Aortic dissection is a tear in the wall of the aorta (the largest artery of the body). This tear causes blood to flow between the layers of the wall of the aorta and dissects the layers apart. Aortic dissection is a medical emergency and can quickly lead to death, even with optimal treatment. If the dissection tears the aorta completely open (through all three layers) massive and rapid blood loss occurs. Aortic dissections resulting in rupture have a near perfect 100% mortality rate even if intervention is timely.
Overview
As with all other arteries, the aorta is made up of three layers. The layer that is in direct contact with the flow of blood is the tunica intima, commonly called the intima. This layer is made up of mainly endothelial cells. Just deep to this layer is the tunica media, known as the media. This "middle layer" is made up of smooth muscle cells and elastic tissue. The outermost layer (furthest from the flow of blood) is known as the tunica adventitia or the adventitia. This layer is composed of connective tissue.
In an aortic dissection, blood penetrates the intima and enters the media layer. The high pressure rips the tissue of the media apart, allowing more blood to enter. This can propagate along the length of the aorta for a variable distance, dissecting either towards or away from the heart or both. The initial tear is usually within 10 cm of the aortic valve.
The risk in aortic dissection is that the aorta may rupture, leading to massive blood loss resulting in death.
Classification systems
Several different classification systems have been used to describe aortic dissections. The systems commonly in use are either based on the anatomy of the dissection or the duration of onset of symptoms prior to presentation.
DeBakey classification system
The DeBakey system is an anatomical description of the aortic dissection. It categorizes the dissection based on where the original intimal tear is located and the extent of the dissection (localized to either the ascending aorta or descending aorta, or involves both the ascending and descending aorta.
Type I - Originates in ascending aorta, propagates at least to the aortic arch and often beyond it distally.
Type II – Originates in and is confined to the ascending aorta.
Type III – Originates in descending aorta, rarely extends proximally.
Stanford classification system
Divided into 2 groups; A and B depending on whether the ascending aorta is involved.
A = Type I and II DeBakey
B = Type III Debakey
Pathophysiology
The initiating event in an aortic dissection is a tear in the intimal lining of the aorta. Due to the high pressures in the aorta, blood enters the media at the point of the tear. The force of the blood entering the media causes the tear to extend. It may extend proximally (closer to the heart) or distally (away from the heart) or both. The blood will travel through the media, creating a false lumen (the true lumen is the normal conduit of blood in the aorta). Separating the false lumen from the true lumen is a layer of intimal tissue. This tissue is known as the intimal flap.
The vast majority of aortic dissections originate with an intimal tear in either the ascending aorta (65%), the aortic arch (10%), or just distal to the ligamentum arteriosum in the descending thoracic aorta (20%).
As blood flows down the false lumen, it may cause secondary tears in the intima. Through these secondary tears, the blood can re-enter the true lumen.
While it is not always clear why an intimal tear may occur, quite often it involves degeneration of the collagen and elastin that make up the media. This is known as cystic medial necrosis and is most commonly associated with Marfan syndrome and is also associated with Ehlers-Danlos syndrome.
In about 13% of aortic dissections, there is no evidence of an intimal tear. It is believed that in these cases the inciting event is an intramural hematoma (caused by hemorrhage within the media). Since there is no direct connection between the true lumen and the false lumen in these cases, it is difficult to diagnose an aortic dissection by aortography if the etiology is an intramural hematoma. An aortic dissection secondary to an intramural hematoma should be treated the same as one caused by an intimal tear.
Signs and symptoms
About 96% of individuals with aortic dissection present with severe pain that had a sudden onset. It may be described as tearing in nature, or stabbing or sharp in character. 17% of individuals will feel the pain migrate as the dissection extends down the aorta. The location of pain is associated with the location of the dissection. Anterior chest pain is associated with dissections involving the ascending aorta, while intrascapular (back) pain is associated with descending aortic dissections. If the pain is pleuritic in nature, it may suggest acute pericarditis due to hemorrhage into the pericardial sac.
While the pain may be confused with the pain of a myocardial infarction (heart attack), aortic dissection is usually not associated with the other signs that suggest myocardial infarction, including heart failure, and EKG changes. Also, individuals suffering from an aortic dissection usually do not present with diaphoresis (profuse sweating).
Individuals with aortic dissection who do not present with pain have chronic dissection.
