Pathophysiology of ureteropelvic junction obstruction: experimental and clinical observations

We compared experimental ligature-induced ureteropelvic junction obstruction in the dog with naturally occurring ureteropelvic junction obstruction in children to determine if clinical behavior and difficulties in diagnosis could be related to different types or components of obstruction at the ureteropelvic junction. Measurements of flow rate out of the ligature-obstructed canine renal pelvis demonstrated a pressure-dependent pattern in which flow increased linearly in response to increasing pressures. In 5 human kidneys with intrinsic ureteropelvic junction obstruction a similar pressure-dependent pattern was demonstrated. This was in contrast to 6 human kidneys with extrinsic mechanical ureteropelvic junction obstruction in which a volume-dependent pressure flow pattern occurred, such that urinary flow rate did not keep pace with increases in pelvic pressure. In some cases flow actually decreased at high pressures because the ureteropelvic junction became self-obstructing as the pelvis enlarged. These findings indicate that the precise pathological anatomy of the ureteropelvic junction defines the pattern of flow across the obstruction. The 2 different types of obstruction, pressure-dependent and volume-dependent flow restrictions, which exist are important determinants of the clinical behavior of the obstructed kidney insofar as its potential for progressive hydronephrosis. They also help to explain why diagnostic tests for assessing obstruction in hydronephrosis are inaccurate at times.