The Kidney and Urinary Tract

Hypoplasia

Renal hypoplasia is a renal parenchymal anomaly in which there are too few nephrons. Renal function depends on the mass of the kidney. True hypoplasia is a rare anomaly. Many patients with unilateral hypoplasia are asymptomatic; the condition is typically an incidental finding. Patients with bilateral hypoplasia often have renal insufficiency. Hypoplasia is believed to result from the ureteral bud making contact with the most caudal portion of the metanephrogenic blastema. This can occur with delayed development of the ureteric bud or from delayed contact of the bud with the cranially migrating blastema. Hypoplasia is established when fewer but otherwise histologically normal renal lobules are identified. At ultrasound, the kidney is small but otherwise appears normal.

Fetal Lobation

Fetal lobation is usually present until 4 or 5 years of age; however, persistent lobation is seen in 51% of adult kidneys. There is infolding of the cortex without loss of cortical parenchyma. At ultrasound, sharp clefts are shown overlying the columns of Bertin.

Compensatory Hypertrophy

Compensatory hypertrophy may be diffuse or focal. It occurs when existing healthy nephrons enlarge to allow healthy renal parenchyma to perform more work. The diffuse form is seen with contralateral nephrectomy, renal agenesis, renal hypoplasia, renal atrophy, and renal dysplasia. Diffuse compensatory hypertrophy is suggested at ultrasound when an enlarged but otherwise normal-appearing kidney is identified. The focal form is seen when residual islands of normal tissue enlarge in an otherwise diseased kidney; focal compensatory hypertrophy may be particularly prominent in the setting of reflux nephropathy. Large areas of nodular but normal renal tissue identified between scars may mimic a solid renal mass.

Ectopia

Failure of the kidney to ascend during embryologic development results in a pelvic kidney; prevalence is 1 in 724 pediatric autopsies. These kidneys are often small and abnormally rotated. Fifty percent of pelvic kidneys have decreased function. The ureters are often short; poor drainage and collecting system dilatation predispose pelvic kidneys to infection and stone formation. The blood supply is often complex; multiple arteries may be derived from regional arteries (typically, internal iliac or common iliac). If the kidney ascends too high, it may pass through the foramen of Bochdalek and become a true thoracic kidney; this is usually of no clinical significance. A search for a pelvic kidney should be performed if the kidney is not identified within renal fossae ( Fig. 9.9 ). If the kidney has ascended too high, ultrasound is helpful to determine if the diaphragm is intact.

Crossed Renal Ectopia

In crossed renal ectopia, both kidneys are found on the same side. In 85% to 90% of cases, the ectopic kidney will be fused to the other kidney (crossed-fused ectopia). The upper pole of the ectopic kidney is usually fused to the lower pole of the other kidney, although fusion may occur anywhere. The incidence is 1 in 1000 to 1 in 1500 at autopsy. Fusion of metanephrogenic blastema does not allow proper rotation or ascent; thus both kidneys are more caudally located, although the ureterovesical junctions (UVJs) are located normally. At sonography, both kidneys are on the same side and are typically fused ( Fig. 9.10 ). In patients with renal colic, knowing that the UVJs are in the normal location is particularly important, since bilateral ureters need to be assessed.

Horseshoe Kidney

The incidence of horseshoe kidneys in the general population is 0.01% to 0.25%. Horseshoe kidneys occur when metanephrogenic blastema fuse prior to ascent; fusion is usually at the lower poles (95%). Typically, the isthmus is composed of functioning renal tissue, although rarely it is made up of fibrous tissue. The horseshoe kidney sits anterior to the abdominal great vessels and derives its blood supply from the aorta and other regional vessels, such as inferior mesenteric, common iliac, internal iliac, and external iliac arteries. Abnormal rotation of renal pelves often results in ureteropelvic junction (UPJ) obstruction; the horseshoe kidney is thus predisposed to infection and stone formation. Additional associated anomalies include vesicoureteral reflux, collecting system duplication, renal dysplasia, retrocaval ureter, supernumerary kidney, anorectal malformation, esophageal atresia, rectovaginal fistula, omphalocele, and cardiovascular and skeletal abnormalities.

