Injury of the thoracic aorta refers to any injury which affects the portion of the aorta which lies within the chest cavity. Injuries of the thoracic aorta are usually the result of physical trauma; however, they can also be the result of a pathological process. The main causes of this injury are deceleration (such as a car accident) and crush injuries. There are different grades to injuries to the aorta depending on the extent of injury, and the treatment whether surgical or medical depends on that grade. It is difficult to determine if a patient has a thoracic injury just by their symptoms, but through imaging and a physical exam the extent of injury can be determined. All patients with a thoracic aortic injury need to be treated either surgically with endovascular repair or open surgical repair or with medicine to keep their blood pressure and heart rate in the appropriate range. However, most patients that have a thoracic aortic injury do not live for 24 hours.
Mechanism Injuries to the aorta are usually the result of trauma, such as deceleration and crush injuries. Deceleration injuries almost always occur during high speed impacts, such as those in motor vehicle crashes and falls from a substantial height. Several mechanical processes can occur and are reflected in the injury itself. A more recently proposed mechanism is that the aorta can be compressed between bony structures (such as the manubrium, clavicle, and first rib) and the spine. In the ascending aorta (the portion of the aorta which is almost vertical), one mechanism of injury is torsion (a two-way twisting). There are clinical predictors of an aortic injury. The predictors include if a patient is older than 50, was an unrestrained patient, has hypotension, has a thoracic injury requiring thoracotomy, has a spinal injury, or has a head injury. If four of these criteria are met their likelihood for an aortic injury is 30% The aortic wall is made up of three different components the inner layer (intima), the muscle layer (media), and the outer layer (adventitia). A traumatic injury to the thoracic aorta can cause disruption of any of these parts. Therefore, aortic injury is on a scale from injury to a part of the inner layer to a complete tear of all three layers. There are 4 grades of aortic injury.
Type I: Intimal tear Type II: Intramural hematoma Type III: Pseudoaneurysm Type IV: Rupture In addition to the 4 grades of aortic injury, the risk of rupture can also be categorized. If both the inner layer and the muscle layer of the aortic wall are both involved in the injury then the injury is categorized as significant aortic injury. If just the inner layer and a portion of the muscle layer are involved in the injury then the injury is characterized as minimal aortic injury. Radiographically this would be seen as an intimal flap less than 1 cm in size. Between the mobile ascending aorta and the relatively fixed descending thoracic aorta is the aortic isthmus. When there is a sudden deceleration the mobile ascending aorta pushes forward creating a whiplash effect on the aortic isthmus. However, a different mechanism is involved when the ascending aorta proximal to the isthmus is torn. When there is a rapid deceleration the heart is pushed to the left posterior chest. This causes a sudden increase in intra-aortic pressure and can cause aortic rupture. This is known as the water hammer effect. Based on the location of the injury in the thorax subsequent injuries can take place. If the injury is in the descending thoracic aorta this could lead to a hemothorax. Where as an injury to the ascending aorta could lead to hemoperricardium and subsequent tamponade or could compress the SVC.
Symptoms It is difficult to rely on symptoms to diagnose a thoracic aortic injury. However some symptoms do include severe chest pain, cough, shortness of breath, difficulty swallowing due to compression of the esophagus, back pain, and hoarseness due to involvement of the recurrent laryngeal nerve. There might be external signs such as bruising on the anterior chest wall due to a traumatic injury. Clinical signs are uncommon and nonspecific but can include generalized hypertension due to the injury involving the sympathetic afferent nerves in the aortic isthmus. A murmur can also be audible as turbulent blood flow goes over the tear.
Diagnosis
Classification There are inconsistencies in the terminology of aortic injury. There are several terms which are interchangeably used to describe injury to the aorta such as tear, laceration, transection, and rupture. Laceration is used as a term for the consequence of a tear, whereas a transection is a section across an axis or cross section. For all intents and purposes, the latter is used when a tear occurs across all or nearly all of the circumference of the aorta. Rupture is defined as a forcible disruption of tissue. Some disagree with the usage of rupture as they believe it implies that a tear is incompatible with life; however, the term accurately gauges the severity of tears in the aorta. A rupture can be either complete or partial, and can be classified further by the position of the tear.
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