Aortic dissection

Author :

Emile R. Mohler, M.D.

Director, Vascular Medicine University of Pennsylvania health system

Penn Web Site: http://www.uphs.upenn.edu/cardio/faculty/mohler.html

2008-07-01

Introduction

The aorta, an artery, is the body’s largest and thickest blood vessel, through which oxygenated blood leaves the heart and courses to the rest of the body. Its wall is composed of three layers. An aortic dissection occurs when the intimal (inner) layer of the aorta tears away from the medial (middle) layer, allowing blood to flow within the wall of the aorta. This causes a dangerous weakening of the aortic wall.

Aortic dissections are described by one of two different classification schemes. The schemes use the site of the intimal aortic tear to classify the syndrome. Tears that involve the ascending aorta, the portion closest to the heart, are called Stanford type A dissections. Tears that do not involve the ascending aorta and are confined to the descending aorta, the segment further from the heart, are called Stanford type B dissections. An older classification scheme was coined by cardiac surgeon Michael DeBakey. The DeBakey type I tear involves both the ascending and descending aorta. The DeBakey type II tear involves only the ascending aorta. The DeBakey type III tear involves only the descending aorta. For clarity, the remainder of this article will use the Stanford system when referring to different anatomic classifications of aortic dissection (Figure 1).

The classification of an aortic dissection is important. Stanford type B dissections have a significantly lower rate of rupture than do Stanford type A dissections, and often, type B dissections can be managed with medications, without the urgent surgical correction that is recommended for type A dissections. Also, complications of dissection are largely based on the segment of aorta involved, as most complications are due to extension of the dissection into adjoining blood vessels or structures.

While the exact incidence of aortic dissection is unknown, population studies estimate it to be roughly 3/100,000 person-years.[1] This translates to about 9,000 cases per year in the Unites States. Acute aortic dissection is a medical emergency and anyone with symptoms of a dissection should call emergency medical services for transport to the nearest hospital emergency room for prompt evaluation.

What are the symptoms of aortic dissection?

Classically, the onset of aortic dissection has been characterized as abrupt, severe, sharp, tearing chest or back pain. The pain may radiate or migrate down the back as the dissection extends down the aorta. This excruciating pain has even been termed “acute aortic agony” in some writings.

However, it is important to recognize this classic description of aortic dissection does not describe all patients with the syndrome. Many patients experience less typical presentations of dissection. These may include less abrupt or waxing/waning pain, less severe pain, abdominal pain, syncope (fainting), or stroke.[2]

Who is at risk for aortic dissection?

Hypertension (elevated blood pressure) is the most common risk factor seen in patients with aortic dissection. Greater than 70% of patients with dissection have a history of hypertension. Men are about two times more likely to experience an aortic dissection than women. While the average age of victims of aortic dissection is around sixty five years, dissections have been reported in a very wide range of patient ages. Patients under forty years of age who sustain aortic dissections often have a predisposing condition apart from hypertension. These include genetic connective tissue disorders such as Marfan’s syndrome and Ehlers-Danlos syndrome that predispose to weakness in the medial layer of the aortic wall and dissection as a result of faulty collagen synthesis. Patients with a bicuspid aortic valve, a congenital abnormality, have aortic root abnormalities and dissect at much higher rates than those with anatomically normal valves. Turner’s syndrome, another genetic illness, predisposes to aortic root problems and dissection. Vasculitides, diseases that cause inflammation of blood vessels such as Takayasu’s arteritis and giant cell arteritis, have been linked to dissection. Cocaine use has been associated with acute aortic dissection. More rarely, aortic dissection can occur as a result of direct traumatic injury to the aorta, such as after a motor vehicle accident. Aortic dissection is also a recognized complication of cardiac catheterization and cardiac surgeries.[3]

How is aortic dissection diagnosed?