At sonography, horseshoe kidneys are usually lower than normal and the lower poles project medially. Transverse imaging of the retroperitoneum will demonstrate the renal isthmus crossing the midline anterior to abdominal great vessels ( Fig. 9.11 ). Hydronephrosis (pyelocaliectasis) and collecting system calculi may be evident.

Renal Agenesis

Renal agenesis may be unilateral or bilateral. Bilateral renal agenesis is a rare anomaly that is incompatible with life. The prevalence rate of bilateral agenesis at autopsies is 0.04%. The condition has a 3 : 1 male predominance. Unilateral renal agenesis is usually an incidental finding; the contralateral kidney of these patients may be quite large secondary to compensatory hypertrophy. Renal agenesis occurs when there is (1) absence of the metanephrogenic blastema, (2) absence of ureteral bud development, or (3) absence of interaction and penetration of the ureteral bud with the metanephrogenic blastema. Renal agenesis is associated with genital tract anomalies, which are often cystic pelvic masses in both men and women. Other associated anomalies include skeletal abnormalities, anorectal malformations, and cryptorchidism.

At ultrasound, although the kidney is absent, a normal adrenal gland is usually found. The adrenal gland will be absent in 8% to 17% of patients with renal agenesis. It may be difficult to differentiate between renal agenesis and a small, hypoplastic or dysplastic kidney. With all these conditions, the contralateral kidney will be enlarged as a result of compensatory hypertrophy. Usually, the colon falls into the empty renal bed. Care should be taken not to confuse a loop of gut with a normal kidney.

Supernumerary Kidney

Supernumerary kidney is an exceedingly rare anomaly. The supernumerary kidney is usually smaller than normal and can be found above, below, in front of, or behind the normal kidney. The supernumerary kidney often has only a few calices and a single infundibulum. The formation of a supernumerary kidney is likely caused by the same mechanism that gives rise to a duplex collecting system. Two ureteric buds reach the metanephrogenic blastema, which then divides, or alternatively, there are initially two blastema. On sonography, an extra kidney will be found.

Duplex Collecting System and Ureterocele

Duplex collecting system is the most common congenital anomaly of the urinary tract, with an incidence of 0.5% to 10% of live births. The degree of duplication is variable. Duplication is complete when there are two separate collecting systems and two separate ureters, each with their own ureteral orifice. Duplication is incomplete when the ureters join and enter the bladder through a single ureteral orifice. Ureteropelvic duplication arises when two ureteral buds form and join with the metanephrogenic blastema or when there is division of a single ureteral bud early in embryogenesis. Normally during embryologic development, the ureteral orifice migrates superiorly and laterally to become part of the bladder trigone. With complete duplication, the ureter from the lower pole of the kidney migrates to assume its normal location, whereas the ureter draining the superior pole of the kidney migrates abnormally to a more medial and inferior ureteral orifice. Patients have an increased incidence of UPJ obstruction and uterus didelphys.

In complete duplication, the ureter draining the lower pole has a more perpendicular course through the bladder wall, making it more prone to reflux. The ectopic ureter from the upper pole is prone to obstruction, reflux, or both ( Fig. 9.12 ). Obstruction can result in cystic dilatation of the intramural portion of the ureter, giving rise to a ureterocele. Ureteroceles may be unilateral or bilateral and may occur in normal, duplicated, or ectopic ureters. Ureteroceles may result in ureteral obstruction and give rise to recurrent or persistent UTIs. If large, they may block the contralateral ureteral orifice and the urethral orifice at the bladder neck. Treatment of these symptomatic ureteroceles is surgical. However, most ureteroceles are transient, incidental, and clinically insignificant.