Less common symptoms that may be seen in the setting of aortic dissection include congestive heart failure (7%), syncope (9%), cerebrovascular accident (3-6%), ischemic peripheral neuropathy, paraplegia, cardiac arrest, and sudden death. If the individual had a syncopal episode, about half the time it is due to hemorrhage into the pericardium leading to pericardial tamponade.
Neurologic complications of aortic dissection (i.e., cerebrovascular accident (CVA) and paralysis) are due to involvement of one or more arteries supplying portions of the central nervous system.
If the aortic dissection involves the abdominal aorta, compromise of the branches of the abdominal aorta are possible. In abdominal aortic dissections, compromise of one or both renal arteries occurs in 5-8% of cases, while mesenteric ischemia (ischemia of the large intestines) occurs 3-5% of the time.
Blood pressure changes
While many patients with an aortic dissection have a history of hypertension, the blood pressure is quite variable at presentation with acute aortic dissection, and tends to be higher in individuals with a distal dissection. In individuals with a proximal aortic dissection, 36% present with hypertension, while 25% present with hypotension. In those that present with distal aortic dissections, 70% present with hypertension while 4% present with hypotension.
Severe hypotension at presentation is a grave prognostic indicator. It is usually associated with pericardial tamponade, severe aortic insufficiency, or rupture of the aorta. Accurate measurement of the blood pressure is important. Pseudohypotension (falsely low blood pressure measurement) may occur due to involvement of the brachiocephalic artery (supplying the right arm) or the left subclavian artery (supplying the left arm).
Chest X-ray
Widening of the mediastinum on an x-ray of the chest has a high sensitivity (81-90%) in the setting of aortic dissection. However, it has low specificity, as many other conditions can cause a widening of the mediastinum on chest x-ray.
The calcium sign is a finding on chest x-ray that suggests aortic dissection. It is the separation of the intimal calcification from the outer aortic soft tissue border by 1 cm.
Pleural effusions may be seen on chest x-ray. They are more commonly seen in descending aortic dissections. If seen, they are typically in the left hemithorax.
About 12% of individuals presenting with an aortic dissection have a "normal" chest x-ray.
EKG
There are no specific electrocardiographic findings associated with aortic dissection. About 1/3 of the time, the EKG will show signs of left ventricular hypertrophy, which is due to the long-standing hypertension seen in these individuals. Another 1/3 of the time the EKG would be considered "normal". If the EKG suggests cardiac ischemia in the setting of aortic dissection, involvement of the coronary arteries should be suspected.
Spiral CT with contrast
The spiral CT scan with contrast is a fast non-invasive test that will give an accurate three-dimensional view of the aorta. It is performed by taking rapid-cut radiographs of the chest and combining them in the computer to create cross-sectional slices of the chest. In order to delineate the aorta to the accuracy necessary to make the proper diagnosis, an iodinated contrast material is injected into a peripheral vein at a properly timed moment so that it will enter the aorta at the time that the aorta is being imaged.
It has a sensitivity of 96 - 100% and a specificity of 96 to 100%. Disadvantages include the need for iodinated contrast material and the inability to diagnose the site of the intimal tear.
Treatment
The risk of death due to aortic dissection is highest in the first few hours after the dissection begins, and decreases afterwards. Because of this, the therapeutic strategies differ for treatment of an acute dissection compared to a chronic dissection. An acute dissection is one in which the individual presents within the first two weeks. If the individual has managed to survive this window period, his prognosis is improved. About 66% of all dissections present in the acute phase.
In all individuals with aortic dissections, medication should be used to control high blood pressure, if present.
In the case of an acute dissection, once diagnosis has been confirmed, urgent surgical consultation is warranted to repair the tear in the aorta. Surgical management is superior to medical management for an acute ascending aortic dissection. In the case of acute distal aortic dissections (abdominal aortic dissections), surgical and medical management are equivalent if there are no complications.
Individuals who present 2 weeks after the onset of the dissection are said to have chronic aortic dissections. These individuals have been self-selected as survivors of the acute episode, and can be treated with medical therapy as long as they are stable.
Medical management is appropriate in individuals with an uncomplicated distal dissection, a stable dissection isolated to the aortic arch, and stable chronic dissections. Patient selection for medical management is very important. Stable individuals who present with an acute distal dissection (typically treated with medical management) still have an 8 percent 30 day mortality.