© Springer-Verlag Italia 2014
J. Hodler, G. K. von Schulthess, R. A. Kubik-Huch and Ch. L. Zollikofer (eds.)Diseases of the Abdomen and Pelvis 2014–201710.1007/978-88-470-5659-6_29Imaging of the Urinary Tract in Children
Jeanne S. Chow1 and J. Damien Grattan-Smith2
(1)
Departments of Urology and Radiology, Boston Children’s Hospital, Boston, MA, USA
(2)
Department of Radiology, Children’s Healthcare of Atlanta at Scottish Rite, Atlanta, GA, USA
Introduction and Techniques
The principle that guides imaging of the pediatric genitourinary tract is the same that guides imaging elsewhere in the body of the child: judicious use of imaging while minimizing ionizing radiation exposure [1] and unnecessary sedation. Ultrasound (US) is the main imaging modality of the genitourinary tract, providing excellent images of the kidneys and bladder. An abnormally dilated ureter is also easily imaged, especially at the levels of the kidney and bladder. Doppler provides additional information regarding vascular flow and is especially helpful in evaluating the main renal artery and veins as well as the arcuate vessels. US contrast agents are used to further evaluate vascular flow of the kidneys, intrarenal masses, and vesicoureteral reflux (VUR) [2]. The main downside of US is that it provides little functional imaging, instead mainly revealing the physical appearance of the urinary tract (UT).
If further imaging is necessary, magnetic resonance urography (MRU) provides exquisite, detailed images of the UT in addition to functional information. MRU has become very popular because it emits no ionizing radiation and yields both anatomical and functional information. When gadolinium is administered, the study provides uptake and excretion rates, relative function and indirect measurements of glomerular filtration rates, and overall provides more information than obtained from a technetium-99m-mercaptoacetyltriglycine (99mTc-MAG)- 3/Lasix renogram alone [3]. However, because MRU is a long procedure that often requires sedation, technetium- 99m dimercaptosuccinic acid (99mTc-DMSA) and MAG- 3 are more commonly used to provide functional imaging of the genitourinary tract. In addition, functional information cannot be obtained by MRI without gadolinium, which places children with chronic renal failure and endstage renal disease at risk for developing nephrogenic systemic fibrosis [4]. Although nephroureterolithiasis may be commonly studied by computed tomography (CT) in adults, US is the first-line imaging modality in children [5].
Key Point
US is the main imaging modality of the genitourinary tract in children
Managing in Utero-Renal-Pelvis Dilatation
Dilatation of the renal pelvis is a common finding on obstetrical US, with a frequency of around 1–4% of all pregnancies. Yet, not every dilatation has the same clinical relevance; furthermore, antenatal and postnatal evolution is variable. This has led to controversy in the literature about the best workup and follow-up after birth [6–8].
Definition
A common criterion for measuring UT dilatation is the anteroposterior (AP) diameter of the renal pelvis, which is measured on a transverse scan of the fetal kidneys. The upper limit for normal (ULN) should be 4 mm in the second trimester and 7 mm in the third trimester. Other US evidence of UT abnormality are visibility of the fetal ureter, dilatation of the renal calyces, abnormal echogenicity of the renal parenchyma, and demonstration of an enlarged bladder.
Findings on Obstetric US
Most mild hydronephrosis is transient and resolves. The greater the degree of hydronephrosis, the greater the likelihood of UT obstruction. The common sites of obstruction in the fetus and child are at the ureteropelvic junction and ureterovesical junctions. Vesicoureteral reflux (VUR) is also associated with UT dilatation; however, the risk does not change significantly with degree of hydronephrosis [9]. In male fetuses, urethral obstruction can cause hydronephrosis. In most cases, US evaluation will differentiate between etiologies. In some patients, especially those with bilateral and complex uropathies, fetal MR imaging (MRI) will provide additional information.
Other organ malformations also can be associated with UT dilatation; therefore, the US survey should be as meticulous and complete as possible. Chromosomal analysis may be indicated in selected patients.
The prognosis of uropathy will depend upon type and extent of anomalies. Amniotic fluid volume is important to the prognosis, as well; oligohydramnios, thought to be related to decreased urine production, is a poor prognostic indicator.
It is of utmost importance that any relevant information is correctly transmitted to the postnatal team that will be in charge of the newborn.
Management of Postnatal Fetal Pelvis Dilatation
Certain conditions require immediate postnatal confirmation and therapeutic maneuvers, such as obstructive posterior urethral valves and prolapsed ectopic ureterocele into the urethra. In those cases, US and voiding cystourethrography (VCUG) should be performed directly after birth. In all other cases, the workup can be planned without urgency, typically in the first month of life. The use of prophylactic antibiotics until the VCUG is performed, once routine, is now variable.
