Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

KIDNEYS AND URETERS

and successful endopyelotomy has been performed in patients with ureteropelvic junction obstruction who had CT evidence of crossing vessels.

Children with unilateral ureteropelvic junction obstruction have not only impaired renal function but also delayed body growth. Longstanding dilation often persists after pyeloplasty, but renal function improves, mostly in younger children. Improvement is slow—only in a minority during the first 6 months, but a majority of the patients improve within 2 years. Long-term follow-up of retrograde balloon dilatation of ureteropelvic junction stenosis reveals variable results; some patients had persistent residual stenosis, but in a few the procedure led to complete ureteropelvic junction obstruction requiring surgical correction.

Ureteral Obstruction

Strictures

The etiologies of a benign ureteral stricture are listed in Table 10.8. Ureteral ischemia is a not uncommon cause of ureteral stricture. Most benign strictures involve a short segment, although an occasional one extends for considerable length.

Benign ureteral strictures are dilated using either an antegrade or retrograde approach.

Table 10.8. Etiologies of benign ureteral strictures (from more common to less common)

Ureteral stones

Ischemia

Thromboemboli

Vascular reconstructive surgery

Sequelae to adjacent organ resection

Polyarteritis nodosa

Trauma

Adjacent Crohn’s disease

Infection

Tuberculosis

Bilharziasis

Retroperitoneal fibrosis

Idiopathic

Success depends not only on the underlying etiology and the length of a stricture but also on relative operator expertise. Balloon dilation using a percutaneous approach has become popular, including dilation of restenosis, although overall the long results of percutaneous dilation have been poor and, at times, even multiple dilations do not achieve ureteral patency.

A rare cause of bilateral ureteral obstruction and hydronephrosis is retroperitoneal systemic sclerosis.

Extrinsic

Ureteral metastases have already been discussed. Occasionally inflammation surrounding an abdominal aortic aneurysm entraps one or both ureters and results in obstruction. Similarly, extrinsic ureteral compression and obstruction occurs with abdominal fibromatosis due to several disorders.

Aberrant arteries may compress a ureter, leading to varying degrees of obstruction. As already mentioned, the most common location is at the ureteropelvic junction, but it can occur anywhere along the ureter.

Ureteral obstruction develops secondary to an adjacent neoplasm, perforated appendiceal abscess, and an adjacent hematoma. An ileocecal phlegmon in Crohn’s disease not uncommonly obstructs the right ureter, but rarely the left. Extensive periureteral venous collaterals due to a thrombosed or absent infrarenal vena cava can result in ureteral obstruction. Anecdotal reports describe an infected urachal cyst, uterine prolapse, and even methacrylate migrating into the pelvic cavity after hip arthroplasty, resulting in ureteral obstruction. Castleman’s disease can present as a recurrent tumor, detected by imaging (103).

Placement of metallic stents in ureters obstructed by an extrinsic tumor, such as an extensive gynecologic cancer, is effective in maintaining renal function during chemotherapy or radiation therapy. If necessary, these stents can be augmented by cystoscopically inserted temporary J endostents. Renal RI, obtained from Doppler US data, was found useful in following patients with unilateral extrinsic ureteral obstruction after therapy with double-J ureteral stents (104).

Retrocaval Ureter

A retrocaval ureter represents an anomalous vena caval development with persistence of the ventral vasculature. A retroiliac ureter is rare. The clinical significance of a retrocaval ureter depends on the degree of obstruction. Urography suggests the diagnosis. Contrast-enhanced CT should be diagnostic.

Reanastomosis using a laparoscopic approach is a viable option.

ADVANCED IMAGING OF THE ABDOMEN

Antegrade ureteral intussusception can develop after antegrade instrumentation. Intussusception can be related to an indwelling endoprosthesis.

A cholesteatoma in the midportion of the left ureter led to obstruction and hydronephrosis (105).

Rarely, polyarteritis nodosa is associated with ureteral obstructions, presumably due to a periureteral vessel vasculitis.

Ovarian Vein Syndrome

Ovarian vein syndrome consists of ureter compression between the external iliac artery and a dilated ovarian vein. It is more common on the right. Occasionally a thrombosed right ovarian vein produces a distinct impression on the ureter, although obstruction is rare.

A similar syndrome on the left side is due to ureter compression between a dilated ovarian vein and the psoas muscle.

Endometriosis

Ureteral endometriosis is generally part of diffuse pelvic involvement. Both focal and diffuse disease lead to ureteral obstruction, generally in the distal ureter. It can be silent and progress to loss of renal function. At times both ureters are involved.

