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Ureteropelvic junction (UPJ) obstruction is the most common cause of antenatal hydronephrosis, occurring when urine flow is impeded at the junction between the renal pelvis and ureter. This condition can be congenital due to intrinsic narrowing or extrinsic compression, or acquired from scarring, stones, or inflammatory processes. Early diagnosis and appropriate management are crucial to prevent progressive renal damage and preserve kidney function.
UPJ obstruction results from functional or anatomical narrowing at the ureteropelvic junction, leading to impaired urine drainage from the renal pelvis into the ureter. Intrinsic causes include aperistaltic ureteral segments, mucosal folds, or strictures, while extrinsic causes involve crossing vessels compressing the UPJ or external masses. The resulting back-pressure causes progressive dilation of the collecting system (hydronephrosis) and can lead to parenchymal thinning, reduced glomerular filtration, and eventual loss of renal function if left untreated.
Diagnosis relies on imaging studies showing hydronephrosis with a dilated renal pelvis and normal-caliber ureter below the UPJ, often identified on prenatal ultrasound or postnatal evaluation for UTIs or abdominal pain. Nuclear medicine scans (MAG3 or DTPA) with furosemide challenge help differentiate true obstruction from non-obstructive dilation by measuring drainage curves and calculating differential renal function. Treatment decisions depend on symptoms, degree of obstruction, and renal function, ranging from observation in asymptomatic cases with good function to surgical intervention (pyeloplasty) for symptomatic or functionally significant obstruction.