Case 88

Type B Aortic Dissection

Question

What do these scans show?

Answer

Answer

Aortic dissection, type B

Echocardiography of aortic dissection

Aortic Dissection

Background: Much has been written on the subject of aortic dissections, from the first well-documented case of aortic dissection, when King George II of England died while straining on the commode, to the first successful operative repairs by DeBakey in 1955, to modern techniques of diagnosing and repairing thoracic aortic dissections.

Aortic dissection is the most common catastrophe of the aorta, 2-3 times more common than rupture of the abdominal aorta. When left untreated, about 33% of patients die within the first 24 hours, and 50% die within 48 hours. The 2-week mortality rate approaches 75% in patients with undiagnosed ascending aortic dissection.

Dissections of the thoracic aorta have been classified anatomically by 2 different methods. The more commonly used system is the Stanford classification, which is based on involvement of the ascending aorta and simplifies the DeBakey classification.

The Stanford classification divides dissections into 2 types, type A and type B.

The DeBakey classification divides the dissections into 3 types.

Thoracic aortic dissections should be distinguished from aneurysms (localized abnormal dilation of the aorta) and transections, which are caused most commonly by high-energy trauma.

Pathophysiology: The essential feature of aortic dissection is a tear in the intimal layer, followed by formation and propagation of a subintimal hematoma. The dissecting hematoma commonly occupies about half and occasionally the entire circumference of the aorta. This produces a false lumen or double-barreled aorta, which can reduce blood flow to the major arteries arising from the aorta. If the dissection involves the pericardial space, cardiac tamponade may result.

Cystic medial necrosis

The normal aorta contains collagen, elastin, and smooth muscle cells that contribute the intima, media, and adventitia to the layers of the aorta. With aging, degenerative changes lead to breakdown of the collagen, elastin, and smooth muscle and an increase in basophilic ground substance. This condition is termed cystic medial necrosis. Atherosclerosis that causes occlusion of the vasa vasorum also produces this disorder. Cystic medial necrosis is the hallmark histologic change associated with dissection in those with Marfan syndrome.

Cystic medial necrosis was first described by Erdheim in 1929. Sources disagree over the accuracy of this term in the elderly, since the true histopathologic changes are neither cystic nor necrotic. Researchers have used the term cystic medial degeneration.

Early on, cystic medial necrosis described an accumulation of basophilic ground substance in the media with the formation of cystlike pools. The media in these focal areas may show loss of cells (ie, necrosis). This term still is used commonly to describe the histopathologic changes that occur.

Dissection sites

The most common site of dissection is the first few centimeters of the ascending aorta, with 90% occurring within 10 centimeters of the aortic valve. The second most common site is just distal to the left subclavian artery. Between 5% and 10% of dissections do not have an obvious intimal tear. These often are attributed to rupture of the aortic vasa vasorum as first described by Krukenberg in 1920.

Pathophysiology: The pathogenesis of the disease is not well understood but the following are associated risk factors:

• Chronic hypertension
• Connective tissue disorders (Marfan syndrome, Ehlers-Danlos syndrome)
• Bicuspid aortic valve
• Coarctation of the aorta
• Turner syndrome
• Takayasu arteritis
• Giant cell arteritis
• Pregnancy
• Trauma
• Crack cocaine use
• Cardiac catheterization
• Metabolic disorders

Frequency:

• In the US: The true incidence of dissection is difficult to estimate. Most estimates are based on autopsy studies. One population-based study estimated the incidence at roughly 6 new aneurysms per 100,000 person years. Evidence of dissection is found in 1-3% of all autopsies.

Mortality/Morbidity:

• From 1-2% die per hour for the first 24-48 hours.

• Aortopathy may be present in heritable diseases such as Marfan syndrome, Ehlers-Danlos syndrome, annuloaortic ectasia, familial aortic dissections, adult polycystic kidney disease, Turner syndrome, Noonan syndrome, osteogenesis imperfecta, bicuspid aortic valve, coarctation of the aorta, and connective-tissue disorders. It is also seen in heritable metabolic disorders such as homocystinuria and familial hypercholesterolemia.

