Mechanisms of Renal Damage

The destructive effects of urinary obstruction on kidney structures are determined by the degree of obstruction (i.e., partial versus complete, unilateral versus bilateral) and the duration of obstruction. The two most damaging effects of urinary obstruction are stasis of urine, which predisposes to infection and stone formation, and development of a back pressure, which interferes with blood flow and destroys renal tissue. 

A common complication of urinary tract obstruction is infection. Stagnation of urine predisposes to infection, which may spread throughout the urinary tract. When present, urinary calculi serve as foreign bodies and contribute to the infection. Once established, the infection is difficult to treat. It often is caused by urea-splitting organisms (e.g., Proteus, staphylococci) that increase ammonia production and cause the urine to become alkaline. Calcium salts precipitate more readily in stag nant alkaline urine; urinary tract obstructions also pre dispose to stone formation. 

In situations of marked or complete obstruction, a back pressure develops because of a combination of continued glomerular filtration and impedance to urinary flow. Prolonged or severe partial obstruction causes irreversible kidney damage. Depending on the degree of obstruction, pressure builds up, beginning at the site of obstruction and moving backward from the ureter or renal pelvis into the calices and collecting tubules. Typically, the most severe effects occur at the level of the papillae, because these structures are subjected to the greatest pressure. Damage to the nephrons and other functional components of the kidneys is caused by compression from increased intrapelvic pressure and ischemia from disturbances in blood flow. Experiments have shown recovery of renal function after release of complete obstruction of up to 4 weeks’ duration. Irreversible damage can, however, begin as early as 7 days. 

Dilatation of the ureters and renal pelvis occurs with prolonged urinary tract obstruction. When the obstruction is in the distal ureter, the increased pressure dilates the proximal ureter, a condition called hydroureter . Hydroureter is also a complication of bladder outflow obstruction owing to prostatic hyperplasia. With increasing pressure, the ureteral wall becomes severely stretched and loses its ability to undergo peristaltic contractions. In extreme cases, the ureter may become so dilated that it resembles a loop of bowel. Hydronephrosis refers to urine-ifiled dilatation of the renal pelvis and calices. The degree of hydronephrosis depends on the duration, degree, and site of obstruction. Bilateral hydronephrosis occurs only when the obstruction is below the level of the ureters. If the obstruction occurs at the level of the ureters or above, hydronephrošis is unilateral. The kidney eventually is destroyed and appears as a thin-walled shell that is filled with fluid.

Manifestations

The manifestations of urinary obstruction depend on the site of obstruction, the cause, and the rapidity with which the condition developed. Most commonly, the person has pain, signs and symptoms of urinary tract infection, and manifestations of renal dysfunction, such as an impaired ability to concentrate urine. Changes in urine output may be misleading, because output may be normal or even high in cases of partial obstruction.

Pain, which often is the factor that causes a person to seek medical attention, is the result of distention of the bladder, collecting system, or renal capsule. Its severity is most closely related to the rate rather than the degree of distention. Pain most often occurs with acute obstruction,. in which the distention of urinary structures is rapid. This contrasts with chronic obstruction, in which distention is gradual and may not cause pain.

Instead, gradual obstruction may produce only vague abdominal or back discomfort. When pain occurs, it is related to the site of obstruction. Obstruction of the renal pelvis or upper ureter causes pain and tenderness over the flank area. With lower levels of obstruction, the pain may radiate to the testes in the male or the labia in the female. With partial obstruction, particularly of the ureteropelvic junction, pain may occur during periods of high fluid intake, when a high rate of urine flow causes an acute hydronephrosis. Because of its visceral innervation, ureteral obstruction may produce reflex impairment of gastrointestinal tract peristalsis and motility with abdominal distention and, in severe cases, paralytic ileus.

Hypertension is an occasional complication of urinary tract obstruction. It is more common in cases of unilateral obstruction in which renin secretion is enhanced, probably secondary to impaired renal blood flow. In these circumstances, removal of the obstruction often leads to a reduction in blood pressure. When hypertension accompanies bilateral obstruction, renin levels usually are normal, and the elevated blood pres. sure probably is volume related. The relief of bilateral obstruction leads to a loss of volume and a fall in blood pressure.11 In some cases, relieving the obstruction does not correct the hypertension.

Diagnosis and Treatment

Early diagnosis of urinary tract obstruction is important because the condition usually is treatable and a delay in therapy may result in permanent damage to the kidneys. Diagnostic methods vary with the symptoms. For example, a distended bladder suggests prostatic hyperplasia in the male. Radiologic methods commonly are used. The opaque kidney stones often are visible on x ray films. Excretory urography, Ct and renal scanning using a radiopharmaceutical agent such as gallium may be used. Ultrasonography has proved to be the single most useful noninvasive diagnostic modality for urinary obstruction. Other diagnostic methods, such as urinalysis, are used to determine the extent of renal involvement and the presence of infection. Treatment of urinary obstruction depends on the cause. Urinary stone removal may be necessary, or surgical treatment of structural defects may be indicated.