![]() However, in cases of bilateral hydronephrosis, severe hydronephrosis in a solitary kidney, elevated creatinine levels, urinary tract infection, suspected perforation, or posterior urethral valve, early neonatal US may require urgency. Therefore, it is more appropriate to perform the first urinary US examination usually after first week of birth ( 10, 11). If US is performed in the first postnatal days, mild hydronephrosis may not be detected or the degree of hydronephrosis may appear milder than the fact due to transient dehydration as a result of physiological oliguria in the early postnatal period and subsequent polyuria. All newborns with a history of antenatal hydronephrosis should be evaluated by US in postnatal period ( 9). ![]() The widespread use of antenatal US screening leads to a significant increase in the detection rate of UPJHN ( 8). This method has lots of advantages such as being safe and non-invasive, cheap, easily accessible in most institutions and also being repeatable with using no radiation exposure. Ultrasonography (US) is the main imaging study used for evaluating the urinary system in the postnatal period in children with suspected or diagnosed prenatal hydronephrosis ( 7). The purpose of this review is to discuss the radiological findings of hydronephrosis related to UPJ obstruction under the title of “ureteropelvic junction type hydronephrosis (UPJHN),” based mainly on ultrasonography and other imaging methods. Imaging methods play an important and crucial role at this point. All efforts are made to recognize which cases to follow and which ones to treat. Therefore, the differentiation of true obstruction from urinary tract dilatation is crucial to avoid unnecessary surgical intervention. Especially in newborns and infants, hydronephrosis develops as a useful adaptation mechanism that protects the kidney from high pressure and damage secondary to the good compliance of the renal pelvis, not as a result of obstruction ( 6). But the majority of cases resolve spontaneously without a real obstruction and renal damage. UPJ obstruction might lead to progressive damage to the renal function by increasing back pressure on the kidney ( 5). As an intrinsic cause of obstruction abnormally developed ureteral smooth muscle at the UPJ resulting in an aperistaltic segment is considered, while extrinsic obstruction is thought to be caused by an overlying renal vessel ( 3, 4). Many theories have been put forward to explain the pathophysiology however, the cause is not clear. Ureteropelvic junction (UPJ) obstruction is the most common cause of pathologic obstructive hydronephrosis in children which is defined as a partial or complete obstruction of the flow of urine from the renal pelvis to the proximal ureter ( 1, 2). This imaging technique currently allows for the detailed assessment of urinary tract anatomy, while also providing information regarding renal function, including differential renal function, and the presence or absence of obstructive uropathy. Magnetic Resonance Urography has progressed significantly in recent years due to the development of both hardware and software that are used to generate high-resolution images. Computerized Tomography angiography with multiplanar reformation and three-dimensional images may be used to depict suspected crossing vessels as a cause of UPJ obstruction in older children and adults. The voiding/micturating cystourethrogram helps exclude other causes of upper tract dilatation, including vesicoureteral reflux, urethral valves, and ureteroceles. Abdominal radiographs may show soft tissue fullness, bulging of the flank, and displacement of bowel loops from the affected side. ![]() This imaging method also has been used differentiating obstructive from non-obstructive hydronephrosis by renal arterial resistive index measurements. Doppler ultrasound may identify a crossing vessel, when present. A properly performed study provides essential information regarding laterality, renal size, thickness, and architecture of the renal cortex and degree of dilatation of the pelvicalyceal system. On ultrasound, abnormal dilatation of the pelvicalyceal system of varying degrees is seen, whereas the ureter is normal in caliber. Ultrasound is the main imaging study used to diagnose ureteropelvic junction (UPJ) obstruction.
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