Diagnosing urinary tract obstruction

2021-07-01 02:18 PM

The purposes of imaging include definite diagnosis of occlusion; find principle causes including nature, size, location; evaluate the effects on the urinary system.


Urinary tract obstruction is the most common pathology of the urinary system. Due to the presence of an obstruction in the urinary tract from the renal calyx to the urethral orifice.

Obstruction may be complete or incomplete, acute or chronic, frequent or infrequent.

The most common cause is urinary tract stones. In addition, it can be caused by other causes from the urinary system such as tumors, blood clots, or outside the urinary system such as abdominal tumors, retroperitoneal fibrosis, trauma scars...

The obstructive syndrome has morphological and functional consequences on the urinary tract.

The purposes of imaging include definite diagnosis of occlusion; find principle causes including nature, size, location; evaluate the effects on the urinary system.

Definite diagnosis

Diagnosis is based on criteria that are to detect pyelonephritis and to find an underlying cause.


It is a very sensitive detection technique for renal calyces. Ultrasound may show pyelonephritis or ureteral stones at the beginning of the ureter or stones where the ureter enters the bladder.

The limitation of ultrasonography is that it does not reveal impaired renal function. Ultrasound does not distinguish between obstructive and nonobstructive pyelonephritis. In addition, in some cases, ultrasound did not find the cause. In case of acute obstruction in the first 6 hours, ultrasound may not show dilatation of the renal pelvis. Often combined with ultrasound and film of the urinary system are not prepared to find stones, the diagnosis of the obstructive syndrome is higher.

Figure: Degrees of renal pelvis dilatation on ultrasound.

(normal; grade I; grade II; grade III).

Venous urography

It is the basic test to help confirm or rule out obstructive syndrome. Intravenous orographs provide functional and morphological information of the urinary system. Indicated in case the diagnosis is in doubt or for patients with indications for surgical intervention.

Delayed excretion: may be subtle or severe, due to increased pressure in the urinary tract and impaired glomerular filtration. The renal pelvis and ureter may show up after many minutes, sometimes after 24 hours.

Dilatation of the renal calyces: If the occlusion is acute and complete, the calyces will be stretched but slightly dilated, showing a rounded angle: grade I fluid retention. If a long-term obstruction, the excretory tract will be dilated, the calyces will have a flat bottom: fluid retention. Grade II, or spherical: Grade III hydronephrosis.

Figure: Three degrees of pyelonephritis on the venous urogram.

(normal; grade I; grade II; grade III).

Delayed excretion: Contrast-enriched urine remains for a long time in the urinary tract above the obstruction.

In addition, some other signs can be seen: stones or other causes of obstruction, large kidney balls. In case of acute obstruction, the contrast-enhanced renal tissue will appear slowly, gradually darken and last a long time. Contrast reflux can be seen due to high pressure in the renal pelvis: reflux into the renal sinus, into the vein, into the lymphatic vessels.

The limitation of intravenous urography is that the use of contrast media can cause intolerance reactions. Intravenous orographs do not show an obstruction cause when renal excretory function is poor, without adequate contrast staining of the urinary tract. This case requires direct staining of the urinary tract or computed tomography.

Computerized tomography scan

Although not indicative of complete renal function, it is the most sensitive technique for detecting contrast-enhanced or non-contrast stones, or other non-stone occlusions, in or out of the urinary system. Computed tomography also showed dilatation of the urinary tract, although not as clearly as in the venous urography.

The limitations of computed tomography are the high cost, cause 3 times more radioactive contamination of the venous urogram, and in many cases they need for contrast injection for a complete diagnosis.

Differential diagnosis

Dilatation of the renal pelvis and ureters due to hypotonicity, after clearance of obstruction or due to vesicoureteral reflux: Image on venous ureter: dilated urinary bladder without stretch; The renal pelvis is straight inside, the outer border rests on the pelvic muscle; dilated ureters with vascular markers; Delayed excretion is usually not severe.

Occlusion infrequently: common in the ureteropelvic junction (high junction syndrome). Often difficult to diagnose, diuresis should be indicated when an intravenous urogram is taken, the urinary tract will be clearly dilated, helping to diagnose the obstructive syndrome.