The most common cause of fetal hydronephrosis is ureteropelvic junction (UPJ) obstruction. This finding is characterized by varying degrees of renal pelvic dilation. The most common cause of UPJ obstruction is an intrinsic narrowing of the proximal ureter. This narrowing can be likened to a scar or stricture in the ureter as it leaves the kidney. Sometimes, the ureter can be obstructed behind a band of tissue or a blood vessel supplying the lower segment of the kidney. This entity is known as a "crossing vessel."
When a dilated ureter is detected along with the renal pelvis, the working diagnosis is vesicoureteral reflux or uretero-vesical junction obstruction. Males with significant hydroureteronephrosis occurring simultaneously with a large thin walled bladder and normal renal parenchyma will often have a condition of massive reflux known as megacystis-megaureter.
Prenatal findings of hydroureteronephrosis can be associated with other diagnoses such as posterior urethral valves, prune-belly syndrome, or duplication anomalies such as ectopic ureter or ureterocele.
The diagnosis of posterior urethral valves is reached when a male fetus has the following findings:
- Bilateral hydroureteronephrosis
- Thick-walled, moderately dilated bladder
- Dilated posterior urethra
- Kidney architectural changes
- Diminished volumes of amniotic fluid
Other genitourinary diagnoses that can be made prenatally include renal dysplasia, multicystic dysplasia, renal agenesis, renal hypoplasia, renal cystic disease, bladder/cloacal exstrophy, and external genital anomalies.
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