LAUSR

The “balloon on a string” (Fig. 1) appearance of the renal collecting system and ureter is a classic imaging sign applied to ureteropelvic junction obstruction (UPJO) [1]. This sign may be seen with traditional radiographic evaluation of the kidney, such as with IV urography (excretory urography) or retrograde pyelography (Fig. 2), but it can also be seen with CT urography (Fig. 3) or MR urography. UPJO is most often congenital and can be separated into intrinsic and extrinsic causes. The reason for intrinsic obstruction is incompletely understood, but it is thought to be an embryologic failure of smooth muscle and/or collagen development around the UPJ, resulting in a short segment of aperistaltic ureter [2]. Etiologies such as decreased density of the interstitial cells of Cajal [3] or abnormalities of intracellular signaling in the involved ureteral segment [4] have been suggested. An extrinsic cause for UPJO is less common, with a vessel crossing over the UPJ as the most common cause. Kinks or valves in the proximal ureter can also create an image similar to a UPJO, although technically these are often just distal to the UPJ. UPJO is often detected prenatally or in childhood [5], but if it escapes detection, it may remain clinically silent. Since the obstruction is only partial, damage to the renal parenchyma is variable and renal function may be relatively preserved. UPJOs that are not detected early may present after an incident in which the rate of urine production/diuresis exceeds the rate of urine exit through the narrowed UPJ, resulting in an episode of painful hydronephrosis (a “Dietl crisis”) [6]. Ultrasound is usually the first modality to show the hydronephrosis from a UPJO, although it may not characterize the narrowing at the UPJ. If the hydronephrosis is discovered in a pediatric patient, then the likelihood that the obstruction is due to a congenital UPJO is very high and diuretic renography may be performed to quantify the rate of outflow [7]. In an adult patient, however, the likelihood of a secondary/ acquired narrowing at the UPJ increases. A urothelial lesion narrowing the lumen (such as urothelial cell carcinoma) should be considered. It could also occur from an acquired benign stricture, such as in a patient with a history of chronic renal stone disease. Imaging with CT urography tends to be performed first in the adult population and retrograde pyelography may be performed as part of a diagnostic or therapeutic procedure.