Fluid losses: GI, renal (polyuria), skin.Fluid intake: detailed breast/formula/PEG feeding history – check feed concentration.Secondary (CCF, nephrotic syndrome, steroids).Iatrogenic (hypertonic saline, sodium bicarbonate).Ingestion of high sodium (inappropriate formula concentration, high osmolality rehydration solutions, salt poisoning).Impaired thirst mechanism secondary to underlying neurological abnormalities or hypothalamic dysfunction.Diabetes insipidus (central, nephrogenic, systemic disease, drugs).Inability to obtain water, including breastfed babies due to inadequate milk supply.Renal losses eg osmotic diuretics, diabetes mellitus, polyuria of acute tubular necrosis.Gastrointestinal loss eg diarrhoea, stomal losses.Chronic hypernatraemia (>48 hours) is often well tolerated and asymptomatic due to cerebral compensation.Infants and small children are more vulnerable to hypernatraemia due to greater insensible losses and inability to communicate their need for fluids or access fluids independently.Moderate to severe hypernatraemia can cause acute brain shrinkage with vascular rupture, haemorrhage, demyelination and permanent neurological injury.All children with moderate or severe hypernatraemia should have a paired serum and urine osmolality, but this should not delay treatment.The rate of correction should not exceed 0.5 mmol/L/hr, ie 10-12 mmol/L per day, to avoid cerebral oedema, seizures and permanent neurological injury.Start treatment early with IV sodium chloride 0.9% + glucose 5%.
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