Hypernatraemia, defined as a serum sodium concentration of >145 mmol/L, is a worrying electrolyte imbalance in children. A level of >150 mmol/L is concerning, and anything >170 mmol/L is considered severe. The UK’s management guidelines emphasise a methodical, careful approach to fluid management to avoid a rapid fall in sodium, which can lead to life-threatening complications.
Aetiology
Hypernatraemia in children results from either a net loss of water or an excess of sodium intake.
Net Water Loss:
Gastrointestinal: The most common cause is severe dehydration from vomiting or diarrhoea, particularly in infants.
Renal: Can be caused by conditions like diabetes insipidus, osmotic diuresis (e.g., in uncontrolled diabetes mellitus), or diuretic use.
Skin: Seen with burns or excessive sweating, especially in hot weather.
Inadequate Intake: This is a major cause in infants due to breastfeeding difficulties or poor feeding.
Excess Sodium Intake:
Iatrogenic: Inappropriate administration of highly concentrated intravenous fluids (e.g., sodium bicarbonate) or enteral feeds.
Dietary: Mixing formula feeds with too little water or accidental salt poisoning.
Endocrine: Rare causes include hyperaldosteronism or Cushing’s syndrome.
Clinical Presentation
The signs of hypernatraemia can be subtle and misleading. The child may appear to be less dehydrated than they are because the high sodium level pulls water from the intracellular to the extracellular space, preserving the intravascular volume. The skin may feel “doughy,” a classic sign of hypernatraemia.
Neurological Symptoms: These are the most prominent and are due to the brain shrinking as water is pulled out of the cells.
Mild: Irritability, lethargy, or weakness.
Severe: Tremors, ataxia, reduced consciousness, and seizures.
Thirst: A high sodium level stimulates the thirst centre, so the child is often very thirsty.
Management
The management of hypernatraemia is a medical emergency that requires a structured and cautious approach to avoid rapid fluid shifts that can cause cerebral oedema and permanent brain damage.
Initial Resuscitation: If the child is in hypovolaemic shock, a bolus of isotonic fluid (0.9% saline) should be given to restore circulation.
Controlled Fluid Replacement: The goal is to correct the sodium level slowly and safely. The serum sodium should not fall by more than 0.5 mmol/L/hour or 12 mmol/L in 24 hours. This allows the brain cells to adapt to the new osmolarity.
Fluid management should be based on the initial serum sodium and the estimated fluid deficit.
A common approach is to use 0.9% saline with 5% glucose for replacement, administered slowly over 48 hours or more.
Monitoring: Frequent monitoring of serum electrolytes (every 4-6 hours initially), fluid intake, and urine output is crucial. A rapid fall in sodium is a red flag and requires a reduction in the rate of fluid infusion and immediate senior advice.
Underlying Cause: Always stop any source of excess sodium intake and treat the underlying cause (e.g., managing diabetes insipidus or infectious gastroenteritis).
Expert Opinion: Any child with severe hypernatraemia (>170 mmol/L) or neurological symptoms should be managed in consultation with a senior paediatrician, the renal team, and PICU.