Renal Trauma

Trauma accounts for almost 50% of deaths of children aged 1 to 14 years in the United States. Over 1.5 million injuries, 500,000 hospitalizations, and 20,000 deaths are sustained by American children as a result of trauma each year.¹ Incidents involving motor vehicles are the predominant cause; pedestrian deaths are prevalent among children aged 5 to 9 years. Males are involved twice as frequently as females; blacks and other minority children in the United States are at increased risk for traumatic injury and death compared with whites.²

The urinary tract is the second most common organ system to suffer substantial injury in children. However, death due to genitourinary trauma is uncommon. Only 5% of trauma-related fatalities are caused by genitourinary injuries.³ Children appear to be more susceptible to major renal trauma than adults.⁴ Blunt trauma is the cause of 90% of renal injuries in children; penetrating injuries are uncommon. Most are due to motor vehicle crashes, vehicle–pedestrian incidents, sports injuries, and assault. The vast majority of blunt renal injuries are contusions and require no invasive therapy.^5–7 Some preexisting renal abnormalities such as hydronephrosis, large cysts, tumors, or ectopia predispose to injury, although the magnitude of the risk is generally low (Figure 51-1).^8,9

Some renal injuries in children are caused by rapid deceleration. Because the kidney is relatively mobile within the Gerota space, it may be thrust laterally against the lower ribs or medially against the vertebral column. The major renal vessels can also be injured in association with major deceleration forces, due to tension on the vessels as the kidney moves relative to the more securely fixed aorta and vena cava. The intima is most susceptible to stretching injury as it is less elastic than the media and adventitia. An intimal tear can lead to dissection, luminal occlusion, or arterial thrombosis. Stretching injury can also cause spasm of the renal artery without a tear. With severe rapid motion of the kidney, vascular avulsion can occur. Tears of the collecting system can also occur by a stretching mechanism.

The clinical diagnosis of renal injury is generally based on a history of trauma in conjunction with flank pain, flank tenderness, and hematuria. The severity of associated hematuria is a useful predictive indicator for renal injury. Major kidney injury occurs in fewer than 2% of children with minimal microhematuria (≤50 RBC/hpf) after blunt trauma, 8% of children with substantial microhematuria (>50 RBC/hpf), and 32% of those with gross hematuria.¹⁰ If the renal artery is completely occluded, hematuria may be minimal or absent despite the presence of severe injury.

Contrast-enhanced CT is the imaging procedure of choice for children who have experienced substantial blunt abdominal trauma and are stable enough to tolerate the examination.^10,11 Penetrating trauma of the thorax or abdomen is also an indication for immediate renal imaging.⁷ Abdominal CT allows staging of clinically stable patients with renal stab wounds who are being considered for nonsurgical management. Approximately 10% to 15% of patients with clinical manifestations of renal involvement after blunt or penetrating trauma have clinically silent abnormalities of other organs on CT.^12,13

Renal pathology related to trauma can be assigned to 3 categories based on the CT findings. Category 1 injuries are relatively minor: contusion, intrarenal hematoma, segmental infarction, and small subcapsular hematoma. About three-fourths of pediatric renal injuries are category 1. Category 2 injuries include laceration of the cortex into the collecting system, renal fracture (a complete laceration), large subcapsular hematoma, ureteropelvic junction disruption, renal vein injury, and multiple lacerations with an intact capsule. Category 3 injuries are rare in children; these include shattered kidney with disruption of the capsule and renal artery occlusion or avulsion.

The most common renal injury is a localized contusion. This category 1 injury accounts for more than half of renal injuries in children. The capsule and collecting system are intact and the renal vessels are unaffected. Hemorrhage and edema increase the pressure within the parenchyma, as the intact capsule is unyielding. Thus, local renal perfusion and function are reduced proportionate to the degree of parenchymal involvement. The major CT finding is poor or delayed enhancement at the site of the contusion. Occasionally, a large or multifocal contusion causes sufficient elevation of parenchymal pressure to reduce renal perfusion globally. CT in this circumstance shows diminished intensity of contrast enhancement throughout the kidney and delayed accumulation of contrast in the collecting system. Delayed accumulation of contrast in the collecting system of an injured kidney is sometimes demonstrable with excretory urography as well. Patients with a renal contusion typically have flank pain and hematuria. In the uncommon instance of extensive involvement, reduction of renal perfusion can be of sufficient severity to cause diminished function of the entire kidney and decreased urine output.

When there is sufficient extravasation of blood into the renal parenchyma to produce a mass, the injury is termed an intrarenal hematoma. A hematoma appears on CT as an area that completely lacks contrast enhancement. The margins of an intrarenal hematoma are usually poorly defined. Displacement of adjacent structures can occur. A contusion can usually be distinguished from a hematoma on CT, as some degree of contrast enhancement usually occurs with the former, at least on delayed images. A localized traumatic renal infarction is distinguished from a hematoma by its typical wedge shape and well-defined margins.

