Pediatric Ureteropelvic Junction Obstruction Workup

  • Author: Sang Won Han, MD, PhD; Chief Editor: Marc Cendron, MD   more...
 
Updated: Jan 25, 2010
 

Laboratory Studies

  • Novel approaches may discern the clinically significant UPJ obstruction. These approaches are currently experimental.
    • Disruption of proximal tubular integrity leads to increased urinary concentrations of beta2-microglobulin (B2M), which normally is resorbed from the tubular lumen via phagocytosis and lysosomal digestion.
    • An increase in urinary concentrations of B2M may indicate tubular dysfunction as a result of the obstructive insult. Functionally significant obstruction and recovery from obstruction may be determined by following the urinary concentration of B2M.
    • The potential for B2M to be a marker for significant obstruction is quite appealing; however, the determination of its levels in obstructed kidneys is not routine, and many different insults other than UPJ obstruction can lead to increased levels of B2M in the urine. In addition, the immaturity of the nephron and the high fractional excretion of water in neonates contribute to elevated B2M levels in the absence of any identifiable renal stress.
    • Further observations of the concentration of this protein in urinary obstruction are necessary before its assessment can have practical application.
  • N -acetyl-β -glucosaminidase (NAG) is a tubular lysosomal enzyme present in the urine of children who have various renal diseases. This is currently experimental.
    • In rats with experimental partial ureteral obstruction, the urinary concentration of NAG increases in the first 2 weeks of obstruction and decreases with the relief of obstruction.
    • In a clinical study, NAG levels in kidneys at the time of pyeloplasty were 7 times higher than those in bladder urine from normal control patients. In addition, enzyme levels in the bladder of patients 6 weeks after surgery suggested normalization of NAG excretion.
  • Urinary biochemical markers of renal damage someday may aid the diagnosis of clinically significant urinary obstruction. These are currently experimental.
    • As described previously, many biologic modulators of glomerular dynamics and renal histology have been identified.
    • The assessment of urine for growth factors (eg, epidural growth factor [EGF], platelet-derived growth factor [PDGF], TGF β), cytokines, and vasoactive substances may be an important adjunct in evaluating obstructive uropathy in the future.
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Imaging Studies

