Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

KIDNEYS AND URETERS

Table 10.2. Conditions associated with a small, smooth kidney(s)

Congenital causes Dysplasia Hypoplasia

Prune belly syndrome (Eagle-Barrett syndrome) Laurence-Moon-Biedl syndrome (?)

Sequelae of multicystic dysplasia Alport’s syndrome

Infection/inflammation

Chronic pyelonephritis

Chronic glomerulonephritis

Post-infection or obstruction

Radiation (late)

Papillary or cortical necrosis

Medullary cystic disease

Amyloidosis (late stage)

Lead poisoning

Hemophilia

Chronic vesicoureteral reflux

Diabetes (late stage)

Collagen vascular disorders

Vascular causes

Associated with hypertension

Renal artery stenosis

Nephroptosis

Dissection

Ischemia

Arterial

Emboli

Thrombosis

Chronic renal vein thrombosis

Hypotension

Nephrosclerosis

Infection/Inflammation

Acute Pyelonephritis

There is no uniform agreement on how to describe an acute bacterial infection of the kidney. Such terms as acute pyelonephritis, acute bacterial nephritis, lobar nephritis, and lobar nephronia abound in the literature and are often applied haphazardly. In addition, some patients have a pyelitis only, with relative parenchymal sparing. The Society of Uroradiology has rec-

ommended that the term acute pyelonephritis be adopted for this condition and this term is used here. Two complementary but distinct definitions are available for acute pyelonephritis: a clinical one and a pathologic finding.

Most acute pyelonephritis is secondary to an ascending infection. The less common hematogenous source of acute pyelonephritis often results in a different appearance from that seen in a setting of reflux; the inflammation initially is not lobar in distribution; rather, multiple foci of involvement predominate in the renal periphery. Eventually, however, most inflammations become confluent, and their appearance is similar regardless of origin.

Pathologically, a focal involved segment is sharply demarcated from adjacent normal parenchyma. In some,tissue necrosis progresses to frank abscess and, with a severe-enough inflammation, the extrarenal space becomes involved.

Table 10.3. Enlarged kidney(s)

Congenital causes Dysplasia (Poly)cystic disease Hemihypertrophy

Trauma

Urinary obstruction

Infection/inflammation

Acute pyelonephritis

Acute glomerulonephritis

Acute interstitial nephritis

Abscess

Chronic infection

Diffuse infiltration

Neoplasia

Amyloidosis (initially)

Sarcoidosis

Lymphangiectasia

Acute tubular necrosis

Diabetes (initially)

Nephrotic syndrome

Vascular causes

Acute renal vein occlusion

Hemolytic-uremic syndrome

Sickle cell disease

Transplanted kidney

Most treated acute pyelonephritis leads to no sequelae. A minority of patients develop cortical scarring, which is detectable with scintigraphy or urography. Renal insufficiency is rare; perhaps more common is renal hypertension.

Children

Clinical

Especially in children, it is reasonable to ascribe acute pyelonephritis to vesicoureteral reflux and retrograde introduction of bacteria. The problem is that in children with acute pyelonephritis only about half are shown to have reflux, while reflux is even less common in adults. Both vesicoureteral reflux frequency and grade progressively decrease with age.

End-stage renal disease secondary to postinfectious nephropathy probably occurs only after pyelonephritis, rather than after such urinary tract infections as cystitis. There is thus a need for a reliable test to detect pyelonephritis, especially in young children where a clinical diagnosis is difficult.

Because congenital urinary tract anomalies are not uncommon in children with a urinary tract infection, the underlying anatomy needs to be studied. Especially in young children, vesicoureteral reflux is common in association with acute pyelonephritis, and thus voiding cystourethrography (or in some centers radionuclide cystography) appears reasonable. After the age of 9 to 10 years or so, reflux in association with acute pyelonephritis is sufficiently uncommon that a need for voiding cystourethrography is individualized.

Imaging

Neither an IV urogram nor US is sufficiently sensitive to serve as a screening test for pyelonephritis in children. Most urograms during an acute attack are normal. In some children considerable time elapses before sufficient renal damage develops to be visible with urography. In a minority, the involved kidney is enlarged and a delay and decreased contrast concentration are evident. The affected kidney returns to normal size in a week or so after the start of treatment. Urography is normally performed primarily to detect other associated conditions, such as obstruction. Computed

ADVANCED IMAGING OF THE ABDOMEN

tomography is also normal in a minority of these children.

Ultrasonography reveals a hyperechoic renal sinus in some children with in acute pyelonephritis (27), yet both US and power Doppler US findings range from normal to focal or multifocal defects, and normal US does not exclude parenchymal damage; even Doppler US underestimates the extent of involvement.

