The causes of obstruction of the ureter at the ureteropelvic and ureterovesical junctions are still controversial. In this study portions of obstructed ureters were excised at operation or autopsy and the suspected sites of obstruction were examined anatomically, radiographically and histologically. A valvular mechanism caused obstruction in 15 of 27 specimens. The valve lay at the junction of the dilated and undilated segments of the ureter. The internal anatomy of this site was hidden from external view by the fascia overlying the ureter and was demonstrated by contrast ureterograms. The axes of the lumina of the dilated an undilated segments were eccentric and the walls of the ureter overlapped at the site of obstruction to form a common wall, which created a valve effect. The valve flaps were visualized radiographically as a filling defect. The valve obstructed antegrade but not retrograde flow. Anatomic dissection or serial sections of blocks of the valve zone demonstrated that either a single or a double flap caused obstruction. The valve was coapted against the opposite wall of the distal segment by the pressure generated in the dilated ureter during antegrade flow. The muscle of the unobstructed segment of ureter distal to the valve appeared normal histologically, whereas the muscle of the obstructed proximal segment was hypertrophic.
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