2. What is obstructive nephropathy?
Long-standing obstructive uropathy may ultimately lead to kidney damage. Obstructive nephropathy is typically caused by elevated pressures in the renal pelvis and calyces due to hydronephrosis and can lead to dilation and ischemia of the distal tubules of nephrons and subsequent interstitial fibrosis and kidney atrophy.
3. What are the most common causes of obstructive uropathy?
The causes of obstructive uropathy vary with age and gender. In older men, benign prostatic hyperplasia (BPH) and prostate cancer are the most common causes. In older women, gynecologic malignancies can cause extrinsic compression on the urinary tract and ureteral obstruction. In younger patients, nephrolithiasis is the most common cause of ureteral obstruction. In children, obstruction is most often a result of congenital anomalies such as ureteropelvic junction obstruction and posterior urethral valves in newborn boys.
4. What is the role of ultrasound in the evaluation of obstructive uropathy?
Ultrasound is the primary modality used for detection and characterization of obstructive uropathy. Hydronephrosis is the most common finding in unilateral upper urinary tract obstruction involving the ureteropelvic junction or ureter. In early obstruction (first 1 to 3 days) or in an anuric or dehydrated state, hydronephrosis may be mild or absent in the setting of obstruction. Duplex Doppler ultrasonography can be used to calculate renal arterial resistive indices, which can improve the sensitivity and specificity in detecting obstruction. In acute obstruction, a resistive index of >0.70 is often indicative of obstruction. In chronic obstruction, there is often a progressive loss of the echo-rich fat in the plane of Gil Vernet. Ultrasonography may also identify a distended urinary bladder in the setting of bladder outlet obstruction.
5. Does hydronephrosis always indicate obstruction?
Mild hydronephrosis found on ultrasound can be a normal physiologic variant; kidney ultrasonography is therefore associated with a high false-positive rate (~24%) in detecting obstruction. Another common “false positive” ultrasonographic finding is hydronephrosis of pregnancy, which is physiologic and rarely clinically significant. This is hypothesized to result from both hormonal changes (elevated progesterone) influencing the ureter and mechanical extrinsic compression on the ureter from the uterus. Hydronephrosis of pregnancy is more common in the right kidney because the sigmoid colon occupies space within the left pelvis and pushes the uterus toward the right ureter.
6. What other imaging modalities are helpful in the evaluation of obstructive uropathy?
A computed tomography (CT) scan can provide a more detailed evaluation of hydronephrosis, hydroureter, and/or bladder distension from outlet obstruction. The noncontrast phase of a CT scan is best for identifying obstruction from stones. If kidney function permits the use of contrast, a delayed contrast phase can show contrast draining from the kidneys to the bladder and identify the cause and location of intrinsic (inside the ureter) and extrinsic (compression of ureter from the outside) forms of ureteral obstruction. A MAG-3 (mercaptoacetyltriglycine) nuclear medicine renal scan can evaluate differential kidney function (kidney uptake) and can quantify drainage times for each kidney unit after administration of a diuretic.
7. What procedures can aid in the diagnosis and treatment of urinary obstruction?
Cystourethroscopy (endoscopic visualization of the urethra and bladder) can identify urethral strictures and/or prostatic hyperplasia that may be causing bladder outlet obstruction. If other imaging studies are equivocal or patients are unable to receive intravenous contrast administered (due to severe chronic kidney disease (CKD) or allergy), a retrograde pyelography (injection of contrast up the ureters during cystoscopy) may be necessary. Retrograde pyelography provides excellent delineation of filling defects within the ureter, renal pelvis, or renal calyces. Collection of urinary cytology is possible during retrograde pyelography. Cystoscopy also allows for intervention to relieve ureteral obstruction in the same setting, such as placement of a ureteral stent. If a ureteral stent is unable to bypass a ureteral obstruction or does not provide adequate drainage of the kidney collecting system, a percutaneous nephrostomy tube may be necessary to optimize kidney decompression.
8. How can urinary retention lead to obstructive uropathy?
Normal patients should have the ability to void at least 80% of their bladder volume, with a postvoid residual of less than 50 mL. In the setting of bladder outlet obstruction (BOO) or poor bladder contractility, the efficiency of bladder emptying may decrease and residual urine may increase. An elevated postvoid residual is associated with increased risk of infection and can lead to a distended and a hypotonic bladder. If high (typically >40 cm H 2 O) intravesical pressures develop due to excessive bladder volumes and/or poor bladder compliance, urine can reflux up the ureters and cause hydroureteronephrosis and obstructive nephropathy.
9. What are common causes of urinary retention?
The most common cause of anatomic cause of urinary retention in men is BOO due to BPH. Urethral strictures can also cause BOO. In females, urinary retention can be caused by the presence of a cystocele or pelvic organ prolapse. Common nonanatomic causes of urinary retention include diabetes, neurologic deficits (spinal cord injury, multiple sclerosis, Parkinson disease), urinary tract infections, and medications (especially anticholinergics).
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