In children, Tc-99m-DMSA scintigraphy is more sensitive in detecting pyelonephritis than is US or IV urography and appears to be even more sensitive than CT and is preferred in detecting and following renal damage due to acute urinary tract infection. While planar DMSA scintigraphy does detect renal cortical defects, DMSA-SPECT detects more and is preferred. Tc-99m-DMSA scintigraphy reveals parenchymal findings suggestive of pyelonephritis in a majority. In some, DMSA scintigraphy simply shows swollen kidneys, with swelling obscuring the focal defects. Nevertheless, although a high sensitivity is achieved in detecting pyelonephritic cortical damage, scintigraphy cannot exclude the diagnosis.

Tc-99m-MAG3 and Tc-99m-glucoheptonate are also used to evaluate pyelonephritis in children. Using a mild (grade 1), moderate (grade 2), and marked (grade 3) scale to grade the severity of decreased radionuclide localization, MAG3 studies were able to detect 79% of grade 2 and 96% of grade 3 cortical lesions but only 59% of grade 1 lesions compared with Tc-99m- glucoheptonate scans (28).

In a study of children with fever-producing urinary tract infection, gadolinium-enhanced MRI detected more pyelonephritic foci than Tc- 99m-DMSA cortical scintigraphy (29). The role of MR in this setting is still evolving.

Adults

Clinical

An unusual organisms, Mycoplasma hominis, is occasionally implicated in acute pyelonephritis. It should be considered if the patient does not improve with conventional antibiotic therapy and no organism is identified; special urine cultures are necessary to grow this organism.

Renal vein thrombosis, a common complication of acute pyelonephritis in the preantibiotic era, currently is rarely encountered.

KIDNEYS AND URETERS

One approach is to treat patients with suspected uncomplicated acute pyelonephritis medically for 72 hours, and limit imaging only to those who do not respond or respond poorly. Diabetics and other high-risk patients should undergo imaging early in the course of infection.

Imaging

Imaging has a limited role during most episodes of clinical acute pyelonephritis in adults; in many patients no imaging abnormality is found, yet imaging is often obtained—not to detect infection, but rather in a search for underlying congenital abnormalities that predispose to infection and detect any obstruction, stasis, calculi, or focus of infection.

Intravenous urography appears indicated in diabetics, patients not responding to antibiotics within 72 hours, and in a setting of an unusual organism. Suspected progression to an abscess or other complication warrants CT. A percutaneous nephrostomy is indicated with the presence of an obstructed collecting system.

The kidneys increase in size during acute pyelonephritis. In some adults focal swelling and decreased attenuation persists for months. If pyelonephritis is severe enough, both in children and adults the final end point is focal scarring of the affected lobe.

Mild acute pyelonephritis does not result in any specific CT findings. Either focal swelling or enlargement of the entire kidney develops with more severe involvement. Precontrast images occasionally show hypodense or hyperdense regions compared to normal parenchyma, with increased attenuation probably representing focal hemorrhage. Postcontrast CT in some patients reveals perfusion defects extending from the papilla to the periphery. At times the involved parenchyma exhibits near-water density, or delayed postcontrast scans show enhancement of involved segments.

Computed tomography detects postcontrast renal perfusion defects in up to two thirds of patients with acute pyelonephritis, depending on the severity of infection. These defects range from unilateral to multifocal and bilateral. The temporal findings are not constant and can change rapidly. Thus a hypodense heterogeneous region can evolve into a streaky,

hyperdense appearance. In general, the CT findings do not change clinical management. Perinephric inflammation is evident with some infections, and CT reveals thickening of Gerota’s fascia and stranding in the perinephric fat.

With a hematogenous origin, instead of wedge-shaped defects, more numerous rounded low-attenuation parenchymal defects predominate. An occasional patient develops a focal intraparenchymal homogeneous tumor mimicking a neoplasm without other parenchymal or perirenal abnormalities.

Ultrasonography identifies hydronephrosis, but is relatively insensitive in detecting renal parenchymal inflammation. A majority of pyelonephritic kidneys appear normal. With a severe inflammation, US identifies hypoto isoechoic and occasionally even hyperechoic regions. The latter presumably represent hemorrhage. Compared to transabdominal US, Doppler US appears to improve the detection of acute pyelonephritis, although it probably is not as sensitive as CT.

Because of cost and restricted availability, MRI currently plays a limited role in most patients with suspected acute pyelonephritis. During an acute attack the kidney is hypointense on T1and hyperintense on T2weighted images.

Tc-99m-DMSA renal scintigraphy either reveals a normal scan or identifies cortical defects; in a setting of acute pyelonephritis, scintigraphy appears to be more sensitive than IV urography or US.