A typical appearance is that of a focal or long infiltrating-appearing ureteral stricture, similar to encasement by extrinsic fibrosis or tumor. Retroperitoneal fibrosis has a similar appearance but the two conditions have different clinical presentations.

Hernia

Ureteral herniation is a rare cause of ureteral obstruction. Possible herniation sites include inguinal, femoral, and sciatic hernias. A number of these obstructions are chronic, manifesting by more proximal hydronephrosis.

Any imaging modality visualizing the ureter should detect such a hernia.

Other Obstructions

During Pregnancy

Proximal ureters dilate during pregnancy due to extrinsic compression, although hormonal factors may also play a role. At times dilation is asymmetrical and a superimposed ureteral calculus is suspected. Dilation gradually resolves after delivery.

With suspected obstruction during pregnancy US evaluates hydronephrosis and at times suggests an etiology. Ureteral jets identified at color Doppler US imply lack of obstruction at the ureterovesicular junction. Occasionally late in a pregnancy no ureteral jet is identified in the absence of obstruction; turning the patient to a contralateral decubitus position generally restores this sign.

Spontaneous urinary tract rupture during pregnancy is rare and generally occurs in a setting of a diseased kidney. Rupture is either in the collecting systems or it involves the kidney parenchyma. Most ruptures occur in the second or third trimester.

If relief of an obstruction is believed necessary, a percutaneous nephrostomy rather than surgical decompression should be considered. Many women with symptomatic acute hydronephrosis are managed with ureteral stenting.

Diabetes Insipidus

Nephrogenic diabetes insipidus is a cause of nonobstructive urinary tract dilatation.

Reflux

Clinical

Ureteral polyps are not common. Occasionally such a polyp intussuscepts and results in an acute obstruction.

Vesicoureteral reflux is abnormal. A number of investigators ascribe most reflux to an abnormal ureterovesical junction, although morphologi-

KIDNEYS AND URETERS

cally this junction is indistinguishable from those without reflux. Nerve density and distribution are similar. Especially in male infants, vesicoureteral reflux tends to be associated with abnormal voiding urodynamics, such as a small voided volume, inadequate voiding dynamics, markedly dyssynergy, and an obstructive pattern. In males an association exists between reflux and a simultaneous increase in detrusor pressure. A familial tendency to vesicoureteral reflux exists.

In children, vesicoureteral reflux is associated with urinary tract infection and impaired growth, although whether reflux per se or the often commonly associated renal tubular acidosis is responsible for growth failure is not clear. Although both reflux frequency and grade tend to decrease with age, reflux remains a common cause of end-stage renal failure requiring renal transplantation. Some of these children also develop hypertension or urinary infections and thus may also require bilateral nephrectomies of the native kidneys.

Reflux tends to be intermittent, and the time to resolution varies widely.

A ureter inserting into a diverticulum is at an increased risk of vesicoureteral reflux. Patients with renal dysplasia are also at an increased risk for reflux, although whether dysplasia is secondary to reflux is not clear.

Screening

In view of renal parenchymal damage associated with vesicoureteral reflux, should US screening for reflux be instituted early in life? Several studies have found that even using a meticulous technique, prenatal and neonatal US achieves a low sensitivity for detecting vesicoureteral reflux. In children, often major renal damage has already occurred when an acute urinary tract infection or a megaureter becomes evident. One proposal is to perform cystography in those at risk for reflux, such as newborns with pyelocaliceal dilation and in siblings of those treated for vesicoureteral reflux, yet such an approach also misses some infants with reflux.

Detection

A search for reflux appears appropriate in infants with a urinary tract infection. Although

often called screening, in a pure sense this is no longer screening but an attempt to detect reflux in a symptomatic patient. Young boys probably should also be studied if they develop an infection because of the relatively high prevalence of posterior urethral valves. Screening of girls with a first infection is somewhat controversial, but a general trend toward earlier study is evolving. In women a search for reflux is appropriate for recurrent infection.

The two primary examinations used to detect vesicoureteral reflux are fluoroscopic voiding cystourethrography and radionuclide cystography, with some centers also having introduced contrast enhanced voiding urosonography. Each study has its proponents. Scintigraphy has a lower radiation dose but also provides fewer anatomic details. One approach is to use voiding cystourethrography in boys, who are more prone to have other underlying anatomic abnormalities such as urethral valves, and radionuclide cystography is girls, in whom anatomic abnormalities are uncommon. Because of its low radiation dose, radionuclide cystography is useful as a follow-up examination, but the lack of anatomic detail limits it as a first study.

