Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

disorder especially prevalent in the province of Quebec. Children with this lethal disease develop central nervous system and liver abnormalities early in life. Renal function is often impaired.

Computed tomography and US in 32 children with tyrosinemia undergoing liver transplantation revealed enlarged kidneys in 47%, a hyperechoic appearance in 47%, nephrocalcinosis in 16%, and delayed contrast excretion in 64% (25); liver transplantation improved renal function in about half of these children, but enlarged kidneys and a hyperechoic appearance persisted.

Congenital Hydronephrosis

and Hydroureter

Many infants with hydronephrosis also have renal tubular dysfunction, with renal tubular acidosis being most common. Hydronephrosis is readily diagnosed by most imaging modalities.

A congenital urinary tract obstruction evolves while a fetal kidney is still developing. The sequelae of a congenital obstruction thus result in findings that tend to be different from those seen with an obstruction developing later in life.

In newborns with unilateral hydronephrosis, the contralateral kidney ranges from larger than normal, to normal in size, to small, and such findings should be interpreted cautiously. Distinguishing obstructive from nonobstructive hydronephrosis in neonates can be difficult. Without an obvious obstructive site, some of these neonates are followed medically until they grow.

At times with known prenatal hydronephrosis, renal US performed immediately after birth is falsely negative for hydronephrosis, probably due to oliguria. Cystourethrography appears indicated in these children because of an increased risk for vesicoureteral reflux.

The term megacystis-megaureter is descriptive and is most often applied to uncorrected vesicoureteral reflux and resultant dilation. In this sense it can be either congenital or acquired. Whether it represents a sequela of uncorrected massive chronic reflux or is a residue of a congenital hydroureter is conjecture. Megacystis-megaureter can be extreme to

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the point that one or both ureters dilate massively. The bladder also enlarges.

Ureteroceles are discussed in Chapter 11.

Megacalyces

An infant or adult occasionally has calyceal dilation without concomitant renal pelvis dilation and without underlying obstruction. Whether this condition, also called Puigvert’s disease, is congenital or acquired is conjecture. Medullary pyramids are thinned and as a result the calyces have a straight or even convex outline rather than their usual concave shape. The number of calyces tends to be increased and they have a polygonal shape. The renal pelvis is not dilated and renal function is normal. Megacalyces is associated with increased stone formation.

Intravenous urography defines the condition and excludes an obstruction.

Ureteropelvic Junction Obstruction

The most common congenital urinary tract obstruction is at the ureteropelvic junction. A ureteropelvic junction stricture in the very young may represent a mild form of multicystic dysplasia. It is not common.

Midureteral Obstruction

A congenital midureteral obstruction is rare, with some of these strictures being classified as ureteropelvic junction strictures. A rare association exists between ureteral valves and ureteral strictures. Congenital adynamic midureteral segments are rare. Resection reveals muscular disarray but a patent ureter lumen.

Primary Megaureter

In congenital or primary megaureter a short nonperistaltic distal ureteral segment results in functional obstruction,while the more proximal ureter dilates and does have peristalsis. Etiology for the nonperistaltic segment is unknown but, unlike rectal aganglionosis, ganglion cells are present in this segment. Some authors subdivide primary megaureter into the following categories: (1) obstructive, (2) refluxing, and (3) nonobstructive and nonrefluxing. The degree of obstruction varies, and calyces are not dilated when obstruction is mild. The condition is more

KIDNEYS AND URETERS

often detected in children than adults and is more common on the left side. In some, ureteral dilation does not progress with age and may even improve. A rare patient first presents with a ureteral stone in a megaureter and an already nonfunctional kidney.

Differential diagnosis includes ureteral dilation secondary to reflux (megacystismegaureter) and distal anatomic ureteral obstruction. Generally the distal ureter is not narrowed in a setting of reflux. At times a retrograde study is necessary to exclude anatomic obstruction.

and, if stable, these patients generally undergo CT. Kidney damage due to anterior abdominal gunshot and knife wounds invariably also involves intraperitoneal structures, and most of these patients are explored. As a result, some surgeons argue against a need for any preoperative abdominal imaging studies. Flank or posterior stab wounds, on the other hand, are often evaluated with contrast-enhanced CT and the results are used to guide therapy.

Blunt Trauma

Clinical

Trauma

A renal injury classification scale, devised by the American Association for the Surgery of Trauma, is outlined in Table 10.1. New classifications of renal trauma continue to be devised, indicating that there are perceived limitations of the prior systems. Radiologic classifications have also been published.

In a setting of abdominal trauma, clinical evaluation of genitourinary trauma is notoriously difficult. As a result, at least in the United States, trauma physicians are rather generous in obtaining screening imaging studies in these patients, thus achieving a low yield for positive studies.

