- Author: Thomas MT Turk, MD; Chief Editor: Bradley Fields Schwartz, DO, FACS more...
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- Laboratory studies in the evaluation of a patient with a possible or known ureteropelvic junction (UPJ) obstruction include routine serum chemistries with measurement of BUN and creatinine, urinalysis, and urine culture. Kidney function, as indicated by the BUN and creatinine values, may be normal or elevated (indicating impairment of renal function) depending on the function of the affected and contralateral kidney.
- Prior to surgical repair, obtain a CBC count and coagulation studies as indicated. One would not expect abnormalities in the CBC count directly related to the UPJ obstruction unless kidney function is severely compromised or an active infection is present. Measures should be taken to correct any abnormality prior to surgical repair. Abnormalities of coagulation parameters would most likely be unrelated to the UPJ obstruction. Uncorrected coagulopathy is a contraindication to surgical repair, and thus, referral to an internist or hematologist would be appropriate before undertaking surgical treatment.
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- Renal ultrasound is usually the first test ordered in children with a urinary tract infection. Also, it is often obtained in combination with an abdominal ultrasound in adults with abdominal pain of unclear etiology.
- Regardless of the indication for renal ultrasonography, the findings of moderate-to-severe hydronephrosis of the renal pelvis and calyces without concomitant hydroureter are indicative of UPJ obstruction.
- If the UPJ obstruction has existed untreated for some time, a thin renal parenchyma may also be apparent.
- In the past, the primary study obtained in adults with a clinical presentation suggestive of renal obstruction has been the intravenous pyelogram (IVP).
- In a patient with UPJ obstruction, the initial finding from an IVP is a delayed nephrogram that may persist for 24 hours or longer.
- Later images demonstrate gradual filling of the collecting system up to the level of obstruction of the urinary tract. The time delay to this opacification is directly proportional to the degree of obstruction.
- Additionally, the amount of hydronephrosis correlates to the completeness and duration of the UPJ obstruction.
- Other findings from an IVP indicative of UPJ obstruction include pyelosinus extravasation and pyelovenous backflow. Pyelosinus extravasation at a ruptured fornix can occur and may correlate with improvement in a patient's symptoms, although the obstruction itself may not have resolved. Pyelovenous backflow may also occur during an acute episode of UPJ obstruction.
- An abdominal and pelvic CT scan with and without contrast is often the first imaging study obtained in adults who present with abdominal or flank pain because of the speed and efficacy at establishing both nonurologic and urologic causes of such pain.
- Although it is not the first-line test for the diagnosis of UPJ obstruction, the images from a contrast CT scan are analogous to the images from an IVP of a patient with UPJ obstruction.
- The noncontrast images demonstrate hydronephrosis without hydroureter and may also demonstrate decreased renal size and parenchymal thickness.
- If intravenous contrast is administered, the affected kidney usually demonstrates delayed cortical and excretory phases, which correlate to the delayed nephrogram and excretory phases of an IVP.
- The UPJ obstruction is often demonstrated by the marked delay of the passage of contrast from a hydronephrotic renal pelvis proximally to a nondilated distal ureter.
- The severity of a UPJ obstruction may prevent any enhancement of the affected kidney during the time of the CT scan. In this instance, a delayed radiograph of the kidneys, ureters, and bladder many hours later may also demonstrate the UPJ obstruction.
- The presence of crossing vessels, an extrinsic cause of UPJ obstruction, may also be apparent on the contrast images of a CT scan. Three-dimensional reconstruction of the CT scan for clarification of the vascular anatomy is of particular value when preoperatively evaluating anomalous renal units such as horseshoe kidneys.
- Although not widespread as yet, the use of magnetic resonance imaging to diagnose and direct therapy of UPJ obstruction will likely increase in the future.
- Magnetic resonance urography (MRU) offers advantages over more conventional imaging modalities because it does not use ionizing radiation and its contrast agents do not cause allergic reactions.
- Further, MRU has recently been demonstrated to have a very high sensitivity for helping detect UPJ obstruction.
- Magnetic resonance angiography is also a useful method to help evaluate for the presence of a crossing vessel at the UPJ, which is an important consideration during endoscopic approaches to the repair of UPJ obstruction.
