Thoracic aortic injury

Thoracic aortic injury is the most common type of traumatic aortic injury and is a critical life-threatening, and often life-ending event. 

Clinical presentation

Approximately 80% of patients with thoracic aortic injury die at the scene of the trauma. In those who make it to hospital, clinical diagnosis is difficult. The signs and symptoms are non-specific and distracting injuries are often present. Clinical presentation may include chest or mid-scapular back pain, signs of external chest trauma or haemodynamic instability. Clinical suspicion is usually based on mechanism and severity of the injury, the haemodynamic status of the patient and/or the presence of related injuries. The diagnosis ultimately relies on appropriate imaging.

Pathology

Trauma to the aorta may result in:

• minimal aortic injury
• aortic laceration: a tear in the intima which may extend through the vessel wall; the tear is typically transverse
• aortic transection: laceration of all three layers of the vessel wall, also known as traumatic aortic rupture
• aortic pseudoaneurysm: aortic rupture contained by adventitia or periaortic tissue
• aortic intramural haematoma: haematoma within the wall of the aorta

An aortic dissection is a longitudinal tear in the aortic wall and is rarely a sequela of trauma.

Blunt thoracic aortic injury is uncommonly an isolated injury. A large autopsy study found that 97% of victims of aortic injury had other traumatic injuries outside the chest ⁵.

Grading 

Thoracic aortic injury can be graded according to the severity of injury. One grading system is ⁶

• grade 1: intimal tear
• grade 2: intramural haematoma
• grade 3: pseudoaneurysm formation
• grade 4: free rupture

Another system is ⁷

• grade I (intimal tears and intramural haematomas)
  • grade Ia: intimal tear
  • grade Ib: intramural haematoma
• grade II: intimal injury with periaortic haematoma
• grade III: transection +/- multiple injuries without rupture
  • grade IIIa: aortic transection with pseudoaneurysm
  • grade IIIb: multiple aortic injuries
• grade IV: free rupture

Aetiology

Thoracic aortic injury can result from either blunt or penetrating trauma:

1. blunt trauma (more common)
   • rapid deceleration (eg. motor vehicle accident, fall from great height)
   • crush injury
2. penetrating trauma
   • stab wound
   • gunshot wound

Location

• aortic isthmus: 90%
• ascending aorta: 5%
• diaphragmatic hiatus: 5%

The isthmus is the portion of the proximal descending thoracic aorta between the left subclavian artery origin and the ligamentum arteriosum. Tethering of the aorta by the ligamentum arteriosum is believed to account for the high frequency of aortic injury in this region.

The above figures represent the site of injury in those patients who present to hospital. The ascending aorta is injured in 20 -25% of cases at autopsy but most of these patients die at the scene from serious complications such as a ruptured aortic valve, coronary artery laceration or haemopericardium with pericardial tamponade.

Radiographic features

Plain radiograph

A supine chest radiograph is the initial screening investigation in the trauma patient. The mediastinum on a portable supine film can be difficult to assess, especially if taken in expiration or if the patient is rotated. Direct signs of aortic injury are not visible on chest radiographs but indirect signs may be detected: either a mediastinal haematoma or signs of concomitant chest injuries.

Signs of mediastinal haematoma:

• widened mediastinum (more than 8 cm when supine, or more than 6 cm when upright)
• indistinct or abnormal aortic contour
• deviation of trachea, ETT or NGT to the right
• depression of left main bronchus
• loss of the aortopulmonary window
• widened paraspinal stripe
• widened paratracheal stripe
• left apical pleural cap
• large left haemothorax

The detection of mediastinal haematoma on a chest radiograph has a high sensitivity for aortic injury but a low specificity because most mediastinal haematomas are due to other causes such as tearing of mediastinal vessels, sternal injury or thoracic spine injury. Only 12.5% of mediastinal haematomas are due to aortic injury. However, the negative predictive value of a normal chest radiograph of good quality is ~97% but this likely reflects the low incidence of blunt thoracic aortic injury.

CT

Non-contrast CT chest

May show indirect signs of aortic injury:

• mediastinal haematoma
• periaortic fat stranding
• other chest injuries

CTA chest

CTA is the investigation of choice as it fast and has sensitivity and specificity of 100% at showing direct signs of aortic injury as well as indirect signs.

Signs of mediastinal haematoma:

• abnormal soft tissue density around mediastinal structures
• location is important – periaortic haematoma much more suggestive of aortic injury than isolated mediastinal haematoma remote from the aorta.

Signs of aortic injury:

• intraluminal filling defect (intimal flap or clot)
• abnormal aortic contour (mural haematoma)
  • sudden change in and/or decreased diameter
• pseudoaneurysm
• extravasation of contrast

Angiography (DSA)

Rarely performed due to the advent of high-quality CTA.

Signs of aortic injury:

• resistance in advancing a guidewire
• intraluminal filling defect (intimal flap or clot)
• abnormal aortic contour (mural haematoma)
• pseudoaneurysm
• extravasation of contrast

Complications:

• general risks of angiography
• dissection or rupture due to guidewire or catheter

Other imaging modalities

Generally not used in the acute setting but can include:

• MRI
• transoesophageal echocardiography
• intravascular ultrasound

Treatment and prognosis

Aortic injury is a surgical emergency. Treatment is with an aortic stent graft or open repair. Mortality is very high ³:

• >95% if untreated
• ~80% die immediately
• >30% if in hospital and treated

Complications

• death from aortic rupture and haemorrhage
• chronic traumatic pseudoaneurysm
• embolisation from pseudoaneurysm

Differential diagnosis

Other causes of widened mediastinum on a chest radiograph:

• technical factors (supine vs erect)
• vascular ectasia
• mediastinal lipomatosis
• mediastinal masses

Mimics of a mediastinal haematoma on CT include:

• artifact
• thymic tissue
• unopacified vessels
• pericardial recesses
• paramediastinal lung atelectasis

Mimics of aortic injury on CTA or conventional angiography:

• aortic pulsation motion artifact (typically left anterior and right posterior aspects of the ascending aorta)
• streaming of contrast
• aortic atheroma
• ductus diverticulum
• infundibulum of branch vessel