  • Prenatal
    • Widespread use of antenatal ultrasonography has opened the new field of perinatal urology; however, even the most modern ultrasonographic techniques only demonstrate the dilation of renal pelvis and ureter and cannot accurately differentiate the true obstruction from a harmless physiologic dilatation.
    • During any session of prenatal ultrasonographic diagnosis, thoroughly investigate the following from the initial study usually performed between 16 and 20 weeks: amniotic fluid volume to rule out oligohydramnios, bladder volume, kidney size, anteroposterior diameter of the renal pelvis, and any associated abnormalities. Functionally significant hydronephrosis can be determined when the anteroposterior diameter of the renal pelvis is more than 10 mm, the ratio of the renal pelvis–to–anteroposterior renal cortex is more than 0.5, or evidence of caliectasis is present after 24 weeks of gestation. Following fetal hydronephrosis also is important to monitor possible progression. A recent meta-analysis of 7 studies of isolated antenatal hydronephrosis showed that 98% of patients with Society of Fetal Urology (SFU) grades 1-2 hydronephrosis (anterior-posterior pelvic diameter [APPD] < 12 mm) resolved, stabilized, or improved during follow-up.
  • Postnatal
    • After the prenatal presumptive diagnosis UPJ obstruction or other conditions causing hydronephrosis is made, the neonate should undergo ultrasonographic evaluation on the second or third day of life. Before this date, results may be false negative because of neonatal dehydration and physiologic oliguria; however, in cases of bilateral hydronephrosis, more rapid evaluation is warranted. Postnatal examination evaluation consists of urinary tract study whether the calyceal pelvic dilation with or without renal cortical thinning is present. Approximately 20% of antenatal hydronephrosis are not found on postnatal ultrasonogram.
    • At the same time, ultrasonographic evaluation on the contralateral kidney, bladder, and ureter is performed. VCUG also is done to rule out vesicoureteral reflux.
    • The renal scan and scintigraphy (ie, diuretic renogram) is the most widely used technique in the presence of hydronephrosis to assess function and obstruction. The rate at which tracer leaves the renal pelvis following diuretic injection, reflected in the slope of the drainage curve and often reported as T1/2 (the time required for 50% of the isotope to exit), is generally viewed as an accurate reflection of the patency of the UPJ. Rapid drainage (low T1/2) indicates no obstruction, while impaired drainage or slow or no washout (T1/2 >20 min) indicates obstruction.
    • The current radiopharmaceutical agent most widely used is technetium 99m diethylenetriamine pentaacetic acid (99mTc-DTPA). It is excreted by glomerular filtration and is not secreted or reabsorbed by the renal tubules. Another much more expensive agent is 99mTc-mercaptoacetyltriglyine (MAG3), which offers better anatomical resolution and can be used in case of decreased renal function. Variables include the use of intravenous hydration, the dosage and timing of administration of diuretic, the requirement for bladder catheterization, the degree of pelvic dilatation, the severity of outflow obstruction, and the method of calculating the clearance after the administration of diuretic.
    • The most useful measure in diuretic renography is the estimate of differential renal function. This is considered significant when it is less than 40%. This percentage usually is well correlated with the half-life (T1/2) washout curve. Therefore, as stated above, many factors must be considered when evaluating the renal scan, especially in neonates. For this reason, the T1/2 of the diuretic renogram cannot be a single indicator to determine surgery, especially in the neonate.
    • Supranormal differential renal function of the affected kidney in UPJO can occasionally be found on renal scans and has been hypothesized to be caused by an increase in single nephron filtration or nephron volume. However, in a study of histopathological changes of hydronephrotic kidneys with supranormal daily replacement factor (DRF) assessed with intraoperative kidney biopsy at the time of pyeloplasty, the glomerular area was not significantly larger than controls, but the probability for a larger renal glomeruli increased with decreasing DRF. Instead of increased nephron volume, the supranormal DRF can be accounted for by an increase in renal blood flow that results from tubuloglomerular feedback, prostaglandins, and the renin-angiotensin system as a protective mechanism from high intrapelvic pressure.
    • The degree of UPJ obstruction is information of utmost importance for patients, but no single test can demonstrate the degree of obstruction accurately and determine which patients require operative intervention. Imaging modalities can exhibit urinary tract dilation, but they offer no conclusive data whether to operate or observe. Radioisotope renogram and the pressure-perfusion test can reproduce significantly impaired washout of urine from the kidney; however, in many instances, kidneys have no measurable deterioration when observed for long periods. Nephrostomy drainage and assessment of creatinine clearance can indicate decreased differential renal function of the involved kidney, which does not necessarily change after surgery.
    • Lastly, the widespread use of modern imaging techniques has not led to an increase in the number of pyeloplasties that are performed. In a multi-institutional study that investigated the total number of pyeloplasties performed in a well-defined region, it was found that the total number of operations has remained constant since the late 1970s. The authors found that the number of pyeloplasties conducted in children aged 1-6 years increased, whereas the number of pyeloplasties in children aged 7-12 years decreased; yet, the total number performed per year stayed the same. In other words, the degree of hydronephrosis, impaired isotope washout, or even reduced differential renal function neither helps to define significant obstruction nor predicts deterioration.
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Other Tests

  • The development of Doppler sonography has become another useful diagnostic modality in the assessment of kidneys with UPJ obstructions. With duplex Doppler sonography, intrarenal vasculature can be assessed to determine the resistive index. Normal kidneys reliably demonstrate resistive indices less than 0.7, and obstructed kidneys show higher values. Administration of diuretics can aggravate the preexisting obstruction, thereby aiding the diagnosis by Doppler sonography. It is especially reliable in the preoperative diagnosis of aberrant-accessory blood vessels associated with UPJ obstruction.
  • A functional study is then necessary to confirm the diagnosis of UPJ obstruction. The excretory urography (ie, IVP) has been used to evaluate UPJ obstruction, but IVP may not provide adequate information to determine the true obstruction, and it is especially difficult to interpret in children.
  • IVP provides information about the obstruction and contralateral side and especially facilitates operative planning; however, infant urograms are compromised by the immature renal function, which impedes adequate visualization of the collecting system. Bowel gas and underlying bony structures also make interpretation of the urogram difficult. Despite such shortcomings, IVP accurately visualizes kidney, renal pelvis, ureter, and the exact point of obstruction. IVP also allows for clear visualizations of malrotated renal units.
  • The drawbacks of IVP include the necessity of dehydration even in infants, which makes it a relatively risky procedure. Of course, a risk of radiation exposure exists, which can be minimized by limiting number of films taken. Problems associated with contrast media exist, such as nephrotoxicity and anaphylactic reactions. These problems can be reduced by the newer nonionic contrast agents that are currently available.
  • CT scan has been used to diagnose UPJ obstruction in children, especially in association with the abdominal trauma. CT scans, like IVP studies, show the dilation of the kidney and collecting system well, and it may be used to estimate the differential renal function by measuring the cortical thickness. Spiral CT provides superior longitudinal resolution; vessels as small as 1 mm may be detected in the UPJ region. New developments in MRI technology have made it possible to image kidneys while assessing intracellular metabolic parameters independent of blood flow and tubular function. Relative high cost and the noise during the procedure limit the routine use of MRI for evaluating urinary obstruction in children.
  • The retrograde pyelogram is one of the first tools that have been used to assess upper ureter and renal pelvis. After the advent of fiberoptic technology, routine endoscopic assessment became possible. The necessity of general anesthesia has made retrograde pyelography an adjunctive role, usually performed in the operating room to confirm the absence of coexisting lower ureteral obstruction.
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Diagnostic Procedures

