Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

KIDNEYS AND URETERS

transplantation. Focal segmental glomerulosclerosis patients who receive a cadaveric transplant have a significantly higher graft loss rate than those with a human leukocyte antigen (HLA)-identical living-related transplant.

Hemolytic-Uremic Syndrome

A number of bacterial and viral infections are associated with hemolytic-uremic syndrome. Thus enterohemorrhagic E. coli infection causes both hemorrhagic colitis and hemolytic-uremic syndrome. Some patients recover with medical therapy, including hemodialysis. In children, E. coli–induced hemolytic-uremic syndrome and resultant renal failure are common factors leading to kidney transplantation.

Scleroderma renal crisis is a rare cause of acute renal failure due to microvascular disease and other factors.

Doppler US reveals an increase both in RI and PI. Renal artery flow is decreased. Renal volume is increased.

Cirrhosis/Hepatorenal Syndrome

Increased renal vascular resistance (RI) develops in patients with progressive cirrhosis. Even in patients with Child’s class A cirrhosis without ascites, renal RI is increased (>0.7) in about a third of patients (129). A greater increase in both RI and PI occurs in those with Child’s B and C cirrhosis. On the other hand, 99mTcDTPA renography identifies a reduced glomerular filtration rate only in more advanced cirrhosis and Doppler US detects earlier than renography those cirrhotic patients at higher risk of developing renal failure.

Hepatorenal syndrome consists of renal failure in association with severe liver disease. The mortality rate is high once this syndrome is established.

Post–Bone Marrow Transplantation

Acute renal failure is a recognized complication after bone marrow transplantation. On an acute basis tumor lysis syndrome and marrow- infusion–associated toxicities occur. Venoocclusive disease leading to hepatorenal-like syndrome occurs within about 1 month and is a common cause of acute renal failure. Late failure includes bone marrow transplanta-

tion–associated nephropathy and cyclosporine nephrotoxicity. These patients also develop sepsis, hypotension, and other related complications, and often renal failure is multifactorial.

Nephrotic Syndrome

Nephrotic syndrome is an end-stage condition induced by a number of entities. Clinically it manifests by hypoproteinemia, proteinuria, and hypercholesterolemia.

Nephrotic syndrome is an occasional manifestation of neoplastic disease. Patients with thymoma are prone to developing a nephrotic syndrome, ranging from minimal histologic change in some to focal segmental glomerulonephritis, proliferative glomerulonephritis, and membranous glomerulopathy. Such diverse conditions as systemic amyloidosis, lupus erythematosus, and renal vein thrombosis can manifest as nephrotic syndrome. Some patients with renal vein thrombosis are asymptomatic, with imaging offering a clue to the underlying condition; the involved renal vein is widened by a thrombus and the affected kidney is enlarged.

Patients with nephrotic syndrome undergoing bicarbonate hemodialysis have developed extensive metastatic calcifications throughout the body. These calcifications are detected by scintigraphy.

Miscellaneous Conditions

Medullary Sponge Kidney

Medullary sponge kidney, or tubular ectasia, refers to cystic collecting tubule dilation (ducts of Bellini) and is believed to be a congenital disorder, although usually manifesting in adulthood. It ranges from focal to generalized. In a minority of patients it is associated with calcifications within collecting tubules.

Tubular ectasia is the early hallmark of this condition, identified by IV urography as con- trast-filled streaks opacifying renal pyramids before or simultaneously as adjacent calyces (Fig. 10.32).A clear border tends to be preserved between the abnormally opacified papillae and calyces. At times faint calcifications develop. This condition should be differentiated from a normal postcontrast papillary blush, seen as a

Figure 10.32. Renal tubular ectasia is identified as barely perceptible streaks of contrast (arrows) in the papillary regions during an IV urogram.

homogeneous papillary enhancement. Some authors use the term benign tubular ectasia to designate early findings and to differentiate them from more advanced medullary sponge kidney, but whether such early findings indeed are part of a spectrum of medullary sponge kidney is not clear.

Occasionally similar findings are detected with CT and MR. Ultrasonography shows increased echogenicity if renal pyramid calcifications have developed.

