Obstruction to the flow of urine, with attendant stasis and elevation in urinary tract pressure, impairs renal and urinary conduit functions and is a common cause of acute and chronic kidney disease (obstructive nephropathy). Early recognition and prompt treatment of urinary tract obstruction (UTO) can prevent or reverse devastating effects on kidney structure and function, and decrease susceptibility to hypertension, infection, and stone formation. Chronic obstruction may lead to permanent loss of renal mass (renal atrophy) and excretory capability. Since obstructive disease may be secondary to serious underlying inflammatory, vascular, or malignant disease, familiarity with clinical findings, appropriate diagnostic testing, and therapeutic approach is of great importance to the clinician.
Obstruction to urine flow can result from intrinsic or extrinsic mechanical blockade as well as from functional defects not associated with fixed occlusion of the urinary drainage system. Mechanical obstruction can occur at any level of the urinary tract, from within the renal tubules, or the renal calyces to the external urethral meatus (obstructive uropathy). Normal points of narrowing, such as the ureteropelvic and ureterovesical junctions, bladder neck, and urethral meatus, are common sites of obstruction. When lower UTO is above the level of the bladder, unilateral dilatation of the ureter (hydroureter) and renal pyelocalyceal system (hydronephrosis) occurs; lesions at or below the level of the bladder cause bilateral involvement.
Common forms of obstruction are listed in Table 313-1. Childhood causes include congenital malformations, such as narrowing of the ureteropelvic junction (UPJ) and abnormal insertion of the ureter into the bladder, the most common cause. Vesicoureteral reflux in the absence of urinary tract infection or bladder neck obstruction often resolves with age. Reinsertion of the ureter into the bladder is indicated if reflux is severe and unlikely to improve spontaneously, if renal function deteriorates, or if urinary tract infections recur despite chronic antimicrobial therapy. Vesicoureteral reflux may cause prenatal hydronephrosis and, if severe, can lead to recurrent urinary infections, hypertension and renal scarring in childhood. Posterior urethral valves are the most common cause of bilateral hydronephrosis in boys. In adults, UTO is due mainly to acquired defects. Pelvic tumors, calculi, and urethral stricture predominate. Ligation of, or injury to, the ureter during pelvic or colonic surgery can lead to hydronephrosis which, if unilateral, may remain undetected. Obstructive uropathy may also result from extrinsic neoplastic (carcinoma of cervix or colon) or inflammatory disorders. Lymphomas and pelvic or colonic neoplasms with retroperitoneal involvement are causes of ureteral obstruction. As many as 50% of men aged >40 years may have lower urinary tract symptoms associated with benign prostatic hypertrophy, but these symptoms may occur without bladder outlet obstruction.
TABLE 313-1Common Mechanical Causes of Urinary Tract Obstruction