Emphysematous Pyelonephritis

Clinical

Emphysematous pyelonephritis is not a separate entity, but a severe type of necrotizing pyelonephritis. The criteria for defining this condition are not settled; some authors define emphysematous pyelonephritis only if gas is found within renal parenchyma, while others expand the definition and include gas not only within the parenchyma but also in the excretory system and perirenal spaces. Emphysematous pyelonephritis should be differentiated from emphysematous pyelitis; the latter is a generally benign condition consisting of gas in the urinary collecting system.

Emphysematous pyelonephritis develops mostly in diabetic patients, both insulin and non–insulin dependent. Concomitant urinary obstruction is more common in nondiabetics. It is a rapidly progressive, life-threatening infection associated with gas-forming coliform bacteria such as Escherichia coli and others. It is almost always unilateral. The simultaneous occurrence of emphysematous pyelonephritis and emphysematous cholecystitis is rare.

Gas is located in the renal parenchyma, perirenal tissues, and at times the collecting system.

Imaging

In spite of some authors’ opinion that a diagnosis of emphysematous pyelonephritis should be made by CT, quite often conventional radiography is sufficient to detect this condition. Thus in patients with emphysematous pyelonephritis diagnosed by CT, abnormal gas was identified with conventional radiography in 66% and US in 88% (30). When localized, emphysematous pyelonephritis can mimic an abscess (Fig. 10.6).

Emphysematous pyelonephritis generally is not associated with ureteral obstruction.

The imaging appearance of emphysematous pyelonephritis consists of either parenchymal destruction with absence of fluid or the pres-

ADVANCED IMAGING OF THE ABDOMEN

ence of streaky or mottled gas, or it contains either renal or perirenal fluid and bubbly or loculated gas or collecting system gas. Such differentiation appears to be of prognostic importance because the former patients have a more fulminant course.

Therapy

Although some of these patients respond to medical management,many undergo a nephrectomy. Some have been successfully treated with imaging-guided percutaneous drainage, but such drainage is not feasible with diffuse involvement. Some drained patients subsequently require elective nephrectomy for adequate therapy.

The serum creatinine level is the most reliable predictor of outcome. Affected kidneys tend to recover function after medical therapy,although many of these patients with underlying diabetes already suffer from chronic renal failure.

Abscess

The term renal carbuncle is used by some radiologists; it has no specific imaging definition and some use it as a synonym for renal abscess. The term is probably best avoided in describing imaging findings.

KIDNEYS AND URETERS

At times acute pyelonephritis progresses to scattered small abscesses, which eventually coalesce into a gross abscess. The involved kidney is often enlarged. An abscess tends to be sharply marginated and has a rim of contrast enhancement. A subcapsular abscess displaces and compresses the adjacent renal parenchyma. Gas within an abscess is not common. Some renal abscesses are associated with extrarenal abscesses in adjacent structures.

In general, CT detects renal and extrarenal abscesses better than US (Fig. 10.7). Still, US detects the larger abscesses and often is more readily available in monitoring resolution. Ultrasonography of an abscess identifies a complex, thick-walled, cystic, fluid-containing mass. Abscesses tend to be hypoechoic or anechoic. With maturity the abscess outline becomes better defined.

Gadolinium-enhanced MRI reveals renal abscesses as heterogeneous hypointense tumors; perinephric inflammatory stranding is common. The MRI findings are similar to those seen with contrast-enhanced CT, and in patients who should not receive iodinated contrast, MRI is a viable alternative.

An abnormal scintigram showing photopenic regions in the kidney is nonspecific and does not differentiate between inflammation and abscess.

The imaging differential for most intrarenal abscesses includes a hemorrhagic or infected cyst and a cystic neoplasm.

Pyonephrosis

Pyonephrosis defines an obstructed and infected renal collecting system. Invariably renal parenchymal damage due to acute pyelonephritis is also present. Clinically these patients range from being asymptomatic to being in septic shock. Stones account for most distal obstructions. Pyonephrosis is not clinically evident in some and only an obstructed urinary tract is suspected until aspiration provides the diagnosis.

In a rare patient infection leads to gas within the excretory system, called emphysematous pyelitis. This condition must be differentiated from bladder reflux, recent instrumentation, or a bowel fistula, conditions that can also introduce gas into the renal excretory system.

The affected kidney usually shows little or no function. Urine in pyonephrosis tends to have increased CT attenuation. At times gray-scale US detects hyperechoic debris or gas, although in many patients only hydronephrosis is found.

An occasional pyonephrosis ruptures and results in a perinephric abscess. Even rarer is for such rupture to result in generalized peritonitis.