At ultrasound, a duplex collecting system is seen as two central echogenic renal sinuses with intervening, bridging renal parenchyma. Unfortunately, this sign is insensitive and is only seen in 17% of duplex kidneys. Hydronephrosis of the upper-pole moiety and visualization of two distinct collecting systems and ureters are diagnostic. The bladder should always be carefully evaluated for the presence of a ureterocele. A ureterocele will appear as a round, cystlike structure within the bladder ( Fig. 9.13 ). Occasionally, it may be large enough to occupy the entire bladder and will cause obstruction of the bladder neck. In female patients, transvaginal sonography can be helpful to identify small ureteroceles ( Fig. 9.14 ). These ureteroceles may be transient. Madeb et al. demonstrated that transvaginal sonography with color Doppler and spectral analysis can provide additional information about flow dynamics, eliminating the need for invasive procedures.

Ureteropelvic Junction Obstruction

UPJ obstruction is a common anomaly with a 2 : 1 male predominance. The left kidney is affected twice as frequently as the right kidney. UPJ obstruction is bilateral in 10% to 30% of cases. Most adult patients present with chronic, vague, back or flank pain. Symptomatic patients and those with complications, including superimposed infection, stones, or impaired renal function, should be treated. Patients have an increased incidence of contralateral multicystic dysplastic kidney and renal agenesis. Most idiopathic UPJ obstructions are thought to be functional rather than anatomic. Histologic evaluation of affected specimens has demonstrated excessive collagen between muscle bundles, deficient or absent muscle, and excessive longitudinal muscle. Occasionally, intrinsic valves, true luminal stenosis, and aberrant arteries are the cause of obstruction. At ultrasound, hydronephrosis is present to the level of the UPJ ( Fig. 9.15 ). Marked ballooning of the renal pelvis is often shown, and if long-standing, there will be associated renal parenchymal atrophy. The caliber of the ureter, on the other hand, is normal. Careful evaluation of the contralateral kidney should be performed to exclude associated anomalies.

Congenital Megacalices

Congenital megacalices refer to typically unilateral, nonobstructive enlargement of the calices. It is nonprogressive; overlying parenchyma and renal function are maintained. Infection and stone formation are increased because of caliceal enlargement. The exact pathogenesis is speculative; the most common association is with primary megaureter. At ultrasound, numerous enlarged clubbed calices are shown. Papillary impressions are absent, and cortical thickness is maintained.

Congenital Megaureter

Megaureter (congenital megaureter, megaloureter) results in functional ureteric obstruction. The most distal segment of ureter is aperistaltic: focal ureteral lack of peristalsis results in a wide spectrum of findings, from insignificant distal ureterectasis to progressive hydronephrosis/hydroureter. As with UPJ obstructions, men are affected more often, and the left ureter is typically involved. Bilateral involvement has been demonstrated in 8% to 50% of patients. The classic finding at ultrasound is fusiform dilatation of the distal third of the ureter ( Fig. 9.16 ). Depending on the severity, associated pyelocaliectasis may or may not be present. Calculi may form just proximal to the adynamic segment.

Aberrant Vessels

As it ascends during embryologic development, the kidney derives its blood supply from successively higher levels of the aorta. Aberrant renal arteries will be present if the vascular supply from the lower levels of the aorta persists. Aberrant vessels can compress the ureter anywhere along its course. Color Doppler ultrasound may be useful to identify obstructing vessels crossing at the UPJ.

Retrocaval Ureter

Retrocaval ureter is a rare but well-recognized congenital anomaly with a 3 : 1 male predominance. Most patients present with pain in the second to fourth decade of life. Normally, the infrarenal inferior vena cava (IVC) develops from the supracardinal vein; if it develops from the subcardinal vein, the ureter will pass posterior to the IVC. The ureter then passes medially and anteriorly between the aorta and IVC to cross the right iliac vessels. It then enters the pelvis and bladder in a normal manner. Sonography shows collecting system and proximal ureteral dilatation. In easy-to-scan patients the compressed retrocaval ureter may be identified.