A subcapsular hematoma is located at the periphery of the kidney and is separated from the Gerota fascia by the renal capsule and the perirenal fat. The thin layer of perirenal fat at the peripheral margin of the hematoma is usually visible on CT. The hematoma typically has a lenticular shape. The central margin indents the adjacent renal parenchyma. If the hematoma contains clotted blood, unenhanced images may show slightly higher attenuation values than the adjacent renal parenchyma. The hematoma does not enhance with IV contrast.

A perirenal hematoma occurs when blood leaks through a laceration in the renal capsule. A subcapsular hematoma frequently occurs concomitantly with a perirenal hematoma. The Gerota fascia serves to confine and tamponade hemorrhage in the perirenal space; therefore, exsanguination from this injury is rare. CT of a perirenal hematoma shows fluid and clotted blood adjacent to the kidney within the Gerota fascia (Figure 51-2). The blood may also extend inferiorly adjacent to the ureter or into the anterior pararenal space; rupture into the peritoneal cavity is rare. Diminished function of the kidney in patients with a perirenal hematoma can be demonstrated on CT or excretory urography by weak and delayed contrast excretion. A large perirenal hematoma can result in a clinically detectable flank mass.

A parenchymal injury that extends into the collecting system and is associated with urine extravasation is termed a complete renal laceration; this is a category 2 injury. The clotted blood within the collecting system in these patients can result in urinary obstruction. Urine can also extravasate into the perirenal space. CT shows poor renal function, a nonenhancing defect at the site of the laceration, and (usually) a nonenhancing perirenal hematoma/urinoma. Delayed images are useful to detect extravasation of contrast-opacified urine into the renal parenchyma or perinephric space and to identify filling defects in the collecting system due to blood clots (Figure 51-3).

A renal fracture is a complete laceration that results in transection of the kidney into 2 pieces. This severe injury tends to occur along interlobar divisions; this pattern of injury frequently results in preservation of blood supply to the 2 fragments. CT shows a nonenhancing defect in the midportion of the kidney and separation between the upper and lower poles. If the kidney is still functioning, findings of extravasation of contrast-opacified urine are nearly always present on delayed images.

A shattered kidney refers to an injury in which there are multiple lacerations and parenchymal hematomas (Figure 51-4). Pulped kidney is similar, with the entire kidney affected by contusions, hematomas, and lacerations. CT of a shattered kidney or pulped kidney typically shows nephromegaly, diminished contrast enhancement, delayed or absent excretion of contrast into the collecting system, poor definition of the renal contour, and extravasation from disruptions in the collecting system (visible on delayed images).

Deficient contrast enhancement of the parenchyma on CT suggests the presence of trauma-related renal arterial injury. With complete disruption of the main renal artery, there is rapid development of a large hematoma; however, as with other renal injuries, the compartmentalized nature of the retroperitoneum (e.g., Gerota fascia) limits the bleeding, and exsanguination is uncommon. Traumatic occlusion of an intrarenal vessel can cause a renal infarction that appears as a wedge-shaped or rounded area of absent enhancement, typically with sharp margins. Disruption, thrombosis, or spasm of the main renal artery results in absence of contrast enhancement of the entire kidney. With partial occlusion, there is diminished intensity of the nephrogram. CT angiography may be of benefit in this circumstance to more accurately define the vascular anatomy. In the nonacute setting, renal scintigraphy can be utilized to assess regional and global kidney function following trauma. Conventional angiography remains the gold standard for the detection of small vessel injuries.

The “cortical rim sign” is a radiographic finding associated with acute renal arterial thrombosis, which is a category 5 injury. This refers to contrast enhancement in the periphery of an ischemic kidney. The mechanism apparently involves collateral perfusion. The cortical rim sign does not develop until at least 8 hours after the onset of ischemia, and in many patients is not present until a few days after the injury. This sign generally indicates that renal salvage is not possible, due to the prolonged nature of the ischemia.^14,15

Ureteral Trauma

Ureteral injuries are uncommon in children. Major deceleration and flexion injuries can lead to ureteropelvic disruption. This injury is more common in children than in adults because of a greater degree of flexibility and renal mobility. In most patients with ureteropelvic disruption, there are at least 3 major associated injuries.¹⁰ The hallmark diagnostic imaging pattern consists of medial periureteral contrast extravasation and nonvisualization of the ipsilateral ureter.^16,17

Unlike most other substantial renal injuries, UPJ disruption can occur in the absence of a retroperitoneal hematoma.¹⁸ The classic radiographic sign of UPJ disruption is lack of contrast-opacified urine in the ureter. Delayed images demonstrate perirenal contrast extravasation.^4,19 A followup standard abdominal radiograph after contrast-enhanced CT is sometimes helpful for additional delineation of ureteral anatomy.

