Hydronephrosis and Hydroureter Workup

Urinalysis is used to assess for signs of infection. Pyuria suggests the presence of infection. Microscopic hematuria may indicate the presence of a stone or tumor.

Complete blood cell count may reveal leukocytosis, which may indicate acute infection.

Serum chemistry studies can reveal an elevation of blood urea nitrogen (BUN) and creatinine levels, which may be the result of bilateral hydronephrosis and hydroureter. In addition, hyperkalemia can be a life-threatening condition.

Imaging in adults

Early diagnosis of urinary tract obstruction is important because most cases can be corrected and a delay in therapy can lead to irreversible renal injury.

Bladder catheterization should be performed initially if there is reason to suspect that bladder neck obstruction leading to acute or chronic urinary retention may be present. Possible clues to this diagnosis include suprapubic pain, a palpable bladder, or unexplained renal failure in an older man.

Radiologic tests are generally used to exclude obstruction at the level of the ureters or above by detecting dilatation of the collecting system. It is important to remember, therefore, that obstruction can occur without dilatation in the following three settings:

• Within the first 1-3 days, when the collecting system is relatively noncompliant and less likely to dilate: In this setting, unilateral obstruction can usually be diagnosed by duplex Doppler ultrasonography, which detects an increased resistive index (a reflection of increased renal vascular resistance) in the affected compared with the contralateral kidney. ^[11] This test is of no value with bilateral involvement because it cannot distinguish obstruction from intrinsic renal disease.

• When the collecting systems are encased by retroperitoneal tumor or fibrosis: In this setting, hydronephrosis may be present in the absence of ureteral dilatation. Retroperitoneal fibrosis can occur in a number of settings, including retroperitoneal fibrosis (most commonly idiopathic or associated with beta-blocker or methysergide use, malignancy, or a connective-tissue disorder) and with the fibrotic reaction that surrounds a renal transplant. ^[12] Thus, the diagnosis of renal insufficiency due to asymptomatic obstruction in a transplanted kidney may be made based on renal biopsy findings, which show diffuse tubular dilatation, rather than the signs of rejection or cyclosporine nephrotoxicity.

• When the obstruction is mild, a setting in which renal function is not usually impaired

Renal ultrasonography is the test of choice to exclude urinary tract obstruction, avoiding the potential allergic and toxic complications of radiocontrast media. ^[13] It can, in the majority of affected patients, help diagnose hydronephrosis and establish its cause; it can also detect other causes of renal disease such as polycystic kidney disease.

Although ultrasound can accurately identify hydronephrosis, it is less sensitive than computed tomography (CT) for detecting stones in the ureters.   In one study, hydronephrosis on ultrasound had a positive predictive value of 0.77 for the presence of a ureteral stone and a negative predictive value of 0.71 for the absence of a ureteral stone. ^[14]  

Indications for CT scanning include the following:

• Ultrasonography results are equivocal
• The kidneys cannot be well visualized
• The cause of the obstruction cannot be identified

The combination of a plain film of the abdomen (including tomographic cuts to detect radiopaque calculi), ultrasonography, and, if necessary, CT scanning is adequate for diagnostic purposes in over 90% of cases. ^[15] It should be noted, however, that the false-positive rate for ultrasonography may be as high as 25% if only minimal criteria (any visualization of the collecting systems) are used to diagnose obstruction.

The advantages of intravenous pyelography (IVP) in relation to ultrasonography are that IVP has a very low false-positive rate, it can identify the site of obstruction, and it can help detect associated conditions such as papillary necrosis or calyceal blunting from previous infection. ^[15] Disadvantages are that IVP is more cumbersome to perform and requires the administration of a radiocontrast agent. On balance, IVP can be used to screen for urinary tract obstruction in the following settings:

• In patients with staghorn calculi or multiple renal or parapelvic cysts, since hydronephrosis is usually not distinguishable from cysts or stones by ultrasonography or CT

• When CT scanning cannot identify the level of obstruction

• With suspected acute obstruction due to kidney stones (or less frequently, to other problems, such as a sloughed papilla or blood clot): Dilatation of the collecting system may not be seen at this time, but the presence and location of the obstructing stone can be identified

Diffusion-weighted magnetic resonance imaging (MRI) may allow noninvasive detection of changes in renal perfusion and diffusion that occur during acute ureteral obstruction. ^[16] The advantage of this technique is that it does not require the use of contrast agents. However, the clinical utility of diffusion-weighted MRI has not been adequately tested.

Hydronephrosis without apparent obstruction or with asymptomatic obstruction

In some cases, one of the above radiologic tests demonstrates hydronephrosis without evidence of obstruction. This is a normal finding in pregnant women. Megaureter due to previous marked vesicoureteral reflux or a dilated but nonobstructed extrarenal pelvis is the most common example of this problem. These patients are often being evaluated for back or flank pain, and the following two questions need to be answered:

• Is obstruction present?
• Is the obstruction responsible for the pain?