Bladder Agenesis

Bladder agenesis is a rare anomaly. Most infants with bladder agenesis are stillborn; virtually all surviving infants are female. Many associated anomalies are often present. At ultrasound, the bladder is absent.

Bladder Duplication

Bladder duplication is divided into three types, as follows :

• Type 1: A complete or incomplete peritoneal fold separates the two bladders.

• Type 2: An internal septum divides the bladder. The septum may be complete or incomplete and may be oriented in a sagittal or coronal plane. There may be multiple septa.

• Type 3: A transverse band of muscle divides the bladder into two unequal cavities.

Bladder Exstrophy

Bladder exstrophy occurs in 1 in 30,000 live births, with a 2 : 1 male predominance. Failure in development of the mesoderm below the umbilicus leads to absence of the lower abdominal and anterior bladder wall. There is a high incidence of associated musculoskeletal, gastrointestinal, and genital tract anomalies. These patients have an increased (200-fold) incidence of bladder carcinoma (adenocarcinoma in 90%).

Urachal Anomalies

Normally, the urachus closes in the last half of fetal life. The four types of congenital urachal anomalies, in order of frequency, are as follows ( Fig. 9.17 ):

• 1

  Patent urachus (50%)

• 2

  Urachal cyst (30%)

• 3

  Urachal sinus (15%)

• 4

  Urachal diverticulum (5%)

Urachal anomalies have a 2 : 1 predominance in males. A patent urachus is usually associated with urethral obstruction and serves as a protective mechanism to allow normal fetal development. A urachal cyst forms if the urachus closes at the umbilical and bladder ends but remains patent in between. The cyst is usually situated in the lower third of the urachus. There is an increased incidence of adenocarcinoma. At ultrasound, a midline cyst with or without internal echoes is seen superior to the bladder. A urachal sinus forms when the urachus closes at the bladder end but remains patent at the umbilicus. A urachal diverticulum forms if the urachus closes at the umbilical end but remains patent at the bladder. Urachal diverticula are usually incidentally found. There is an increased incidence of carcinoma and stone formation.

Acute Pyelonephritis

Acute pyelonephritis is a tubulointerstitial inflammation of the kidney. Two routes may lead to inflammation: ascending infection (85%; e.g., Escherichia coli ) and hematogenous seeding (15%; e.g., Staphylococcus aureus ). Women age 15 to 35 years are most often affected ; 2% of pregnant women will develop acute pyelonephritis. Most adults present with flank pain and fever and can be diagnosed clinically with the aid of laboratory studies (bacteriuria, pyuria, and leukocytosis). With appropriate antibiotics, both clinical and laboratory findings show rapid improvement. Imaging is necessary only when symptoms and laboratory abnormalities persist: imaging is useful to identify potential causes of insufficiently treated infection, including renal and perirenal abscesses, calculi, and urinary obstruction. The Society of Uroradiology proposed using acute pyelonephritis to describe acutely infected kidneys, eliminating the need for terms such as bacterial nephritis, lobar nephronia, renal cellulitis, lobar nephritis, renal phlegmon, and renal carbuncle.

At ultrasound, the majority of kidneys with acute pyelonephritis appear normal. However, ultrasound findings of pyelonephritis include ( Fig. 9.19 ) renal enlargement, compression of the renal sinus, decreased echogenicity (secondary to edema) or increased echogenicity (potentially from hemorrhage), loss of corticomedullary differentiation, poorly marginated mass(es), gas within the renal parenchyma, and focal or diffuse absence of color Doppler perfusion corresponding to the swollen inflamed areas.

If the pyelonephritis is focal, the poorly marginated masses may be echogenic, hypoechoic, or of mixed echogenicity. Echogenic masses may be the most common appearance of focal pyelonephritis.

