Figure 3.4 gives a schematic diagram of some of the more common anomalies of the urinary system. Renal anomalies occur in some 3–4% of newborn infants and are usually those of number of kidneys, rotation and position.

Figure 3.4 Congenital anomalies of the kidneys and ureters. Some of the more commonly encountered renal anomalies. (A) A rotated kidney and bifid ureter. (B) A crossed-fused ectopic kidney. (C) A pelvic pancake kidney.

RENAL ANOMALIES

Anomalies of the upper urinary tract

Anomalies of the lower urinary tract

Anomalies of the distal ureter

Primary megaureter

Megaureter is a general term used to describe the presence of a dilated ureter with or without associated dilation of the upper collecting system. The normal ureter in children is rarely more than 5 mm ¹ and if a ureter is detected on ultrasound is generally considered to be abnormal. The term primary megaureter is a term that includes all abnormalities related to a congenital alteration at the vesicoureteric junction (VUJ) and may be obstructed, refluxing or non-refluxing non-obstructed.

Typically there is a short, narrowed non-peristaltic section of ureter next to the bladder which inserts into the normal position. The normal ureter proximal to this segment dilates. The cause is unknown.

A refluxing megaureter is due to a short or absent intravesical ureter or an abnormality of the VUJ.

Secondary megaureter occurs as a result of some other abnormality involving the bladder or urethra such as urethral obstructions, for example posterior urethral valves (PUV), or bladder abnormalities, for example neuropathic bladders.

Nowadays most infants are detected prenatally and all will require cystography to exclude secondary causes and VUR.

Ectopic ureter

An ectopic ureter refers to the abnormal insertion of the ureter into the bladder. This is related to the abnormal intrauterine development of the ureteric bud where it fails to separate from the mesonephric duct (Wolffian duct) and is carried caudally. In males it may empty into the posterior urethra, lower bladder, seminal vesicle or ejaculatory duct. In females it may insert into the lower bladder, urethra, vestibule or vagina. The most important feature that differentiates ectopic ureters in girls and boys is that girls become incontinent whereas boys remain continent. A history of 24-hour wetting in a girl is highly suspicious of an ectopic ureter. Ectopic ureters are most commonly associated with ureteral duplication so that the upper moiety ureter inserts below and medial to its normal location and to that of the lower moiety ureter. For this reason it is frequently associated with ureterocele. Ectopic ureters can also drain a single kidney. They are associated with dysplasia of the draining moiety, most commonly the upper moiety.

Ureterocele

Ureteroceles are cystic dilations of the intravesical segments of the ureter and may be associated with either single or duplex ureters. The wall is composed of bladder and ureteral epithelium. Ureterocele may be small or fill the whole bladder and even prolapse out of the urethra. When they are large and fill the bladder they may obstruct the contralateral ureter and may be missed on ultrasound. They may be simple or ectopic. A simple ureterocele refers to a normal position in the bladder with a stenosis of the ureteral orifice. This is more common in adults and is thought to be related to infection.

On ultrasound the ureterocele appears as a cystic structure with a thin membrane within the bladder almost always associated with a dilated ureter. If large enough it will obstruct the bladder, and in boys may even be mistaken for PUV with an irregular trabeculated bladder wall. In nearly three quarters of pediatric patients an ectopic ureterocele is associated with a duplex collecting system and is the distal portion of the upper moiety ureter. The upper moiety is usually obstructed by the ureterocele. Due to the stasis of urine, urinary tract infections and stones in the ureterocele may occur. VUR may be associated with the lower moiety duplex due to the shorter and more vertical intravesical course of the ureter as mentioned previously (Fig. 3.8)

Figure 3.8 Ureterocele mimicking posterior urethral valves. (A) Longitudinal sonogram of the bladder in a male infant showing the thin wall of the ureterocele and a large full bladder. (B) Cystogram on the same infant showing the filling defect of the ureterocele obstructing the bladder outlet. The bladder wall is trabeculated as one might expect to see in posterior urethral valves. This infant was mistakenly thought to have posterior urethral valves prenatally.

Primary vesicoureteric reflux (VUR)

Vesicoureteric reflux is the flow of urine from the bladder into the ureter and upper collecting system. It may be isolated but is commonly associated with other abnormalities, in particular bladder and bladder outflow obstructions. There is also a known association with duplex systems and multicystic kidneys, and it is known to be a familial condition (Table 3.1).

