Minimal aortic injury

Minimal aortic injury (MAI) is a mild form of blunt traumatic aortic injury which are limited to the aortic intima and are recognized more frequently due to the use of high-resolution vascular imaging in trauma.

Epidemiology

Minimal aortic injuries account for 10-28% of all blunt traumatic aortic injuries ^1,6,7. The proportion of this type of aortic injury among overall blunt traumatic aortic injuries is higher in more recent studies due to the widespread use of multidetector computed tomography (MDCT) in trauma. The majority of cases are due to restrained front-on motor vehicle collisions, while unrestrained passengers in front-on or side motor vehicle collisions are associated with high grade blunt traumatic injuries of the aorta.

Pathology

Minimal aortic injuries are on the low end of the spectrum of blunt traumatic aortic injury.  Various definitions of minimal aortic injury have been used in literature. They were originally defined as small (<10 mm) intimal flaps with no or minimal periaortic hematoma ⁸. Some authors have further defined the injuries as post-traumatic abnormalities of the inner (internal) aortic wall projecting into the lumen with no evidence of an abnormality to the outer (external) contour of the aorta. Currently (late 2020), the definition is "sub-centimeter intimo-medial abnormality with no external contour deformity' and now includes small intramural hematomas ⁸.

Like other blunt traumatic aortic injuries, the proposed mechanisms include:

• rapid deceleration
• shearing forces
• osseous pinch
• sudden increase in hydrostatic forces or water-hammer phenomenon (thump effect) ⁸

Injuries limited to the intima usually do not compromise the resistance of the aortic wall and may heal spontaneously by endothelialization.

Most minimal aortic injuries occur in the upper descending thoracic aorta at the aortic isthmus. Other sites affected are located where vessel branches arise or non vascular structures are tethered, including:

• aortic arch branches
• intercostal arteries
• diaphragmatic crura

Radiographic features

CT

Multidetected CT angiography is the standard investigation and with improvements in imaging technology which allow a thinner CT slice thickness,  MAI are being diagnosed more frequently. As with other vascular pathologies, the use of thicken MIPs aids in diagnosis.

Minimal aortic injuries findings can be categorized as direct and indirect.

Direct signs

• intimal irregularity or flap under 10 mm in size with no external wall contour abnormality ⁸
  • most common imaging appearance (~80%) ¹
  • rounded, triangular or focal thin membrane-like linear filling defects
• small intramural hematoma (~10%) ¹ without outer contour abnormality of the aortic wall ⁸
  • if a non contrast series has been performed, a IMH may demonstrate slight crescentic hyperdensity
• small pseudoaneurysm, usually in the anteromedial aorta at the isthmus

Indirect signs

• more commonly seen with significant aortic injury rather than MAI
• periaortic hematoma - despite being included in the original definition as excluding MAI, a study has found a large proportion of MAI can have a coexisting periaortic hematoma ⁸
• mediastinal hematoma
• intraluminal aortic thrombus
• associated injuries ⁸
  • sternal fractures
  • upper rib fractures
  • scapular injuries
  • upper thoracic vertebral fractures

Treatment and prognosis

Since intimal injuries may resolve spontaneously, minimal aortic injuries are amenable to non-operative or medical management and can be treated conservatively with anti-hypertensives, anticoagulants or both depending on the coexisting injuries. Limited data on the natural history of this type of aortic injury necessitates close imaging surveillance for detection of any potential complication or adverse evolution of the minimal aortic injury. There is currently (late 2020) no establish follow up guidelines. Suggested follow up is for CTA at 48-72 hours then repeat CTA at one month ⁸.

Gunn et al ¹ reported distal emboli to the kidneys or spleen in 17% of patients in a cohort of 23 MAI cases.

Pseudoaneurysms are treated (surgically or with endovascular repair) due to the high risk of progress.

Differential diagnosis

Normal anatomic variants and conditions that can mimic minimal aortic injuries on CT include ⁸:

• breathing, arterial pulsation and other motion artefacts
• streaming/streaking of contrast
• aortic dissection
• ductus diverticulum and ductus remnant - calcification is often present
• aortic spindle
• infundibulum of a branch vessel
• atheroma