Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

KIDNEYS AND URETERS

eral,they tend to be asymmetrical.A duplication is considered incomplete if the two ureters join before inserting at one bladder orifice. In a complete duplication both ureters insert separately. Duplications are familial and are more common in women. In most patients a functioning duplicated system is a curiosity, and whether to consider an otherwise unremarkable duplicated system as part of a normal spectrum is a matter of opinion.

Partial duplications have a Y configuration. Much rarer are inverted-Y partial ureteral duplications, with the two duplicated lower ureteral segments inserting either in an orthotopic or ectopic location. Perhaps rarest of all is ureteral duplication with blind superior and inferior ends (17).

Duplications are difficult to detect on precontrast axial CT. Postcontrast visualization of two ureters provides a clue to the presence of a duplication. Postcontrast CT coronal views and conventional radiographs offer another clue: the uppermost lower pole calyx (which is generally the kidney’s middle calyx) is located more lateral than expected, and as a result the kidney axis is more vertical than expected. Considerable calyceal distortion occurs with grade 5 reflux, and counting the number of calyces is an unreliable way to detect a duplication.

Neither US nor scintigraphy has sufficient resolution to reliably detect most duplications. Magnetic resonance coronal views are useful in a setting of poor renal function; thus T2weighted images reveal fluid-filled upper and lower pole structures.

With a complete duplication the lower renal pole is generally larger and its draining ureter inserts in a normal position at the bladder trigone. The upper pole ureter inserts abnormally in a more caudal position in the bladder, urethra, or adjacent structures. A ureterocele is a common associated anomaly. The upper pole ureter obstructs more often than the lower pole ureter; most obstructions are in the distal portion of the ureter, although at times an obstruction occurs at the ureteropelvic junction and involves either ureteral segment. Similar to a normal system, a transitional cell carcinoma can develop in a completely duplicated collecting system, leading to a complex imaging appearance.

Therapy for obstruction or reflux of a single ureter in a duplicated system is common sheath

reimplantation, with some surgeons performing an ipsilateral ureteroureterostomy.

Ectopia

Kidneys

Ectopia means an abnormal location for a structure. Ectopic kidneys range in location anywhere from just inferior to the diaphragm to the pelvis. A rare renal ectopia is intrathoracic; these are more common on the left and in males. The lack of imaging detection of a kidney in its usual location should not be assumed to represent agenesis; the entire abdomen needs to be scanned for an ectopic kidney, which at times is small and poorly functioning.

Malrotation of a normally positioned kidney is generally a curiosity of little significance.

An ectopic kidney undergoes incomplete rotation around its axis (it is thus malrotated), and the renal pelvis tends to face more anterior than usual. The renal collecting system does not drain as readily as in a normally situated kidney, relative stasis ensues, and thus an increased prevalence of stone formation is found in these kidneys. Ureteropelvic junction obstruction is also relatively common. These kidneys tend to be dysplastic and some are associated with other malformations.

In renal ectopia the ureter length is appropriate for kidney position; in renal ptosis the ureter appears more tortuous and is more appropriate to a normally positioned kidney.

A normal US renal sinus echo complex is absent in two thirds of ectopic kidneys, while in the other one third it is eccentric in location.

Crossed Ectopia

In crossed renal ectopia both kidneys are on one side of the abdomen. They are usually partly fused and the ectopic kidney malrotated. The ureter from the ectopic lower kidney crosses the midline and usually inserts in its normal position. Such crossed fused renal ectopia is an uncommon congenital anomaly generally of little or no clinical significance. In some, however, renal ectopia is associated with other congenital abnormalities, including ectopic ureteral insertion.

Ultrasonography of crossed fused renal ectopia identifies an empty renal fossa and a

distorted, enlarged kidney with a duplicated renal sinus. At times the ectopic kidney is difficult to identify with US due to malrotation.

Technetium-99m-DMSA shows an empty renal fossa and outlines the functional status of an ectopic kidney.

Horseshoe Kidney

A horseshoe kidney is a type of ectopia consisting of partial fusion of the two kidneys occurring in about 0.25% of the population. The lower poles of both kidneys extend more medially than usual and either have fused renal parenchyma or are connected by fibrosis. Fusion is anterior to the aorta, and the kidneys tend to be more inferior in location than usual. Each kidney has its own collecting system. Often multiple and complex renal arteries are present. Rarely, associated inferior vena cava anomalies or a retrocaval ureter are also present.

Neoplasms do develop in a horseshoe kidney, probably at the same incidence as in normal kidneys although transitional cell carcinomas occur somewhat more often than expected. The diagnosis of a transitional cell carcinoma is generally made at an advanced stage in these patients. Patients with a horseshoe kidney also appear more prone to developing a primary carcinoid tumor.

