Epidemiology of Kidney Injury

Renal injuries are relatively uncommon and are present in about 1% of patients hospitalized after traumatic injury. Of all genitourinary trauma, however, the kidney is by far the most commonly injured organ in civilian institutions. Extrapolations based on large renal trauma series estimate there to be an annual incidence of 245,000 traumatic renal injuries worldwide. In large population-based studies, blunt injury is the leading mechanism of injury and accounts for between 81% and 95% of cases. The mechanism of trauma can vary significantly, however, and single institutional series in urban centers demonstrate a much greater predominance of penetrating injury to the kidney. Although the incidence of renal trauma is lower compared with other solid organ injuries regardless of the mechanism, these injuries can be associated with life-threatening complications of which renal hemorrhage is the most acute and dramatic.

Conservative Management of High-grade Injuries

Multiple studies have shown that high-grade renal trauma can be successfully managed in most cases conservatively. This is true even in cases of penetrating trauma in patients that do not require laparotomy for other abdominal injuries, have undergone appropriate radiologic imaging, and have adequate renal injury staging. The absolute indications for renal exploration are life-threatening hemorrhage from renovascluar injury; ureteropelvic junction avulsion; and urinoma unresponsive to minimally invasive procedures (ureteral stenting or perinephric drainage). The relative indications for exploration are laparotomy for other abdominal injuries, concomitant pancreatic or bowel injury, and large devascularized segments of kidney.

The importance of nonoperative management of renal injury is avoiding laparotomy and more importantly avoiding iatrogenic or unnecessary nephrectomy. At trauma centers with interest in renal trauma, nephrectomy is rare and reserved for renovascular injuries. However, nationally studies have shown that the most common operation in the management of renal trauma remains nephrectomy. In series where nonoperative management has been rigorously adopted, nephrectomy in high-grade renal trauma has dropped and proportionally renal salvage has risen. It is intuitive and actually observed that after trauma nephrectomy renal function is halved, although it is debated whether this leads to worse outcomes. Despite this debate, it is obvious to any trauma surgeon that unnecessary nephrectomy in the setting of severely traumatized patients is harmful.

Because the most common management of renal injury remains nephrectomy, conservative management of renal injuries, often with angioembolization when feasible, has been shown to decrease the rate of nephrectomy and increase renal salvage.

Epidemiology of Kidney Injury

Renal injuries are relatively uncommon and are present in about 1% of patients hospitalized after traumatic injury. Of all genitourinary trauma, however, the kidney is by far the most commonly injured organ in civilian institutions. Extrapolations based on large renal trauma series estimate there to be an annual incidence of 245,000 traumatic renal injuries worldwide. In large population-based studies, blunt injury is the leading mechanism of injury and accounts for between 81% and 95% of cases. The mechanism of trauma can vary significantly, however, and single institutional series in urban centers demonstrate a much greater predominance of penetrating injury to the kidney. Although the incidence of renal trauma is lower compared with other solid organ injuries regardless of the mechanism, these injuries can be associated with life-threatening complications of which renal hemorrhage is the most acute and dramatic.

Conservative Management of High-grade Injuries

Multiple studies have shown that high-grade renal trauma can be successfully managed in most cases conservatively. This is true even in cases of penetrating trauma in patients that do not require laparotomy for other abdominal injuries, have undergone appropriate radiologic imaging, and have adequate renal injury staging. The absolute indications for renal exploration are life-threatening hemorrhage from renovascluar injury; ureteropelvic junction avulsion; and urinoma unresponsive to minimally invasive procedures (ureteral stenting or perinephric drainage). The relative indications for exploration are laparotomy for other abdominal injuries, concomitant pancreatic or bowel injury, and large devascularized segments of kidney.

The importance of nonoperative management of renal injury is avoiding laparotomy and more importantly avoiding iatrogenic or unnecessary nephrectomy. At trauma centers with interest in renal trauma, nephrectomy is rare and reserved for renovascular injuries. However, nationally studies have shown that the most common operation in the management of renal trauma remains nephrectomy. In series where nonoperative management has been rigorously adopted, nephrectomy in high-grade renal trauma has dropped and proportionally renal salvage has risen. It is intuitive and actually observed that after trauma nephrectomy renal function is halved, although it is debated whether this leads to worse outcomes. Despite this debate, it is obvious to any trauma surgeon that unnecessary nephrectomy in the setting of severely traumatized patients is harmful.

Because the most common management of renal injury remains nephrectomy, conservative management of renal injuries, often with angioembolization when feasible, has been shown to decrease the rate of nephrectomy and increase renal salvage.

Modifications of AAST Grading

To eliminate this discrepant grading of renal injury prevalent throughout the trauma literature, Buckley and McAninch recently proposed changes to the AAST grading system. The authors proposed that grade 5 injuries should include only those injuries that involved a main renal artery or renal vein injury and would cause in most cases total devascularization of the kidney by either renal hilar avulsion or an obstructive intimal arterial flap ( Fig. 1 ). Examples of grade 4 and 5 injuries with the modified AAST grading system, referred to by the authors as the revised Renal Injury Staging Classification (RISC), are illustrated in Fig. 2 .

Modified AAST grading intended to eliminate inconsistent reporting of grade 4 and 5 renal injuries. Grade 5 injuries, with this modification, would consist of those injuries with arterial thrombus or a renal hilar injury, such as renal hilar avulsion. The previous grade 5 “shattered kidney” is now referred to as a grade 4 injury if there is vascularized kidney.

Computed tomography of modified AAST grading of renal trauma. ( A ) Grade 4 injury with urinary extravasation. ( B ) Severe grade 4 laceration previously often categorized as grade 5 “shattered kidney.” ( C ) Grade 5 injury with intimal flap from deceleration injury. ( D ) Grade 5 renal artery hilar avulsion.

( Courtesy of Dr Jack McAninch, San Francisco General Hospital, San Francisco, CA.)

Adopting the revised RISC means that most injuries that demonstrated any vascularized and viable kidney are designated as a grade 4 injury. These changes expand the breadth of grade 4 injuries and also increase the spectrum of hemorrhage risk in grade 4 injury, the subject of this article. It is very important to understand the discrepancies in the reported outcomes of high-grade renal injury within the literature to interpret the results from published series including prognostic nomograms and models.

Although the revised RISC encourages uniform reporting and a greater ability to compare results between studies, it does not increase the prognostic value of the system and the grade of renal injuries still remains very broad. When Buckley and McAninch applied their proposed modifications to the large series from San Francisco General Hospital they found that only 2 of 52 patients with grade 5 injuries were downgraded to grade 4 and the changes made very little difference in the renal salvage rate (4.3% vs 4.2% in grade 5 injuries). However, this series was graded with a prejudice toward the modifications suggested in the recent article throughout its maintenance over the last four decades. In other series, such as Altman and colleagues, where a salvage rate of grade 5 injuries was 46%, there is little doubt that reclassification of grade 5 injuries into the grade 4 category dramatically decreases the grade 5 renal salvage rates to make them more comparable with San Francisco General Hospital’s series.

Thus, although these changes encompassed in the revised RISC may add uniformity within the literature they do little to increase the value of AAST grading to predict renal injuries that fail conservative management. Conversely, they act to broaden the definition of grade 4 injuries and the probable spectrum of clinical behavior of these injuries. Recognizing the wide spectrum of hemorrhage risk within grade 4 injuries, expanded by the revised RISC, highlights the importance of developing a predictive model in high-grade renal injury, especially given the greater contemporary role of conservative management of these injuries.