Purpose: During the last decade it has become apparent that prenatally detected, unilateral severe hydronephrosis does not necessarily represent obstruction and may spontaneously improve or resolve postnatally. To define its natural history better we performed a long-term (mean 78 months) followup study of infants with hydronephrosis.
Materials and methods: A total of 104 newborns with antenatally diagnosed, primary, unilateral severe hydronephrosis were followed nonoperatively unless evidence of renal deterioration occurred for which pyeloplasty was performed.
Results: All 23 infants (22%) who required pyeloplasty were younger than 18 months and had progressive hydronephrosis and/or reduction in differential renal function. Differential function exceeded predeterioration levels in all kidneys postoperatively. Of those cases followed nonoperatively hydronephrosis resolved in 69% and improved in 31%. Mean time to maximum improvement of hydronephrosis was 2.5 years. In 76% of those cases followed nonoperatively initial differential function was greater than 40% and final function averaged 49%. In the remaining 24% of cases differential function was less than 40% (mean 23%), and in an average of 18 months differential function increased to a mean of 47%. Initial half-time in nonoperative cases was greater than 30 minutes in 37%, 20 to 30 in 21% and less than 20 in 42%. Final half-time was greater than 30 minutes in 16%, 20 to 30 in 17% and less than 20 in 67%. Half-time was greater than 30 minutes in 87% of the patients and 20 to 30 in 4% before, and greater than 30 in 10%, 20 to 30 in 27% and less than 20 in 63% after pyeloplasty.
Conclusions: Unilateral newborn hydronephrosis appears to be relatively benign and in most instances dilatation and renal function improve with time. However, close followup is necessary to identify the subgroup of less than 25% of infants with obstruction because prompt pyeloplasty will prevent permanent loss of renal function. Standard tests for assessing obstruction in older patients appear to be invalid in infants because prolonged half-time and/or high grade hydronephrosis is neither an indicator of obstruction or surgery. Nonoperative treatment with close followup especially during the first 2 years is safe and recommended for these children.