• Incidence is increased in pregnancy and syphilis. Thoracic aortic dissection also is associated with crack cocaine use and iatrogenic causes, such as cardiac catheterization.

Race: Aortic dissection is more common in blacks than in whites and less common in Asians than in whites.

Sex: Male-to-female ratio is 3:1.

Age: Approximately 75% of dissections occur in those aged 40-70 years, with a peak in the range of 50-65 years.

Clinical Details:

• Ripping or tearing pain in the intrascapular area
• Abrupt onset of the pain
• Acute, severe chest pain (Anterior chest pain can mimic acute myocardial infarction)
• Pain extending to the neck or jaw
• Altered mental status
• Cerebrovascular accident symptoms
• Syncope
• Limb paresthesias
• Horner syndrome
• Dyspnea
• Dysphagia
• Flank pain if the renal arteries are involved
• Hypertension
• Hypotension if associated with cardiac tamponade, hypovolemia, excessive vagal tone

Dissections of the aorta can be classified into types.

• DeBakey classification

   

  • Type I: The entire aorta is involved.

   

  • Type II: Only the ascending aorta is involved.

     

  • Type III: Only the descending aorta is involved. Type IIIA involves the descending aorta as far as the diaphragm. Type IIIB involves the descending aorta below the diaphragm.

   

• Stanford classification

   

  • Type A: The ascending aorta is involved.

   

  • Type B: The descending aorta is involved.

Note that isolated dissections that begin in the aortic arch but do not involve the ascending aorta do not fit neatly into these classifications

AIH is classified in a similar fashion

Preferred Examination: Preferred examinations include contrast-enhanced spiral CT transesophageal echocardiography (TEE) in the emergency setting and MRI for hemodynamically stable patients. The ability of TEE to evaluate the status of the aortic valve and the ostia of the coronary arteries is an advantage over CT and MRI. CT and MR angiography has largely replaced conventional diagnostic angiography in the assessment of aortic dissection.

Several factors determine the best modality for the initial evaluation as well as postoperative follow-up. These factors include the following: stability of the patient's condition, patient's renal function, suspected postoperative complication, and availability of each imaging modality.

Limitations of Techniques: Three noninvasive studies are associated with high specificity and sensitivity. CT and MRI are associated with high sensitivity and specificity of 94-100% and 95-100%, respectively. TEE is less sensitive and specific than spiral CT or MR, and TEE is operator-dependent. In addition, due to tracheal interposition, there is a 2 cm "blind spot" for TEE just proximal to the innominate arteries. Also, approximately 1% of patients have a contraindication to TEE (eg, esophageal varices).

Differential diagnosis

Aortic Regurgitation 
Aortic Stenosis 
Back Pain, Mechanical 
Gastroenteritis 
Hernias 
Hypertensive Emergencies 
Myocardial Infarction 
Myocarditis 
Myopathies 
Pancreatitis 
Pericarditis and Cardiac Tamponade 
Peripheral Vascular Injuries 
Pleural Effusion 
Pulmonary Embolism 
Shock, Cardiogenic 
Shock, Hemorrhagic 
Shock, Hypovolemic 
Thoracic Outlet Syndrome 



Other Problems to be Considered: 

Musculoskeletal chest pain

Lab Studies:

Blood studies

• Usually, the diagnosis is made before the blood work is returned; however, leukocytosis may be present.

• BUN and creatinine are elevated if the dissection involves the renal arteries.

• Creatine phosphokinase (CPK) is elevated if the dissection has caused myocardial ischemia.

• Decreases in the hemoglobin and hematocrit are ominous findings suggesting the dissection either is leaking or has ruptured.

