Hyponatraemic hypertensive syndrome in two extremely low birthweight infants

Two cases of hyponatraemic hypertensive syndrome occurring in extremely low birthweight infants are presented. Both infants experienced unilateral renal ischaemia resulting in hyponatraemia and hypertension. A proposed pathophysiological mechanism, namely unilateral renal ischaemia leading to a pressure-natriuresis in the contralateral kidney, is presented. This is associated with an increase in plasma renin and aldosterone, with a paradoxical increase in urinary sodium loss. Immature renal tubular function and relative aldosterone resistance could place the extremely low birthweight infant at increased risk for the condition. The paucity of reports suggests that the condition might be under-recognized.