Table 3.1 Grades of VUR based on guidelines of the International Reflux Study Committee ²⁰(Lebowitz)

Grade	Characteristics
I	Ureter only
II	Ureter, pelvis, and calyces; no dilation; normal calyceal fornices
III	Mild or moderate dilation or tortuosity of the ureter and moderate dilation of the renal pelvis; no or slight blunting of the fornices
IV	Moderate dilation or tortuosity of the ureter and moderate dilation of the renal pelvis and calyces; complete obliteration of the sharp angle of the fornices but maintenance of the papillary impressions in the majority of calyces
V	Gross dilation and tortuosity of the ureter; gross dilation of the renal pelvis and calyces; papillary impressions are no longer visible in the majority of the calyces

Cystography, that is the catheterization of the bladder and the instillation of radiographic contrast (the conventional method), radioisotope or ultrasound contrast agents is the only method of directly detecting VUR.

Anomalies of the bladder

Bladder agenesis is extremely rare and most infants are stillborn.

Urachal abnormalities

The urachus is situated in the midline superficially and just anterior to the peritoneum in the space of Retzius. It extends from the dome of the bladder to the umbilicus. It is obliterated before birth but in some it may persist as a cyst or sinus which is a result of failure of closure. This is associated with obstruction to the lower urinary tract or ventral abdominal wall defects. It may be diagnosed in the neonate as urine leaking from the umbilicus. A urachal cyst forms when the umbilical and bladder section of the lumen close, leaving fluid in the intervening portion. This looks like a midline cyst on ultrasound and may present clinically due to infection or bleeding.

Bladder duplication

This is also an extremely rare anomaly. It may occur in the sagittal or coronal plane, most commonly sagittal, with two bladders lying side by side. Each bladder receives the ureter of the ipsi-lateral kidney and is drained by its own urethra. Associated congenital anomalies are duplication of the external genitalia and lower gastrointestinal (GI) tract together with spinal duplication.

Bladder diverticulae

Large congenital bladder diverticulae are found in approximately 1.7% of children, almost exclusively boys. They may occur as a periureteric diverticulum classically known as a Hutch diverticulum which is associated with VUR. They are caused by a congenital bladder muscular anomaly. They may be unilateral or bilateral and are not associated with bladder and bladder outflow abnormalities. They are generally diagnosed at cystography when the child is being evaluated for a urinary tract infection (UTI) or incontinence. They may be quite difficult to appreciate on ultrasound, only appearing as an unusual contour to the bladder. Post-micturition they do not always empty and may in fact enlarge, which will be the clue to the diagnosis. When associated with VUR they are generally excised as they alter the ureteric insertion and VUR will not spontaneously cease.

Prune belly syndrome (Eagle–Barrett syndrome)

The term ‘prune belly’ refers to the prune-like, wrinkled abdominal wall in these children. It is a syndrome which is associated with other anomalies of the respiratory, gastrointestinal, musculoskeletal and cardiovascular systems. The prune-like appearances of the anterior abdominal wall is thought to be related to the intrauterine ascites which stretches the abdominal wall. There are bilateral undescended testes, varying degrees of dysplasia and dilation of the upper urinary tract and a large bladder. It occurs almost exclusively in males.

Cloacal abnormalities

The term cloaca means ‘sewer’ in Latin. Cloacal malformations refer to the common channel of the urinary, genital and gastrointestinal tracts so that there is a single perineal opening. This anomaly is a result of the failure of these structures to separate and have their own opening onto the perineum. The anterior abdominal wall is intact. Abnormalities of the bladder, uterus, vagina and bony pelvis together with the lower spinal cord are common associations. This only occurs in girls.

Cloacal exstrophy is a different condition where the lower abdominal wall fails to close and the bladder opens onto the anterior abdominal wall. It occurs in both girls and boys.

Urogenital sinus is where the urinary and genital tracts have a common orifice with the GI tract separate so that there are two orifices, one for the bladder and vagina and one for the anus. The anterior abdominal wall is normal. This only occurs in girls.

Urethral abnormalities

Posterior urethral valves

This is the commonest urethral abnormality occurring in boys. There is a thick membrane in the posterior urethra which behaves as a valve so that while a catheter can be passed into the bladder, the infant is unable to adequately void and empty the bladder. This causes a typical dilation of the posterior urethra; i.e. dilation proximal to the obstructing valve (Fig. 3.9).

Figure 3.9 Posterior urethral valves on MCU. Image of the posterior urethra in a male infant while voiding. The posterior urethra is markedly dilated with a sharp transition to the anterior urethra (arrow). The bladder neck appears hypertrophied with a trabeculated bladder wall. These are the typical appearances of posterior urethral valves.