A number of vesicoureteral reflux grading systems have been proposed. One classification, based on the results of voiding cystourethrography, is outlined in Table 10.9. Reflux can be missed with incomplete bladder distention. At times reflux is observed only after several episodes of bladder filling to capacity and having the patient void. If reflux is detected during a voiding cystourethrogram, the study should be modified so the degree of calyceal and ureteral dilation can be evaluated and any

Table 10.9. Classification of vesicoureteral reflux

Grade Description

IReflux into ureter

VSevere ureter, renal pelvis and caliceal

dilation, papillary outline not preserved

underlying obstruction detected. Often a duplicated collecting system can be suspected from this examination.

What is the role of US in detecting reflux? Intermittent renal collecting system dilation and contraction during real-time renal US is an indirect sign of vesicoureteral reflux (106). Likewise, renal pelvis dilation during bladder contractions points to reflux, yet a large minority of children with negative renal US have reflux at voiding cystourethrography.

In spite of some rather optimistic claims, it is still difficult to gauge the accuracy of intravesical contrast enhanced voiding US in detecting and grading vesicoureteral reflux in children. Although presence of microbubbles in the ureters or renal pelves does signify reflux, in one study 38% of such reflux was graded either lower or higher than with voiding cystourethrography (107). Additional work is necessary to place contrast enhanced voiding US in a proper clinical perspective.

An occasional author suggests air as a US bladder contrast agent; although uncommon, intravasation of an instilled contrast agent does occur, especially with the distention pressures generally employed to detect reflux, and air thus cannot be recommended.

Mild renal pelvic dilation detected at renal US is not a reliable sign of vesicoureteral reflux. An increase in renal pelvic size on postvoid US also does not signify vesicoureteral reflux.

Upper urinary tract wall thickening in children occurs secondarily to urinary tract infection, chronic obstruction, and chronic vesicoureteral reflux. Thus voiding cystourethrography appears reasonable to evaluate for reflux in these children if US detects upper urinary tract wall thickening.

Radionuclide cystography is performed with Tc-99m–sulfur colloid (it is not absorbed from the bladder). Reflux is detected as any activity superior to the bladder. A reflux grading system of mild, moderate and severe is generally employed. This radionuclide test provides grossly similar information to a voiding cystourethrogram regarding reflux, although continuous monitoring during scintigraphy may better detect intermittent reflux. Its disadvantage is a poorer spatial resolution. At times vesicoureteral reflux is not evident during an initial fluoroscopic or scintigraphic study, but reflux

ADVANCED IMAGING OF THE ABDOMEN

becomes apparent during a second cycle of bladder filling. When to perform such a second cycle is not clear; a positive study is more common in patients strongly suspected of having reflux.

The SPECT Tc-99m-DMSA is a noninvasive test useful in assessing functional kidney impairment in children due to pyelonephritis or in those with vesicoureteral reflux.

Therapy

Due to increased preand postnatal US screening, more infants with vesicoureteral reflux are being detected. Reflux is being managed both medically and surgically. Endoscopic therapy consists of injecting polytetrafluoroethylene (Teflon) or polydimethylsiloxane (Macroplastique). Resection after the failure of endoscopic therapy reveals that both substances induce an inflammatory giant cell macrophage reaction, new vessel formation, and fibrosis (108).

Among 302 children under 11 years of age with urinary tract infection and grade III or IV vesicoureteral reflux enrolled in the European branch of the International Reflux Study in Children group and randomly assigned to surgical or medical therapy, follow-up for 10 years with serial IV urography revealed no significant difference in mean renal growth between the two groups (109). Debate continues, however, about the relative merits of conservative versus surgical therapy.

Conventional Surgery

The classic surgical therapy for vesicoureteral reflux is ureteral reimplantation. Infants with reflux typically undergo surgical correction at about the age of 6 months, although in a setting of hydronephrosis or renal damage correction as early as 1 month of age is performed. In some surgical centers a very high success rate in correcting reflux is achieved.

A minority of patients with unilateral vesicoureteral reflux treated by reimplantation develop postoperative contralateral vesicoureteral reflux; most such reflux resolved spontaneously. Although it is tempting to attribute any new contralateral reflux to the surgical procedure, one should keep in mind that new reflux develops in some children.