A ureterocele is a cystic dilatation that occurs at the distal end of the ureter at its junction with the bladder. Simple ureteroceles are those that are located in the normal location of the ureteral orifice in the trigone of the bladder. Simple ureteroceles are more commonly detected in adults and usually are associated with a single collecting system. Simple ureteroceles are rarely associated with the upper pole ureter of a complete duplication of the collecting system. Simple ureteroceles may be associated with a varying degree of obstruction, but this is not significant in most patients. In contrast, with an ectopic ureterocele the ureteral orifice is located in an ectopic position, usually distal to the trigone, and this form is typically associated with a duplex collecting system. Ectopic ureterocele is also associated with an increased incidence of duplication in the contralateral kidney. The ureterocele may vary in size, from a tiny cystic lesion within the submucosal ureter, to that of a large cystic balloonlike structure that fills the bladder. The size of the ureterocele may fluctuate from one examination to the next. Histologically, the ureterocele is covered by the mucosa of the bladder and lined by the mucosa of the ureter, with varying degrees of attenuated smooth muscle bundles and connective tissue between the two layers of mucosa.

Several theories have been proposed to account for the embryonic development of a ureterocele. In one theory, a membrane covering the ureteral orifice persists for a long period, leading to the development of the ureterocele (Chwalle, 1927). In another theory, a ureterocele forms as a result of a stimulus to an expansion that transforms the bladder into a globular cap, creating an expanded thin-walled distal ureter (Stephens, 1971). Lastly, a localized embryonic arrest has been hypothesized as the cause of the ureterocele (Tokunaka et al., 1981).

Ectopic ureterocele has been classified anatomically by Stephens (1958, 1983). In this classification scheme, stenotic ectopic ureterocele is characterized by a small stenotic orifice and accounts for approximately 40% of cases. In sphincteric ectopic ureterocele, the orifice of the ureterocele is within the internal sphincter and accounts for an additional 40% of cases. In sphincteric ectopic ureterocele the ureteral orifice may be of normal caliber or may be enlarged and may open either in the posterior urethra in males or distal to the external sphincter in females. Sphincterostenotic ectopic ureterocele accounts for approximately 5% of cases. The stenotic orifice of this type of ectopic ureterocele is located within the urethral floor. There are other rarer types of ectopic ureterocele, including cecoureterocele, in which the lumen extends distal to the level of the orifice as a long tongue or cecum; blind ectopic ureterocele, in which there is no orifice so that the ureter is completely obstructed; and nonobstructive ectopic ureterocele, in which the orifice is enlarged within the bladder.

In complete ureteral duplication, the ureter draining the upper pole moiety usually opens caudal and medial to the ureter draining the lower pole moiety. This upper pole moiety ureter is commonly associated with ureterocele. When a duplex system is found to be present on one side, there is contralateral duplication in approximately 50% of the cases. This occurs far more frequently in females and can potentially result in bladder outlet obstruction if the ureterocele is sufficiently large. The orifice of the ureterocele is usually obstructed either by stenosis or because it opens at the level of the urogenital diaphragm and is obstructed by the closed bladder neck. Not only can the ureterocele cause obstruction of the ureter from which it arises, but the ureterocele can also result in obstruction of the ureter draining the lower pole moiety, placing the entire kidney at risk. In ectopic ureterocele the upper pole moiety is commonly dysplastic and has poor or no function because of long-standing high-grade obstruction. The ectopic ureterocele may also affect the contralateral ureter and renal unit. Large ectopic ureteroceles may obstruct the ipsilateral lower pole moiety and, if extensive enough, they may even obstruct the contralateral side or induce vesicoureteral reflux (Koyanagi et al., 1980; Harrison and Filly, 1991).

The incidence of ureterocele is estimated to be 1 in 4000 autopsies in children (Brock and Kaplan, 1978). Ureteroceles are far more common in whites than in blacks. Ureteroceles are also five times more common among girls than among boys (Brock and Kaplan, 1978). Approximately 80% of ureteroceles are associated with a duplex collecting system. No information is available about the prenatal incidence of ectopic ureteroceles.

The common mode of presentation for a fetus with ectopic ureterocele is unilateral hydronephrosis. The dilated upper pole of a duplex collecting system and a normal-appearing lower pole collecting system are key features that permit an accurate sonographic diagnosis of an obstructed duplex collecting system (Mahony, 1994). The dilated upper pole moiety may enlarge sufficiently so that it displaces the nondilated lower pole of the kidney inferiorly and laterally. Sonographic detection of two distinct ureters may be quite difficult in utero. It is important to search for a thin-walled fluid-filled ectopic ureterocele within the bladder (Figure 83-1). This may be very challenging, as the ureterocele may expand and decompress with ureteral peristalsis and bladder emptying. The ureterocele may also prolapse into the urethra if intravesical pressure exceeds intraurethral pressure. The ureterocele will initially collapse then subsequently prolapse or herniate into the urethra (Cremin et al., 1977; Fitzsimmons et al., 1986). Ultrasonographers should be mindful of this potential diagnostic pitfall, especially since ultrasonographic urinary bladder examinations with a full bladder are the very condition most likely to cause difficulty in identifying ureteroceles because of the flattening or intraurethral prolapse it causes. In every patient in whom hydronephrosis is antenatally diagnosed, a detailed examination of the fetal bladder should be performed to look for an intravesical cystic structure or the appearance of an extravesical diverticulum due to the intraurethral prolapse of the ureterocele during micturition (Fitzsimmons et al., 1986).