Urogenital ImagingHydronephrosis
dilatation of the intrarenal collecting system. It may be divided into obstructed and nonobstructed. In the former group, the
obstruction may be mechanical or functional and may occur anywhere in the collecting system. If the
obstruction occurs more distally in the
ureter, then hydroureter (dilatation of the
ureter due to a distal
obstruction) is also seen. Among the diverse causes, the more common ones include: urolithiasis,
tumour, stricture, vesicoureteric reflux,
congenital abnomalities (ureteropelvic junction
obstruction, posterior urethral valves), and extrinsic compression (retroperitoneal
fibrosis or lymphandenopathy). Nonobstructed hydronephrosis, also known as "urinary ileus", may be associated with previous
obstruction or urinary infection.
In the acute state, the affected kidney is enlarged and of smooth contour. Atrophy and fibrosis are seen with long-standing hydronephrosis. Infection is a serious complication of hydronephrosis. An infected and obstructed collecting system is known as pyonephrosis, and the affected patient can quickly succumb to sepsis.
Hydronephrosis is easily detected by ultrasound (Fig.1). A dilated intrarenal collecting system is imaged as enlarged anechoic fluid structures, which communicate with each other to form the renal pelvis. In contrast, the multiple cysts in polycystic renal disease are separate and do not communicate with each other. Internal echoes are seen in the dilated collecting system in the setting of pyonephrosis. While ultrasound usually cannot distinguish between obstructed and nonobstructed hydronephrosis, excretory urography may provide additional information in that it may demonstrate the level of obstruction (Fig.2). Forniceal rupture is a complication of hydronephrosis in excretory urography; it may result from a sudden increase in urine volume due to contrast-induced diuresis.
Hydronephrosis can also be imaged by CT (Fig.3) or MRI, though the expense of MRI precludes its use given the usefulness of the other imaging modalities for this purpose. CT is particularly helpful when imaging hydronephrosis due to obstruction from an abdominal or pelvic tumour. Not only can it depict the level of obstruction, CT can often demonstrate the cause and its extent, as well as mass effect upon the adjacent structures.
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