The kidney is the most frequently injured genitourinary organ, typically as a result of blunt trauma and present in 2–3 % of all pediatric trauma-related injuries. The pediatric kidney is at increased susceptibility to trauma secondary to multiple anatomic protective factors: softer rib cage, less mature torso musculature, proportionally larger kidneys extending below the protection of the rib cage, and less cushioning perinephric fat [9]. There has been a steady shift over the past 30 years toward conservative therapy for renal trauma, with intervention reserved only for selected patients. Multiorgan trauma is common and present in 50–80 % of patients with any renal injury [10, 11].

Congenitally abnormal kidneys are more susceptible to trauma. Unrecognized renal anomalies are far more common in children and are present in approximately 10 % of children with renal injuries (Figs. 20.3 and 20.4). The most common congenital abnormalities associated with trauma are hydronephrosis, horseshoe kidney, and renal ectopia [12–14]. Despite making the kidney more prone to injury, there does not appear to be an increase in morbidity or mortality [15, 16].

Since the late 1980s CT scan has become the imaging modality of choice for the initial evaluation of suspected renal trauma. CT allows for assessment of injury to the renal parenchyma and hilum, extent of renal hemorrhage, extravasation of urine, as well as associated injuries to other abdominal organs and vasculature structures.

The American Association for the Surgery of Trauma (AAST) has developed a universally accepted grading scale (I–V) to quantify the severity of renal trauma and the likelihood of surgical intervention (Fig. 20.5). There is no consensus on an imaging algorithm for the evaluation of the pediatric trauma patients [17]. The most comprehensive and most algorithm was proposed by McAninch in 2004 and based on observation and management of 374 pediatric patients [18]. This algorithm utilizes the etiology of blunt or penetrating trauma, stability of patient, coupled with the number of red blood cells (RBCs) found on urinalysis (UA) to determine if abdominal CT scan is warranted. They concluded that Abdominal CT scan should be performed in all stable patients with penetrating trauma who have greater than five RBCs on UA and selectively in stable patients with blunt abdominal trauma greater than 50 RBC on UA. The selection criteria to obtain a abdominal CT scan in patients with blunt abdominal trauma, include positive clinical findings of renal trauma (e.g., flank pain, ecchymosis) and rapid deceleration injury. Management of unstable patients depends on the expertise of the medical center. In level 1 trauma center more patients are treated with conservative or minimally invasive procedures (e.g., angiographic embolization, nephrostomy, ureteral stent) as compared to abdominal exploration and nephrectomy. This approach results in a sensitivity for detecting Grade II or greater injuries in over 98 % of blunt trauma and greater than 95 % of penetrating trauma [18]. However, the yield of abdominal CT scan in pediatric patients with microscopic hematuria is very small, and its use should be considered carefully in these patients [19].

Some groups have raised concerns that this approach (CT scan for microscopic hematuria >5 RBC/hpf) results in the performance of many unnecessary CT scans which have no impact on clinical course. These groups support the use of established adult trauma criteria for CT scan in the pediatric population; i.e., CT scan is reserved for patients with gross hematuria or are that are clinically unstable [19, 20]. A study simulating this approach in 27 children with microscopic hematuria after blunt trauma showed that only one patient had a Grade IV injury; the kidney was found to be uninjured at surgery [11] and in retrospect misdiagnosed by CT scan.

Ultrasonography (US) evaluation for renal injuries is enticing, as it does not require radiation and can be performed at the bedside, but it lacks sensitivity for detecting solid-organ and vascular injuries. However, if the decision is made to limit CT scanning only to children with gross hematuria, major trauma, or clinical instability, then one may consider use of Doppler US for screening patients with blunt abdominal trauma and microscopic hematuria [19]. At our institution we mainly use US for long-term follow-up of renal injuries.

A single portal venous phase IV contrasts abdominal and pelvis study is performed for any abdominal trauma including for renal trauma. If any renal injury is detected, 5 min delayed CT scan is performed to evaluate for contrast extravasation. To reduce the radiation dose, a split-bolus CT urographic technique has been introduced [21]. It is possible to use three split contrast boluses and scan only once and get all three phases which is our current preference for evaluating vascular, parenchymal, and collecting system injuries (Fig. 20.6) and for follow-up. We use three contrast phases in a single CT scan (Fig. 20.7).

Grade I injuries are defined as hematuria with a normal CT, renal parenchymal contusion, or nonexpanding subcapsular hematoma without parenchymal laceration. This is the least severe renal injury and accounts for approximately 80 % of all renal injuries [18]. Typical CT findings consist of ill-defined or discrete areas of decreased contrast enhancement or with subcapsular hematoma. A subcapsular hematoma is found between the renal capsule and the parenchyma and demonstrates a crescent or elliptical shape, often deforming the parenchyma beneath. A large subcapsular hematoma may cause severe enough compression to impair renal perfusion, resulting in hypertension and a “Page” kidney. Patients with Grade I renal injuries may be observed but frequently do not require admission, bed rest, or any serial laboratory investigations. No follow-up imaging is typically necessary [22–24].

