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Obstructive uropathy
1. What is obstructive uropathy?
Obstructive uropathy is structural or functional interference of normal urine flow anywhere along the urinary tract. Obstructive uropathy can be acute or chronic, partial, or complete, and unilateral or bilateral.
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.
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