 Hydronephrosis can be classified by cause and by unilateral versus bilateral involvement. Obstructive hydronephrosis can be further classified by level of obstruction, complete versus partial obstruction, and intrinsic versus extrinsic obstruction [1]. Causes of hydronephrosis include urolithiasis, malignant obstruction, stricture, upper urinary tract infection, traumatic or ischemic injury, postradiation changes, retroperitoneal fibrosis, supravesical or bladder dysfunction, bladder outlet obstruction as with prostatic hyperplasia, mechanical compression as with enlarged uterus or pelvic organ prolapse, endometriosis, schistosomiasis, drug effects as with cyclophosphamide or ketamine, vessel-related and/or congenital ureteropelvic junction obstruction, and congenital posterior urethral valves [1-3]. Hydronephrosis also occurs frequently during pregnancy; asymptomatic hydronephrosis occurs in an estimated 70% to 90% of pregnant patients, typically asymmetrically prominent on the right, due to a combination of mechanical obstruction from an enlarged uterus and collecting system smooth muscle relaxation due to progesterone [4]. Approximately 0.2% to 4.7% of pregnant patients experience symptomatic hydronephrosis, with the prevalence higher in advancing trimesters and in multiparous patients [5-9]. Symptomatic hydronephrosis may lead to preterm labor or maternal/fetal death when left untreated [9]. Special Imaging Considerations Scintigraphic diuresis renography (DRG) uses nuclear medicine and the administration of a diuretic to differentiate nonobstructive hydronephrosis from hydronephrosis due to true functional obstruction. Administration of a selected radiotracer such as diethylenetriamine pentaacetic acid (DTPA) or mercaptoacetyltriglycine (MAG3) is used to demonstrate flow during the perfusion, extraction, and excretion phases. Delayed planar imaging can also be performed.
Hydronephrosis on Prior Imaging Unknown Cause.  Hydronephrosis can be classified by cause and by unilateral versus bilateral involvement. Obstructive hydronephrosis can be further classified by level of obstruction, complete versus partial obstruction, and intrinsic versus extrinsic obstruction [1]. Causes of hydronephrosis include urolithiasis, malignant obstruction, stricture, upper urinary tract infection, traumatic or ischemic injury, postradiation changes, retroperitoneal fibrosis, supravesical or bladder dysfunction, bladder outlet obstruction as with prostatic hyperplasia, mechanical compression as with enlarged uterus or pelvic organ prolapse, endometriosis, schistosomiasis, drug effects as with cyclophosphamide or ketamine, vessel-related and/or congenital ureteropelvic junction obstruction, and congenital posterior urethral valves [1-3]. Hydronephrosis also occurs frequently during pregnancy; asymptomatic hydronephrosis occurs in an estimated 70% to 90% of pregnant patients, typically asymmetrically prominent on the right, due to a combination of mechanical obstruction from an enlarged uterus and collecting system smooth muscle relaxation due to progesterone [4]. Approximately 0.2% to 4.7% of pregnant patients experience symptomatic hydronephrosis, with the prevalence higher in advancing trimesters and in multiparous patients [5-9]. Symptomatic hydronephrosis may lead to preterm labor or maternal/fetal death when left untreated [9]. Special Imaging Considerations Scintigraphic diuresis renography (DRG) uses nuclear medicine and the administration of a diuretic to differentiate nonobstructive hydronephrosis from hydronephrosis due to true functional obstruction. Administration of a selected radiotracer such as diethylenetriamine pentaacetic acid (DTPA) or mercaptoacetyltriglycine (MAG3) is used to demonstrate flow during the perfusion, extraction, and excretion phases. Delayed planar imaging can also be performed.