Sonography, including power Doppler, is less sensitive than CT, magnetic resonance imaging (MRI), or technetium-99m single-photon emission computed tomography ( ^99m Tc-DMSA SPECT) renal cortical scintigraphy for demonstrating changes of acute pyelonephritis. However, ultrasound is more accessible and less expensive and thus an excellent screening modality for monitoring and follow-up of complications, as well as in the assessment of pregnant patients with acute pyelonephritis because of its lack of ionizing radiation.

A unique renal infection known as alkaline-encrusted pyelitis has been described in renal transplants and native kidneys of debilitated and immunocompromised patients. This entity is most frequently caused by Corynebacterium urealyticum, a urea-splitting microorganism. Urothelial stone encrustation develops in the kidney and bladder. If the kidney is affected, the patient may present with hematuria, stone passage, or an ammonium odor to the urine. Dysuria and suprapubic pain are the most common clinical signs if the bladder is involved. Treatment is with antibiotics and local acidification of the urine. On sonography, alkaline-encrusted pyelitis is suggested if thickened, calcified urothelium is identified. The calcification can be thin and smooth or thick and irregular. Care should be taken to distinguish urothelial calcification from layering of collecting system calculi.

Renal and Perinephric Abscess

Untreated or inadequately treated acute pyelonephritis may lead to parenchymal necrosis and abscess formation. Patients at increased risk for renal abscesses include those with diabetes, compromised immunity, chronic debilitating diseases, urinary tract obstruction, infected renal calculi, and intravenous drug abuse. Renal abscesses tend to be solitary and may spontaneously decompress into the collecting system or perinephric space. Perinephric abscesses, also a complication of pyonephrosis, may result from direct extension of peritoneal or retroperitoneal infection or interventions. Small abscesses are treated conservatively with antibiotics, whereas larger abscesses often require percutaneous drainage and, if drainage is unsuccessful, surgery.

At ultrasound, renal abscesses appear as round, thick-walled, hypoechoic complex masses with through transmission ( Fig. 9.20 ). Internal mobile debris and septations may be seen. Occasionally, “dirty shadowing” may be noted posterior to gas within the abscess. The differential diagnosis includes (1) hemorrhagic or infected cysts, (2) parasitic cysts, (3) multiloculated cysts, and (4) cystic neoplasms. Although not as accurate as CT in determining the presence and extent of perinephric abscess extension, sonography is an excellent modality for following conservatively treated patients with abscesses to document resolution.

Pyonephrosis

Pyonephrosis implies purulent material in an obstructed collecting system. Depending on the level of obstruction, any portion of the collecting system, including the ureter, can be affected. Early diagnosis and treatment are crucial to prevent development of bacteremia and life-threatening septic shock. The mortality rates of bacteremia and septic shock are 25% and 50%, respectively ; 15% of patients are asymptomatic at presentation. In the young adult, UPJ obstruction and calculi are the most frequent cause of pyonephrosis, whereas malignant ureteral obstruction is typically the predisposing factor in older patients. Pyonephrosis is suggested when ultrasound shows mobile collecting system debris (with or without a fluid-debris level), collecting system gas, and stones ( Fig. 9.21 ).

Emphysematous Pyelonephritis

Emphysematous pyelonephritis (EPN) is an uncommon, life-threatening infection of the renal parenchyma characterized by gas formation. Most patients are women (2 : 1) and diabetic (90%), with a mean age of 55 years. In diabetic patients, EPN tends to occur in nonobstructed collecting systems; the reverse is true in nondiabetic patients. Bilateral disease occurs in 5% to 10% of EPN patients. Escherichia coli is the offending organism in 62% to 70% of cases; Klebsiella (9%), Pseudomonas (2%) Proteus, Aerobacter, and Candida are additional causative organisms. At presentation, most patients are extremely ill with fever, flank pain, hyperglycemia, acidosis, dehydration, and electrolyte imbalance ; 18% present only with fever of unknown origin.