These infants are primarily detected prenatally but a significant number are still seen with a late diagnosis postnatally. Dilation and dysplasia of the upper urinary tract is variable and dependent on when the valve became effective in intrauterine life.

Other urethral abnormalities

Anterior urethral valve, urethral duplication, megalourethra and congenital urethral stricture are all rare urethral abnormalities which are generally diagnosed by cystography.

ULTRASOUND PREPARATION AND TECHNIQUE

Ultrasound is the first investigation in all children suspected of having any urinary tract abnormality. The findings of the ultrasound will then direct further investigation so it is crucial the sonographer performs a meticulous examination with a properly prepared child.

The anatomical information given by ultrasound is independent of function. Functional information is generally obtained from nuclear medicine studies. Sometimes further anatomical information, particularly of the ureters and calyces, is needed and for this an intravenous urogram (IVU) is performed (Table 3.2) (Fig. 3.10).

Table 3.2 Renal investigations in children

Investigation	Description	Indication
Micturating cystourethrogram (MCU)	Bladder catheterization. Demonstrates bladder size and shape. VUR if present. Only test which will show urethral abnormalities in boys	Suspected PUV. Thick-walled bladder. Ureteric dilation. UTI in boy < 1 year. Prenatal diagnosis of bilateral renal tract dilation
Direct isotope cystogram (DIC)	Bladder catheterization and isotope contrast	First investigation in girls and for follow-up in toddlers who are not potty- trained. Lower radiation dose than IRC
Voiding urosonography	Bladder catheterization and ultrasound contrast	Similar to DIC Not universally accepted yet
Indirect radioisotope cystograms (IRC)	Dynamic renogram followed by voiding to look for VUR	Follow-up of VUR in older continent children. Gives functional information about kidneys
Dimercaptosuccinic acid (Tc 99m DMSA)	Isotope gets fixed in the kidney	To look for scars in the kidney following UTI To search for functioning renal tissue not seen on US To assess differential renal function
Diethylenetriaminepentaacetate (Tc 99m DTPA)	Dynamic renogram	Assess differential renal function and drainage
Mercaptoacetyltriglycine (Tc 99m MAG3)	Isotope taken up by kidney and passed in urine. After 30 min micturition views to look for VUR (IRC)	Postoperative evaluation of the collecting system: Indirect cystography Following renal transplantation Renography with captopril stimulation for renovascular hypertension
Intravenous urogram (IVU)	Contrast injected intravenously. Series of views of the kidneys and bladder as the contrast passes through the system	To demonstrate calyceal anatomy In the work-up of calculi To demonstrate ureteric anatomy Difficult duplex systems

IRC, indirect radioisotope cystogram; PUV, posterior urethral valves; UTI, urinary tract infection; VUR, vesicoureteric reflux.

Figure 3.10 Nuclear medicine cystography. (A) Direct isotope cystogram (DIC). This form of cystography is used in children who are not potty-trained. The bladder has been catheterized and isotope instilled into the bladder. This example shows bladder filling and bilateral vesicoureteric reflux into the kidneys. (B) MAG3 and indirect radioisotope cystogram (IRC). In this examination a formal MAG3 study is performed. This image shows isotope in the bladder at the end of the examination (top left image). The child is then asked to void and vesicoureteric reflux is seen into both kidneys (bottom right image).

Preparation

Examine the child well hydrated and with a full bladder wherever possible. Failure to have a full bladder could result in the sonographer missing an intermittent PUJ obstruction, dilated lower ureters, intravesical pathology such as ureteroceles, pelvic masses and assessing bladder emptying.

Start with the patient supine and examine the bladder first as the infant may micturate and vital information will be lost (Box 3.1).

BOX 3.2 Normal renal tract ultrasound series

• Supine longitudinal and transverse full bladder views. Bladder wall measurements away from the trigone

• Post-micturition supine longitudinal of the right kidney and left kidney comparing the echogenicity to the liver and spleen

• Prone bipolar measurement of maximal length of both kidneys

• Prone transverse pelvic diameter at the point of maximal width (either intra- or extrarenal). Use Doppler to identify the renal vessels

• Supine post-micturition longitudinal and transverse views of the bladder and a volume measurement of post-micturition residue if present