Penetrating Injuries

Intravenous urography is insensitive in evaluating renal damage from a penetrating injury,

Blunt renal injuries consist of contusion, laceration, rupture, and injury to the renal pedicle. Ureter rupture due to blunt trauma is rare. Most ureteral injury is from gunshot and stab wounds.

Renal trauma should be suspected in a setting of shock, hematuria, or adjacent fractures. Several studies have correlated the lack of hematuria with absence of major urinary tract injury, yet occasional patients without hematuria do have urinary tract injury, especially to the renal pedicle and ureter, and the degree of hematuria does not correlate with the extent of the injury eventually detected. Even a major ureteral laceration does not lead to hematuria in about one third of patients. Leakage from the urinary tract results in an accumulation of urine and either a urinoma or urinary ascites ensue.

The presence of hematuria, on the other hand, is generally investigated further with imaging studies. Often not only is urinary tract

injury present but also multiorgan damage. Incidentally, not all red urine represents blood. Porphyrins and certain drugs and foods lead to red-colored urine. About 15% of the population has a genetic predisposition to beeturia, due to the betalaine red pigment found in beet root.

In children, the kidney is the most common organ injured in blunt abdominal trauma. Complicating the issue is that congenital malformations need be considered. Thus underlying hydronephrosis or even an extrarenal pelvis predisposes to a laceration or avulsion.

A rare cause of nontraumatic ureteral hematoma is overcoagulation.

Imaging

Intravenous urography is no longer the primary imaging modality for renal trauma, having been supplanted by CT, which also evaluates other intraabdominal structures. If CT is not readily available, urography is an alternate for evaluating the upper urinary system. A normal IV urogram essentially excludes major renal and ureteral injury, although rare instances of severe renal laceration have not been detected with an urogram.

Limited urography, consisting of a scout radiograph, a radiograph shortly after contrast

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injection, and perhaps another one in 5 minutes or so (called “one-shot urography” by some clinicians) is a fast study to assess gross renal function and establish that there are indeed two kidneys in an unstable patient, but it is inadequate to assess for underlying renal trauma and should be discouraged in stable patients. It is a useful study in unstable patients who then undergo immediate exploration; their renal status is determined intraoperatively.

Many trauma patients have multiple injuries and, in the United States and some other countries, if they are hemodynamically stable, contrast-enhanced helical CT has become the screening tool of choice. Precontrast images are generally not obtained. An early-phase CT evaluates the renal pedicle and overall vascularity, a parenchymal-phase CT detects renal lacerations, and a delayed-phase CT provides information about contrast excretion and possible extravasation (Fig. 10.4). Such a technique also evaluates injury to the liver, spleen, pancreas, and adjacent structures. In particular, one should not rely on early-phase images to exclude collecting system injury with contrast extravasation; delayed scans are necessary. At times delayed serial postcontrast views reveal an increasing CT density or MR signal intensity in adjacent fluid, confirming extravasation.

In some countries US is the primary imaging modality in evaluating renal trauma. Ultra-

KIDNEYS AND URETERS

sonography can detect major vascular injury, renal fracture, or gross extravasation. It is a common modality in children. Nevertheless, accuracy of US appears to be less than with CT and even iv urography.

Currently MRI is not the initial imaging modality in suspected renal trauma, with availability, cost, and imaging time delays being some of the factors involved. Nevertheless, MRI complements CT in some trauma patients, especially those with equivocal CT findings, those who need repeat imaging studies, and those with an iodine allergy.

Ureteropelvic junction avulsion consists of complete ureteral transection, and thus no contrast is identified in the distal ureter, but contrast extravasation is evident, often perirenal in location. Although ureter nonvisualization is at times even a normal finding, in the appropriate clinical setting such a finding suggests avulsion, and a more specific study of ureter integrity, such as retrograde pyelography, should be considered. Both contrast extravasation and contrast in the ureter distally signify an ureteropelvic junction laceration. These findings are detected with both urography and postcontrast CT, but in any one patient the differentiation between avulsion and laceration can be difficult. Differentiation between avulsion and laceration has therapeutic implications because the latter is often managed conservatively.

At times a preexisting abnormality alters an otherwise more typical appearance of renal trauma. A renal cyst is prone to rupturing during blunt abdominal trauma, and the patient develops either hematuria or retroperitoneal hemorrhage. Such trauma-induced rupture of a renal cyst modifies the appearance of an associated hematoma. Ultrasonography identifies an acute hematoma as an isoechoic or hyperechoic tumor mimicking a neoplasm; it becomes more heterogeneous during resolution.

Renal scintigraphy has a role in detecting urinary leaks in patients with a contraindication to IV contrast. It is also often employed in renal transplant patients.

