemia (e.g., in nephrotic syndrome), liver failure, or congestive heart failure.
Management and Prevention
The management of hyponatraemia is dependent on the severity of symptoms and the underlying cause.
Medical Emergency: A child with severe hyponatraemia (Na <125 mmol/L) presenting with seizures or an altered conscious state is a medical emergency.
Treatment: The first-line treatment is to give a bolus of hypertonic 3% saline (3-5 mL/kg) over 30-60 minutes. This raises the serum sodium quickly to reverse cerebral oedema. This should be a controlled correction to avoid the risk of Central Pontine Myelinolysis (CPM).
Consultation: This is a critical situation that requires immediate discussion with a senior paediatrician, the renal team, and PICU.
General Management:
Fluid Restriction: For euvolaemic and hypervolaemic hyponatraemia, the primary management is fluid restriction.
Sodium Replacement: For hypovolaemic hyponatraemia, rehydration with an isotonic fluid (e.g., 0.9% saline) is needed.
Treat the Underlying Cause: Management is futile without addressing the root cause, such as discontinuing a causative drug, treating an infection, or managing an endocrine condition.
Prevention: The UK’s NICE guidelines have revolutionised the prevention of hyponatraemia in hospitalised children. The key principle is to use isotonic IV fluids for maintenance, rather than the previously used hypotonic fluids. This prevents dilutional hyponatraemia in children who are at risk of SIADH. All children on IV fluids should have regular monitoring of electrolytes and fluid balance.