Chapter 16. Obstructive Uropathy

• Urinary tract obstruction, not an uncommon problem, can occur at any age.
• It can be acute or chronic, partial or complete, with unilateral or bilateral renal involvement.
• The diagnosis of obstructive uropathy usually requires the presence of hydronephrosis (dilation of the renal pelvis and calyces), hydroureter, and/or bladder distention.

Patients presenting with unexplained acute or chronic kidney disease, with or without obstructive symptoms, should be evaluated for the possibility of obstructive uropathy. Chronic obstruction can result in chronic tubulointerstitial disease. Bladder outlet obstruction is common in older males due to prostatic hypertrophy or carcinoma. Urinary retention can be seen in both genders postoperatively and as a complication of urinary tract infection.

Patients complaining of voiding dysfunction including hesitancy, decreased force of stream, interruption in stream, or postvoid dribbling should be evaluated for the presence of urinary retention. These symptoms are usually due to prostate disease, urethral stricture, or neurogenic bladder. Medications can often be used to treat many of these symptoms and prevent complications. Patients with bladder spasticity can benefit from anticholinergic agents such as oxybutinin and propantheline bromide. Patients with bladder outlet problems can be treated with α-antagonists, which act by relaxing the smooth muscle of the bladder neck and prostate. Patients with severe bladder atony may require intermittent bladder catheterization.

Symptoms and Signs

Patients with obstruction of a solitary kidney or bilateral obstruction present with acute oligoanuric renal failure. Incomplete obstruction can result in fluctuating urine output. Pain is related to the location, duration, and severity of obstruction. Acute obstruction can result in severe pain due to distention of the collecting system or renal capsule. Renal colic due to calculi is often sudden and severe, with pain beginning in the flank and radiating into the ipsilateral groin. This can be accompanied by nausea and vomiting. Patients with renal colic prefer to be in motion, compared to patients with peritonitis, whose pain is worsened with movement.

A distended bladder on physical examination or the presence of a flank mass is suggestive of obstruction. Hypertension can be seen in obstructive uropathy due to volume expansion and activation of the renin–angiotensin–aldosterone system.

Laboratory Findings

Table 16–1 lists the common laboratory abnormalities in obstructive uropathy. The kidneys lose their ability to concentrate the urine early in obstruction. Later, they cannot concentrate or dilute urine well (isosthenuria). Defects in distal urinary acidification result in a hyperchloremic metabolic acidosis (distal renal tubular acidosis). This can be accompanied by hyperkalemia. Patients will have renal insufficiency if there is bilateral obstruction or obstruction to a solitary functioning kidney. Often the ratio of blood urea nitrogen to serum creatinine will be greater than 10:1 due to increased urea reabsorption throughout the collecting system. Patients with a partial obstruction can have nephrogenic diabetes insipidus (resistance to antidiuretic hormone) and develop hypernatremia. Patients may have polycythemia due to excess erythropoietin production, or may be anemic with more advanced renal impairment. Urinary stasis can result in urinary tract infection with urea splitting bacteria like Proteus and Staphylococcus. This results in an alkaline urine pH and is associated with struvite (magnesium, ammonium, phosphate) calculi.

Imaging Studies

Abdominal X-Ray

Abdominal x-ray is useful in identifying radiopaque calculi. However, it is often omitted as part of the evaluation for obstructive uropathy because spiral computed tomography (CT) without contrast is often performed as part of the initial evaluation.

Ultrasound

Ultrasound is 90% sensitive and specific for detecting hydronephrosis. It can be falsely negative in the presence of an early obstruction, volume depletion, or if the ureters are encased due to a retroperitoneal process. Ultrasound of the bladder reveals a thickened bladder wall with trabeculations in chronic bladder outlet obstruction. Duplex Doppler can also be performed. A high resistive index is seen from increased vascular resistance in obstruction.

Spiral Computed Tomography

Spiral CT (noncontrast) has become the radiographic study of choice to evaluate a patient for stone disease. It is a very short procedure that is highly sensitive for detecting renal and ureteric calculi.

Magnetic Resonance Urography (MRU)

MRU is available at some centers and can be performed without the use of contrast agents. Although T2-weighted MRU can determine the severity and location of obstruction, it has a sensitivity of only 70% for detecting calculi. This fact combined with its cost will likely limit the utilization of MRU.

Radioisotope Renography

Radioisotope renography with the administration of furosemide can be used to differentiate mechanical from functional obstruction.

