Kidney stone disease affects equally boys and girls, accounting for 1:1000–1:7600 hospital admissions in the USA. UTI and genetic/metabolic diseases are the leading causes for their development in children (in western world countries). Flank pain with vomiting, hematuria, and UTI with or without fever are the main symptoms. The UT is most of the time dilated because of the obstruction. Imaging, especially US, will demonstrate the dilatation as well as the lithiasis (Fig. 20.5). A lithiasis stuck within the lumbar ureter is more easily demonstrated by CT than by US (See Chap. 19). Noteworthy, several medications are lithogenic and knowing the history of the disease and the various treatments is essential [7, 8].

UTI may lead to ureteral obstruction especially in cases of fungal infections (candidiasis) (see Chap. 18). Imaging, especially US will demonstrate the dilatation as well as echogenic urine corresponding to pyonephrosis (Fig. 20.5) and thickening of the ureteral wall to be noted, as mentioned, the presence of an underlying congenital uropathy as well as the presence of urolithiasis increases the risk of ureteral obstruction [9, 10].

Some benign neoplasms of the ureter such as fibro-epithelial polyps or ureteral webs may induce ureteral obstruction. More generally any large malignant pelvic tumor may invade the ureters or the posterior part of the bladder (see below) and determine uphill dilatation of the UT [11–13].

In case of marked urinary tract dilatation due to obstruction or to reflux, on the basis of clinical, scintigraphy, and other imaging findings, surgery may be elected. A ureteral stent may or not be left after the procedure. US will be performed to assess post-operative follow-up. In most cases, the urinary tract dilatation will resolve progressively (in several weeks). A moderate persisting dilatation is acceptable and will be related to post-operative edema at the site of surgery (Fig. 20.6). This dilatation is transitory. In some other cases, hemorrhage (Fig. 20.7) or a hematoma may develop and induce pseudo-obstructive dilatation. The hematoma will appear as a collection of various echogenicity around the site of anastomosis; with time this might resolve as well. Rarely, a leakage may occur at the site of operation with a urinoma (Fig. 20.8). A persisting or increasing dilatation with reduction of the renal function on renal scan is most of the time related to a postoperative stenosis. It will be treated with a JJ stent or a redo procedure [14, 15].

Finally, acute and delayed ureteral obstruction may occur after endoscopic treatment of vesico-ureteral reflux by injections of hyaluronic polymer (Deflux^®) (Fig. 20.8). The obstruction is usually transitory and due to associated edema at the site of injection. The incidence of real obstruction is evaluated between 1–6%. On US, the Deflux^® appears as a hyperechoic nubbin located at the uretero-vesical junction (Fig. 20.8). There is no need for other imaging as US is sufficient to demonstrate post injection dilatation and the potential resolution with time [16, 17].