Papillary Necrosis

The term papillary necrosis describes specific damage to the papilla detected by imaging. Common conditions associated with papillary necrosis include analgesic and NSAID abuse, diabetes mellitus, some infections, and sickle cell anemia. The final pathway probably is ischemia. Findings develop either acutely or on a chronic, indolent basis.

Imaging findings tend to be rather specific and are best seen with IV pyelography: papillary erosions, necrosis, sloughing, and eventual passage of necrotic debris distally, with an

eventual residual blunted calyx (Fig. 10.33). Calcifications develop in the papillary region. Either a portion or the entire papilla is involved; necrosis involves either one or several papilla; it is either unilateral or bilateral.

Papillary necrosis should be differentiated from medullary sponge kidney.

Radiation Nephritis

Damage occurs directly to the renal tubules and glomeruli and is due to radiation vasculitis. Some patients develop renal failure or hypertension.

A sufficient radiation dose results in a shrunken kidney. The renal outline becomes irregular. Damage is limited to the radiation port.

Amyloidosis

Both primary and secondary amyloidosis affect the kidneys. Affected patients are prone to developing renal vein thrombosis, and the sudden onset of nephrotic syndrome should suggest this diagnosis. Renal failure is common in patients with extensive renal amyloidosis.

Benign lymph node hyperplasia (Castleman’s disease) is associated with renal amyloidosis. Occasionally a patient with Crohn’s disease and renal amyloidosis develops nephrotic syndrome.

Initially the kidneys enlarge, but in time cortical atrophy ensues and the kidneys shrink. Focal amyloid infiltration mimics a neoplasm. Occasionally scattered, amorphous calcifications develop. At times US reveals a prominent medulla and a hyperechoic renal parenchyma.

Lupus Nephritis

Lupus nephritis is a manifestation of systemic lupus erythematosus. These patients develop a nonspecific sclerotic change and immunoglobulin deposition within glomerular small arterioles and capillaries. A patient with lupus erythematosus who develops a nephrotic syndrome should be investigated for renal vein thrombosis.

Renal Doppler US is useful in these patients. A normal RI predicts a better renal outcome regardless of creatinine level.

Pseudoxanthoma Elasticum

Ultrasonography in pseudoxanthoma elasticum reveals highly reflective foci in the renal parenchyma. While this finding is suggestive of pseudoxanthoma elasticum, it is not pathognomonic and is also found in other conditions.

in the renal cortex. Thus conditions associated with intravascular hemolysis and the resultant renal cortical iron storage produce a hypointense cortical signal on T2-weighted MRI.

Intravascular Hemolysis

Hemoglobin from intravascular hemolysis is bound to plasma proteins and metabolized by the liver. Excess unbound hemoglobin is filtered by the kidney, some is excreted, and some is reabsorbed by tubules and stored

Dialysis-Associated

Findings

Using typical doses of gadolinium contrastagents in dialysis patients, 79% of the contrast agent was dialyzed after the first dialysis and

99.6% after the fourth (130); the authors conclude that being on hemodialysis is not a contraindication for the use of these contrast agents.

Dialysis-Acquired Cysts

A better name for this condition is acquired cystic disease of the kidney because the onset of cysts begins in patients with chronic renal failure even before dialysis is started.A common condition in patients with end-stage renal disease undergoing long-term hemoor peritoneal dialysis, cyst prevalence increases with duration of dialysis. Why these cysts form is not known, but they tend to involute after renal transplantation. Some of these cysts from dialysis patients are lined with cuboidal epithelium.

Most patients with these cysts are asymptomatic. Pain and hematuria are mostly due to cyst rupture and bleeding.

Imaging of these often small, distorted native kidneys is difficult, especially with US. Computed tomography using IV contrast is feasible if the patient is on dialysis. These patients are at increased risk of developing renal cell carcinoma, and any contrast enhancement suggests a malignancy. Imaging detects numerous cortical cysts in a small kidney. Intracystic hemorrhage and its sequelae tend to mimic a neoplasm. Cyst wall calcifications develop, presumably from prior hemorrhage, but limited urinary output makes kidney stones uncommon.