  • The antegrade pressure-flow study was introduced by Whitaker and has proven useful in the equivocal obstruction in children. Koff and colleagues have characterized the volume-dependent changes in pressure and have classified patterns of pressure exit flow curves as simple or complex. The Whitaker measurement records the response of the renal pelvis to distention, which does not truly define obstruction. In the complex cases in which intrinsic and extrinsic obstruction coexist, this test does not provide conclusive evidence.
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Histologic Findings

  • This intrinsic obstruction is evident as the ureteral narrowing with angulation is found. During exploration, the catheter usually is passed to the renal pelvis without resistance, and this is evidence of the fact that the true narrowing is not a main pathologic change in UPJ obstruction. Some claimed the presence of remnant valve and others claimed the disproportionate abundance of longitudinal muscles as the cause of this condition. The most attractive theory is the obstruction secondary to muscular discontinuity. This absence or disorientation of smooth muscle fibers at UPJ is clearly evident on electron microscope evaluation with the findings of increased ground substance and collagen fibers; therefore, conduction of the peristatic wave is impeded.
  • One study had identified altered expression of interstitial Cajal cells in obstructed UPJ specimens, which are normally intercalated between nerve terminal and smooth muscle cells, providing a means of transducing signals from neurotransmitters and mediating neurotransmission.[1] This suggests that UPJO may cause the failure of transmission of peristaltic waves across the UPJ, resulting in the failure of urine to be propelled from the renal pelvis into the ureter.
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Contributor Information and Disclosures
Author

Sang Won Han, MD, PhD  Professor, Department of Urology, Yonsei University College of Medicine, Korea

Sang Won Han, MD, PhD is a member of the following medical societies: European Society for Paediatric Urology, International Continence Society, Korean Medical Association, and Korean Urological Association

Disclosure: Nothing to disclose.

Coauthor(s)

Koon Ho Rha, MD, PhD  Fellow, Instructor, Department of Urology, Yonsei University and Severance Hospital, Seoul, Korea

Disclosure: Nothing to disclose.

Hye-young Lee, MD  Clinical Research Assistant Professor, Department of Urology, Yonsei University College of Medicine/Severance Hospital

Hye-young Lee, MD is a member of the following medical societies: Korean Medical Association and Korean Urological Association

Disclosure: Nothing to disclose.

Specialty Editor Board

Bartley G Cilento, Jr, MD  Instructor, Department of Surgery, Division of Urology, Children's Hospital of Boston and Harvard Medical School

Bartley G Cilento, Jr, MD is a member of the following medical societies: American Academy of Pediatrics, American Urological Association, and Massachusetts Medical Society

Disclosure: Nothing to disclose.

Mary L Windle, PharmD  Adjunct Associate Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Nothing to disclose.

Harry P Koo, MD  Chairman of Urology Division and Director of Pediatric Urology, Professor of Surgery, Virginia Commonwealth University School of Medicine, Medical College of Virginia; Director of Urology, Children's Hospital of Richmond

Harry P Koo, MD is a member of the following medical societies: American Academy of Pediatrics, American College of Surgeons, and American Urological Association

Disclosure: Nothing to disclose.

Daniel Rauch, MD, FAAP  Director, Pediatric Hospitalist Program, Associate Professor, Department of Pediatrics, New York University School of Medicine

Daniel Rauch, MD, FAAP is a member of the following medical societies: Ambulatory Pediatric Association, American Academy of Pediatrics, and Society of Hospital Medicine

Disclosure: Baxter Honoraria Consulting

Chief Editor

Marc Cendron, MD  Associate Professor of Surgery, Harvard School of Medicine; Consulting Staff, Department of Urological Surgery, Children's Hospital Boston

Marc Cendron, MD is a member of the following medical societies: American Academy of Pediatrics, American Urological Association, European Society for Paediatric Urology, Johns Hopkins Medical and Surgical Association, New Hampshire Medical Society, Society for Fetal Urology, and Society for Pediatric Urology

Disclosure: Nothing to disclose.

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