More severe renal injuries occur when the parenchyma tears or splits and include both Grade II and III injuries, determined by depth of laceration. A Grade II injury is defined as either a nonexpanding perirenal hematoma or renal cortical laceration that extends less than one cm (Fig. 20.1). A parenchymal laceration appears as a linear or wedge-shaped defect in the renal enhancement. On CT scan Grade III lacerations extend greater than 1 cm into the renal cortex but do not extend into the collecting system. A perinephric hematoma appears on CT as an ill-defined, high-density fluid collection between renal parenchyma and Gerota’s fascia, rarely deforming the renal contour (Figs. 20.3, 20.8, and 20.9). Throughout multiple series Grade I–III injuries account for over 90 % of all renal injuries and are without question managed conservatively, e.g., bed rest and serial laboratory investigations, and do not often necessitate follow-up imaging nor intervention [22–24].

Grade IV renal injuries extend through the renal cortex, through the medulla, and into the collecting system, possibly resulting in urinary extravasation. CT delayed phase may show extravasation of urine into the perirenal space (Figs. 20.4, 20.6, and 20.10). Urinary extravasation does not necessitate surgical intervention as over 80 % spontaneously resolve. Follow-up by US can be performed to evaluate change in the extrarenal fluid collection (Fig. 20.9). An injury to the hilum, either venous or arterial, is also considered a Grade IV renal injury. Grade IV injuries including thrombosis, dissection, or injury to the renal vasculature result in segmental infarcts. CT scan may demonstrate active arterial extravasation, which can be managed with angiographic embolization (Fig. 20.10). Infarcts appear as well-circumcised linear or wedge-shaped areas without enhancement.

Grade IV renal injuries involve any injury to collecting system, including the renal pelvis. Ureteropelvic junction (UPJ) disruption is more commonly seen in pediatric patients, typically resulting from a deceleration injury. A complete UPJ avulsion is diagnosed by lack of contrast in the distal ureter, but a partial UPJ avulsion may demonstrate ureteral contrast. Contrast-laden urine located medial to the renal pelvis and UPJ is often seen in injury to UPJ or the renal pelvis.

Treatment of nonvascular Grade IV renal injury varies, but most can be managed conservatively and will not require intervention. Urinomas may require a ureteral stent or percutaneous drainage in approximately 15 %, while only 10 % require open surgical exploration. One study has shown that medial urinary extravasation on initial CT scan increases risk for necessitating urologic intervention. This same study suggested that lack of filling of the ipsilateral ureter is concerning for UPJ disruption, which likely will require surgical intervention.

Grade V injuries denote a shattered or devascularized kidney or complete avulsion of the renal hilum resulting in devascularization of the kidney. Radiographic appearance of a shattered kidney demonstrates multiple Grade IV injuries, with infarcted, devitalized areas coupled with urinary extravasation secondary to trauma to the collecting system (Fig. 20.11).

Injuries to the renal hilum may not result in hematuria but are almost always associated with significant injuries to other organs. The most common hilar injury is renal artery occlusion secondary to acute stretching of the renal artery causing intimal tearing, arterial dissection, and vessel occlusion. CT will demonstrate a well-defined segment or global absence of renal enhancement but reniform contour is maintained (Figs. 20.12 and 20.13). Right renal artery injuries may also demonstrate retrograde filling of the right renal vein from the inferior vena cava. Collateral perfusion may result in enhancement to the capsule and subcapsular areas, also known as the cortical rim sign, but several hours are required for the appearance of this sign. Isolated renal vein injuries are particularly uncommon, and renal vein thrombosis is often seen in concert with arterial or parenchymal injuries. CT will show renal enlargement, decreased early nephrogram, and prolonged corticomedullary nephrogram with delayed or absent opacification of the collecting system. Laceration of the renal vein demonstrates medial or circumferential subcapsular or perinephric hematoma.

The management of a Grade V renal injury is complex and should be tailored to each patient, as each individual represents a unique injury. Conservative management is possible in Grade V renal injuries, but often an interventional radiology or surgical intervention is necessary. Follow-up imaging is necessary to assess for expanding hematoma or urinoma. Findings on early follow-up imaging may necessitate percutaneous drains, percutaneous nephrostomy tubes, or ureteral stents for further management. Nephrectomy may be required for unstable patients or unsalvageable renal injuries; however, renorrhaphy should always be attempted prior to nephrectomy.