Magnetic resonance imaging is an alternative imaging modality. The content of some cysts is hyperintense on T1-weighted MRI due to past hemorrhage. No enhancement is evident post–MR contrast, thus differentiating these cysts from neoplasms.

Eventually the imaging appearance approaches that seen with autosomal-dominant polycystic disease, but unlike polycystic disease these patients do not develop cysts in other organs.

Malignancies

Long-term dialysis is associated with several benign and malignant renal neoplasms,

ADVANCED IMAGING OF THE ABDOMEN

occasionally developing even in children. Some malignant-appearing tumors histologically exhibit growth patterns suggesting a benign tumor, including renal cell carcinomas and urothelial carcinomas. Some surveys suggest that especially young patients undergoing prolonged dialysis who develop renal cysts are prone to renal cell carcinomas and they should undergo regular screening by imaging.

Early postcontrast helical CT achieved a 96% sensitivity and 95% specificity in detecting a carcinoma in chronic hemodialysis patients, while delayed images had an 83% sensitivity and 94% specificity (131); early postcontrast images revealed significant enhancement differences between carcinoma and adjacent renal parenchyma, a finding not seen on delayed images.

Other Findings

Urinary stones are uncommon in native kidneys in these patients, presumably due to their decreased urine production, but renal calcifications are common.

On rare occasions emphysematous pyelonephritis develops in a nonfunctioning renal allograft of a diabetic patient.

Mesenteric ischemia appears to be more common than expected in patients with endstage renal disease.

Pyeloureteritis Cystica

Of unknown etiology but probably due to degeneration, pyeloureteritis cystica consists of numerous fluid-filled cysts in the collecting system or ureter wall. Associated chronic infection is common. It is not premalignant. The condition is generally detected incidentally.

Imaging shows multiple small, uniform size, and round soft-tissue tumors in a ureter or renal pelvis (Fig. 10.34). Occasionally malacoplakia is in the differential diagnosis, but tumors in the latter condition tend to be more irregular in outline and the involved ureter somewhat dilated—findings not seen in pyeloureteritis cystica. Ureteral varices tend to be more serpiginous in appearance.

KIDNEYS AND URETERS

Figure 10.34. Ureteritis cystica.

Perinephric Conditions

Thickening

The right anterior extrarenal space extends from the perirenal fascia to the liver capsule. Conditions that widen this space and are hyperechoic include acute inflammatory conditions such as pancreatitis, appendicitis, acute cholangitis, adjacent abscess, ischemic bowel disease, and even a penetrating ulcer. An increased width with normal echogenicity is seen with chronic inflammation, peritoneal metastases, previous surgery, and steroid therapy. Left perirenal fat necrosis develops with severe pancreatitis; CT and MR reveal a fat-containing infiltrate mimicking a retroperitoneal tumor, but associated pancreatic findings should suggest the correct diagnosis.

Urinomas and lymphoceles are discussed in Chapter 14.

Hematoma

Spontaneous nontraumatic subcapsular or perirenal hemorrhage, also called Wunderlich syndrome, is not common. Clinically, an underlying neoplasm or bleeding diathesis needs to be excluded, although occasionally polycystic kidneys, a vasculitis, glomerulonephritis, or pyelonephritis is responsible.

Computed tomography or angiography should suggest an etiology for most of these hematomas. Acute hemorrhage may hide an underlying neoplasm, and some of these patients require exploration or follow-up study after the hematoma resolves.

Abscess

Most perinephric abscesses are associated with renal or adjacent organ infection. Some abscesses are confined by Gerota’s fascia, while others extend into the adjacent retroperitoneal structures.

Most of these abscesses are readily detected with imaging. An occasional perinephric abscess mimics a neoplasm.

Ultrasonography of a perinephric abscess often reveals a complex fluid collection. These abscesses can be successfully drained during laparoscopy, although many are drained percutaneously.

Tumors

Similarly to infection, tumors spread readily in the perirenal space. Most primary tumors are of mesenchymal origin. Lymphoma either involves this space diffusely, with little displacement of adjacent structures, or develops in a nodular pattern. A similar diffuse involvement is also occasionally found with metastatic melanoma. Differentiation from an adrenal tumor can be difficult.