Urolithiasis developing in patients with a horseshoe kidney is treated with either percutaneous nephrolithotomy or extracorporeal shock-wave lithotripsy (ESWL), similarly to those in patients with normal kidneys.

A bone scan in a patient with a horseshoe kidney can mimic bone metastases.An apparent horseshoe kidney (pseudo–horseshoe kidneys) is seen in patients with spina bifida; gibbus deformity causes lower renal pole medial migration. In these patients, however, the prevalence of true horseshoe kidneys is also somewhat increased.

Ureters

Ureteral Bud Remnants

A ureteral bud remnant associated with renal agenesis or dysplasia is usually identified with a retrograde study, provided reflux exists. In the absence of reflux or with an ectopic insertion,

ADVANCED IMAGING OF THE ABDOMEN

detection is difficult at best. At times an ectopic insertion is suggested indirectly by severely deformed adjacent structures.

Ectopia

A right retrocaval ureter passes posterior to the vena cava and courses inferiorly in a more medial position than usual. Most patients with a retrocaval ureter are asymptomatic, and this condition is discovered incidentally during either urography or CT. A rare anomaly is a left retrocaval ureter associated with a left inferior vena cava.

Ectopic insertion of a single ureter is into any adjacent structure proximal to the external urethral sphincter. It occurs more often in males and occasionally is associated with prostatitis or epididymitis. Some are associated with other anomalies, such as a crossed single ectopic ureterocele. The kidney drained by such an ectopic ureter tends to be dysplastic.

An ectopic duplicated ureter is more common in females and is more often on the left. The ectopic ureter usually drains the upper pole of a duplex kidney and tends to insert into the urethra distal to the urethral sphincter, thus accounting for urinary incontinence. Occasionally found are intraprostatic and vaginal ectopic insertions.

Even if the distal end of an ectopic ureter is not stenotic, a low-inserting ectopic ureter can be obstructed by an adjacent urethral sphincter. Huge ureters terminating in the posterior urethra are occasionally found.

Detecting most ectopic ureters with imaging should be straightforward, yet delayed detection of an infrasphincteric ectopic ureter is not uncommon, especially in girls. Delayed, inappropriate and misleading imaging contribute to this problem. Technically excellent excretory urography is generally diagnostic, although problems occur with a ureter draining a dysplastic kidney. Contrast enhanced CT or 99mTcDMSA scintigraphy aid detection of an ectopic insertion if a dysplastic kidney is present.

With a sufficiently small and poorly functioning renal segment, an occult ectopic ureter and its insertion can be difficult to visualize with imaging. Problems also arise with unusual insertions and with a poorly functioning kidney. Some insertions are only

KIDNEYS AND URETERS

transiently visible on one of several imaging studies. Vaginography defines the insertion of some ectopic vaginal ureters because these ureters tend to reflux. Occasionally MRI is useful, such as with an ectopic ureter from a poorly functioning upper pole duplicated kidney that is not detected with IV pyelography and US.

A triple ureter, with an ectopic opening into the vagina has been described (18).

Ectopic ureteroceles are discussed in Chapter 11.

Nephroptosis

One can be criticized for including nephroptosis, or floating kidney, as an abnormality. Older physicians are undoubtedly familiar with this condition and the resultant attempts to cure it by nephropexy, a procedure that is considerably less common today. Recently, however, a few reports have described laparoscopic nephropexy as therapy for symptomatic nephroptosis, and this procedure is again somewhat in vogue.

Occasionally nephroptosis is a cause of chronic ureteral obstruction and associated symptomatology.

Multicystic Dysplasia

Clinical

Fetal kidney nephrons form from fetal metanephric blastema surrounding the ureteral bud. This fetal blastema normally matures during gestation, but occasionally some persists after birth as nephrogenic rests. These nephrogenic rests are associated with an increased risk of multicystic dysplasia, multilocular cystic nephroma, and Wilms’ tumors. They are more common in several syndromes, such as Beckwith-Wiedemann syndrome (discussed in a later section), hemihypertrophy, and sporadic aniridia; children with these syndromes should be screened for Wilms’ tumors.

Complete multicystic dysplasia (also called multicystic kidney, renal dysplasia, and renal dysgenesis) is found only unilaterally— complete bilateral involvement is incompatible with life. Depending on the extent of involve-

ment, dysplasia is limited to the infundibula, renal pelvis, and proximal ureter, or it involves a kidney to the point that dilated calyces appear as intrarenal cysts. Segmental multicystic dysplasia occurs in a setting of a duplex collecting system. At times a hypoplastic renal artery is identified.

Most often multicystic dysplasia is detected as an abdominal tumor in infancy. Some older patients present with ureteropelvic junction obstruction. The condition is discovered in an occasional adult as an incidental finding.

