INTRODUCTION	Section 2 of 10
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Background: Traumatic aortic disruption is a time-sensitive injury requiring rapid and accurate diagnosis to prevent mortality. Although the clinical, or mechanism, score is of primary importance in the prompt diagnosis of patients with traumatic aortic injury (TAI), the radiologic findings play a vital supportive role.

TAI syndrome has a variable length relatively clinically silent contained rupture (pseudoaneurysm) followed by a rapid transition to free, uncontained pseudoaneurysm rupture, exsanguination, and death. The clinical challenges are to rapidly stabilize and evacuate the patient to a level I trauma center for evaluation, diagnosis, and definitive treatment before free rupture occurs.

Pathophysiology: Blunt aortic injury is part of the spectrum of blunt or deceleration polytrauma. The major regions of concern in the thorax are the heart, major vessels, abdominal organs, neck, spine, and aerodigestive tract. Penetrating injury (eg, stabbing or gunshot injury) is a different clinical entity from blunt or deceleration trauma.

Blunt or deceleration injury to the aorta is mostly confined to the thoracic aorta, except in the seat-belt injury, which involves the abdominal aorta. The radiologic evaluation must be as accurate as possible. Thoracotomy entails a significant risk, and a missed diagnosis is life threatening.

For this discussion, aortic rupture or disruption includes the aorta, the proximal portion of the great vessels, and the sinuses of Valsalva. The most common location for TAI is at the isthmus, just beyond the origins of the great vessels. In decreasing order of frequency, other locations are the descending thoracic aorta, the ascending aorta, the aortic arch, and the abdominal aorta. Ruptures at the aortic hiatus (diaphragm level) are less common. Competing theories of the mechanism of TAI have been proposed. Suffice it to say that the deceleration or crush forces interact with the asymmetric aortic fixation, causing translational and rotational forces that result in injury.

Pathologically, an aortic tear is usually transverse and involves the layers of the aorta to varying degrees. A complete tear through the intima, media, and adventitia usually leads to rapid exsanguination and death. In aortic rupture survivors, the pseudoaneurysm is contained by the adventitia and occasionally mediastinal structures.

Frequency:

• In the US: Abdominal aortic TAI is extremely rare, with only 46 cases reported as of 1990.
  
  

  The distribution of posttraumatic aortic pseudoaneurysms differs in the classic surgical literature and the pathology (autopsy) literature. These differences are likely due to differences in survival rates with injuries in various locations. Locations of TAI, as adapted from the report by Parmley et al in 1958, are as follows: ascending aorta, 17 (9.9%) of 171 cases; arch, 16 (9.4%); isthmus, 95 (55.6%); thoracic aorta, 27 (15.8%); abdominal aorta, 11 (6.4%); and multiple locations, 5 (2.9%). Locations of TAI, as adapted from Kadir's report in 1986, are as follows: ascending aorta, 1%; innominate artery, 4.5%; left common carotid artery, 0.8%; left subclavian artery, 2.7%; aortic isthmus, 88.2%; descending aorta, 1.8%; and aortic hiatus (diaphragm level), 0.4%.
  

  Investigators from the Medical College of Wisconsin (unpublished data, 1990) performed a retrospective review of 181 consecutive patients with blunt aortic trauma (BAT) in January 1986 through August 1990. The patients underwent angiography, for which the yield for TAI was 17%; this finding corresponded to the historic rate of 10-20%. Locations of TAI were as follows: isthmus, 23 (74%) of 31 (circumferential artery, 16 [52%] and anteromedial artery, 7 [23%]); innominate artery, 4 (13%); subclavian artery, 1 (3%); undersurface of the arch, 1 (3%); and other, 10 (32%).
  

  Traditional clinical series report TAI to occur at the aortic isthmus in 95% of patients surviving long enough to undergo surgery versus 45-66% found in autopsy studies. The speed and efficiency of the trauma response system affects the overall survival rate and distribution of the rupture.

Mortality/Morbidity: Traumatic aortic rupture is an extremely unstable condition. The patient's survival depends on rapid diagnosis and treatment. Regional trauma centers and rapid patient transport have dramatically improved survival rates.

