Fetal congenital midaortic syndrome with unilateral renal artery stenosis prenatally presenting with polyhydramnios and postpartum as hyponatremic hypertensive syndrome.

The midaortic syndrome (MAS) is a rare anomaly, characterised by narrowing of the distal aorta and its major branches. The most common symptom is severe arterial hypertension. The combination of hyponatremia, polyuria and renovascular hypertension caused by a unilateral renal artery stenosis is described as hyponatremic hypertensive syndrome. We report a case of MAS with unilateral renal artery stenosis in a preterm female neonate. A pregnant woman at 34 weeks of gestation was referred with fast growing abdominal circumference and pain. The ultrasound revealed severe polyhydramnios and fetal myocardial hypertrophy. Within the first 48 hours of the neonatal period, the diagnosis of MAS was made. We conclude that symptomatic MAS, caused by unilateral renal artery stenosis, resulting in increased renin-angiotensin-aldosterone system activity and subsequent polyuria of the non-stenotic kidney, lead to clinically significant polyhydramnios.