Aetiology

Water overload:

• secondary to ADH secretion commonly occuring in hyaline membrane disease, ventilation pneumothorax or intraventricular haemorrhage
• in the first 24 hours due to excess fluid administration to the mother or child
• in acute renal failure before fluid restriction

Increased sodium losses:

• excess renal loss in the preterm baby is exacerbated by diuretic or aminophylline therapy
• inherited tubular disorders
• adrenal insufficiency - with dehydration and hyperkalaemia
• excess gastrointestinal losses in gastroenteritis

Inadequate sodium intake:

• late hyponatremia may be caused by inadequate sodium content during intravenous nutrition
• in the preterm after 2 weeks of low solute breast milk or formula. Preterm milk has a higher sodium concentration and should not be used after 4 to 6 weeks unless high sodium losses continue