Definition

Hyponatraemia is defined as serum sodium  <135mmol/L. It represents an excess of water in relation to sodium in extracellular fluid.  Symptoms are likely with Na <125 mmol/L or if the serum sodium has fallen rapidly.

Background

The main causes of hyponatraemia in children are:  

• Administration of hypotonic fluids, intravenous or enteral (e.g. excessively dilute formula or 0.18% NaCl)
• Conditions with impaired free water excretion and high anti-diuretic hormone levels
  • Meningitis, encephalitis, pneumonia, bronchiolitis, sepsis
  • Surgery, pain, nausea and vomiting
• Relative excess fluid intake in a child receiving exogenous anti-diuretic agents (eg Desmopressin for nocturnal enuresis)
• Gastrointestinal fluid losses

Less common but important causes are:

• Adrenal insufficiency (Congenital Adrenal Hyperplasia, Addison's Disease )
• Defect in renal tubular absorption, including obstructive uropathy  
• Psychogenic polydipsia

Prevention

Special attention should be paid when administering intravenous fluid to children with conditions associated with high ADH levels and impaired free water excretion (see above). (see IV Fluids guideline)

• Never give 0.18% NaCl or 5% dextrose in water iv as maintenance fluids
• Give enteral feeding, rather than iv fluids, wherever possible, and include the volume of enteral fluid in calculations of fluid intake.
• Use Plasma-Lyte 148 and 5% Glucose OR 0.9% sodium chloride (normal saline) and 5% Glucose as 'maintenance fluid'.
• Give fluid volumes that take into account the reduced volumes required in children with impaired free water excretion.
• For children who are on more than 50% iv maintenance fluids check the serum electrolytes prior to starting fluids and at least once in the following 12 hours. 
  Electrolytes may need to be checked more frequently if the starting sodium was abnormal. Pay attention to major changes in sodium even when it is still within normal range.

Assessment

Most children with mild to moderate hyponatraemia are asymptomatic or manifest the symptoms of the underlying disease (bronchiolitis, meningitis etc).  The symptoms and signs of severe hyponatraemia are predominately neurological:

• Headache
• Nausea, vomiting 
• Lethargy or irritability  
• Hyporeflexia  
• Decreased conscious state 
• Seizures

Assess the patient's hydration state. (see Gastroenteritis guideline)

If Na+ <130 mmol/L: measure serum potassium, chloride, urea, creatinine and glucose.

Measure the urinary sodium and osmolarity.

In the presence of hyperlipidaemia (e.g. nephrotic syndrome) or hyperpoteinaemia, some laboratories produce falsely low measurements of serum sodium (this is not the case at RCH).  This is sometimes called pseudohyponatraemia.  Contact your laboratory.

Management

The ideal rate of serum sodium correction depends on the presence and severity of symptoms.  Correction that is too rapid (>8 mmol/L Na+/24h) can result in cerebral demyelination, especially of the pons, with risk of severe and lasting brain injury.  This is especially a risk if hyponatraemia has been present for more than 5 days and is rapidly corrected.

The hyponatraemic child with seizures or CNS depression

• Notify ICU urgently.
• Resuscitation (ABC) and intravenous anticonvulsants as clinically indicated. Hyponatraemic seizures often respond poorly to conventional anticonvulsants, and sodium correction should not be delayed.  The sodium should be raised until it reaches 125mmol/L or until seizures stop, whichever occurs first. 
• Use intravenous 3% NaCl solution.  It is stored in the resuscitation trolleys on the wards, in the ICU and ED.  Give 4ml/kg of 3% NaCl.  Give over 15-30 minutes.  This will raise the serum sodium by 3 mmol/L and will usually stop the seizures.  3% saline is hypertonic and should be given through a central venous line where possible - however do not delay administration in a fitting child to put in a central line. Careful use of a peripheral IV line is suitable in an emergency.
• Measure the serum sodium after the first bolus.  Ongoing seizures and persistent hyponatraemia will require more 3% NaCl.
• Many children with hyponatraemia and seizures will have other reasons for seizures (fever, meningitis, hypoglycaemia), and these should also be addressed
• After the seizures have resolved the total sodium correction (including the bolus) should not exceed 8 mmol/L per day (e.g. from 122-130mmol/L).
• Measure electrolytes every 2 hours until stable, then every 4-6 hours until the serum sodium is normal and the child is off iv fluids

The child with no symptoms of hyponatraemia

Management of children without specific symptoms of hyponatraemia depends on volume status.  They may be normally hydrated, moderately dehydrated or severely dehydrated (see Gastroenteritis guideline).

Active correction of hyponatraemia (e.g. with 3% NaCl) is not necessary.  Allow the plasma sodium concentration to rise at no more than 8 mmol/L per day using the guidelines below, based on hydration state.  Continue correction to 135 mmol/L.

1. The child with normal or increased volume status

• Restrict water to slowly remove the increased body water.
• Do not use hypotonic solutions (see prevention, above)
• See, for example,  meningitis fluid guidelines.

2. The child with moderate dehydration and serum sodium 130-135mmol/L

Try nasogastric rehydration.  When using Gastrolyte, remember that it contains 60mmol/L of sodium; rapid re-hydration may make the Na+ fall faster than is safe.

If NG rehydration is not possible or results in a too rapid fall in sodium give iv Plasma-Lyte 148 and 5% Glucose OR 0.9% sodium chloride (normal saline) and 5% Glucose (see severe dehydration below).

3. The child with severe dehydration or serum sodium <130mmol/L

Give iv Plasma-Lyte 148 and 5% Glucose OR 0.9% sodium chloride (normal saline) and 5% Glucose until the child can take enteral feeds.

Measure electrolytes every 4 hours until stable, whether on iv or nasogastric rehydration.

Hyponatraemia and severe hyperglycemia

Hyponatraemia occurs because high plasma glucose increases serum osmolarity, causing a shift of water from the intracellular space into extracellular fluid.  The reduction in blood glucose after beginning treatment may correct the hyponatraemia, through a shift of water back to the intracellular space.  However if the serum sodium fails to increase as the glucose falls hyponatraemia should be actively corrected.  This will prevent a reduction in serum osmolality, which carries an increased risk of cerebral oedema.  Using 0.9% sodium chloride (normal saline) as the fluid for DKA resuscitation will generally maintain the osmolarity.  (see DKA guideline).