The assessment of obstruction in the newborn with unilateral hydronephrosis by measuring the size of the opposite kidney.

We measured the size of the normal kidney opposite a unilateral hydronephrotic kidney in infants to determine if compensatory changes occurred and could be used as a diagnostic test for defining or excluding obstruction. Comparison of subgroups of neonates with unilateral hydronephrosis or multicystic renal dysplasia to normal controls demonstrated that compensatory changes do occur in the normal kidney. Normal kidneys opposite obstructed hydronephrotic kidneys requiring surgery became larger than normal for age. Normal kidneys opposite nonobstructed poorly functioning hydronephrotic kidneys whose function rapidly improved were smaller than normal for age. These changes in renal growth by the normal newborn kidney reflect renal counterbalance, which is exaggerated in this age group and which may be used to corroborate rapid changes in renal function caused by the presence or absence of obstruction. By plotting serial measurements of normal renal length on a renal growth chart, the diagnosis of obstruction in newborn hydronephrosis can be facilitated and the clinical management of the patient improved.