Vascular Injury

Renal pedicle injury consists of renal artery or major vein laceration, avulsion, or thrombosis.

Figure 10.5. Aortography in this patient after a motorcycle accident reveals complete left renal artery occlusion (arrow) due to a subintimal tear. No renal function was evident. No extravasation was seen. (Courtesy of David Waldman, M.D., University of Rochester.)

An associated extraperitoneal hematoma is common. In some centers angiography is the imaging modality of choice with suspected renal pedicle injury; in others CT is preferred (Fig. 10.5). The lack of contrast enhancement of renal parenchyma is a hallmark of pedicle injury, a finding that is not always reliable. Some patients have partial parenchymal enhancement in spite of main renal artery and vein disruption, at times due to an intact accessory renal artery, extensive capsular collaterals or even flow being maintained by an adjacent hematoma. Computed tomographic detection of an extensive extraperitoneal hematoma is common with a pedicle injury.

An arterial intimal tear can initially lead to an intimal flap and evolve into a thrombus. Extensive involvement results in a lack of perfusion and excretion by the kidney involved, although distal ischemia is usually segmental rather than the entire kidney being involved. Occlusion of the main renal artery or of a main branch is equally well seen either by contrast-enhanced helical CT or angiography in most, but not all, patients. Occasionally helical CT identifies an

abrupt cut-off to the renal artery. Contrastenhanced helical CT should also detect any pseudoaneurysms.

Retrograde flow of IV contrast into the renal vein suggests renal artery obstruction.

Avulsion results in renal infarction. A hematoma surrounds the site of rupture and at times a postcontrast study detects contrast extravasation. Renal artery thrombosis or renal pedicle avulsion should be suspected with postcontrast nonvisualization of a kidney, although an absent kidney, vascular spasm, or simply an ectopic kidney outside the imaged field results in similar findings.

Ultrasonography of some renal pedicle ruptures is initially noncontributory, and either CT or renal arteriography is required to exclude major pedicle injury.

Occasionally occult renal injury results in posttraumatic arterial hypertension. Whether computed tomography angiography (CTA) or arteriography should be performed is debatable, although arteriography is considered to be the gold standard. Renal artery or major branch laceration or intrarenal artery constriction is detected in some of these patients.

Renal vein thrombosis results in an enlarged kidney and a delayed and diminished nephrogram. Renal vein laceration leads to a perinephric hematoma that tends to mask the seriousness of an underlying vascular injury. Quite often CT suggests a laceration only indirectly if a follow-up scan reveals an increasing perinephric hematoma.

Therapy

A rather conservative therapeutic approach is evolving for CT-detected renal trauma in stable patients. For similar injuries some urologists favor exploration while others manage them conservatively as long as the patient is hemodynamically stable. When contemplating nephrectomy versus conservative therapy of a damaged kidney, a postcontrast CT finding of enhancing parenchymal rim, even if thin, and excretion of contrast into calyces should suggest a more conservative approach. Percutaneous interventional techniques suffice for some compli-

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cations. With continued bleeding, after diagnostic angiography establishes a site of hemorrhage in patients with traumatic kidney injury, transarterial superselective embolization should stop the bleeding in most patients. Most localized extravasations resolve spontaneously. On the other hand, patients with renal pedicle trauma require prompt revascularization to avoid future kidney loss.

Most unrecognized complications manifest within the first 4 weeks of injury; the exceptions are hypertension, hydronephrosis due to partial obstruction, infected urinoma, and pyelonephritis. A devitalized kidney segment predisposes to subsequent infection.

Injuries to the main renal artery are treated either by surgical revascularization or conservatively. Percutaneous insertion of an endovascular stent suffices for some vascular injuries. Posttherapy hypertension can be due to either an initial injury or a therapy complication.

Posttherapy renal function can be confirmed by scintigraphy.

Small or Large Kidney(s)

At times imaging reveals a kidney that is smaller or larger than normal. Some of the conditions associated with smaller than normal kidneys having a smooth outline are listed in Table 10.2. It is difficult in an older patient to determine if a small kidney is secondary to congenital hypoplasia or dysplasia or is due to an acquired condition such as ischemia or infection.

Hypotension results in an increasingly dense and persisting bilateral nephrogram postcontrast in kidneys that appear smaller than usual. A similar but unilateral nephrogram occurs with urinary tract obstruction and acute renal vein thrombosis.

Conditions leading to an enlarged kidney are limited (Table 10.3). Diffuse tumor infiltration results in a poorly defined border between tumor and normal parenchyma, making detection difficult. A reniform shape is generally preserved. In children, the most common cause of bilateral enlarged kidneys is leukemic infiltration.