In this setting, 3 different tests have been used: diuretic renography, IVP (less often), and perfusion pressure flow studies. ^[3]

Diuretic renography involves the administration of a loop diuretic (eg, 0.5 mg/kg of furosemide) prior to radionuclide renal scanning or during IVP, while the latter involves percutaneous insertion of a catheter into the dilated renal pelvis, followed by fluid perfusion into the pelvis at a rate of 10 mL/min. The marked increase in urine flow should, if obstruction is present, slow the rate of washout of the radioisotope during renal scanning, further increase the size of the collecting system on IVP, or elevate the renal pelvic pressure to above 22 mm Hg during a perfusion study. Furthermore, any of these procedures may precipitate pain similar to the patient's initial complaint.

Noninvasive diuretic renography is generally preferred. However, optimal interpretation of any of these test results is uncertain, because both false-positive and false-negative results may be seen. Nevertheless, the following general recommendations have been made:

• Surgical correction should be considered in a patient with pain and positive diuretic renography findings.

• No therapy is necessary in an asymptomatic patient with positive diuretic renography findings but normal renal function. These patients often are adults and have therefore had the partial obstruction for many years without apparent damage to the kidney. Hydronephrosis may first be noted after a radiologic study is performed for some other reason, or, as noted above, obstruction may be suspected when pain is induced after a period of high fluid intake leads to a diuresis that exceeds the rate at which urine can flow through the obstructed area. Similarly, the decreased washout observed on renography occurs only at a urine flow rate much higher than the patient is likely to achieve on his or her own. Periodic monitoring of renal function and renal parenchymal size (by ultrasonography) is indicated in these patients to exclude progressive renal injury.

• No therapy is indicated in an asymptomatic patient with negative renography findings. Long-term follow-up has demonstrated stable renal function in most of these patients.

• A perfusion pressure flow study should be performed in a symptomatic patient with negative or equivocal diuretic renography findings. Some nonrenal cause for the pain is probably present if the perfusion study is negative. On the other hand, a positive study is suggestive of obstruction and the need for surgical correction.

• A perfusion pressure flow study may also be performed in patients with hydronephrosis and poor renal function. The diuretic renogram may be falsely negative in this setting, because the diuretic may not sufficiently raise the urine flow.

In general, approximately 50% of patients with positive diuretic renography findings eventually require surgery, either for pain or progressive parenchymal loss. ^[17]

Imaging in children

The following grading systems have been developed for evaluating hydronephrosis severity in infants ^[18] :

• Anterior-posterior (AP) diameter of renal pelvis
• Society for Fetal Urology (SFU)
• Radiology
• Urinary Tract Dilatation (UTD)
• Onen

The Onen grading system applies to both prenatal and postnatal ureteropelvic junction–type hydronephrosis and includes two categories of findings: dilation of the pelvicalyceal system and, most important, the quality of the renal parenchyma (thickness and appearance). AP diameter is not important. ^[18]

The Onen system has four grades. Grade 1 criterion is renal pelvic dilatation. Grade 2 criteria are as follows:

• Pelvis and caliceal dilatation
• Renal parenchyma (medulla and cortex) are normal (> 7 mm)

Grade 3 criteria are as follows:

• Pelvis and caliceal dilatation
• Medulla is short and thin
• Cortex is normal
• Total parenchymal thickness 2-5 mm second trimester, 2.5-6 mm third trimester, 3-7 mm postnatal
• Corticomedullary differentiation is normal

Grade 4 criteria are as follows:

• Pelvis and caliceal dilatation
• No medulla is present (total loss)
• Cortex is thin (second trimester, < 2 mm; third trimester, < 2.5 mm; postnatal, < 3 mm)
• No corticomedullary differentiation is visible
• Recesses between calyxes are significantly short and thin

Detection of antenatal hydronephrosis by ultrasound usually occurs in the second trimester. In this setting, hydronephrosis is defined as renal pelvic dilation (RPD) of 4 mm or more. Mild hydronephrosis (RPD of 4-10 mm or SFU grade 1 or 2) can be associated with Down syndrome or other chromosome anomalies. More severe dilatation increases the risk of renal and/or urinary tract disorders.

During the ultrasonography, the appearance of the fetal renal system can vary in both healthy fetuses without hydronephrosis and those with hydronephrosis. Therefore, this diagnosis should not be based on a single measurement. ^[19] An increase of maternal hydration can also increase the RPD in both healthy fetuses and those with hydronephrosis. ^[20]

If fetal hydronephrosis is detected, the following parameters need to be evaluated using ultrasonography, as they guide further need for evaluation and are helpful in determining the cause of hydronephrosis:

• Severity of hydronephrosis: The likelihood of a congenital kidney or urinary tract anomaly increases with the severity of RPD.

• Unilateral versus bilateral involvement: Bilateral involvement increases the risk of a significant renal abnormality and the risk of impaired postnatal renal function.

• Ureter: Dilatation of the ureter can be consistent with vesicoureteral reflux (VUR) or obstructive uropathy distal to the ureteropelvic junction.

Postnatal radiologic studies

Postnatal radiologic evaluation of a newborn with antenatal hydronephrosis begins with an ultrasonography examination. The timing of ultrasonography and the need for other studies depend on the severity of postnatal hydronephrosis and whether there is bilateral involvement or an affected solitary kidney.

