[The prenatal diagnosis of hydronephrosis: when and why NOT to operate?]

To determine the proper timing and indications for pyeloplasty in infants with prenatally diagnosed unilateral hydronephrosis.

Examination of the 1) biological features of hydronephrosis which are unique to the newborn, 2) natural history of non-operated newborn hydronephrosis, and 3) accuracy of tests which assess obstruction in hydronephrosis.

Prenatally diagnosed unilateral hydronephrosis is a benign disease in most infants because obstruction is not present: the hydronephrosis will improve or disappear spontaneously, and impaired renal function will improve or normal renal function will remain stable. Consequently surgical treatment is not indicated; it will neither hasten nor improve the outcome. Unfortunately, the conventional tests for assessing obstruction are invalid in the infant and obstruction must be diagnosed or excluded by newly described protocols which use serial measurement of pelvic dilation, differential renal function and renal size.

UPJ obstruction presents a diagnostic but not a therapeutic dilemma in infancy. True UPJ obstruction should be operated upon as soon after birth as possible to prevent renal deterioration. However, over 85% of infants with prenatally diagnosed hydronephrosis do not have a true obstruction and do not require surgery; they will improve spontaneously. The difficulty in deciding which patients require surgery can be minimized by a careful diagnostic protocol.