A variant of multicystic dysplasia includes an associated hydronephrosis, with renal cysts communicating with the renal pelvis. This condition presumably develops from incomplete ureteral obstruction. Often some renal function is evident.

Among term neonates and infants with unilateral multicystic dysplastic kidneys, in those with no vesicoureteral reflux the contralateral kidney was more than 1 standard deviation longer than the mean for age in 54% (19). Contralateral vesicoureteral reflux is detected in 15% to 25% of children with a newly diagnosed multicystic kidney, and these refluxing kidneys are significantly shorter than nonrefluxing ones. Other anomalies include ectopic ureters, ureterocele, bladder diverticula, and urethral duplication. An association exists among renal dysplasia, Gartner’s duct cyst, and ipsilateral müllerian duct obstruction. An investigation for lower urinary tract abnormalities is thus in order in these infants. Extraurinary anomalies include bowel malrotation and congenital cardiomyopathy.

Renal malignancy is rare in multicystic dysplasia, and current opinion is that this condition is not premalignant, although the kidney appears prone to nodular renal blastema and reports suggest that renal cell carcinoma, Wilms’ tumor,and even mesothelioma appear to be more common than by chance alone. Complicating the issue is that occasionally other lesions, including tumors are misdiagnosed as multicystic dysplasia.

The current trend is to follow segmental multicystic kidneys nonoperatively because these cysts tend to involute and the kidney decreases in size. With age, some patients develop extensive calcifications. At times by adulthood only a cystic remnant with a calcified rim is apparent.

Figure 10.1. Right multicystic dysplastic kidney. Noncontrast computed tomography (CT) identifies a small, irregular right kidney (arrow). (Courtesy of Nancy Curry, M.D., Medical University of South Carolina.)

Imaging

A voiding cystourethrogram and renal US appear appropriate in newborns with suspected multicystic kidney disease to confirm the diagnosis and detect any associated anomalies. The grape-like intrarenal cysts do not communicate with the ureter. Little normal renal parenchyma is detected in the involved kidney (Fig. 10.1). A characteristic finding is a club-like deformity of a rudimentary ureter. A small dysplastic kidney is more echogenic than a normal kidney; the US differential diagnosis also includes hydronephrosis due to other etiologies.

Occasionally during renal scintigraphy a multicystic dysplastic kidney accumulates a radiopharmaceutical agent; such uptake correlates with the presence of mature renal cortical tissue in the diseased kidney.

Tc-99m-DMSA renal uptake is decreased in the contralateral kidney in a majority of patients with a multicystic kidney, reflecting tubular and glomerular damage.

Polycystic Disease

Autosomal Dominant

Clinical

Autosomal-dominant polycystic disease, also called adult polycystic kidney disease and Potter type III, is the most common hereditary renal disorder. It is not a simple entity; no family

ADVANCED IMAGING OF THE ABDOMEN

history of renal disease is obtained in about half the patients, and variable penetrance and expressivity are apparent. Many of these patients develop hypertension. Renal failure is a late finding. These patients are also prone to developing aortic and cerebral aneurysms with their related complications.

A rare disorder called oral-facial-digital syndrome type 1 includes polycystic kidney disease as one component. The syndrome is X-linked, with affected males dying before birth. Although renal findings resemble autosomal polycystic disease and a dominant inheritance pattern is evident, histopathology reveals mostly glomerular cysts (20).

There are two subtypes of autosomaldominant polycystic disease. Polycystic kidney disease type 2 (PKD2) has a milder clinical phenotype than PKD1; the median age at death or onset of end-stage renal disease is 53 years for individuals with PKD1 and 69 years for those with PKD2 (21).About half of these patients also develop liver cysts; far fewer develop pancreatic or splenic cysts or even adrenal cysts. A rare patient also has Caroli’s disease, but this association is less common than with autosomalrecessive polycystic disease. Polycystic kidney disease can develop in a horseshoe kidney. Histologically, cystic changes occur both in nephrons and collecting ducts. Eventually multiple cysts of varying size are found in both the renal cortex and the medulla. Subcapsular cysts also develop.

Abdominal pain is a common presentation. Many patients develop hypertension. An occasional cyst rupture into the collecting system leads to hematuria. Enlarged kidneys are palpable in some individuals. Renal stones with their related complications are more prevalent than in the general population. Eventually renal failure ensues. The rare emphysematous pyelonephritis is probably due to superimposed infection rather than polycystic disease. Likewise, renal cell carcinoma developing in a setting of polycystic disease is probably fortuitous.

Typically few cysts are detected early in life, but gradually multiple bilateral cysts enlarge and interstitial fibrosis develops with loss of normal renal tissue. Not uncommonly progressive worsening of renal function manifests in early adulthood. The cysts compress and distort adjacent calyces and eventually little renal