An overfull bladder may cause a mild fullness of the collecting system, so it is best to start the examination of the full bladder and then get the patient to micturate. After micturition, examine the kidneys. If the kidneys appear dilated with the full bladder, then it is generally best to wait at least 15 minutes before examining the kidneys again. Establish a local protocol so that the referring nephrourologists know exactly when (i.e. before or after micturition) and where the renal pelvis is measured (Box 3.2)

BOX 3.1 Tips

• Always start with the bladder first

• If the kidneys are tucked underneath the ribs try using deep inspiration or expiration, or by asking the child to blow their tummy up like a balloon, i.e. Valsalva

• Dilated ureters in a neonate are best seen in the supine position

• Try scanning the upper ureter by angling the probe 45° medial of the maximum renal length in the prone position

• Bipolar length must be measured prone. Supine renal lengths may be falsely high due to image magnification

• Bladder wall should be measured away from the trigone as the musculature here is slightly thicker than the rest of the bladder wall and may give falsely high measurements

• A renal ultrasound examination is incomplete if the bladder has not been examined full and after micturition

Doppler evaluation is integral to the examination but particularly important in conditions such as:

• renal transplant assessment (to evaluate blood flow to the kidney and areas of hypo- and hyperperfusion)

• essential in primary non-function of the renal transplant

• suspected renal vein thrombosis (to evaluate the renal vein and inferior rena cava)

• renal tumor (to evaluate renal vein and IVC for tumor thrombus)

• hypertension (to look at the caliber of the aorta, aneurysmal dilation of the main renal vessels and particularly to assess the intrarenal spectral trace).

Normal ultrasound appearances

The normal ultrasound appearances in a neonate typically show an increase in the cortical echogenicity. Normally the parenchymal echogenicity is equal to or increased as compared to the liver and spleen. This can persist for up to 6 months and is thought to be related to the greater volume of glomeruli occupying the renal cortex. The medullary pyramids are prominent, hypoechoic, triangular structures with base on the renal cortex and regularly arranged around the central collecting system. There is an echogenic ‘dot’ on the base of the triangle which is the arcuate artery passing between the cortex and medulla. This is an easily identifiable structure on dynamic scanning and will accurately differentiate between calyceal dilation and cysts. The central sinus echoes in a neonate are much less evident than in an adult or older child. Fetal lobulation may still be present.

After 6 months and in the older child the echogenicity of the cortex becomes more hypoechoic and the cortex appears thicker in relation to the medullae. The central sinus echoes become more prominent with age and body fat. Normally no calyceal dilation is seen. The renal pelvis shows some variation in size and a transverse pelvic diameter of <10 mm is considered to be within the normal range. Good hydration of the child and a very full bladder may result in it being more prominent ¹ (Figs 3.11 and 3.12).

Figure 3.11 Fetal lobulation. Prone longitudinal sonogram of the right kidney in a neonate. The outline of the kidney has regular indentations of fetal lobulation. The indentations occur between the medullae. This appearance must not be mistaken for renal scarring, which usually extends over a longer area, and the angle at indentation of the cortex is not so acute.

Figure 3.12 Normal neonatal kidney. Longitudinal sonogram of the right kidney in a normal neonate. The parenchyma of the cortex is the same echogenicity or of increased echogenicity as that of the liver. The cortex is thinner than the older child and adult and there are very few central sinus echoes. The medullae are triangular in shape and regularly arranged around the central collecting system. The capsule is not visible.

When imaging the renal tract it is important to have the children fully hydrated in order to accurately assess any degree of pelvicalyceal dilation. A grading system of hydronephrosis has been proposed ² such that:

• Grade 0—no hydronephrosis

• Grade 1—renal pelvis only visualized

• Grade 2—hydronephrosis is present when a few but all not all calyces are identified in addition to the renal pelvis

• Grade 3—hydronephrosis requires that all calyces are seen

This system should only be used after vesico-ureteric reflux has been excluded. The emphasis of this system is on evaluating the intrarenal portion of the pelvicalyceal system rather than unduly emphasizing an extrarenal pelvis. Evidence suggests that calyceal dilation has a poorer prognosis for the kidney and generally the threshold for further imaging is lower in these children.

