Aortic dissection
Aortic dissection is a tear 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 an extremely serious medical emergency and can quickly lead to death.
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| Aortic dissection |
| A=Adventitia, M=Media, I=Intima, T=Intimal tear, FL=False lumen, TL=True lumen |
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 10cm of the aortic valve.
The risk in aortic dissection is that the aorta may rupture, leading to massive blood loss resulting in death. Actor John Ritter died in 2003 from this complication of aortic dissection.
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.
Pathophysiology
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| Aortic dissection | |
| An echocardiogram displaying the true lumen and false lumen of an aortic dissection. In the image to the left, the intimal flap can be seen separating the two lumens. In the image to the right, color flow during ventricular systole suggests that the upper lumen is the true lumen. | |
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.
Etiology
Aortic dissection is associated with hypertension (high blood pressure) and many connective tissue disorders. Vasculitis (inflammation of an artery) is rarely associated with aortic dissection. It can also be the result of chest trauma. 72 to 80% of individuals who present with an aortic dissection have a previous history of hypertension.
The highest incidence of aortic dissection is in individuals who are 50 to 70 years old (in their 6th and 7th decade of life). The incidence is twice as high in males as in females (male-to-female ratio is 2:1). Half of dissections in females before age 40 occur during pregnancy (typically in the 3rd trimester or early postpartum period).
A bicuspid aortic valve (a type of congenital heart disease involving the aortic valve) is found in 7-14% of individuals who have an aortic dissection. These individuals are prone to dissection in the ascending aorta. The risk of dissection in individuals with bicuspid aortic valve is not associated with the degree of stenosis of the valve.
Marfan syndrome is noted in 5-9% of individuals who suffer from aortic dissection. In this subset, there is an increased incidence in young individuals. Individuals with Marfan syndrome tend to have aneurysms of the aorta and are more prone to proximal dissections of the aorta.
Chest trauma leading to aortic dissection can be divided into two groups based on etiology: blunt chest trauma (commonly seen in motor vehicular accidents) and iatrogenic. Iatrogenic causes include trauma during cardiac catheterization or due to an aortic balloon pump.
Aortic dissection may be a late sequalae of cardiac surgery. 18% of individuals who present with an acute aortic dissection have a history of open heart surgery. Individuals who have undergone aortic valve replacement for aortic insufficiency are at particularly high risk. This is because aortic insufficiency causes increased blood flow in the ascending aorta. This can cause dilatation and weakening of the walls of the ascending aorta.
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).
Aortic insufficiency
Aortic insufficiency (AI) occurs in 1/2 to 2/3 of ascending aortic dissections, and the murmur of aortic insufficiency is audible in about 32% of proximal dissections. The intensity (loudness) of the murmur is dependant on the blood pressure and may be inaudible in the event of hypotension.
There are multiple etiologies for AI in the setting of ascending aortic dissection. The dissection may dilate the annulus of the aortic valve, so that the leaflets of the valve cannot coapt. Another mechanism is that the dissection may extend into the aortic root and detach the aortic valve leaflets. The third mechanism is that if there was an extensive intimal tear, the intimal flap may prolapse into the LV outflow tract, causing intimal intussusception into the aortic valve preventing proper valve closure.
Myocardial infarction
Myocardial infarction (heart attack) occurs in 1-2% of aortic dissections. The etiology of the infarction is involvement of the coronary arteries (the arteries that supply the heart) in the dissection. The right coronary artery is involved more commonly than the left coronary artery. If the myocardial infarction is treated with thrombolytic therapy, the mortality increases to over 70%, mostly due to hemorrhage into the pericardial sac causing pericardial tamponade.
Because aortic dissection may present to the emergency room physician similar to a myocardial infarction, the physician must be careful to make the proper diagnosis prior to initiating treatment for myocardial infarction, since the treatment regimen for myocardial infarction can be lethal to an individual presenting with aortic dissection.