• Hematuria, oliguria, and even anuria (<50 mL/d) may occur if the dissection involves the renal arteries.
•  

Plain X-Ray

Findings: Findings of aortic dissection on plain film images include the following

• Mediastinal widening (most common plain radiographic finding in aortic dissection, noted in 80% of patients)
• Double aortic knob sign (in 40% of patients)
• Diffuse enlargement of the aorta with poor definition or irregularity of the aortic contour
• Inward displacement of aortic wall calcification of more than 10 mm
• Tracheal displacement to the right
• Pleural effusion (more common on the left side, suggests leakage)
• Pericardial effusion
• Cardiac enlargement
• Displacement of a nasogastric tube
• Left apical opacity

All findings on plain images are nonspecific but may help in determining the need for further workup.

False Positives/Negatives: Mediastinal fat can commonly cause a widened mediastinum and a false-positive diagnosis of aortic dissection

CAT Scan

Findings: Since its introduction in the 1970s, CT has become a widely used technology, particularly in the ED. With the advent of spiral CT, the studies can be performed in less time than before with less patient discomfort, greater accuracy, and lower iodine load. Spiral CT permits patient translation and data acquisition simultaneously. A major advantage of this technology is in the evaluation of thoracic trauma, which enables the rapid diagnosis of thoracic injury. Multislice or multidetector CT can be used for faster imaging or to acquire thinner slices that can be reconstructed in multiple planes

A typical helical scanning protocol for aortic dissection includes the following parameters: 5-mm collimation, 1.5 pitch, and 7.5-mm imaging spacing. Multidetector CT can be performed with 1-2.5 mm collimation. Initial nonenhanced CT is used for the diagnosis of acute hemorrhage and aortic rupture. This is followed by helical CT performed approximately 25-30 seconds after the injection of contrast material. Nonionic contrast material (120-135 mL) is power injected via a peripheral intravenous site at a rate of 3-4 mL/s. Because cardiac output is quite variable in these sick patients, use a test injection of contrast to determine circulation time or an automated bolus detection scheme. One advantage of the test injection method is that one may visually differentiate the true and false lumen based on contrast arrival time.

Usually, scanning is performed from the thoracic inlet to the common femoral arteries. When a dissection is identified, repeat scanning can be performed to obtain delayed images of the false lumen and aortic branches. Multiplanar reformation images are obtained in sagittal, coronal, oblique sagittal, and curved projections generated with an independent workstation. The use of volume rendering can be helpful for planning surgery

Typical CT findings in acute dissection or intramural hematoma include the following:

• Aortic intramural hematoma: Crescentic high-attenuating clot within the media, with internally displaced calcification
• Intimal flap separating the two aortic channels

Hemorrhagic pleural and pericardial effusions and mediastinal hemorrhage may be seen.

CT is also helpful in postoperative follow-up. It can accurately depict associated complications, including the following:

• Thrombosis
• Hemorrhage
• Infection
• Pseudoaneurysms
• Aortoenteric fistula
• Ureteral obstruction

Degree of Confidence: The sensitivity of CT is 87-94%, and the specificity is 92-100%.

False Positives/Negatives: Inadequate contrast opacification can lead to false-negative findings. Aortic intramural hematoma can be misinterpreted as an aneurysm with thrombus or arteritis.

Spiral CT artifacts include perivenous streaks and motion artifacts. The perivenous streaks are caused by beam hardening and motion due to transmitted pulsation in a vein that carries undiluted contrast medium into the heart. Some authors recommend injecting the contrast agent at a rate of 2 mL/s via a peripheral intravenous site in the right arm. Aortic motion artifact is produced by the aortic wall motion from the end of diastole to the end of systole. Typically, this artifact is seen in the left anterior and right posterior margins of the aortic circumference.

In some patients, especially those with cystic medial necrosis, the intimal flap may be subtle.

MRI

Findings: MRI is an accurate tool for use in diagnosis but it may not be readily available in the acute setting. In addition, unstable patients with Swan-Ganz catheters should not be studied with MR.