Normative charts must be available and used for all examinations. The normal renal length relative to age should be quoted in all reports. However, renal patients are often poorly grown and small so that it is our preference to use charts that also allow us to compare weight and height rather than age alone. The length of the kidney is the easiest parameter to assess but volume has a more accurate correlation with most body size parameters (i.e. weight and height, which is important in children) The renal length in normal term neonates is 3.4–5.0 cm and the renal volume is 5.7–14.3 cm³. As a rule of thumb in premature infants use 1 mm renal length per week gestation in order to assess renal size.³^–⁵ In children who have had a kidney removed or only have a single functioning kidney, due to a multicystic dysplastic kidney for example, use charts for a single kidney (Fig. 3.13).⁶

Figure 3.13 Renal size in children (Han and Babcock, Sonographic measurements and appearance of normal kidneys in children. Reprinted with permission from the American Journal of Roentgenology). These charts represent the maximum renal length (A) for age, (B) for weight, (C) for height.

The normal bladder wall when distended is 0.04–0.27 cm (90–100% of capacity) and when empty is 0.16–0.39 cm (0–10% of capacity) (Fig. 3.14).⁷

Figure 3.14 Normal bladder wall. Longitudinal sonogram of the normal bladder. Bladder wall measurements should be taken from the echogenic mucosa to the echogenic serosa and away from the trigone (between calipers).

In little girls a normal phenomenon occurs of vaginal filling post-micturition. This is occasionally observed in cystography and is a normal finding (Fig. 3.15).

Figure 3.15 Fluid in the vagina. (A) Longitudinal and (B) transverse sonogram of the bladder in a female infant. There is fluid in the vagina (arrow). Fluid in the vagina can occur normally in little girls and is occasionally found as an incidental finding in the absence of other pathology.

Doppler examination

Doppler examination is integral to a full examination of the kidneys. Often children are unable to lie still for a long examination, so it is wise to have manufacturer presets readily accessible on the equipment and good entertainment and distraction for the child. Rarely does the information obtained on Doppler warrant sedation that a quick experienced operator cannot obtain. Equipment which incorporates both color energy, spectral and color Doppler is essential in pediatric ultrasound practice, with easy switching between the three and simultaneous display of color and spectral Doppler.

The color Doppler should be used as an adjunct when assessing the renal pelvis and hilar vessels and for a quick overview of the blood flow to a kidney. It is particularly useful in renal transplantation. It should be used in suspected renovascular hypertension both to evaluate the aorta and the kidneys. The appearance of the normal intrarenal Doppler waveform is shown and reflects the low resistance of the renal bed (Fig. 3.16).

Figure 3.16 Doppler spectral trace. Line diagram demonstrating the normal renal Doppler spectral trace and intrarenal waveform. There is forward flow in systole and diastole and the systolic upstroke is well-defined and sharp. The acceleration time (AT) is defined as (y−x) while the acceleration index is (z−x)/(y−x)

PRENATALLY DIAGNOSED UROLOGICAL ABNORMALITIES

Since the introduction of routine prenatal scanning and latterly more detailed anomaly scanning, the prenatal ultrasound diagnosis of renal abnormalities is now well established. This prenatal detection has resulted in a whole new group of patients presenting to pediatric urologists in the last decade. These infants are primarily asymptomatic, and are referred for urological opinion and radiological investigation and monitoring (see Ch. 2). Treatment is mainly preventive and relies on close follow-up and timely intervention when necessary.

Accurate postnatal ultrasound, in conjunction with knowledge of the prenatal findings, is of fundamental importance in these infants and guides further imaging. Practice varies but it is generally accepted that the first postnatal ultrasound should be when the baby is well hydrated and this is generally once the mother’s milk has come on at around 2 to 3 days of age. Depending on resources, some centers would repeat the ultrasound at 6 weeks to confirm normality. Babies suspected of having posterior urethral valves, duplex systems and complex anomalies, i.e. conditions requiring urgent investigation and treatment, should be referred without waiting.

Infants referred for postnatal follow-up are those who at the 18-week scan have a renal pelvic diameter of 5 mm or more, which either enlarges or remains static during pregnancy. Some would also include those with a measure of 8 mm at eight months: 1 mm per month of gestation is a good rule of thumb.

The role of ultrasound postnatally is to:

• confirm the prenatal diagnosis

• detect infants that may need more urgent investigation

• provide baseline measurements for long-term follow-up.

The differential diagnosis is listed below and the echogenic or bright kidney will be dealt with later (Table 3.3) (Box 3.3).

Table 3.3 Differential diagnosis of prenatal hydronephrosis

Unilateral	Bilateral
RPD	RPD
VUR	VUR
Megaureter (± VUR)	Megaureters (± VUR)
MCK (simple)	MCK (complicated); cystic dysplastic or dilated (PUJ) kidney on the opposite side
Complicated duplex kidney, i.e. obstructed upper moiety with ureterocele and/or refluxing lower moiety	Complicated duplex kidneys
	Bladder or outlet pathology, e.g. neurogenic bladder or PUV with bilateral upper tract dilation

RPD, renal pelvic dilation; VUR, vesicoureteric reflux; MCK, multicystic dysplastic kidney; PUJ, pelviureteric junction; PUV, posterior urethral valves.