Figure 10.7. Right renal abscess. Contrast-enhanced CT reveals a huge fluid-filled structure at the superior pole of the right kidney (arrows). It was drained percutaneously. (Courtesy of Patrick Fultz, M.D., University of Rochester.)

Fungal Infection

Urinary fungal infections are encountered with increasing frequency both because of the wider use of broad-spectrum antibiotics and an increasing number of immunocompromised patients. The most common organism is Candida albicans, followed by Torulopsis glabrata. Some fungal infections develop bilaterally.

Most renal candidiasis occurs in a setting of systemic involvement. Typically numerous abscesses are scattered throughout the kidneys, similar to bacterial infections. Occasionally collecting system involvement results in a fungus ball, an appearance similar to an intraluminal tumor, but a fungus ball moves with a change in patient position and can even obstruct outflow. Fungus balls tend to be heterogeneous and hypodense, with no contrast enhancement, while US reveals a hyperechoic tumor with no posterior acoustic shadowing (31).

Tuberculosis

Most urinary tuberculosis is due to hematogenous spread. At times the primary site is not identified, although active or inactive lung disease is found in a majority. Although initially both kidneys are infected, disease often progresses unilaterally.

Among 57 patients with urogenital tuberculosis, fever, anorexia, and weight loss were evident only in 11%, 16% had an isolated genital lesion, and, surprisingly, 14% presented with renal failure (32); bacilluria was identified only in 5%. These patients have a sterile pyuria, microscopic hematuria, and an acidic urine. The diagnosis is confirmed by urine culture, a procedure taking up to 4 weeks.At times US-guided aspiration is helpful in establishing the diagnosis. A polymerase chain reaction to detect mycobacterial DNA has a high sensitivity and establishes the diagnosis.

Urography was abnormal in 80% of affected individuals, with the most frequent abnormality being a nonfunctioning silent kidney found in 40% (32).

Focal infections, or tuberculomas, develop initially. Tuberculomas tend to be hypointense on both T1and T2-weighted MRI, provided they contain no calcifications, hemorrhage, or fibrosis. These either resolve spontaneously or enlarge, become necrotic, and communicate with the collecting system, spreading infection distally. Imaging at this stage reveals irregular, necrotic, and moth-eaten papilla, at times mimicking papillary necrosis due to other causes. Some calyces become obstructed due to infundibular stenosis. Strictures range in location from calyces to ureters, leading to obstruction and further damage. Cortical scarring, calyceal distortion, and strictures with caliectasis develop. In some, CT simply identifies small hypodense tumors along the renal cortex (33). Ulcerations, abscesses, and fibrosis dominate the late stages of infection. Extension of infection outside the kidney leads to perinephric abscesses. Calcifications vary from curvilinear to diffuse, from homogeneous to granular. If neglected, eventually the small, calcified kidney loses all function.

Ureteral involvement results in a shaggy, irregular lumen. A dilated ureter ensues with ureterovesical junction obstruction. In time, the ureter wall thickens, ulcers develop, and exten-

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sive fibrosis leads to foreshortening. Ureteral calcifications are uncommon, thus aiding in distinguishing tuberculosis from the more common ureteral calcifications found in schistosomiasis.

Some patients with bladder trigone involvement develop upper tract obstruction and eventually renal function is lost even if the obstruction is relieved by stenting or a percutaneous nephrostomy; a kidney can deteriorate from an acceptable pretherapy glomerular filtration rate to becoming nonfunctional after relief of an obstruction. Predictors of renal recovery after diversion are an adequate residual renal cortex and a glomerular filtration rate of >15mL/min.

At the more advanced stages CT and urography should suggest the diagnosis. In some patients xanthogranulomatous pyelonephritis is in the differential. At times a tuberculous kidney develops a fistula to an adjacent organ.

Schistosomiasis

Schistosomiasis generally affects the lower portion of both ureters. Involvement is usually bilateral and results in stenosis and a thickened ureteral wall. Often multiple strictures are evident and proximal ureteral dilation develops. Early in the course a contrast study often reveals soft tissue polyps. Calcifications develop with chronic infection and have a characteristic linear appearance; a diagnosis of schistosomiasis can be strongly suggested if such calcifications are seen in a young adult from endemic regions.

Curiously, in distinction to bladder involvement, ureteral schistosomiasis is not associated with a high risk of ureter cancer.

Echinococcal Cyst

Although uncommon, hydatid cysts do occur in the kidneys, with Echinococcus granulosus being more common than Echinococcus multilocularis. In rare instances renal cysts develop bilaterally. Ureteral cysts are rare. No age group is immune, including children.

Patients are often asymptomatic until the pressure on an adjacent structure or cyst rupture develops. Rupture into a collecting system leads to hydatiduria and possible renal colic due to cyst debris.