Obstruction of the Urinary Tract

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Chapter 534 Obstruction of the Urinary Tract

Urinary tract obstruction can result from congenital (anatomic) lesions or can be caused by trauma, neoplasia, calculi, inflammatory processes, or surgical procedures, although most childhood obstructive lesions are congenital. Obstructive lesions occur at any level from the urethral meatus to the calyceal infundibula (Table 534-1). The pathophysiologic effects of obstruction depend on its level, the extent of involvement, the child’s age at onset, and whether it is acute or chronic.

Table 534-1 TYPES AND CAUSES OF URINARY TRACT OBSTRUCTION

LOCATION CAUSE
Infundibula
Renal pelvis
Ureteropelvic junction
Ureter
Bladder outlet and urethra

Etiology

Ureteral obstruction occurring early in fetal life results in renal dysplasia, ranging from multicystic kidney, which is associated with ureteral or pelvic atresia (see Fig. 531-2), to various degrees of histologic renal cortical dysplasia that are seen with less severe obstruction. Chronic ureteral obstruction in late fetal life or after birth results in dilation of the ureter, renal pelvis, and calyces, with alterations of renal parenchyma ranging from minimal tubular changes to dilation of Bowman’s space, glomerular fibrosis, and interstitial fibrosis. After birth, infections often complicate obstruction and can increase renal damage.

Diagnosis

Urinary tract obstruction may be diagnosed prenatally by ultrasonography, typically showing hydronephrosis. More complete evaluation, including imaging studies, should be undertaken in these children in the neonatal period.

Urinary tract obstruction is often silent. In the newborn infant, a palpable abdominal mass most commonly is a hydronephrotic or multicystic dysplastic kidney. With posterior urethral valves, which is an infravesical obstructive lesions in boys, a walnut-sized mass representing the bladder is palpable just above the pubic symphysis. A patent draining urachus also can suggest urethral obstruction. Urinary ascites in the newborn usually is caused by renal or bladder urinary extravasation secondary to posterior urethral valves. Infection and sepsis may be the first indications of an obstructive lesion of the urinary tract. The combination of infection and obstruction poses a serious threat to infants and children and generally requires parenteral administration of antibiotics and drainage of the obstructed kidney. Renal ultrasonography should be performed in all children during the acute stage of an initial febrile UTI.

Imaging Studies

Renal Ultrasonography

The presence of a dilated urinary tract is the most common characteristic of obstruction. Hydronephrosis is a common ultrasonographic finding (Fig. 534-1). Dilation is not diagnostic of obstruction and can persist after surgical correction of an obstructive lesion. Dilation can result from vesicoureteral reflux, or it may be a manifestation of abnormal development of the urinary tract, even when there is no obstruction. Renal length, degree of caliectasis and parenchymal thickness, and presence or absence of ureteral dilation should be assessed. Ideally, the severity of hydronephrosis should be graded from 1 to 4 using the Society for Fetal Urology grading scale (Table 534-2). The clinician should ascertain that the contralateral kidney is normal, and the bladder should be imaged to see whether the bladder wall is thickened, the lower ureter is dilated, and bladder emptying is complete. In acute or intermittent obstruction, the dilation of the collecting system may be minimal and ultrasonography may be misleading.

Table 534-2 SOCIETY FOR FETAL UROLOGY GRADING SYSTEM FOR HYDRONEPHROSIS

  RENAL IMAGE
GRADE OF HYDRONEPHROSIS Central Renal Complex Renal Parenchymal Thickness
0 Intact Normal
1 Slight splitting Normal
2 Evident splitting, complex confined within renal border Normal
3 Wide splitting pelvis dilated outside renal border, calyces uniformly dilated Normal
4 Further dilatation of pelvis and calyces (calyces may appear convex) Thin

After Maizels M, Mitchell B, Kass E, et al: Outcome of nonspecific hydronephrosis in the infant: a report from the registry of the Society for Fetal Urology, J Urol 152:2324–2327, 1994.

Radioisotope Studies

Renal scintigraphy is used to assess renal anatomy and function. The 2 most commonly used radiopharmaceuticals are mercaptoacetyl triglycine (MAG-3) and technetium-99m-labeled dimercaptosuccinic acid (DMSA). MAG-3, which is excreted by renal tubular secretion, is used to assess differential renal function, and when furosemide is administered, drainage also can be measured. An alternative to MAG-3 is diethylene tetrapentaacetic acid (DTPA), which is cleared by glomerular filtration. The background activity of DTPA is much higher than that of MAG-3. DMSA is a renal cortical imaging agent and is used to assess differential renal function and to demonstrate whether renal scarring is present. It is used infrequently in children with obstructive uropathy.

In a MAG-3 diuretic renogram, a small dose of technetium-labeled MAG-3 is injected intravenously (Figs. 534-2 and 534-3). During the first 2-3 min, renal parenchymal uptake is analyzed and compared, allowing computation of differential renal function. Subsequently, excretion is evaluated. After 20-30 min, furosemide 1 mg/kg is injected intravenously, and the rapidity and pattern of drainage from the kidneys to the bladder are analyzed. If no obstruction is present, half of the radionuclide should be cleared from the renal pelvis within 10-15 min, termed the half-time (t1/2). If there is significant upper tract obstruction, the t1/2 usually is >20 min. A t1/2 of 15-20 min is indeterminate. The images generated usually provide an accurate assessment of the site of obstruction. Numerous variables affect the outcome of the diuretic renogram. Newborn kidneys are functionally immature, and, in the first month of life, normal kidneys might not demonstrate normal drainage after diuretic administration. Dehydration prolongs parenchymal transit and can blunt the diuretic response. Giving an insufficient dose of furosemide can result in inadequate drainage. If vesicoureteral reflux is present, continuous bladder drainage is mandatory to prevent the radionuclide from refluxing from the bladder into the dilated upper tract, which would prolong the washout phase.

image

Figure 534-2 Same patient as in Figure 534-1. 6 wk old, MAG-3 diuretic renogram. The right kidney is on the right side of the image. A, Differential renal function: left kidney 70%, right kidney 30%. B, After administration of furosemide, drainage from the left kidney was normal and drainage from the right kidney was slow, consistent with right ureteropelvic junction obstruction. Pyeloplasty was performed on the right kidney.

image

Figure 534-3 Same patient as in Figure 534-1. A, MAG-3 diuretic renogram at 14 mo of age shows equal function in the two kidneys. B, Prompt drainage after the administration of furosemide.

The MAG-3 diuretic renogram is considered superior to the excretory urogram in infants and children with hydronephrosis, because bowel gas and immaturity of renal function often cause the intravenous pyelogram (IVP) images to be suboptimal. The diuretic renogram provides an objective assessment of the relative function of each kidney.