BOX 3.3 General concepts for postnatal imaging protocol

• Use of antibiotics in all patients is controversial. Some advocate prophylactic trimethoprim at least until VUR has been excluded

• Postnatal ultrasound after 2–3 days when the infant is well hydrated

• Follow-up examination after 6 weeks to confirm the perinatal ultrasound findings

• Urgent urological referral for infants suspected of having PUV and complex duplex systems

• Conventional contrast cystography on dilated ureters, duplex systems and suspected PUV ‘thick-walled bladders’

• Functional imaging generally after 3 months (after period of transitional nephrology), usually MAG3

Prenatal detection of renal pelvic dilation and normal postnatal scan

The commonest abnormality in general obstetric practise is mild renal pelvic dilation which on postnatal ultrasound is found to be normal. This is the largest group of children detected: 30% have been shown to have vesicoureteric reflux and it is for this reason that some centers advocate the routine use of cystography. However, this is controversial as it imposes a large workload on radiology departments and some argue that there will be too many false-negative results as VUR resolves anyway. Long-term studies do not exist in this group but it would appear that while vesicoureteric reflux exists, these kidneys are normal and not scarred or dysplastic. However, their potential for renal damage in the presence of infection must be high (Fig. 3.17).

Figure 3.17 Normal kidney and vesicoureteric reflux (VUR). (A) Longitudinal sonogram on an infant showing a normal right kidney. The left kidney also appeared normal with no dilation of the collecting systems. The renal outline and parenchyma appear normal. (B) A micturating cystourethrogram examination performed just after the ultrasound. There is bilateral VUR into the upper tract. It is well recognized that approximately 30% of infants with an prenatal diagnosis of renal pelvic dilation will have a normal scan postnatally and will have VUR.

There is another group of patients who have an prenatal diagnosis of hydronephrosis but whose kidneys are not entirely normal on ultrasound, with irregular outlines and areas of cortical thinning, increased echogenicity and dilation of the collecting system. These patients appear to have damaged or dysplastic kidneys in the presence of prenatal vesicoureteric reflux and have a different prognosis with a much higher potential for further renal damage should infection occur (Fig. 3.18).⁸^–¹³

Figure 3.18 Abnormal kidney and vesicoureteric reflux (VUR). (A) & (B) Longitudinal sonogram of left and right kidneys in an infant with prenatally diagnosed renal tract dilation. There is asymmetry in size of the kidneys and abnormal outline. (C) DMSA (see Table 3.2) on the same infant showing the grossly abnormal renal outlines of both kidneys. The right kidney is smaller than the left. (D) There was bilateral VUR at cystography. These prenatally diagnosed infants are a well-defined group and have never had a urinary infection. This is termed fetal reflux nephropathy and they are at greater risk of long-term renal damage if infection occurs.

Interestingly, this whole group is more common in male infants.¹⁴ Further imaging will depend on management and the postnatal ultrasound examination. Depending on local protocols, in some centers contrast cystography will be performed on these infants. Those with VUR will be placed on antibiotic cover and have a DMSA performed. If after the normal postnatal ultrasound no further imaging including a cystogram is undertaken, then it is reasonable for these children to be placed on antibiotic prophylaxis at least for the first year of life while the potential for renal damage is at its highest.

Continuing renal pelvic dilation and/or calyceal dilation postnatally is still the most important finding in suspected VUR but other indicators such as irregular outlines, small size, loss of the corticomedullary differentiation, any ureteric dilation or an abnormal thick-walled bladder must be carefully looked for. Further investigation is warranted in these high-risk infants such as a micturating cystourethrogram (MCU) and DMSA scan.

Unilateral pelvic dilation

Mild renal pelvic dilation is one of the commonest prenatal diagnoses the sonographer will encounter. When reporting the examination it should be clearly stated whether there is calyceal dilation and the size of the renal pelvis. Simply calling the dilation a hydronephrosis is not sufficient and poorly describes the state of the collecting system. This is particularly important for monitoring and accurate long-term follow-up. Care should be taken in not mistaking the renal pelvis for renal vasculature at the hilum and a quick Doppler to confirm the position of the vasculature is sometimes needed (Fig. 3.19).^15,¹⁶

Figure 3.19 Renal pelvis vasculature. (A) Transverse sonogram of the renal pelvis, which appears to be dilated on the gray scale image. (B) Transverse sonogram with color Doppler of the same renal hilum showing that the ‘pelvis’ is in fact the renal artery and vein. A Doppler of the pelvis is very useful to clearly define the real renal pelvis.