Aortic dissection can be difficult to diagnose, particularly because many other potentially life-threatening chest pain syndromes, such as myocardial infarction (heart attack) and pulmonary embolism, are significantly more common, sometimes making the clinical diagnosis confusing. Many other common, non-emergent causes of chest pain such as pericarditis (inflammation of the lining of the heart), gastroesophageal reflux disease (heartburn), and musculoskeletal pain can share similar symptoms with dissection as well. Especially with atypical presentations, it takes a high degree of clinical suspicion to make the diagnosis of aortic dissection.

Initial work-up of a patient suspected to have an aortic dissection based on symptoms starts with a careful physical examination and chest x-ray. On physical examination, patients with an acute aortic dissection may have hypertension (elevated blood pressure), tachycardia (elevated heart rate), and, in a minority of cases, a significant blood pressure difference between the two arms. Cardiac auscultation with a stethoscope may reveal a tell-tale murmur that points toward a malfunctioning aortic valve, the connection between the heart and the aorta. In the majority of cases of aortic dissection, chest x-ray will reveal a widened mediastinum (Figure 2).

The mediastinum (as shown in above x-ray) is an area in the middle of the chest that houses the heart, aorta, pulmonary arteries, esophagus, trachea, thymus, and thoracic lymph nodes. However, there are instances when dissection exists without an obviously widened mediastinum on chest x-ray. Also, a widened mediastinum is often seen on chest x-ray in the absence of dissection. There are no electrocardiographic (ECG) findings that assist in diagnosing aortic dissection. Ultimately, no specific symptom, physical examination sign, or chest x-ray finding reliably makes or excludes the diagnosis of aortic dissection. In patients who have a combination of symptoms and signs that are suspicious for aortic dissection, the definitive diagnosis can be made by one of three methods, computed tomographic angiography (CTA), magnetic resonance angiography (MRA), or transesophageal echocardiography (TEE). All of these methods reliably diagnose aortic dissection with greater than 90% accuracy.[4,5]

Computed tomographic angiography (CTA)

CTA is the preferred method for diagnosis of aortic dissection in most US hospitals due to its speed, availability, and non-invasiveness. A patient is taken for a computed tomographic scan, also called a CT scan, that is an x-ray specifically focused on the aorta. This special CT scan involves injection of iodinated contrast dye through a small intravenous catheter in the arm that allows for visualization of the aorta and arteries. The CTA scan can be performed and interpreted within minutes. This technology is widely available in most US emergency rooms. Limitations can include the availability of a scanner and an experienced radiologist available to read the study. Also, patients with a history of kidney disease are at risk of worsening their kidney function through exposure to the iodinated contrast dye needed to perform the scan (Figure 3).

Magnetic resonance angiography (MRA)

Similar to the above CTA, MRA involves the patient having a non-invasive magnetic resonance scan that is specifically focused on the aorta. MRA is not as widely available as CTA. It also requires injection of dye. Also, MRA takes somewhat longer to perform (usually at least thirty minutes) than does CTA. Like CTA, an experienced radiologist is necessary to interpret the study. Additionally, the scanning machine is a small, enclosed space that can be difficult for patients with claustrophobia. Unstable patients are inaccessible when in the scanner and this can make MRA unsuitable in this group. Patients with metal devices in their body (pacemakers, defibrillators, ear implants, etc.) are also precluded from having magnetic resonance scans. Rarely, a severe skin problem can arise in patients with a history of kidney failure when they are exposed to the dye used in MRA scans.


Transesophageal echocardiography (TEE)

TEE involves the passage of an ultrasound probe through the mouth and into the esophagus in order to visualize the heart and aorta. Limitations include the availability of an experienced cardiologist or anesthesiologist who is able to perform and interpret the study. Additionally, significant sedation is usually necessary to be able to perform the procedure. Patients with a history of severe esophageal disease are precluded from having the procedure done. Rarely, TEE can result in severe complications such as oropharyngeal trauma, bleeding, aspiration, esophageal perforation, and death. Practically, a TEE can be difficult to obtain promptly on nights and weekends.