Additional VCUG is recommended if US shows a significant anomaly, such as greater than mild to moderate pelvicaliceal dilatation, ureteral dilatation, duplex kidney with dilatation, renal size discrepancy, or dysplasia. If VUR is not present, complementary imaging is necessary to determine the precise origin of the dilatation. Renal function is assessed through isotopic studies, whereas function and anatomy are best evaluated by MRI.
Treatment type (conservative or surgical) will depend upon diagnosis, renal function on follow-up, and complications. The trend is increasingly toward a conservative, nonsurgical approach, as many causes of UT dilatation resolve with time. The length of follow-up must be adapted to anomaly type and clinical and imaging follow- up.
Key Points
Fetal renal dilatation is a common finding during obstetric US.
Thresholds of 4 and 7 mm during the second and third trimester, respectively, are commonly used.
Postnatally, these patients must be further evaluated by US; VCUG is performed only if an anomaly is present on postnatal US.
The trend is toward a more conservative approach to treatment based on clinical and imaging follow-up.
Imaging Cystic Kidneys in Children
Renal cystic diseases may be discovered or suspected at any stage during fetal life or at any age in childhood. Such diseases encompass a large number of conditions that can be separated into those with or without hereditary transmission. Imaging, mainly US, plays an important role in differentiating between the various types of cystic diseases, as it shows the features of renal involvement as well as associated anomalies.
Cystic Kidneys in the Fetus
In the fetus (and during the perinatal period), cystic renal disease should be suspected whenever bilateral hyper-echoic kidneys or cysts are discovered during an US examination. Many of these renal cystic diseases are ciliopathies, which are due to dysfunction of hair-like organelles called cilia [10]. The imaging approach to the diagnosis should be based on detailed sonographic analysis that includes measurement of renal length, presence or absence of normal corticomedullary differentiation (CMD), and presence, number, size, and location of cysts. This evaluation should be completed with an analysis of the entire fetus, looking for associated anomalies. Timing of detection and the amount of amniotic fluid are the most important prognostic factors. Furthermore, a detailed clinical and familial inquiry is essential in disease evaluation.
Autosomal recessive polycystic kidney disease (ARPKD) is the main diagnosis to consider in case of bilateral, markedly enlarged, hyperechoic kidneys without normal CMD. The prognosis is usually poor if amniotic fluid volume is markedly decreased.
In case of moderately enlarged hyperechoic kidneys, three diagnoses must be considered: (1) nephropathy due to a mutation in the TCF2 gene, (2) a milder form of ARPKD, and (3) autosomal-dominant polycystic kidney disease (ADPKD).
In TCF2-mediated nephropathy, CMD can be normal or abnormal, and whenever cysts are visible, they are typically subcortical.
In the milder form of ARPKD, cysts may be observed in the medullary area.
The main sonographic feature of ADPKD is a striking cortical hyperechogenicity associated with increased CMD.
Whenever cysts are the main US finding in the fetal kidney, number and location are the main criteria for differential diagnosis.
The diagnosis of unilateral multiple cysts suggests a multicystic dysplastic kidney.
Bilateral multiple cysts can be visualized in a large number of renal diseases, including bilateral multicystic dysplastic kidney, ADPKD, bilateral obstructive dysplasia, and glomerulocystic kidneys [10].
Renal Cystic Diseases in Children
In children, renal cystic diseases are usually discovered during US examination performed in the follow-up of a known perinatally diagnosed disease, during the workup of syndromes diagnosed after birth, during screening in an at-risk family, or as an incidental finding. The US approach is the same as that described for the fetus. The role of imaging in the diagnosis and follow-up of renal involvement is to search for complications such as hemorrhage or urolithiasis. Specifically, an important role for US is detecting hepatic-biliary complications.
Key Points
Renal cystic diseases can be diagnosed prenatally or in childhood.
Hyperechogenicity or cysts are cardinal findings.
Familial history, detailed clinical inquiry, and associated findings help establish the diagnosis.
Renal Ectopia and Duplications
One of the most interesting areas of pediatric uroradiology is studying and understanding the multitude congenital abnormalities of the UT. During normal renal development, the kidneys ascend from the renal pelvis while rotating medially. If the kidneys do not ascend or ascend past their normal location in the renal fossae, they are ectopic. In some cases, they are as low as the pelvis and in others as high as the thoracic cavity. If the kidneys fuse during ascent, pelvic cake kidneys, midline horseshoe kidneys, or left- or right-sided cross-fused ectopic kidneys form. As the embryological origin of the kidneys (metanephros) is separate from that of the ureters (ureteric buds), the site of ureteral insertion is normal, even if the kidney is ectopic. However, renal blood supply from the aorta will vary depending on the level of ectopia.