A renal pelvic diameter of 10 mm and less is generally considered to be within the normal range.¹⁷ Isolated RPD <10 mm is virtually always benign and no aggressive investigations should be undertaken. Urologists vary but some would even take renal pelvic diameters of <15 mm to be benign and treat conservatively. Most would not undertake further imaging and would simply follow with ultrasound examinations. However, if there is calyceal dilation the prognosis for the kidney is generally worse and the threshold for imaging is lower. It is essential that the sonographer is able to recognize calyceal dilation. Calyceal and/or ureteric dilation irrespective of renal pelvis diameter are always significant and both reflux and obstruction must be excluded. This dilation of the renal pelvis is sometimes called PUJ (pelviureteric junction) obstruction (Fig. 3.20).¹⁸

Figure 3.20 Calyceal and renal pelvis dilation. (A) Prone longitudinal scan of the left kidney with mild calyceal dilation (arrow). (B) Supine longitudinal scan of the right kidney showing severe calyceal dilation. (C) Prone transverse scan of the same kidney. The dilated calyces and renal pelvis look like ‘mickey mouse’. This is the position to measure the dilated renal pelvis (between calipers). It is measured at its widest point in this prone position.

All these infants are asymptomatic and are now generally managed conservatively. Documented increase in renal pelvic dilation at follow-up requires further investigation such as a MAG3 and referral to a urologist.

Bilateral renal pelvic dilation

Mild renal pelvic dilation may be bilateral. No general agreement on how to manage these patients exists. Postnatal imaging should be the same as that for unilateral renal pelvic dilation. Some centers include a cystogram (Table 3.4).

Table 3.4 Imaging protocol for RPD

Unilateral	Bilateral
Postnatal ultrasound after 2–3 days	Same as unilateral but add MCUG
MAG3 renogram after 2–3 months if RPD > 15 mm
No MCUG if no dilated ureter

MCU, micturating cystourethrogram.

Megaureter

A megaureter simply refers to an enlarged ureter which may or may not reflux. The kidney has good function and the degree of calyceal and upper ureteric dilation are less than the degree of lower ureteric dilation. When undertaking the sonogram this condition will be suspected when the lower ureter is seen to be markedly dilated, typically tapering down to the vesicoureteric junction (VUJ) in the presence of a relatively normal or mildly dilated upper collecting tract. It is important to ensure on ultrasound that there is not a ureterocele in the bladder and that the bladder wall appears normal. When unilateral, the investigative protocol is similar to that of renal pelvic dilation with the addition of an MCU because of the dilated ureter and in order to exclude vesicoureteric reflux.

Multicystic kidney (MCK)

This is a non-hereditary cystic kidney which is sometimes otherwise known as multicystic dysplastic kidney. The characteristic feature of a multicystic kidney is the atretic ureter, and there is no continuity between the glomerulus and the calyces. On functional radionuclide studies they are non-functioning. The condition is more common in males and the natural history is to involute with time either prenatally or postnatally. Urological opinion varies and nowadays these kidneys are not removed unless they are causing symptoms or evidence of respiratory embarrassment. There have been reported associations with hypertension and malignancy, although generally these are not used as indications for early removal of the MCK (Fig. 3.21).

Figure 3.21 Variations in appearance of the multicystic kidney (MCK). (A) Longitudinal sonogram of the right kidney. There are a number of cysts of varying sizes. This is the typical appearance of a MCK. If the renal cortex can be identified, then the diagnosis of MCK must be in doubt. (B) Longitudinal sonogram of the right kidney (between calipers). There is a single large cyst. (C) Prone longitudinal sonogram of the left flank. This is a small involuted MCK. Small cysts are measured between calipers. Since prenatal detection, the spectrum of appearances in MCK is wide from a small single cyst to the large multiple cysts.

If they are present bilaterally, it is incompatible with life. The prognosis depends on the function of the contralateral kidney, for which there is a high association with abnormalities, usually PUJ or ureteric stenosis (30%).