Ultrasonography of kidneys and bladder should be performed in the postnatal period on affected infants. The timing of the study depends on the severity of the antenatal hydronephrosis. In general, examination should be avoided in the first 2 days after birth because hydronephrosis may not be detected because of extracellular fluid shifts that underestimate the degree of hydronephrosis. However, infants with bilateral hydronephrosis and those with a severe hydronephrotic solitary kidney require urgent evaluation on the first postnatal day because of the increased likelihood of significant disease and a possible need for early intervention. For unilateral hydronephrosis without antenatal bladder pathology, performing postnatal sonography 1-4 weeks after birth is recommended.

A voiding cystourethrography (VCUG) is performed to detect VUR and, in boys, to evaluate the posterior urethra. For this procedure, a urinary catheter is inserted into the bladder and contrast material is instilled. Fluoroscopic monitoring is performed while the bladder is filling and during voiding. Infants usually tolerate this procedure well. Although the duration of fluoroscopy is minimized, the gonads, especially the ovaries, are exposed to radiation. ^[21]

Diuretic renography is used to diagnose urinary tract obstruction in infants with persistent hydronephrosis and is usually ordered after a VCUG has demonstrated no vesicoureteral reflux. ^[22] It measures the drainage time from the renal pelvis and assesses total and individual kidney renal function. The test requires insertion of a bladder catheter to relieve any pressure that can be transmitted to the ureters and kidneys. Intravenous access is needed for hydration and the administration of the radioisotope and diuretic. The preferred radioisotope is technetium Tc 99m-mercaptoacetyltriglycine (Tc99mMAG3), which is taken up by the renal cortex, filtered across the glomerular basement membrane to the renal tubules, and excreted into the renal pelvis and urinary tract. ^[23]

Diuretic renography includes two phases. First, radioisotope is injected intravenously and renal parenchymal (cortical) uptake is measured during the first 2-3 minutes. The relative contribution of each kidney to overall renal function (called the split renal function) is assessed quantitatively and is useful as a baseline study. Subsequent studies can be compared to assess whether kidney function remains stable or has deteriorated, suggesting true obstruction. ^[24]

Second, at peak renal uptake, intravenous furosemide is administered and the excretion of isotope from the kidney is measured, referred to as the washout curve. This phase indicates the extent of obstruction, if present. In a healthy kidney, furosemide administration results in a prompt washout. In a dilated system, if washout occurs rapidly after diuretic administration (< 15 min), the system is not obstructed. If washout is delayed beyond 20 minutes, the pattern is consistent with obstructive uropathy. However, a delayed washout must be interpreted with caution. ^{[25, 26]}

In a series of 39 infants with antenatal unilateral hydronephrosis followed without surgery, diuretic renography indicated obstruction in 24 patients whose renal function never decreased and who thus did not have obstruction. ^[26] These results may be due, in part, to the normally low neonatal GFR that can be refractory to diuretic therapy. If washout is from 15-20 minutes, the study is indeterminate.

Gravity-assisted drainage imaging may assist in the assessment of pediatric hydronephrosis. Unlike customary diuretic renography, in which the patient remains supine, with gravity-assisted renography a single, static image is obtained after positioning the child in the upright position for 5 minutes to promote additional drainage of tracer from the collecting system. Notable improvement in drainage with this maneuver suggests that slowness in drainage is not due to urinary tract obstruction. ^[27]

Split renal function results are the most useful criteria to evaluate a decrease in renal function. In patients with unilateral hydronephrosis (which is the most common clinical scenario), if the normal nonhydronephrotic kidney and hydronephrotic kidney both have equal function, conservative management without surgery is a safe option. In a cohort of 831 cases of antenatal hydronephrosis, renal scanning performed in 229 newborns demonstrated that 16% of patients had a significant decrease in renal function of one kidney, defined as 35% or less differential renal function. ^[28] A decrease in differential renal function was associated with severe antenatal hydronephrosis (ie, renal pelvic diameter >10 mm at 20-24 wk gestation and >16 mm at 33 wk gestation).

Magnetic resonance urography (MRU) in children is becoming more commonly used in the diagnosis and management of congenital uropathies such as UPJ obstruction. ^{[29, 30]} MRU is especially useful in the management of obstructed kidneys that have rotation or ascent anomalies, or are solitary. MRU can more clearly define the anatomy and delineate the proper surgical approach (ie, retroperitoneal vs transperitoneal). Newer MRU technology may even define obstruction, eliminating the need for diuretic renal scanning.

The disadvantage of MRU is that the study often requires general anesthesia or heavy conscious sedation in children. Furthermore, the contrast agent gadolinium can only be used if renal function is normal (requiring a preprocedure serum creatinine test) because of reports of irreversible renal fibrosis in patients with renal insufficiency.

Antegrade or retrograde pyelography is usually used to relieve, rather than diagnose, urinary tract obstruction. However, these procedures can also be performed for diagnosis when the history is highly suggestive (eg, unexplained acute kidney injury in a patient with known pelvic malignancy), even though hydronephrosis may be absent (due to possible ureteral encasement) on ultrasonography and CT scanning. ^[15]