Genitourinary > Renal obstruction

Renal Collecting System (Ureteropelvic junction) Obstruction

Although some use the terms hydronephrosis and hydroureter to describe dilatation of the collecting system secondary to obstruction, and the terms caliectasis and pelviectasis denote dilatation, but not necessarily obstruction, others use the term hydronephrosis to describe non-specific collecting system dilation. Causes of hydronephrosis and hydroureteronephrosis include

• Obstruction
• Previous obstruction
• Vesicoureteral Reflux
• Recent urinary tract infection
• Congenital malformations
• Noncompliant bladder

Asymptomatic hydronephrosis without obstruction is not associated with increased morbidity. When the obstruction is complete, however, the renal parenchyma atrophies and renal size decreases (obstructive nephropathy). In partial obstruction, the degree of parenchymal thinning is directly proportional to the severity of obstruction. Glomerular function (evaluated with Tc-DTPA) is more sensitive to ureteral obstruction than is tubular secretion (evaluated with MAG3), although once tubular changes occur, they take longer to reverse.

Prenatal hydronephrosis (collecting system dilatation) can be identified on prenatal ultrasound in about 1% of patients (0.3-4.5% [9]) [6] and is bilateral in 37-57% of cases [9]. Followup is generally recommended if the AP diameter of the collecting system is 7-10 mm during the 3rd trimester of pregnancy [9]. The first follow up exam should be performed during the 1st week after birth- but not during the first 72 hours because of reduced urine output after delivery [9]. On post-natal follow-up, the dilatation will resolve in 20-50% of cases [6]. The most common cause of persistent hydronephrosis is ureteropelvic junction obstruction (UPJ), while vescioureteral reflux accounts for 9-15% of cases [9](although some authors report that less than 15% of cases will be related to a ureteropelvic junction obstruction [6]). UPJ can be bilateral in 20-25% of cases and is more frequent in males [9].

The goal of post-natal management of asymptomatic unilateral hydronephrosis is to identify patients with severe urinary flow restriction and compromised renal function so that decisions can be made about further treatment [6]. Although there is general agreement that diuretic renography is accurate in adults, recent opinion is changing in neonates and infants with congenital unilateral ureteropelvic junction obstruction [9]. The recent trend in infants is toward less aggressive/conservative management of the dilated asymptomatic collecting system, even when unilateral poor function or obstructive diuretic renography findings are present on initial scanning [1]. Careful followup can be performed safely as the natural history of dilation seems to be resolution or stability of dilation and renal function in this group (only 20-25% if infants managed conservatively require surgery during the first 2 years of life [9]). However, others prefer to operate before there is loss of function and avoid repetitive invasive imaging studies [7].

The Whitaker test (pressure perfusion study) has been considered the gold standard for distinguishing obstructive from non-obstructive hydronephrosis. However, this status has been questioned because of non-physiologic flow rates, lack of pediatric normal limits, presence of an indeterminate range in a purported gold standard, and lack of studies correlating its results with deteriorating function. The exam measures the pressure within the renal pelvis under conditions of increased flow. A pressure greater than 22 cm H₂O at a flow rate of 10 mL/min. is felt to be representative of obstruction (normal is less than 15 cm H₂O). Unfortunately, the examination is invasive (a percutaneous nephrostomy is required) and fails to provide any information regarding renal function. At the present, many believe that a prospective imaging gold standard does not exist for chronic obstruction, and that the diagnosis is made retrospectively by demonstrating loss of function or relief of symptoms with an intervention. 

Furosemide Renography

Technique

Non-diuretic renography is extremely sensitive but very non-specific for obstruction. Furosemide renography is based on the principle that prolonged retention of tracer in a dilated, non-obstructed collecting system is due to a reservoir effect and that diuresis will cause increased urine flow through the collecting system which should "wash out" the activity. In the presence of significant obstruction, the urinary tract is unable to clear and activity will not change significantly.

The examination can be performed on neonates less than one month old [4], but it is usually recommended to delay the exam until the age of 4 weeks as the immature renal tubules may not respond to lasix [9]. For infants and young children, prior to the exam oral hydration should be offered as formula or water beginning 2 hours before the study and throughout the exam [4]. IV fluid can be given as a dilute saline solution (D5 0.3 NS or D5 0.25 NS) at a rate to deliver 15 mL/kg over a 30 minute interval, beginning 15 minutes before injecting the radiopharmaceutical [4]. The infusion is continued for the remainder of the study at a maintenance fluid volume at a rate of 200 cc/kg/24 hours [4]. Adults can be given oral hydration prior to the exam.