Ultrasound and postnatal imaging

Postnatal ultrasound should make the diagnosis of a multicystic kidney and, in terms of management and prognosis, cases are then divided into two groups. An MCK and normal contralateral kidney is considered to be simple, while an abnormal contralateral kidney is considered to be a complicated MCK. The typical appearances of an MCK are those of a single large cyst with multiple smaller cysts. However, there is a wide variation in size and appearance of these kidneys. Normally no renal parenchyma should be seen around the edge of the cysts. If parenchyma is seen then the possibility of a large PUJ obstruction causing similar ‘cystic’ appearances should be considered. Nuclear medicine studies will not discriminate between the two, as neither will function, and prognostically there will be the same outcome. A small proportion of multicystic kidneys will have a ureterocele in the bladder. Multicystic kidneys may also affect part of a kidney.

The initial postnatal ultrasound examination should confirm the diagnosis and provide a baseline for follow-up.

The examination should include a bipolar renal length of the multicystic kidney, measurement of the largest cyst and approximate number of cysts within the kidney. In so doing, follow-up examinations will be able to assess whether the multicystic kidney is involuting. A small number may enlarge.

The contralateral kidney should be carefully examined, looking for dilation of the calyces, pelvis or ureter and increased parenchymal echogenicity which may reflect renal dysplasia (Fig. 3.22).

Figure 3.22 Contralateral kidney abnormalities in multicystic kidney (MCK). (A) Longitudinal sonogram of the right kidney showing dilation of the calyces. The left kidney was multicystic. (B) The dynamic renogram in the same child demonstrates the non-function on the side of the MCK and delayed transit of isotope through the dilated contralateral kidney. PUJs and ureteral stenoses are a well-recognized association with MCK. (C) This is another child with an MCK and this is the contralateral kidney. It shows a global increase in echogenicity with loss of the corticomedullary differentiation and a small subcortical cyst (between calipers) which is typical of cystic dysplasia.

There is a known association of ipsilateral and contralateral vesicoureteric reflux. It is for this reason that cystography was originally advocated. However, it is well recognized that the grades of reflux are low and the contralateral kidney is unlikely to be damaged, so the current role of cystography is controversial.

In some centers a DMSA or MAG3 study is performed in order to document the normality of the contralateral kidney. This is also controversial and some would only perform these examinations on the abnormal (so-called ‘complicated’) contralateral kidney detected by ultrasound (Table 3.5).

Table 3.5 Imaging protocol for MCK

Simple	Complicated
Postnatal ultrasound after 2–3 days No need for prophylactic antibiotics Repeat scan at 6 weeks DMSA or MAG3 renogram. If any function on these tests, MCK is unlikely. Refer to urologist	Contralateral kidney dilated >15 mm Exclude obstruction MAG3 renogram after 2–3 months Prophylactic antibiotics Contralateral kidney <15 mm with ureteric dilation MCUG and, if VUR, then follow as for VUR. If no VUR, then follow as for simple kidney

Posterior urethral valves

The clinical presentation is very variable, with the most obstructed systems detected prenatally and the less severely affected in early infancy.

Posterior urethral valves refers to a condition where quite simply a flap of mucosa obstructs the urethra. The mucosal flap extends from the wall of the urethra to the verumontanum and effectively acts as a ‘windsock’. There is a pinhole orifice, so that a urinary catheter can be passed up the urethra; however, on micturition, the valve balloons so that there is an obstruction to the passage of urine out of the bladder and down the urethra. The appearances seen on ultrasound are related to a hypertrophy of the bladder wall as it tries to overcome the outflow obstruction. The kidney changes depend on the time of onset of the obstruction in intrauterine life, so that the earlier the onset, the higher the risk of severe consequences to the kidneys such as dysplasia. The later the onset, the more likely the kidneys will be simply hydronephrotic or in older boys may even be normal. Urinomas, or collections of urine around the kidney, develop when a calyx ruptures due to the high pressure in the collecting system. This is a protective event for the kidney as it allows the kidney to decompress.

The presence of bilateral hydronephrosis and a full bladder during intrauterine life associated with oligohydramnios is the essential prenatal diagnosis (see Ch. 2). These infants need urgent postnatal imaging in order to make the diagnosis and allow early treatment of the bladder outflow obstruction. Also these infants have a high risk of developing a urinary tract infection which could further compromise the kidneys and even prove fatal.

Postnatal ultrasound findings would include bilateral hydronephrosis and hydroureters, echogenic cystic dysplastic kidneys, a urinoma where the urine has ruptured out of the kidney from a calyx and a thick-walled bladder (Fig. 3.23).