Historically, aortography was the preferred method for diagnosing an aortic dissection but as other imaging technologies have matured it has fallen out of favor due its invasiveness and the time needed to perform it. Additionally, some studies indicate it is less accurate than the above three modalities. The procedure involves the puncture of the femoral artery (a large blood vessel in the leg) and the advancement of a thin catheter up the femoral artery and aorta. Iodinated contrast dye is directly injected through the catheter into the aorta under fluoroscopic (X-ray) imaging. Limitations include the availability of an experienced cardiologist, interventional radiologist, or vascular surgeon to perform and interpret the study. Also, complications of the procedure include bleeding, damage to the aorta from catheter advancement, damage to the femoral artery, and worsening of kidney function from exposure to iodinated contrast dye.

 What are the complications of aortic dissection?

In addition to aortic rupture resulting in massive hemorrhage, there are a number of complications of aortic dissection that are based upon the location of the dissection.

           

Complication	Mechanism
Pericardial tamponade	Dissections near the heart can rupture into the space between the heart and its lining, the pericardium.  This results in the rapid accumulation of blood around the heart, squeezing it and impairing its function.
Aortic insufficiency	Dissections that involve a segment of aorta very near the heart often cause dysfunction in the valve connecting the heart to the aorta.  This is manifest as backward flow of blood into the heart from the aorta and, when severe, can result in respiratory failure.
Myocardial infarction	Dissections near the heart can extend into coronary arteries, the blood vessels that supply the heart, causing myocardial infarctions (heart attacks).  The right coronary artery is more at risk from aortic dissection due to its location on the aorta than its left-sided counterpart.  When dissection and myocardial infarction coexist, the diagnosis of dissection is particularly difficult to make.  Additionally, the common treatment of myocardial infarction with blood thinners and powerful clot-busting thrombolytic drugs is contraindicated in dissection as it can precipitate aortic rupture, pericardial tamponade, and death.
Paraplegia	Dissections can progress into the arteries that supply the spinal cord, potentially causing paralysis.
Renal ischemia	Dissections that involve the descending aorta can progress into arteries supplying the kidneys, causing a loss of blood flow to one or both kidneys.
Mesenteric ischemia	Dissections that involve the descending aorta can progress into the mesenteric arteries, those that supply the gut, causing a loss of blood flow to the intestines.
Lower extremity ischemia	Dissections that involve the descending aorta can progress into the arteries that supply the legs, causing loss of blood flow to the legs.

   

What is the treatment of aortic dissection?

Definitive treatment of an acute Stanford type A aortic dissection is the urgent surgical replacement of severely damaged segments of aorta with Dacron (a synthetic fiber) grafts. If the dissection has caused aortic valvular dysfunction, the operation often includes an aortic valve replacement as well. These surgeries are high-risk with operative mortality rates in the 10-30% range depending on a patient’s age and co-morbidities.[3] However, attempted medical management of acute type A aortic dissections has been shown in studies to be inferior to urgent surgery, despite the high peri-operative mortality associated with surgery. It is a common axiom that, without surgery, the hourly mortality of an acute type A dissection is about one percent per hour over the first 48 hours.6 More recent registry data has confirmed in-hospital mortality rates of greater than 50% in patients with acute type A dissections who are treated only with medications.[3]

While awaiting surgical evaluation, patients with type A aortic dissections should be placed on powerful intravenous medications to control blood pressure and heart rate in order to reduce stress on the aortic wall. The most effective medications to reduce shear stress on the aortic wall are beta-blockers administered via continuous infusion through an intravenous pump. These medications should be titrated for goal heart rates of 55-65 beats per minute. If a patient remains hypertensive even after an intravenous beta-blocker is maximally titrated, intravenous sodium nitroprusside should be added to target goal systolic blood pressures below 120 mm Hg and mean arterial pressures of 60-70 mm Hg. Intravenous calcium channel blockers can also be used in patients with contraindications to beta-blockers. Patients should be cared for in monitored intensive care units, ideally with continuous blood pressure monitoring via an intra-arterial catheter.[7]