Tc-MAG3 is the agent used for the examination at a dose of 1.85 MBq/kg (50uCi/kg) and a minimum dosage of 37MBq (1mCi). The adult dose is 10 mCi. The patient is supine during the exam. Digital data (at 20 sec/frame) and analog images are acquired [4]. A variety of protocols are used for timing of the lasix injection [8]. We prefer to perform a baseline renal exam for 20 or 30 minutes. A normal baseline exam will remove the need for a lasix study and to allow activity to clear from the renal parenchyma. At the end of the standard exam a non-catheterized patient should stand up, go to the toilet, and void. A post void image with the patient in the supine position should then be obtained. This has two purposes- the collecting system may clear (in which case a lasix exam is not required) and it also empties the bladder (a distended ballder can slow collecting system drainage). If collecting system activity persists, then a lasix examination is performed. We inject lasix IV at a dose of 1mg/kg (maximum 20 mg in children and 40 mg in adults [9]) at the start of the diuretic portion of the study (when the entire collecting system is believed to be full). Images are acquired for 30 minutes following lasix administration.

Although the practice is not universal, some centers routinely catheterize patients for the exam to ensure adequate drainage throughout the study [4]. Catheterization is particularly useful in adults or children who cannot empty their bladder. Catheterization may also be required for children with grade II vesicoureteral reflux or higher as reflux can cause "saw-tooth" waves on the down-sloping portion of the renogram curve that can affect T1/2 determination [4,5].

A wide clinical variance in the performance furosemide renography has led to the publication of three consensus statements which are very similar in their methods and interpretive criteria:

• SNM Nuclear Medicine Procedure Guidelines or Pediatric Diuretic Renography
• The "Well Tempered Diureitic Renogram" [2]
• Consensus statement from the 9th International Radionuclides in Nephrourology Meeting, Committee on Diuretic Renography [3]. This statement had the most detailed recommendations for interpretation.

Following the lasix exam, a ROI should be drawn around the dilated collecting system and a curve generated of the rate of clearance of activity from the collecting system. A collecting system without significant obstruction will have a clearance half-time (ie: the time for half of the activity present to clear) following lasix administration of less than 10 minutes. Greater than 20 minutes is abnormal and is associated with high grade obstruction. Clearance half-times between 10 and 20 minutes are considered indeterminate, but the patient should be followed closely. The T_1/2 is best measured from the point of response to furosemide (not necessarily from the point at which furosemide is given). The response to furosemide usually begins 1 to 2 minutes after injection, but maximum diuresis may not be reached until 15 minutes in some patients. It is important to document that the patient did have a good diuretic response to lasix as dehydration or poor renal function can affect collecting system clearance. Adults should produce 300-400 ml of urine at the end of 30 minutes. A post void volume should be recorded after completion of the lasix exam.

The T_1/2 should not be the sole criterion upon which a determination of obstruction is based. Its value should be interpreted in conjunction with the other data available from the exam- curve analysis, differential function, and visual analysis of the acquired images. The washout curve from a non-obstructed kidney is more likely to have an exponential or concave shape, even if washout is prolonged. A convex shape is more indicative of obstruction. A biphasic curve (one which drops quickly, but then levels off at a high level or even rises) is almost always indicative of a flow dependent obstruction- particularly when the curve levels off above 30% of retained activity. A differential renal function below 40% or a decrease of differential renal function of more than 5% on successive diuretic exams is considered indicative of renal function deterioration and is often used as the threshold for surgery [9].

Improvement in the washout of the tracer after pyeloplasty varies greatly. In many cases, the half-time remains in the obstructed or equivocal range for at least 6 months. If function remains stable during this interval, however, there is no need for concern.

Dilated non-obstructed right collecting system: The right collecting system demonstrates prompt washout of tracer with a T1/2 of 6.2 minutes. The washout curve is somewhat convex in shape, but there is only about 11.5% retained collecting system activity and the findings were most consistent with a dilated non-obstructed system.

