Renal pelvis volume during diuresis in children with hydronephrosis: implications for diagnosing obstruction with diuretic renography.

PURPOSE: We measured the volume of the renal pelvis during diuretic renography (DR) in children with normal and hydronephrotic kidneys to determine if changes in pelvic volume could affect the accuracy of DR in diagnosing obstruction.

MATERIALS AND METHODS: We studied 18 patients 1 month to 10 years old with unilateral hydronephrosis ultimately proved to be either obstructive or nonobstructive. Simultaneous DR and ultrasound were performed with patients supine using the gamma camera. Ultrasound measurements of the renal pelvis in 3 dimensions, obtained before and at intervals after diuretic injection, were used to calculate renal pelvic volume. The contralateral normal kidneys were used as controls.

RESULTS: Between 15 and 60 minutes after diuretic injection the renal pelvis enlarged to a maximum volume in all hydronephrotic and normal kidneys and then gradually decreased in size. Mean average increase in volume for hydronephrotic kidneys ranged from 46% in obstructed kidneys to 88% in nonobstructed kidneys. Volume expansion caused dilution of isotope within the renal pelvis, which resulted in prolongation of elimination half-time (T1/2) in 42% of nonobstructed hydronephrotic kidneys sufficient to register an obstructed washout pattern. However, there were no differences in the initial pelvic volume or the rate or extent of increases or decreases in pelvic volume that would permit nonobstructed hydronephrotic kidneys to be distinguished from obstructed ones.

CONCLUSIONS: The renal pelvis enlarges during diuresis in children with hydronephrosis. This enlargement causes dilution of isotope within the renal pelvis during DR, which prolonged the isotope washout rate or T1/2 sufficiently to produce an obstructed washout pattern in more than 40% of hydronephrotic kidneys that were ultimately proved to be nonobstructed. This misdiagnosis of obstruction is particularly likely to occur in children younger than 2 years because pelvic volume expansion is so exaggerated. Consequently, T1/2 appears to be particularly vulnerable to inaccuracy in diagnosing obstruction in this age group, and, therefore, it should not be relied on as an operative determinant.