Acute Stanford type B aortic dissections do not have the same mortality risk as type A dissections, in the absence of surgical correction. The decision to go to surgery for type B dissections must be determined on a case-by-case basis. Factors involved in making the decision include the age and health of the patient, the predicted complication rate of surgery, the precise location of the dissection, and the presence or likelihood of complications from the dissection. In many cases, type B dissections can be managed medically. The goals for heart rate and blood pressure are the same as stated above for initial type A dissection management. With an acute type B dissection, patients usually require initial intravenous continuous infusions of beta-blockers and sodium nitroprusside in order to optimally decrease stress on the aortic wall. However, over the course of a few days to a week, these medications can be transitioned to oral hypertensive agents that the patient can continue to take as an outpatient.[8,9]

Chronic aortic dissections, those greater than two weeks old, can usually be managed medically regardless of their location. The determination that a dissection is chronic is reached by the lack of clear presenting symptoms and sometimes by anatomical characteristics seen on imaging. These dissections may be found incidentally when a patient is getting a CT or MRI scan for other indications.

What is the long-term management and prognosis of aortic dissection?

Survivors of acute aortic dissection must be carefully managed with medication after their initial hospitalization. First, blood pressure must be tightly controlled with goals identical to those in the acute setting. The class of medication called beta-blockers is the first-line drug to control blood pressure and reduce stress on the aorta. The classes of drugs called ACE-inhibitors and calcium channel blockers are often added to improve blood pressure control. Also, patients should be monitored carefully with imaging to identify extension of the dissection, complications related to it, and aneurysm formation in the aorta. Generally, patients are imaged with MRA or CTA every three to six months for the first one to two years after an acute dissection. After this, re-imaging usually occurs every one to two years.

The statistics regarding mortality and re-operation rates for patients who survive acute aortic dissection are disparate. Overall, survivors of aortic dissection clearly have increased rates of death compared to those who have never had the syndrome but complications are minimized by aggressive medical management and monitoring.

References

1)      Meszaros, I, Morocz, J, Szlavi, J, et al. Epidemiology and clinicopathology of aortic dissection. Chest 2000; 117:1271.

2)      Nallamothu, BK, Mehta RH, Saint S, et al. Syncope in acute aortic dissection: diagnostic, prognostic, and clinical implications. Am J Med 2002; 113:468.

3)      Hagan, PG, Nienaber, CA, Isselbacher, EM, et al. The International Registry of Acute Aortic Dissection (IRAD): new insights into an old disease. JAMA 2000; 283:897.

4)      Cigarroa, JE, Isselbacher, EM, DeSanctis, RW, Eagle, KA. Diagnostic imaging in the evaluation of suspected aortic dissection. Old standards and new directions. N Engl J Med 1993; 328:35.

5)      Hayter, RG, Rhea, JT, Small, A, et al. Suspected aortic dissection and other aortic disorders: multi-detector row CT in 373 cases in the emergency setting. Radiology 2006; 238:841.

6)      Hirst AE Jr, Johns VJ Jr, Kime SW Jr. Dissecting aneurysm of the aorta: a review of 505 cases. Medicine 1958;37:217-79. 

7)      Tsai, TT, Nienaber, CA, Eagle, KA. Acute aortic syndromes. Circulation 2005; 112:3802.

8)      Umana, JP, Lai, DT, Mitchell, RS, et al. Is medical therapy still the optimal treatment strategy for patients with acute type B aortic dissections? J Thorac Cardiovasc Surg 2002; 124:896.

9)      Estrera, AL, Miller CC, 3rd, Safi, HJ, et al. Outcomes of medical management of acute type B aortic dissection. Circulation 2006; 114:I384.