Percutaneous Endopyelotomy for the Treatment of Ureteropelvic Junction Obstruction

Flavio Castaneda, M.D.


Ureteropelvic junction (UPJ) obstruction occurs where the ureter joins the kidney. It may be attributable to extrinsic factors, but more often the cause is faulty development. Because the helical arrangement of muscle fibers needed for urine transport develops progressively, arrest of this process results in only circularly oriented fibers at the UPJ, an arrangement more likely to be obstructive than conductive. Elongation of the ureter, leaving predominantly longitudinal fibers, would likewise be a poorly conducting arrangement. Over developmental errors, such as agenesis or reduction in fiber numbers, also leads to a nonconducting segment. Extrinsic causes of UPJ obstruction are usually an aberrant vessel or a band passing anterior to the junction, although this structure may contribute to intrinsic causes of UPJ obstruction.

PATHOLOGY AND ITS IMPLICATIONS FOR TREATMENT
INTRINSIC CAUSES

Adynamic Segment
There is increasing evidence that most idiopathic UPJ obstructions are functional rather than anatomic. (2-7) In the majority of cases, the obstructive segment is probably patent and calibrates normally to 14 Fr. or 16 Fr. Functional obstruction results from either failure to propagate the peristaltic wave across the UPJ (3,5) or lack of coordination between the contractions of the renal pelvis and the upper ureter. (2) Overdistention of the renal pelvis contributes to peristaltic ineffectiveness and probably makes functional obstruction worse. (8,9)

The most common microscopic abnormalities in an adynamic segment are excessive collagen between muscles bundles, (4-6) deficient or absent muscle, (3) and an abnormal preponderance of longitudinal muscle fibers. (2) Thus, there are interesting similarities between an obstructive UPJ and primary obstructive megaureter.

Intrinsic Stenosis
An inuinsic stenosis is a true narrowing of the ureteral lumen, ranging in length from 1 or 2 mm to several centimeters. (3) The tissue abnormalities are similar to those found in the adynamic segment. (3,5,6)

Valves and Folds
A true valve is a structure that mechanically impairs antegrade flow of urine but allows free passage in the opposite direction. Valves are an occasional cause of congenital UPJ obstruction. (3,10,11) More commonly, valves occur in the lower ureter. (1) Fetal folds are almost always non-obstructive (12) and disappear as the child grows, but occasionally overdeveloped folds cause a valve-like obstruction at or near the UPJ.

Tumors
Benign and malignant tumors occasionally mimic idiopathic UPJ obstruction. Fibroepithelial polyps, (13-18) transitional cell papilloma and carcinoma, (19-21) and renal cell carcinoma can all obstruct the UPJ.

Organ-Compressing Lesions
Retroperitoneal fibrosis, carcinomatous nodial disease, metastatic tumors, and inflammatory bowel disease are potenial causes of intrinsic UPJ obstruction.

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