# Solid Organ Injury Grading: AAST Classification

## Introduction

The **American Association for the Surgery of Trauma (AAST) Organ Injury Scale (OIS)** is the standard classification system for grading solid organ injuries following blunt and penetrating trauma. Accurate CT-based grading directly informs clinical management decisions -- specifically, whether a patient can be managed nonoperatively or requires surgical or interventional radiology intervention. The 2018 AAST revision incorporated **vascular injury criteria** detected on contrast-enhanced CT into the grading system.

## General Principles of Solid Organ Injury on CT

A **laceration** appears as a linear or branching hypoattenuating defect within the parenchyma. A **hematoma** may be subcapsular (lens-shaped, conforming to the organ surface) or intraparenchymal (round or irregular hyperattenuating collection). **Active hemorrhage** is a focus of contrast extravasation (hyperattenuating, greater than 85 HU) that does not conform to a normal vessel. A **pseudoaneurysm** is a contained, round focus of contrast enhancement isodense to the aorta. **Devascularization** is a non-enhancing segment of parenchyma indicating disrupted arterial supply.

## Splenic Injury Grading (AAST 2018 Revision)

| Grade | Criteria |
|-------|----------|
| I | Subcapsular hematoma <10% surface area; capsular laceration <1 cm depth |
| II | Subcapsular hematoma 10-50%; intraparenchymal hematoma <5 cm; laceration 1-3 cm depth |
| III | Subcapsular hematoma >50% or expanding; ruptured hematoma; laceration >3 cm or involving trabecular vessels; intraparenchymal hematoma >=5 cm |
| IV | Laceration involving segmental/hilar vessels with >25% devascularization; active bleeding confined within capsule |
| V | Shattered spleen; hilar vascular injury with complete devascularization; active bleeding into peritoneum |

**Grade I** includes subcapsular hematoma less than 10% surface area or capsular laceration less than 1 cm depth. **Grade II** includes subcapsular hematoma of 10-50% surface area, intraparenchymal hematoma less than 5 cm, or laceration 1-3 cm depth. **Grade III** includes subcapsular hematoma greater than 50% or expanding, ruptured subcapsular or parenchymal hematoma, laceration greater than 3 cm depth or involving trabecular vessels, or intraparenchymal hematoma 5 cm or greater. **Grade IV** includes laceration involving segmental or hilar vessels with major devascularization (greater than 25% of the spleen) or active bleeding confined within the splenic capsule. **Grade V** includes a shattered spleen, hilar vascular injury with complete splenic devascularization, or active bleeding extending into the peritoneum.

### Management Pearls

Grades I-III in hemodynamically stable patients are managed with **nonoperative management** and serial clinical and imaging follow-up. **Active extravasation** or **pseudoaneurysm** (Grade IV-V) should prompt consideration of **angioembolization** in hemodynamically stable patients. Hemodynamic instability with splenic injury warrants operative management (splenectomy).

![CT images demonstrating AAST splenic injury grades II, III, and IV with corresponding lacerations, hematomas, and active hemorrhage](splenic-injury-grading-ct.png)

## Hepatic Injury Grading (AAST 2018 Revision)

| Grade | Criteria |
|-------|----------|
| I | Subcapsular hematoma <10% surface area; laceration <1 cm depth |
| II | Subcapsular hematoma 10-50%; intraparenchymal hematoma <10 cm; laceration 1-3 cm depth, <10 cm length |
| III | Subcapsular hematoma >50% or expanding; ruptured hematoma; laceration >3 cm depth; intraparenchymal hematoma >=10 cm |
| IV | Parenchymal disruption 25-75% of hepatic lobe; active bleeding confined within parenchyma |
| V | Parenchymal disruption >75% of hepatic lobe; juxtahepatic venous injury (IVC/central hepatic veins); active bleeding into peritoneum |

**Grade I** includes subcapsular hematoma less than 10% surface area or laceration less than 1 cm depth. **Grade II** includes subcapsular hematoma of 10-50% surface area, intraparenchymal hematoma less than 10 cm, or laceration 1-3 cm depth and less than 10 cm length. **Grade III** includes subcapsular hematoma greater than 50% or expanding, ruptured subcapsular or parenchymal hematoma, laceration greater than 3 cm depth, or intraparenchymal hematoma 10 cm or greater. **Grade IV** includes parenchymal disruption of 25-75% of a hepatic lobe or active bleeding confined within the liver parenchyma. **Grade V** includes parenchymal disruption of greater than 75% of a hepatic lobe, juxtahepatic venous injury (retrohepatic vena cava or central major hepatic veins), or active bleeding extending into the peritoneum.