Intravenous Pyelography (IVP)

IVP should be avoided in patients with renal insufficiency due to the risk of radiocontrast-induced acute renal injury. It may be used in patients in whom ultrasound or noncontrast spiral CT has been unable to determine the exact location of obstruction, and is helpful in diagnosing papillary necrosis. However, the use of IVP has been largely replaced by contrast-enhanced CT, which has been found to be more sensitive than IVP in determining the cause of obstruction.

Special Examinations

Documenting postvoid residual urine by bladder catheterization or ultrasound is an integral part of the initial evaluation of a patient with suspected obstructive uropathy. Patients may also need rectal and/or pelvic examinations to determine the presence of a mass (cervix, rectal, prostate) and the size of the prostate. Cystoscopy with retrograde ureterograms and percutaneous nephrostomy with antegrade ureteropyelograms are diagnostic and therapeutic procedures done to determine the site of and potentially relieve the obstruction.

As mentioned earlier, obstructive uropathy should be included in the differential diagnosis of acute or chronic renal failure. Table 16–2 lists the common causes of obstructive uropathy according to location. In men, bladder outlet obstruction due to prostate disease or urethral stricture is the most common cause of lower tract obstruction and is easily diagnosed by the assessment of postvoid residual urine (by catheter or ultrasound). In women, cervical cancer, uterine prolapse, and pregnancy are common causes of obstructive uropathy. Stones often cause unilateral upper tract obstruction. Urine flow is usually obstructed due to the presence of stones at one of the three places: The ureteropelvic junction, where the ureter crosses over the iliac vessels, and at the ureterovesicular junction. An ultrasound often reveals hydroureter and/or hydronephrosis. Papillary necrosis occurs when there is sloughing of renal papillae due to ischemia or toxins. This can be complicated by ureteric obstruction. These patients often have gross or microscopic hematuria.

Pathologic processes in the retroperitoneal space (radiation injury, trauma, infection, granulomatous diseases, fibrosis) can cause ureteric obstruction. Retroperitoneal fibrosis can result in obstructive uropathy without evidence of obstruction on ultrasound. Retroperitoneal fibrosis is an insidious process resulting from a local or systemic inflammatory process. Intravenous pyelography reveals external compression of the ureters.

Urinary tract dilation can occasionally be seen without obstruction. A classic example is pregnancy, where there is a progesterone-mediated decrease in ureter peristalsis and hydroureter, more commonly involving the right collecting system. Other examples include a dilated extrarenal pelvis and hydroureter due to vesiculoureteral reflux.

Obstruction results in decreased renal blood flow, decreased glomerular filtration rate, and impaired tubular function. There is some controversy as to the duration of complete obstruction after which correction will result in substantial renal recovery. Complete obstruction for greater than 1 week will likely result in some permanent renal damage. Complete obstruction for greater than 8–12 weeks is unlikely to result in renal recovery. Obstruction may be complicated by urinary tract infection, which requires emergent relief of obstruction.

Postobstructive diuresis can occur after relief of obstruction. This large volume of urine output after relief of obstruction can result in volume and electrolyte (potassium, sodium, and magnesium) depletion. Urine output should be measured hourly. Intravenous fluids (IVF) are required if the patient has evidence of volume depletion (orthostatic hypotension, tachycardia), is unable to maintain adequate oral intake, or has urine output greater than 200 mL/hour. Urine and serum electrolytes can be measured every 6 hours to determine the IVF composition, but 0.5 normal saline soluction (NSS) with KCl and sodium bicarbonate if needed is often most appropriate. Urine output should be replaced 0.5 mL IVF/mL urine output unless the patient is volume depleted. This will prevent continued diuresis due to IVF administration.

Obstruction should be relieved as indicated by its location. Obstructive uropathy complicated by acute renal failure and hyperkalemia requires emergent intervention. Bladder outlet obstruction can be relieved with a transurethral or suprapubic catheter. Ureteric obstruction can be relieved via cystoscopy with retrograde ureteroscopy and stent placement or with percutaneous nephrostomy. Stones that are less than 5–6 mm usually pass spontaneously and can be observed. These patients are treated with increased fluid intact and analgesic medications. Stones larger than 7 mm are unlikely to pass spontaneously. Stones in the distal ureter can be treated with ureteroscopic stone extraction. Extracorporeal shock wave lithotripsy (ESWL) can also be performed.

Some patients may require intermittent straight catheterization or a chronic indwelling Foley catheter. Careful technique must be used to prevent urinary tract infection with bladder instrumentation. If possible, definitive therapy should be performed to limit the use of catheters.