Pleural effusion
A pleural effusion (fluid collection in the space between the lungs and the chest wall or diaphragm) can be due to either blood from a transient rupture of the aorta or fluid due to an inflammatory reaction around the aorta. If a pleural effusion were to develop due to aortic dissection, it is more commonly in the left hemithorax rather than the right hemithorax.
While taking a good history from the individual may be strongly suggestive of an aortic dissection, the diagnosis cannot always be made by history and physical signs alone. Often the diagnosis is made by visualization of the intimal flap on a diagnositic imaging test. Common tests used to diagnose an aortic dissection include a CAT scan of the chest with iodinated contrast material and a trans-esophageal echocardiogram. Other tests that may be used include an aortogram or magnetic resonance angiogram (MRA) of the aorta. Each of these test have varying pros and cons and they do not have equal sensitivities and specificities in the diagnosis of aortic dissection.
In an individual with chest pain radiating to the back, the differentials to consider include:
Diagnosis
Because of the varying symptoms and signs of aortic dissection depending on the initial intimal tear and the extent of the dissection, the proper diagnosis is sometimes difficult to make.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.
The aortogram was previously considered the gold standard test for the diagnosis of aortic dissection, with a sensitivity of up to 88% and a specificity of about 94%. It is especially poor in the diagnosis of cases where the dissection is due to hemorrhage within the media without any initiating intimal tear.
The advantage of the aortogram in the diagnosis of aortic dissection is that it can delineate the extent of involvement of the aorta and branch vessels and can diagnose aortic insufficiency.
The disadvantages of the aortogram are that it is an invasive procedure and it requires the use of iodinated contrast material.
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.
The disadvantage of the MRI scan in the face of aortic dissection is that it has limited availability and is often located only in the larger hospitals, and the scan is relatively time consuming. In addition, many individuals cannot the MRI scan due to sensations of claustrophobia while in the scanning tube.Biochemical markers
While there are currently no blood tests that can accurately diagnose aortic dissection, research has been performed into the serial measurement of monoclonal antibodies to smooth muscle myosin heavy chains that appears to be both sensitive and specific for aortic dissection.1 The sensitivity of this test is about 90% and the specificity is 97% within the first 12 hours of the beginning of the dissection, and this assay can accurately differentiate myocardial infarction from aortic dissection. This test is not currently available for the diagnosis of aortic dissection in the clinical setting.Aortogram
An aortogram involves placement of a catheter in the aorta and injection of contrast material while taking x-rays of the aorta. The diagnosis of aortic dissection can be made by visualization of the intimal flap and flow of contrast material in both the true lumen and the false lumen.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.MRI
Magnetic resonance imaging (MRI) is currently the gold standard test for the detection and assessment of aortic dissection, with a sensitivity of 98% and a specificity of 98%. An MRI examination of the aorta will produce a three-dimensional reconstruction of the aorta, allowing the physician to determine the location of the intimal tear, the involvement of branch vessels, and locate any secondary tears. It is a non-invasive test, does not require the use of iodinated contrast material, and can detect and quantitate the degree of aortic insufficiency.
| Risk of death in untreated aortic dissection |
| 25% in first 24 hours |
| 50% in first week |
| 75% in first month |
| 90% in first year |
First Aid
It is unlikely that a first-aider will recognize this condition. Call for help and arrange for immediate transport to advanced medical care. In wilderness first aid, immediate evacuation is imperative, by MEDEVAC to an advanced medical facility if available.
Field Care (for EMTs)
Transport immediately to a facility capable of emergency cardiac surgery such as a trauma center, not a community hospital without a surgical service. "Load and go". Provide oxygen therapy and supportive care.
Medication may be used to control high blood pressure if present. Supportive care should be instituted.
In the case of an acute dissection, once diagnosis has been confirmed, urgent surgical consultation is warranted to repair the tear in the aorta.
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 without death, his prognosis is improved. About 66% of all dissections present in the acute phase.Related topics
References
1. Suzuki T, Katoh H, Nagai R. Biochemical diagnosis of aortic dissection: from bench to bedside. Jpn Heart J. 1999 Sep;40(5):527-34. (Medline abstract)External links