MRI findings of aortic dissection include the following:

• MRI shows an intimal flap of medium–signal intensity surrounded by a signal void of fast-flowing blood on "black blood" echocardiogram (ECG)-gated spin-echo or double inversion recovery single shot fast spin-echo
• With cine gradient echo imaging, the intimal flap is a dark line against the high -signal intensity of the flowing blood and may change configuration during the cardiac cycle. Careful examination of the aortic flap during the cardiac cycle on cine MR imaging is important to detect the presence of "true lumen collapse," which may be associated with end-organ ischemia. When the intima is stripped 360° from the media and is essentially "free floating," this may result in catastrophic intimo-intimo intussusception.

Newer pulse sequences such as True Fisp or Fiesta offer very fast cine imaging.

Basic MRI sequences to evaluate for aortic dissection include spin-echo T1-weighted or breath-hold double inversion recovery sequences, cardiac-gated gradient-echo sequences, and three-dimensional (3D) thin-section MR angiography with a bolus injection of a single or double dose of gadolinium-based contrast agent

MRI findings of AIH include a crescent of blood surrounding but not compressing the aorta. The signal intensity of the crescent varies with age on T1-weighted imaging: it is isointense to muscle in the acute setting and markedly hyperintense after 3-7 days.

MRI is also helpful in postoperative follow-up. It can accurately depict associated complications, including the following:

• Thrombosis
• Hemorrhage
• Infection
• Pseudoaneurysms
• Aortoenteric fistula
• Ureteral obstruction

Degree of Confidence: The sensitivity and specificity are both more than 90%.

False Positives/Negatives: Potential drawbacks of MRI include reported artifacts on cardiac-triggered thoracic spin-echo phase images. These can appear as an artifactual borderlike feature across the aorta (due to helical flow in the aorta) that can be interpreted as a dissection. Other potential causes of misinterpretation include an atypical configuration of the intimal flap seen in short dissections and multiple false channels where the flaps are complex. Aortic anomalies also can cause confusion. False positives seen in gadolinium-enhanced MRA include a central line or "maki" artifact. This occurs when the acquisition is performed too early as intraortic gadolinium concentration is rising. This artifact can be readily differentiated from an aortic dissection as it does not take a spiral course like a true intimal flap.

Ultrasound

Findings: ECG is helpful in the diagnosis of aortic dissections. It is particularly helpful with ascending thoracic dissections, cardiac tamponade, and aortic regurgitation where TEE has a greater sensitivity and specificity than CT or MR in detecting coronary arterial occlusion, aortic insufficiency, and cardiac tamponade. The sensitivity is 97-99%. The specificity is in the range of 97-100%.

TEE was the favored study for the evaluation of aortic dissection according to a study by Mastrogiovanni et al from Salerno, Italy. In their report of 54 patients, TEE findings confirmed the diagnostic dissection in all patients but one. The site of the intimal tear; the extension of the dissection, pericardial effusion, aortic incompetence; and left ventricular function was noted. Because of the high level of correspondence between the diagnosis made at TEE and the surgical anatomic findings, the authors favor the use of TEE, often as the sole diagnostic modality.

False Positives/Negatives: A tortuous aorta may result in a false-positive diagnosis of dissection. In massive dilated ascending aortas (usually due to cystic medial necrosis) it may be difficult to identify a small intimal flap.

Angiography

Findings: Aortography was the reference standard for the preoperative evaluation and diagnosis of aortic dissection. With the advent of TEE, CT, and MRI, its role has become important only if nonsurgical interventional procedures are indicated. It is quite controversial whether coronary angiography should be performed prior to sternotomy in a stable patient with aortic dissection, as concomitant coronary bypass grafting can be performed if diseased vessels are present.

