▶ Longitudinal ultrasound image (left) reveals a markedly dilated ureter (UR) behind the bladder (BL) with a small focus of acoustic shadowing (arrowhead).
▶ Transverse image of the bladder (right) shows a tiny echogenic focus with posterior shadowing representing a stone (arrowhead).
Dilation of the ureter may be due to vesicoureteric reflux, obstruction, or primary megaureter. In young children, the most common cause is reflux, but in this case a stone is visualized. The degree of dilation of the ureter is too great to be explained by obstruction due to ureterolith alone. Rather than the cause of dilation, the stone is the result of primary megaureter with stasis leading to stone formation.
▶ The term “primary megaureter” describes ureteral dilation due to an idiopathic congenital abnormality of the ureterovesical junction. These conditions may be further categorized as obstructed, refluxing, or nonrefluxing, nonobstructed primary megaureter.
▶ Obstructed primary megaureter is a functional obstruction with a short aperistaltic segment of distal ureter with a normal ureteral insertion analogous to Hirschsprung disease of the GI tract. As in Hirschsprung disease, it is the normal proximal ureter that becomes dilated due to the distal obstruction, while the aperistaltic segment maintains a normal caliber. Nonrefluxing, nonobstructed primary megaureter is idiopathic, but one theory is that it represents an ultra-short form of obstructed primary megaureter.
▶ Urine stasis predisposes to infection and stone formation. The stone lodges in the nondilated, aperistaltic segment.
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