Hyponatraemia in children

Determination of total body fluid status is key to investigate and manage hyponatraemia.

 

The commonest cause of hyponatraemia is not a deficiency of total body sodium, but an excess of total body water, as in SIADH.

 

Causes of hyponatraemia

Hypovolemic hyponatraemia

Renal loss (Urine Na >20mmol/L)

 – Diuretics- thiazide or loop

 – Osmotic diuresis

 – Renal Tubular damage e.g. obstructive

 – Renal salt wasting

 – Adrenal insufficiency- CAH / Addisons

 

Extra-renal loss (Ur Na <20mol/L)

 – GI losses due to Diarrhea, Vomiting, drains, fistula

 – Third-space losses like effusions, ascites

 – Skin losses due to burns or excess sweat in CF

 

Normovolemic hyponatremia

SIADH (Ur Na >20mol/L) due to

 – CNS diseases (Meningo-encephalitis), brain tumors

 – Lung diseases (Bronchiolitis, Severe asthma, Pneumonia)

 – Post-operative state

 – Drugs (Carbamazepine, Valproate, Cyclophosphamide, Vasopressin, etc)

 

Non-SIADH (Ur Na <20mol/L)

 – Hypothyrpodism

 – Excess enteral fluids- psychogenic polydipsia, dilute milk formula

 

Hypervolemic hyponatremia (Ur Na <20mol/L)

 – Fluid overload (Hypotonic IV fluids)

 – Hypoalbuminemia, Nephrotic syndrome

 – Congestive heart failure

 – Cirrhosis

 – Acute or chronic renal failure (Ur Na >20mol/L)

 

Clinical presentation

Hyponatraemia is defined as serum sodium < 135mmol/L, but generally asymptomatic unless Na <125mmol/L

 

 – Review fluid intake and output from history & charts

 – Enquire symptoms of conditions or drugs that may contribute to hyponatraemia

 – Neurological features emerge due to fluid shifts

 – Nausea, lethargy or irritability, headache, reduced consciousness and seizures

 – Seizures generally appear if Na <120mmol/L

 

Perform a thorough assessment of hydration status & systemic examination including CNS

Maintain GCS chart if disturbance of consciousness or seizures

 

Investigations 

 – Find underlying cause

 – Paired serum + urine electrolytes and osmolality

 – Glucose (severe hyperglycaemia causes pseudo-hyponatraemia)

 – Consider blood gas if child unwell

 

Prevention of Hyponatraemia

 – Only give isotonic fluid (e.g. 0.9% Saline + 5% glucose) as maintenance fluids

 – Give partial / restricted maintenance fluids if child is euvolaemic & at risk of SIADH. Measure U&E as baseline, then monitor daily while on fluids

 

Management

Hyponatraemic seizures and/or altered conscious state are a medical emergency and can cause irreversible neurological damage.

 

 – Seizures may be refractory. Inform PICU & administer 3 – 5ml/kg of hypertonic 3% Saline over 30 – 60 minutes.

 

 – Fluid restriction alone is often sufficient for SIADH; but sodium and water restriction is required in hypervolemic hyponatraemia.

 

 – If dehydrated and/or hypotensive, treat with IV Normal Saline or Oral Rehydration Solution

 – All children should have a strict fluid balance including daily weight

 – Remember to treat the underlying cause.

 – Rapid decline of serum Na is associated with risk of central pontine myelinosis. Correction in the first 48 hr should not exceed 15-20 mmol/L

 

 – Discuss with senior doctor, Renal team and PICU as appropriate