Wan et al. retrospectively studied 38 patients with EPN and identified two types of disease: EPN1, characterized by parenchymal destruction and streaky or mottled gas, and EPN2, characterized as renal or perirenal fluid collections, with bubbly or loculated gas or with gas in the collecting system. Mortality rates for EPN1 and EPN2 were 69% and 18%, respectively. The authors postulated that the different clinical outcomes of EPN1 and EPN2 result from the patient's immune status and the vascular supply of the affected kidney. Emergency nephrectomy is the treatment of choice for EPN1, whereas percutaneous drainage is recommended for EPN2. CT is the preferred method to image patients with EPN, to determine the location and extent of renal and perirenal gas. Sonographic evaluation of EPN1 or EPN2 may be difficult because dirty shadowing from parenchymal gas will obscure deeper structures; shadowing might also prompt an erroneous interpretation of renal calculi or bowel gas ( Fig. 9.22 ).

Emphysematous Pyelitis

Emphysematous pyelitis refers to gas localized within the urinary collecting system. This disease entity is seen most often in women with diabetes or obstructing stone disease; a mortality rate of 20% has been reported. It is important to exclude iatrogenic causes of gas within the collecting system. At ultrasound, nondependent linear echogenic lines with dirty distal posterior acoustic shadowing, indicative of gas are seen within the collecting system ( Fig. 9.23 ). As with EPN, CT is often required to identify emphysematous pyelitis because dirty acoustic shadowing from gas at ultrasound may obscure the exact extent of renal and perirenal disease.

Chronic Pyelonephritis

Chronic pyelonephritis is an interstitial nephritis often associated with vesicoureteric reflux. Reflux nephropathy is believed to cause 10% to 30% of all cases of end-stage renal disease (ESRD) ( Fig. 9.24 ). Chronic pyelonephritis usually begins in childhood and is more common in women. The renal changes may be unilateral or bilateral but usually are asymmetric. Reflux into the collecting tubules occurs when the papillary duct orifices are incompetent. This reflux occurs more often in compound papillae, which are typically found at the poles of the kidneys. Cortical scarring therefore tends to occur over polar calices. There is associated papillary retraction with caliceal clubbing. At ultrasound, a dilated blunt calix is seen, associated with overlying cortical scar or cortical atrophy ( Fig. 9.25 ). These changes may be multicentric and bilateral. If the disease is unilateral, there may be compensatory hypertrophy of the contralateral kidney. If the disease is multicentric, compensatory hypertrophy of normal intervening parenchyma may create an island of normal tissue simulating a tumor.

Xanthogranulomatous Pyelonephritis

Xanthogranulomatous pyelonephritis (XGP) is a chronic, suppurative renal infection in which destroyed renal parenchyma is replaced with lipid-laden macrophages. XGP is typically unilateral and may be diffuse, segmental, or focal. XGP is typically associated with nephrolithiasis (70%) and obstructive nephropathy. The disease most commonly occurs in middle-aged women and diabetic patients. Presenting signs are nonspecific: pain, mass, weight loss, and UTI ( Proteus or E. coli ). The diffusely involved kidney is usually nonfunctional. Ultrasound findings of diffuse XGP include renal enlargement, maintenance of a reniform shape, and lack of corticomedullary differentiation. Multiple hypoechoic areas correspond to dilated calices or inflammatory parenchymal masses. Through transmission is variable and depends on the degree of liquefaction of the parenchymal masses. Occasionally, the large, complex cystic masses may mimic pyonephrosis. A staghorn calculus will result in extensive shadowing from the central renal sinus ( Fig. 9.26 ). Perinephric extension may occur, but this is often best appreciated with CT. Diffuse XGP has no specific sonographic features but is suggested when parenchymal thinning, hydronephrosis, stones, debris in a dilated collecting system, and perinephric fluid collections are present. Segmental XGP will be seen as one or more hypoechoic masses, often associated with a single calyx. An obstructing calculus may be seen near the papilla. Focal XGP arises in the renal cortex and does not communicate with the renal pelvis. It cannot be distinguished sonographically from tumor or abscess.