Factors which affect half-time clearance include:

1- Urine flow rate response to the diuretic:

• Patient hydration status: Dehydrated patients will have less of a response. Therefore, adequate hydration is essential for diuresis. Adults should consume 300-500 ml of water prior to the exam. In infants and children, 5% dextrose in one-third normal saline (15 mL/kg) is infused over 30 minutes prior to the exam (WTDR)
• Diuretic dose: 1mg/kg up to 40 mg has been shown to be optimal. In very obese patients or in patients with impaired renal function, doses larger than 40 have been proposed, although the Radionuclides in Nephrology consensus statement points out that there is no data to support increased furosemide doses in azotemic patients. Maximum diuresis usually occurs within a few minutes after furosemide injection in the hydrated patient.
• Renal immaturity: In neonates, renal immaturity may produce false positive results [7].
• Degree of renal insufficiency: The urine flow rate following furosemide is roughly proportional to the creatinine clearance. Adequate diuresis is possible even at creatinine clearances as low as 10-15mL/min. In general, when the collecting system does not completely fill within one hour after injection (ie: persistent parenchymal activity) or when the affected kidney has less than 20% of the total renal function, a prolonged washout is not be a reliable indicator of obstruction. The Radionuclides on Nephrology consensus statement recommends that a single kidney GFR (SKGFR) be used in interpreting the significance of abnormal T1/2 and renogram curve shapes:
  • The negative predictive value for a normal T1/2 and curve shape is high in all cases.
  • SKGFR > 15ml/min with an obstructive curve should be interpreted as obstructed.
  • SKGFR < 15ml/min with obstructive or indeterminate curve should be interpreted as indeterminate, not partially obstructed.
  • SKGFR > 15ml/min with an indeterminate curve should be interpreted as probably obstructed.

2- Volume and compliance of the dilated collecting system

Massive hydronephrosis can result in apparently abnormally prolonged clearance. T_1/2 is directly proportional to the collecting system volume:

T_1/2 = k (Collecting system volume)/flow rate

3- Distortion of true activity in the collecting system

Background activity or prolonged cortical retention of tracer can both affect the calculated clearance time (ie: retained parenchymal activity will wash into the collecting system and artifactually prolong the T1/2).

4- Functional Obstruction

a. Distended Bladder

A distended bladder may prolong clearance due to the increased intravesicular pressure which will slow collecting system drainage or the presence of vesicoureteral reflux. An indwelling foley catheter will eliminate this effect, as well as permit measurement of urine output and decrease the radiation dose to the gonads. If a catheter is not used, patients should void prior to the initiation of the lasix portion of the exam. A post void image should also be obtained if the bladder is distended at the completion of the lasix portion of the exam [9].

b. Postural stasis

Certain patients may drain completely when erect. It is essential that a standing/upright post-void view be obtained [9]. In children, prone imaging should be performed if there is delayed clearance on supine images. Markedly improved drainage in the prone position makes high grade obstruction unlikely, and is usually associated with preservation of renal function.

REFERENCES:

(1) J Urol 1994;152: 593-5

(2) Semin Nucl Med 1992; Conway JJ. "Well-tempered" diuresis renography: its historical development, physiological and technical pitfalls, and standardized technique protocol. 22: 74-84

(3) J Nucl Med 1996; O'Reilly P, et al. Consensus on diuresis renography for investigating the dilated upper urinary tract. Radionuclides in Nephrourology Group. Consensus Committee on Diuresis Renography. 37(11):1872-6

(4) J Nucl Med 2001; Rossleigh MA. Renal cortical scintigraphy and diuresis renography in infants and children. 42: 91-95

(5) Semin Nucl Med 1994; Valkema R, et al. Sharp peaks in the downslope phase of the diuresis renogram. 24: 350-353

(6) J Nucl Med 2001; Boubaker A, et al. F + 0 renography in neonates and infants younger than 6 months: an accurate method to diagnose severe obstructive uropathy. 42: 1780-1788

(7) J Nucl Med 2003; Moon DH, et al. Value of supranormal function and renogram patterns on ^99mTc-mercaptoacetyltriglycine scintography in relation to the extent of hydronephrosis for predicting ureteropelvic junction obstruction in the newborn. 44: 725-731

(8) J Nucl Med 2005; Liu Y, et al. The F+0 protocol for diuretic renography results in fewer interrupted studies due to voiding than the F+15 protocol. 46: 1317-1320

(9) J Nucl Med 2006; Boubaker A, et al. Radionuclide investigations of the urinary tract in the era of multimodality imaging. 47: 1819-1836

(10) AJR 2011; Bao J, et al. Key variables for interpreting ^99mTc-mercaptoacetyltriglycine diuretic scans: development and validation of a predictive model. 197: 325-333