The ureteric bud must meet the metanephros in order for the kidney to form. Without this interaction, kidney formation is not induced. If two ureteric buds meet at the metanephric blastema, then the kidney becomes duplex. Ureteral duplication may be complete or, more commonly, incomplete.
Incomplete Ureteral Duplication
In incomplete ureteral duplication, a single ureteric bud bifurcates and meets the metanephros during approximately the fifth to sixth week of gestation. The two branches of the ureter may join at the level of the renal pelvis (bifid pelvis) or at the proximal, mid, or distal ureter (bifid ureter) and terminate in a single distal ureter that inserts orthotopically into the bladder. As the two moieties of the kidney share a common distal ureter, they behave similarly and usually appear normal. Rarely, one of the ureteral buds may be blind-ending and never appear to reach the kidney (blind-ending ureteral duplication). The associated kidney has a single collecting system.
Complete Ureteral Duplication
In complete ureteral duplication, two separate ureteric buds arise from the Wolffian duct. The lower-pole ureter is considered the analogue to the normal, single-system ureter. Thus, the lower pole of the kidney has all the same abnormalities that can affect a single-system kidney, including VUR, ureteropelvic junction obstruction (UPJO), and UVJO. The upper-pole ureter is abnormal and ectopic (Weigert-Meyer rule).
Ectopic Ureter
The ectopic ureter inserts medially and inferiorly to the normal ureteral orifice, usually in the bladder. In girls, the ectopic ureter may insert below the bladder base, into the urethra or vagina. A vaginal ectopic ureter can cause constant urinary dribbling and is a cause of incontinence [11]. In boys, ectopic ureters never terminate below the urinary sphincter and thus never cause incontinence; however, the ureter can terminate in Wolffian duct derivatives, including seminal vesicals and vas deferens. Very rarely, three completely or incompletely separated ureters form, resulting in ureteral triplication [12].
Ectopic ureters are often obstructed but rarely reflux. If the ectopic ureter inserts into the urethra at the level of the urinary sphincter, urinary flow is obstructed or refluxes depending whether the sphincter is closed or open [13]. The more distal the ureteral insertion, the more dysplastic and dysfunctional the associated renal parenchyma. Ectopic ureters, and all the associated abnormalities, can also occur in single-system kidneys (single ectopic ureter) [14].
Ureterocele
A ureterocele is the dilated submucosal terminal segment of the ureter. It causes varying degrees of ureteral obstruction and subsequent dilatation of the renal pelvis and calyces. In girls, ureteroceles are most commonly seen in association with ectopic upper-pole ureters. In boys, they are most commonly associated with singlesystem kidneys and are orthotopic. Although ureteroceles protrude into the bladder, when the intravesical pressure equals that of the ureterocele, the ureterocele can flatten and become imperceptible (efface). When the intravesical pressure exceeds that of the ureterocele, the latter everts or intussuscepts into its ureter. Ectopic bladderneck ureteroceles or large, simple ureteroceles can prolapse into the urethra and cause bladder outlet obstruction.
Key Points
Renal ectopia is due to abnormalities in the normal ascent of the kidney.
Ureteral duplication may be incomplete (more common) or complete.
The Weigert-Meyer rule states that the upper-pole ureter of a duplex kidney inserts ectopically, medially, and inferiorly to the orthotopic location.
The lower-pole ureter is the analogue of the single-system kidney.
Urinary Tract Obstruction
Urinary tract obstructions occur at three main areas: the ureteropelvic junction, the ureterovesical junction, and the bladder outlet (i.e., urethra). Rarely, the midureter can be obstructed by webs, fibrosis, or compression from the inferior vena cava; or there may be obstruction at the level of the infundibula in the kidney. On US, the normal hypoechoic medullary pyramids seen routinely in infancy and childhood should not be confused with dilated calyces or a sign of obstruction.
Related posts:
Enterography: Small Bowel Imaging That Impacts Patient Management
to Perform and Report Magnetic Resonance Imaging for Pelvic Floor Dysfunction: An Interactive Case-Based Approach
and Therapy of Neuroendocrine Tumors of the Abdomen
in Hepatobiliary-Pancreatic Tumors
Abdominal Disease: Diagnosis and Therapy
Urography in Children
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