### Management Pearls

**Nonoperative management** is successful in more than 80% of blunt hepatic injuries in hemodynamically stable patients. **Hepatic arterial embolization** is effective for active hemorrhage from hepatic arterial branches. **Hepatic venous and IVC injuries** (Grade V) have high mortality and usually require operative management. Delayed complications include **biloma**, bile leak (peritonitis), hepatic abscess, and delayed hemorrhage. Follow-up imaging is recommended for high-grade injuries.

## Renal Injury Grading (AAST 2018 Revision)

| Grade | Criteria |
|-------|----------|
| I | Subcapsular hematoma; renal contusion; non-expanding perirenal hematoma |
| II | Laceration <1 cm depth without collecting system involvement; contained hematoma |
| III | Laceration >=1 cm without collecting system rupture; vascular injury/active bleeding confined within Gerota fascia |
| IV | Laceration into collecting system with urinary extravasation; renal pelvis/UPJ disruption; segmental vascular injury; active bleeding beyond Gerota fascia |
| V | Main renal artery/vein thrombosis or avulsion; shattered kidney |

**Grade I** includes subcapsular hematoma, renal contusion, or non-expanding perirenal hematoma. **Grade II** includes laceration less than 1 cm depth without collecting system involvement or contained renal hematoma. **Grade III** includes laceration 1 cm or greater depth without collecting system rupture or urinary extravasation, or any vascular injury or active bleeding confined within Gerota fascia. **Grade IV** includes laceration extending into the collecting system with urinary extravasation, laceration of the renal pelvis or complete ureteropelvic junction disruption, segmental renal artery or vein injury with devascularization, or active bleeding beyond Gerota fascia into the retroperitoneum or peritoneum. **Grade V** includes main renal artery or vein thrombosis or avulsion (devascularized kidney) or a shattered kidney.

### Management Pearls

**Delayed phase CT** (5-10 minutes) is essential to detect urinary extravasation and collecting system injury. Grades I-III are managed nonoperatively. Grade IV with active hemorrhage may require angioembolization, with ureteral stenting for collecting system injury. Grade V with a devascularized kidney often requires exploration or nephrectomy; warm ischemia tolerance is approximately 60-90 minutes.

![CT showing Grade IV renal injury with laceration extending to the collecting system, perirenal hematoma, and contrast extravasation on delayed phase images](renal-injury-grade-iv-ct.png)

## Pancreatic Injury

Pancreatic injury is less common but clinically significant and often difficult to detect on initial CT. The **key finding** is a laceration across the pancreatic body, especially if it involves the **main pancreatic duct**, which differentiates low-grade from high-grade injury. AAST Grades I-II include contusion or minor laceration without duct injury. Grade III is distal transection with duct injury. Grade IV is proximal transection. Grade V is massive disruption of the pancreatic head. **MRCP** or **ERCP** may be needed to evaluate duct integrity when CT is equivocal.

## Adrenal Injury

Adrenal injury is often associated with other severe injuries (ipsilateral hepatic or renal injury, spine fracture). CT findings include adrenal enlargement, periadrenal fat stranding, and a round, hyperattenuating hematoma. It is usually managed conservatively. **Bilateral adrenal hemorrhage** may cause adrenal insufficiency, which is rare but life-threatening.

![Comparison panel of hepatic and splenic lacerations at multiple AAST grades on contrast-enhanced CT](liver-spleen-injury-comparison.png)

## Key Clinical Pearls

The **2018 AAST revision** integrates vascular findings (active hemorrhage, pseudoaneurysm, devascularization) into the grading system, which better predicts need for intervention than morphologic criteria alone. **Active hemorrhage on CT** does not automatically mandate surgery; **angioembolization** is the first-line intervention in hemodynamically stable patients with solid organ bleeding. Always obtain **delayed phase images** for renal injuries to evaluate for collecting system disruption. A **sentinel clot sign** (highest density hematoma immediately adjacent to the injured organ) localizes the source of bleeding in patients with hemoperitoneum. Injury grading should be performed on the **initial CT**; subsequent imaging may show evolution of findings that do not reflect acute injury severity.

## References

1. Kozar RA, Crandall M, Shanmuganathan K, et al. Organ Injury Scaling 2018 Update: Spleen, Liver, and Kidney. *J Trauma Acute Care Surg*. 2018;85(6):1119-1122.
2. Coccolini F, Montori G, Catena F, et al. Splenic Trauma: WSES Classification and Guidelines for Adult and Pediatric Patients. *World J Emerg Surg*. 2017;12:40.
3. Stassen NA, Bhullar I, Cheng JD, et al. Nonoperative Management of Blunt Hepatic Injury: An Eastern Association for the Surgery of Trauma Practice Management Guideline. *J Trauma Acute Care Surg*. 2012;73(5 Suppl 4):S288-S293.
4. Morey AF, Brandes S, Dugi DD, et al. Urotrauma: AUA Guideline. *J Urol*. 2014;192(2):327-335.