• In 1958, Parmley et al reported that 85% of patients with aortic injury died prior to reaching hospital and that their survival was dependent on the time from injury to surgery. Although some traumatic aortic pseudoaneurysms stabilize, mortality from untreated injuries continues. Of the subset of patients surviving at least 1 hour, death rates from TAI varied with the time until surgery, as follows (Parmley, 1958): 15% with an interval of 6 hours; 27%, 24 hours; 61%, 1 week; and 88%, 4 months.

• In 1976, Kirsh et al reported a survival rate of approximately 70% with prompt and aggressive treatment.

• In 1997, Fabian et al reported a 31% mortality rate in 274 patients (207 were operated on) who presented to the hospital with TAI. Approximately two thirds of deaths were due to the aortic rupture, and the rest were due to associated injuries.

Race: No race-related difference in anatomy or physiology causes a predisposition to TAI. Differences in the incidence of TAI are strictly due to the likelihood of severe blunt injury.

Sex: No sex-related difference in anatomy or physiology causes a predisposition to TAI. Differences in the incidence of TAI are strictly due to the likelihood of severe blunt injury.

Age: TAI is essentially an entity of the adult. Pediatric cases are extremely rare.

Anatomy: The most common diagnostic challenge is differentiating a traumatic aortic isthmus pseudoaneurysm from a normal ductus diverticulum (type III ductus). Ductus diverticulum is commonly thought of as the remnant of the ductus arteriosum, although Grollman theorized that the ductus is a remnant of the right dorsal aortic root. The aortic isthmus is located anteromedially, just distal to the origin of the left subclavian artery. This location is most commonly ruptured. It is also the location of the ductus diverticulum; therefore, diagnostic confusion can result. Smooth anteromedial outpouching of the aortic isthmus represents the normal ductus diverticulum and should not be confused with a pseudoaneurysm.

The Goodman classification (from 1982) describes the contour of the isthmus as follows: Type I involves a concave isthmus contour; type II, mild straightening or convexity without a discreet bulge; and type III, a discrete focal bulge of the aortic isthmus called the ductus diverticulum.

Another normal variant is a fusiform enlargement or prominence of the proximal descending aorta. It is considered normal unless other findings of TAI are present.

Clinical Details: BAT is usually the result of a crush or deceleration injury, which often occurs during a motor vehicle accident or a fall from substantial height. Clinical evaluation and triage are vital to the patient's survival. The severity, likelihood, and clinical impact of potential injuries must be considered in the context of the patient's polytrauma as a whole.

The clinical signs and symptoms associated with TAI are nonspecific and for the most part indirect. The exception is the rapid onset of severe hypovolemic shock associated with a free (noncontained) aortic rupture. The signs and symptoms of contained TAI are more related to the associated injuries of polytrauma, such as fractures, spinal cord, and head injury. Some centers use criteria such as mechanism or visible steering wheel injury in their polytrauma protocols. Fortunately, the evaluation for TAI is fairly easily incorporated into trauma center workflow.

The integrity of the airway, cardiovascular system, and spinal cord must be addressed before a potential blunt aortic injury is considered. Splenic or aortic disruption must be considered in patients with blunt trauma who present in hypovolemic shock. The choice and timing of plain radiography, CT, and/or angiography must be considered in context of the patient's condition as a whole.

Preferred Examination: CT and angiography are the prime imaging modalities in planning treatment for blunt trauma.

As yet, no ideal diagnostic algorithm is available for TAI. The clinical history (mechanism score) is often used for the prompt diagnosis of TAI. Plain radiographic evaluation is usually the first to be performed. The optimal upright posteroanterior (PA) chest evaluation is often deferred for a portable examination with the patient still on the backboard.

Either CT or angiography may be performed, depending on institutional preferences, the patient's condition, and the likelihood of other injuries. For example, a patient with hemorrhage from a crushed pelvis would undergo angiography first, whereas a hemodynamically stable patient with a suspected renal or splenic injury would undergo CT first. Diagnostic imaging should be deferred in patients presenting in hypovolemic shock or cardiac arrest.