What causes bilateral hydronephrosis?

Bilateral hydronephrosis may be caused by kidney stones, retroperitoneal lesions leading to ureteral or renal pelvis obstruction, or kidney tumors. Another possible cause is ureteral narrowing due to bilateral kidney inflammation, although this is relatively rare. In such cases, once the inflammation resolves, the bilateral hydronephrosis typically disappears. Moreover, the accumulation of fluid is usually mild and can sometimes occur with excessive bladder distension, resulting in mild bilateral hydronephrosis. This type of hydronephrosis caused by mild bladder overfilling resolves after the bladder is emptied.