Diagnostic criteria include visualization of a lucent flap and delayed filling and washout of the false lumen. The expanding false lumen may compress the true lumen and cause it to become narrowed. A dual lumen aorta is noted when both the true and false lumens are opacified. AIH is almost impossible to diagnose with aortography as no compression of the lumen exists

During aortography, overinjection of the false lumen should be avoided if it is entered during the procedure. The operator should be suspicious if he or she has difficulty advancing the guidewire into the aortic valve. Abdominal and pelvic aortography should be included in the diagnostic study to assess the level of the reentry site. Obstruction of the aortic branches may be noted (most commonly in the left renal artery in approximately 25-30% of patients). Visceral and extremity ischemia can occur when the superior or inferior mesenteric arteries and the iliac arteries are compromised.

Degree of Confidence: Aortographic findings are less sensitive than those of newer noninvasive techniques, especially for aortic intramural hematoma.

False Positives/Negatives: Pitfalls of angiography include a lack of visualization of the false lumen because of thrombosis or inadequate opacification with contrast material. Streak artifacts secondary to aortic or cardiac motion or opacification of the sinus of Valsalva can be confused with thrombus. Pitfalls also include missing the diagnosis of an intramural hematoma (frequently associated with progression to frank dissection) and misdiagnosis when the false lumen is thrombosed.

Treatment

Prehospital Care:

• Assure adequate breathing, maintain oxygenation, treat shock, and obtain useful historic information.

• Establishing the diagnosis in the field is usually difficult or impossible, but certain salient features of aortic dissection may be observed. It is life threatening if not quickly recognized and treated.

• Radio communication with the receiving hospital permits the medical control physician to direct care and select a capable destination hospital while permitting the ED to mobilize appropriate resources.

• In the rare event that the diagnosis can be made based on prehospital information, the physician directing prehospital care should request transport to a facility capable of operative treatment of an aortic dissection.

Emergency Department Care:

• The mortality rate of patients with aortic dissection is 1-2% per hour for the first 24-48 hours. Initial therapy should begin when the diagnosis is suspected. This includes 2 large-bore intravenous lines (IVs), oxygen, respiratory monitoring, and monitoring of cardiac rhythm, blood pressure, and urine output.

   

• Clinically, the physician should reassess the patient frequently for hemodynamic compromise, mental status changes, neurologic or peripheral vascular changes, and development or progression of carotid, brachial, and femoral bruits.

• Urgent surgical intervention is required in type A dissections.

• The area of the aorta with the intimal tear usually is resected and replaced with a Dacron graft.

   

• The operative mortality rate is usually less than 10% and serious complications are rare with ascending aortic dissections.

   

• The development of more impermeable grafts, such as woven Dacron, collagen-impregnated Hemashield (Meadox Medicals, Oakland, NJ), aortic grafts, and gel-coated Carbo-Seal Ascending Aortic Prothesis (Sulzer Carbomedics, Austin, TX) has greatly enhanced the surgical repair of thoracic aortic dissections.

   

• With the introduction of profound hypothermic circulatory arrest and retrograde cerebral perfusion, the morbidity and mortality rates associated with this highly invasive surgery have decreased.

   

• Dissections involving the arch are more complicated that those involving only the ascending aorta, as the innominate, carotid, and subclavian vessels branch from the arch. Deep hypothermic arrest usually is required. If the arrest time is less than 45 minutes, the incidence of central nervous system complications is less than 10%.

• Retrograde cerebral perfusion may increase the protection of the central nervous system during the arrest period.

• Mortality rate of aortic arch dissections is about 10-15%, with significant neurologic complications occurring in another 10%.

   

• The mortality rate is influenced by the patient's clinical condition.

• The definitive treatment for type B dissections is less clear.

• Uncomplicated distal dissections may be treated medically to control blood pressure.

   

• Distal dissections treated medically have the same or lower mortality rate as those treated surgically.

   

• Surgery is reserved for distal dissections that are leaking, ruptured, or compromising blood flow to a vital organ.

