Pathophysiology of Penetrating Trauma
Low‐velocity weapons, such as knives, lead to local tissue damage and effects along the tract of penetration. High‐velocity projectiles, such as bullets and shrapnel, result in wider tissue injury. This is governed by the formula KE = ½ MV2, where KE is kinetic energy, M is the bullet mass, and V is the velocity. On average, a rifle (such as the AK‐47 or M‐16A1) has a bullet velocity 2–3 times greater than a standard handgun. This translates into 4–9 times greater kinetic energy (holding mass constant), thus causing greater damage. There are two areas of projectile‐tissue interaction in missile wounds – the permanent and temporary cavity. The permanent cavity is due to local tissue damage and necrosis due to the projectile, whereas the temporary cavity is caused by the transient lateral displacement of tissue [16]. Due to varying bullet characteristics, such as fragmentation, weight, and yaw patterns, bullets can cause variable and significant damage in the temporary cavity that may seem out of proportion to the entry or exit wound.
Injury from explosions are classified into: (i) primary BI due to the interaction of the blast wave with gas‐filled structures; (ii) secondary BI due to ballistic trauma resulting from fragmentation wounds from the explosive device or the environment; (iii) tertiary BI due to displacement of the victim or environmental structures, which are largely blunt injuries; and (iv) quaternary BI or burns, toxins, and radiation contamination [17]. Most primary BI do not result in surviving causalities because these patients would have been so close to the blast epicenter that they likely sustained lethal injuries. The pressure wave caused by blasts cause damage primarily to gas‐containing organs, such as the lung; the kidneys are remarkably resilient to the pressure effects of blasts, although renal pelvis injuries have been documented [18]. The kidneys are mainly injured by the secondary and tertiary mechanisms. The PRI from blasts have pathophysiology similar to more common injury patterns, such as GSW. Although these fragments are often much smaller than bullets, they may cause more tissue damage due to the sheer number of fragments and because the velocity of these fragments can be over twice that of a rifle.
Anatomy
It is imperative to have a sound understanding of renal anatomy, as it is foundational for understanding associated injury patterns, surgical approach/reconstruction, and comprises the basis of non‐operative management (NOM). The kidneys are paired retroperitoneal organs extending from vertebral levels T12 to L3. From deep to superficial, the layers surrounding the kidney are as follows: renal capsule, peri‐renal fat, renal fascia (Gerota's fascia), and para‐renal fat. Both Gerota's fascia and the renal capsule are responsible for tamponade of renal hematomas and they are both critical layers for renorrhaphy. The vascular supply consists of one main renal artery and vein, although about 25% of kidneys have accessory vessels. The internal structure can grossly be divided into the renal parenchyma and collecting system. The latter is comprised of minor and major calyces that coalesce into the renal pelvis. This distinction between the parenchyma and collecting system is important in renal injury grading (Table 2.1).
Table 2.1 AAST Renal injury classification, revised in 2018.
| Grade I | Contusion or nonexpanding subcapsular hematoma |
| Grade II | Nonexpanding perirenal hematoma |
| <1 cm cortical laceration without urinary extravasation | |
| Grade III | Cortical laceration >1 cm without urinary extravasation |
| Any injury in the presence of a kidney vascular injury or active bleeding contained within Gerota's fascia | |
| Grade IV | Laceration into collecting system |
| Segmental renal artery or vein injury | |
| Active bleeding beyond Gerota's fascia | |
| Segmental or complete kidney infarction due to vessel thrombosis without active bleeding | |
| Grade V | Main renal artery or vein laceration or hilar avulsion |
| Devascularized kidney with active bleeding | |
| Shattered kidney with loss of identifiable parenchymal renal anatomy |
Figure 2.1 The kidneys and their association with adjacent organs.
Source: figure courtesy of Daniel Burke, University of Washington.
The diaphragm, 11th and 12th ribs, quadratus lumborum, and psoas major surround both kidneys. Anteriorly, the right kidney is associated with the liver, duodenum, and right colic flexure; the left kidney is associated with the spleen, stomach, pancreas, left colic flexure, and jejunum (Figure 2.1). Even with the safeguards of their retroperitoneal location, they are susceptible to penetrating trauma and it is due to their close anatomical relationship with other organs that isolated PRI is rare.
Evaluation
The initial evaluation and management of trauma patients has been standardized according to set protocols with the development of the Advance Trauma Life Support (ATLS) guidelines. Thus, the initial management of the trauma patient has often been completed by the trauma team prior to the involvement of a urologist [19, 20]. Vitals sign monitoring is imperative in patients with PRI, as patient stability dictates management.
For suspected renal trauma, the evaluation should include a thorough history and physical examination to evaluate for penetrating entry and exit wounds, flank ecchymosis, rib fractures, and gross hematuria. In addition to standard laboratory testing, a urinalysis should be obtained to evaluate for microscopic hematuria – defined as three or more red blood cells per high power field. Hematuria is the best indicator of significant renal trauma; however, it is not a sensitive marker, as up to 20.8% of patients with renal trauma lack hematuria [21, 22].
Imaging
The goals of imaging are to grade the renal injury, identify injuries to other organs, and demonstrate the presence of a functioning contralateral kidney should operative management be necessary. The stability of the patient determines the initial imaging; unstable patients cannot obtain computed tomography (CT) scans if they require immediate intervention and the kidneys and retroperitoneum can be assessed in the operating room at time of laparotomy. In military trauma, due to forward deployment of combat support hospitals and the technological progression of expeditionary medicine, CT capabilities are available in war zones and the imaging principles remain congruent with civilian trauma [23].
All stable patients with penetrating abdominal trauma should get diagnostic imaging with IV contrast enhanced CT. To fully evaluate and stage renal trauma (Table 2.1), the American Urological Association (AUA) and European Association of Urology (EAU) recommend a three‐phase CT
