Hyponatraemia and Hypernatraemia in Children
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Direct answer
Hyponatraemia and hypernatraemia are disorders of serum sodium, defined as levels below 135 and above 145 mmol/L, and both cause neurological harm when corrected too fast. Hyponatraemia commonly follows hypotonic fluid intake or gastroenteritis, while hypernatraemia usually reflects water loss or diabetes insipidus. The key safety rule is a correction of no more than about 8 to 10 mmol/L in 24 hours, because rapid correction risks osmotic demyelination.
What you must remember
- Cut-offs: serum sodium below 135 mmol/L defines hyponatraemia, and above 145 mmol/L defines hypernatraemia.
- Hyponatraemia symptoms: irritability, vomiting, seizures and coma from cerebral oedema, with severity tied to how quickly sodium fell.
- Hypernatraemia symptoms: irritability, lethargy, spasticity and seizures, with intracerebral haemorrhage in severe cases.
- Correction limit in hyponatraemia: no more than about 8 to 10 mmol/L in 24 hours, with a lower limit for high-risk chronic cases.
- Osmotic demyelination: central pontine myelinolysis from over-rapid correction, appearing after a lucid interval with dysarthria and quadriparesis.
- Hypernatraemia causes: water loss in diarrhoea, diabetes insipidus and excess sodium from concentrated feeds or home remedies.
- SIADH: hyponatraemia with inappropriately concentrated urine, often from CNS infection or pulmonary disease.
Correcting sodium safely
Sodium correction is planned by duration, volume status and symptoms, and the rate is then set so the brain is not shocked. The steps below move from classification to the controlled rise.
- Define the severity: classify by serum sodium and by the presence of seizures or reduced consciousness, since these set the urgency.
- Judge the duration: separate acute hyponatraemia from chronic, because the brain adapts over days and is more vulnerable to rapid correction in chronic cases.
- Assess volume status: use isotonic saline for hypovolaemic children and fluid restriction for hypervolaemic children, so the choice follows the physical examination.
- Treat severe symptoms: give a small bolus of hypertonic 3 percent saline for seizures or severe neurological features, with the dose set by current paediatric guidance.
- Set the correction limit: plan a rise of no more than about 8 to 10 mmol/L in 24 hours, and check serum sodium every few hours.
- Correct hypernatraemia slowly: reduce sodium by no more than about 10 to 12 mmol/L in 24 hours, replacing free water at a controlled rate.
A rise that looks safe over one hour can still exceed the daily limit, so the 24-hour total must be tracked.
Where the exam tries to catch you
Sodium questions turn on the direction of urine concentration and the speed of correction. Each trap below mixes a cause or a rate with its opposite.
- Trap: correcting chronic hyponatraemia by 15 mmol/L overnight. Correct: the limit is about 8 to 10 mmol/L in 24 hours, because faster rises risk osmotic demyelination.
- Trap: giving rapid free water to a child with diarrhoeal hypernatraemia. Correct: free water must be given slowly, because a rapid fall in sodium can cause cerebral oedema.
- Trap: attributing hyponatraemia with concentrated urine to diabetes insipidus. Correct: diabetes insipidus gives dilute urine with high serum sodium, whereas SIADH gives hyponatraemia with concentrated urine.
Frequently asked questions
What is the safe rate of sodium correction in chronic hyponatraemia?
Serum sodium should rise by no more than about 8 to 10 mmol/L in any 24-hour period in chronic hyponatraemia. Faster correction risks osmotic demyelination, and high-risk children need a lower limit. Serum sodium should be checked regularly throughout the correction.
Why does rapid correction of hyponatraemia cause osmotic demyelination?
Brain cells adapt to chronic hyponatraemia by losing osmolytes, so a rapid rise in serum sodium draws water out of them. This shrinkage damages myelin, most notably in the pons, producing central pontine myelinolysis. Its features appear after a lucid interval of days.
What are the causes of hypernatraemia in children?
Hypernatraemia in children is caused mainly by water loss, such as in diarrhoea and vomiting with inadequate fluid intake, or by diabetes insipidus. Excess sodium from concentrated feeds or salt-containing home remedies can also cause it. Hypernatraemia with dilute urine points towards diabetes insipidus.
Which features suggest that hyponatraemia is severe?
Seizures, reduced consciousness and persistent vomiting suggest severe symptomatic hyponatraemia, because cerebral oedema may be present. These children need a small bolus of hypertonic 3 percent saline with close monitoring. Severity depends on both the sodium level and how quickly it fell.
What is the difference between SIADH and diabetes insipidus?
SIADH causes hyponatraemia with inappropriately concentrated urine, because antidiuretic hormone is released despite low serum osmolality. Diabetes insipidus causes hypernatraemia with dilute urine, because antidiuretic hormone is deficient or the kidney is resistant to it. The two are opposite disorders of water balance.