A diagnosis of perirenal lymphoma is not always straightforward; at times even a needle biopsy is not diagnostic.

Vascular Disorders

Arteriovenous malformations and hemangiomas were discussed earlier (see Vascular Tumors).

Ischemia

One result of renal ischemia is renovascular hypertension (discussed in Chapter 17), but in some patients ischemia manifests primarily as renal failure (also called ischemic nephropathy). Also, renal artery stenosis is but one etiology of renal ischemia, with other etiologies being vessel compression by extrinsic tumors, artery thrombosis, emboli, vasculitides, and venous occlusions. Major renal vein obstruction eventually leads to the ischemia–renal failure cycle but not to hypertension. In addition, renal vein obstruction has different imaging manifestations.

Renal ischemia is incompletely understood. Normal kidneys receive more blood than necessary for functioning and tolerate considerable underperfusion as long as systolic blood pressure is above 70 or 80mmHg or so. Ischemia is less common in a setting of fibromuscular dysplasia than with atherosclerotic disease.

A sufficient decrease in renal blood supply, regardless of etiology, will evolve into renal infarction. Acute infarction has developed in Behçet’s disease, primary renal artery dissection or thrombosis, cocaine abuse, and a subcapsular hematoma secondary to metastasis to the kidney,and has even been idiopathic.In a setting of renal artery stenosis, acute renal failure has developed after some medications, including diuretics and other antihypertensive drugs.

Clinically, acute nontraumatic renal artery thrombosis and resultant ischemia typically manifest by sudden onset of flank pain. On a more chronic basis decreased arterial flow is less symptomatic, and some of these patients simply develop a small, nonfunctioning kidney.

The onset of flank pain due to acute renal ischemia/infarction mimics that of renal colic. Among 300 patients admitted with renal colic, renal parenchymal infarction was eventually diagnosed in three patients (132).

Intravenous urography of acute renal artery obstruction reveals a nonfunctioning but normal-sized kidney. Ultrasonography also shows a normal kidney. No major renal blood flow is identified with Doppler US. In such a clinical setting postcontrast helical CT or angiography simply confirms the diagnosis.

Computed tomography performed without contrast in a search for calculi will miss renal arterial ischemia. Thus in such a clinical sce-

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nario, if nonenhanced CT does not reveal an abnormality to explain the patient’s clinical findings, a postcontrast CT is indicated to evaluate for a possible renal infarct.

Postcontrast CT of a total infarct shows a hypodense, nonfunctioning kidney. Prominent capsular collaterals, called the cortical rim sign, often lead to surrounding vascular enhancement. A focal infarction appears as a wedge-shaped region of decreased or absent contrast enhancement with sharply defined margins, most often extending to the capsule. Ultrasonography reveals a wedge-shaped hypoechoic region. Eventually the involved parenchyma atrophies, a scar forms, and an irregular renal outline remains as evidence of prior infarction.

Early intraarterial fibrinolysis therapy is indicated if recovery is anticipated. Renal ischemic due to renal artery stenosis is treatable by angioplasty (discussed in Chapter 17).

Nephrosclerosis

Arteriolar spasm, endothelial spasm, muscle hypertrophy, and eventual fibrosis develop in hypertension-induced nephrosclerosis. The final pathway is renal ischemia. Proteinuria is common. If untreated, renal failure ensues. The kidneys gradually shrink and cortical thinning becomes evident. Calyces are not affected.

Immunosuppression/Acquired

Immunodeficiency Syndrome

General

Renal abnormalities are common in HIVinfected patients. Ultrasoaography findings cousist of enlarged kidneys, decreased corticomedullary definition, decreased renal sinus fat, and a heterogeneous parenchyma, some with hyperechoic striations (133). In general, increased renal insufficiency in AIDS patients is associated with pelvocalyceal thickening and a hyperechoic appearance.

Cerebral toxoplasmosis in AIDS patients is treated with sulfadiazine, which is associated with acute renal failure due to sulfadiazineinduced crystalluria. Ultrasonography findings consist of hyperechoic foci in renal parenchyma and hyperechoic debris in collecting systems.