- The premier radiographic test to confirm the diagnosis of UPJ obstruction is a diuretic renogram. This study entails the administration of a radiopharmaceutical tracer such as technetium Tc 99m mercaptoacetythiglycine (MAG-3) or technetium Tc 99m diethylenetriaminepentaacetic acid (DTPA).
- The concept of a "well-tempered" renogram involves adequate patient hydration with a combination of oral and intravenous fluids, as well as bladder catheterization for continuous drainage throughout the study.
- The MAG-3 diuretic renogram is the criterion standard test because the agent is secreted by the renal tubules. This enables an interpretation of both the relative function of each kidney and the presence of any urinary tract obstruction, including UPJ obstruction
- The DTPA diuretic renogram findings can also lead to a diagnosis of UPJ obstruction, but this agent does not measure tubular function because it is only filtered at the glomerulus and not secreted by renal tubules. Thus, no determinations can be made regarding relative renal function.
- Additionally, conditions that inhibit the glomerular filtration rate, such as the renal immaturity of neonates or other nephropathies (eg, diabetic nephropathy), may make interpretation of DTPA renogram findings difficult.
- As implied by its name, a diuretic agent, such as furosemide, is administered during the study. Furosemide is a loop diuretic, with peak effect occurring 15-18 minutes after administration. Although the exact regimen may vary by institution, the classic well-tempered renogram involves the administration of the diuretic agent 20 minutes after the radiopharmaceutical agent is administered.
- Under this protocol, the initial images and the corresponding tracer counts indicate relative renal function. In unobstructed systems, the secretion and passage of MAG-3 to the bladder is visualized. In an obstructed urinary tract, such as in UPJ obstruction, the secretion and passage of the tracer is delayed.
- At 20 minutes, the furosemide is administered and further images are obtained.
- If the tracer clears with the induced diuresis, obstruction can be excluded.
- If the tracer does not clear, then the images demonstrate continued holdup of the tracer.
- The corresponding renogram curve also demonstrates the poor clearance of the tracer from the renal pelvis.
- From these curves, the reaction half time (t1/2) of the clearance of the pharmaceutical tracer can be determined.
- The upper limit of normal t1/2s has been determined to be either 10 or 15 minutes.
- Obstructed systems have t1/2s greater than 20 minutes.
- Systems with t1/2s between 10 and 20 minutes are deemed indeterminate.
- Although not necessary to establish the diagnosis of UPJ obstruction, retrograde pyelograms and antegrade pyelograms may both help confirm the presence of obstruction.
- An antegrade pyelogram may be obtained if the patient presented with an infection that required nephrostomy tube placement. Once the infection has cleared, the administration of contrast through the nephrostomy tube demonstrates the same hydronephrotic collecting system and UPJ obstruction that is seen on IVP findings.
- Similarly, a retrograde pyelogram demonstrates similar findings. Retrograde pyelography is often performed immediately prior to the definitive repair of a UPJ obstruction in the operative suite to help confirm the site of obstruction and to help rule out any other concomitant obstruction that may be present in the urinary tract.
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- In those cases that are indeterminate, a Whitaker test may be appropriate to establish the diagnosis of UPJ obstruction. The idea of the test is that obstruction produces a constant impediment to the flow of urine that requires an elevated pressure gradient to drive these higher flow rates. Therefore, the test attempts to measure this elevated pressure gradient.
- The Whitaker test requires a nephrostomy tube and the ability to simultaneously measure intrapelvic and cystometric pressures.
- Once these devices are present, fluid is administered via the nephrostomy tube at a pressure sufficient to establish a flow rate of 10 mL/s, and the resulting pressure gradient between the renal pelvis and the bladder is measured.
- Whitaker proposed ranges of normal pressure differential as high as 12 cm water and obstructive pressure differentials greater than 20 cm water.
- Although this test is helpful, it frequently is not obtained in clinical practice because it is highly invasive and often requires anesthesia.
The histologic findings at the UPJ can be variable, depending on the etiology of the obstruction. In general, derangement of the normal architecture is thought to exist with an increase in collagen, atrophy, or abnormal arrangement of smooth muscle cells and aberrant innervation. The net result of the changes can result in a functional, but not a structural, obstruction. The abnormal segment can lack the normal peristaltic ability of the ureter and produce the normal passage of urine across the narrowed segment.
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