Bladder Trauma

Symptomatic bladder trauma in children is uncommon, accounting for only 2% of abdominal injuries requiring surgery.²⁰ Blunt trauma is the most common cause of bladder injury in children.²¹ The bladder is relatively protected from injury by the overlying bony pelvis and by its mobile character. However, a full bladder is susceptible to rupture as a consequence of blunt trauma. In addition, the incompletely developed pubic bones of young children provide limited protection. Patients with bladder rupture usually have associated injuries; 97% have a pelvic fracture. Extraperitoneal bladder rupture is somewhat more common than intraperitoneal rupture. Conservative therapy is usually sufficient for extraperitoneal bladder rupture, whereas intraperitoneal rupture often requires surgical therapy.

CT and cystography are the most useful imaging techniques for patients with suspected bladder trauma. With cystography, sequential images should be obtained during bladder filling. Anteroposterior (AP) and oblique radiographs are obtained when the bladder is full. Additional images are obtained after drainage of the bladder, to detect extravasation. CT imaging for bladder trauma can be performed either by instillation of contrast via a catheter or by obtaining delayed images following IV contrast administration (Figure 51-5). As with standard cystography, images should be obtained both with the bladder full and following drainage.

Simple extraperitoneal bladder rupture refers to extravasation that is confined to the perivesical space. Imaging studies show a focal contrast collection adjacent to the bladder, often with a “flame-shaped” appearance. Complex extraperitoneal bladder rupture involves contrast extension beyond the perivesical space into the thigh, scrotum, perineum, abdominal wall, or retroperitoneum. The contrast collections usually have a streaky character. More extensive extravasation can mimic the appearance of intraperitoneal contrast on radiographs; however, outlining of loops of bowel is lacking (Figure 51-6). Intraperitoneal bladder rupture results in free-flowing extravasation that outlines intraperitoneal organs. Combined bladder injury refers to coexistent intraperitoneal and extraperitoneal bladder rupture.

Urethral Trauma

Nearly all posterior urethral injuries are associated with severe blunt trauma and the shearing and stretching effects of a concomitant pelvic fracture. An estimated 3% to 5% of male patients with a pelvic fracture have an associated posterior urethral injury; 10% to 30% of these patients also have bladder rupture.²² Blood at the urethral meatus and a distended bladder are the most useful clinical signs of posterior urethral injury.

Posterior urethral injuries can be classified into 3 types on the basis of the findings with retrograde ureterography; this is the “Colapinto–McCallum classification.”²³ Type I injury refers to rupture of the puboprostatic ligaments, with stretching but maintenance of continuity of the urethra. Type II injury refers to tearing of the membranous urethra above an intact urogenital diaphragm. Extravasated contrast material during urethrography accumulates in the pelvic extraperitoneal space, but does not extend into the perineum. Type III is most common, with rupture of the membranous urethra and extension into the proximal aspect of the bulbous urethra due to laceration of the urogenital diaphragm. Urethrography shows contrast extravasation into the perineum as well as into the pelvic extraperitoneal space.

The “Goldman classification” adds 3 types of urethral injury to the standard classification system.²⁴ The type IV injury involves the bladder neck and the internal sphincter. The type IVa injury is an extraperitoneal bladder rupture at the base of the bladder, without extension into the bladder neck. The type V injury is a straddle injury, with compression of the bulbous urethra and corpus spongiosum between an external hard object and the pubic bones. Urethrography may show an intact urethra (contusion only) or extravasation at the sump of the bulbous urethra.²⁵

Trauma to the anterior urethra is rare because most of the anterior urethra is in a freely movable portion of the penis. The most common mechanism of anterior urethral trauma is a straddle injury of the perineum. The bulbar urethra is the most commonly injured segment, with forceful compression against the posterior margin of the pubic symphysis.²⁶

Urethral injuries in girls are rare. Most urethral injuries in female children are due to a pelvic fracture; concomitant injuries of the bladder neck or vagina are common.

Vascular Complications of UPJ Repair

Vascular complications can occur during procedures for repair of ureteropelvic junction obstruction. Injury to a crossing vessel is reported to occur in up to 9% of patients treated with endopyelotomy. Preoperative vascular imaging to detect crossing vessels at the site of a UPJ obstruction may diminish this risk. Only rarely is imaging required to evaluate the patient with evidence of hemorrhage postoperatively. CT is useful for detecting a hematoma. Angiography can be utilized to detect active arterial extravasation or an iatrogenic arteriovenous fistula.

Urine Leak

Urine leak can occur following urinary tract surgery or various forms of instrumentation. A urine leak can be detected with contrast-enhanced CT or MR, retrograde urography, or antegrade urography (Figure 51-7). Sonography shows a urinoma as a confined perinephric fluid collection (Figure 51-8). Occasionally, percutaneous nephrostomy placement is indicated for urinary diversion to facilitate healing of an iatrogenic urine leak.

Urinary Diversion