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). With early relief of obstruction, the defects in function usually disappear completely. However, chronic obstruction may produce permanent loss of renal mass (renal atrophy) and excretory capability, as well as enhanced susceptibility to local infection and stone formation. Early diagnosis and prompt therapy are, therefore, essential to minimize the otherwise devastating effects of obstruction on kidney structure and function.
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 the renal calyces to the external urethral meatus. Normal points of narrowing, such as the ureteropelvic and ureterovesical junctions, bladder neck, and urethral meatus, are common sites of obstruction. When obstruction 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 343-1. Childhood causes include congenital malformations, such as narrowing of the ureteropelvic junction 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 and renal scarring in childhood. Posterior urethral valves are the most common cause of bilateral hydronephrosis in boys. In adults, urinary tract obstruction (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 over 40 years old may have lower urinary tract symptoms associated with benign prostatic hypertrophy, but these symptoms may occur without bladder outlet obstruction.
TABLE 343-1Common Mechanical Causes of Urinary Tract Obstruction |Favorite Table|Download (.pdf) TABLE 343-1 Common Mechanical Causes of Urinary Tract Obstruction
|Ureter ||Bladder Outlet ||Urethra |
|Congenital || || |
Ureteropelvic junction narrowing or obstruction
Ureterovesical junction narrowing or obstruction and reflux
Bladder neck obstruction
Posterior urethral valves