   

• Acute distal dissections in patients with Marfan syndrome usually are treated surgically.

   

• Inability to control hypertension with medication is also an indication for surgery in those with a distal thoracic aortic dissection.

   

• Patients with a distal dissection are usually hypertensive, emphysematous, or older.

   

• Long-term medical therapy involves a beta-adrenergic blocker combined with other antihypertensive medications. Avoid antihypertensives (eg, hydralazine, minoxidil) that produce a hyperdynamic response that would increase dP/dT (ie, alter the duration of P or T waves).

   

• Survivors of surgical therapy also should receive beta-adrenergic blockers.

• Definitive treatment involves segmental resection of the dissection with interposition of a synthetic graft.

• When thoracic dissections are associated with aortic valvular disease, replace the defective valve.

   

• With combined reconstruction-valve replacement, the operative mortality rate is approximately 5% with a late mortality rate of less than 10%.

   

• Operative repair of the transverse aortic arch is technically difficult, with an operative mortality rate of 10% despite induction of hypothermic cardiocirculatory arrest.

   

• Repair of the descending aorta is associated with a higher incidence of paraplegia than repair of other types of dissections because of interruption of segmental blood supply to the spinal cord.

• Operative mortality rate is approximately 5%.

Consultations:

• Once a thoracic dissection is suspected, consult a thoracic surgeon.

• Since many patients with this disorder have concomitant medical illness, consult the patient's primary care physician to expedite preoperative preparation.

• Early consultation is encouraged when ordering further imaging studies if the patient requires rapid operative intervention.

• Consult a radiologist prior to obtaining aortography.

Follow up

Further Inpatient Care:

• Patients with symptomatic dissection should undergo immediate repair, especially if it is leaking or expanding

• Symptomatic patients require admission to a center experienced in cardiopulmonary bypass and operative care.

• Completely asymptomatic patients may have their repair performed electively but may require admission to expedite their evaluation or for preoperative stabilization of their condition.

• Patients with chest pain should undergo serial ECGs and CPK determinations if AMI is indicated.

Further Outpatient Care:

• Follow-up examinations with radiologic studies are recommended at 3-month intervals for the first year and every 6 months for the next 2 years.

• After this, follow up annually.

Transfer:

• Symptomatic patients require care at a facility equipped to perform cardiopulmonary bypass with aortic and/or valvular repair.

• Contact the receiving physician as soon as possible to transfer patients before their condition deteriorates.

• Early airway management is indicated in the presence of hemoptysis or stridor.

• If coronary insufficiency is suspected, nitrates may be used, but therapy with thrombolytic agents and aspirin should be avoided.

• Patients should be monitored and accompanied by personnel capable of resuscitation.

• If a prolonged ground transport time is anticipated, consider air transport.

Prognosis:

• On the basis of his experience, Crawford has stated that "no patient should be considered cured of the disease."

• The 5-year survival rate is about 75% whether the patient is treated medically or surgically.

• The 10-year survival rate is between 40% and 69% for both surgically and medically treated dissections.

• In the pretreatment era, the 1-year survival rate was 5-10%.

• Reoperation may be necessary for late complications.

Medical/Legal Pitfalls:

• Failure to identify the imaging features of a dissection
• Delaying treatment by performing unnecessary imaging studies
• Death during an aortic fenestration performed without appropriate informed consent
• Failure to diagnose the disease in patients presenting with chest pain
• Failure to avoid using thrombolytics in the patient presenting with chest pain and ECG changes
• Multiple case reports describe patients who received thrombolytics and were found later to have a dissection. The diagnosis of aortic dissection can be very subtle.

• The diagnosis depends on clinical suspicion, with contributory history, physical exam, and imaging studies.

• Obtaining a chest radiograph prior to thrombolytics is considered prudent.

• Checking blood pressures in both arms and listening for carotid bruits also can help diagnose aortic dissection prior to thrombolytics. The entire clinical picture must be taken into account.