Aortic dissection overview
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Dr. Frank Nicholls described the first case of aortic dissection in 1761. Later, in 1819, Dr. René Laennec used the term dissecting aneurysm for the first time. The first successful surgical intervention for aortic dissection was done by Dr. Cooley and Dr. Creech in 1954. In 1965, Dr. DeBakey suggested the first classification system for aortic dissection. In 1970, a group of surgeons suggested the simplified classification system of Stanford. In 1996, an international, referral center was registered for acute aortic dissection with the goal of registering patients' presentation, treatment, and outcome. This registry provides information of more than 1500 patients with acute aortic dissection and may be used as a resource for this disorder. In 2006, another international registry was established in Germany for acute aortic dissection type A (AADA) with the goal of collecting patients' information for structured follow-up. This registry with more than 2000 patients is now the largest database on AADA worldwide.
Several different classification systems have been used to describe aortic dissections. The systems commonly in use are either based on either the anatomy of the dissection (proximal, distal) or the duration of onset of symptoms (acute, chronic) prior to presentation. DeBakey and Stanford's systems are commonly used classification systems for aortic dissection. European society of cardiology defined another classification system for aortic dissection in 2014.
Aortic dissection begins as a tear in the aortic wall in > 95% of patients. The tear is usually transverse, extends through the intima and halfway through the media and involves ~50% of the aortic circumference. An inherent weakness in the tunica media layer predisposes patients to develop a tear in the intima layer. Two thirds of dissections originate in the ascending aorta, and 20% are in the proximal descending aorta.
Age-related changes due to atherosclerosis and uncontrolled hypertension are associated with spontaneous dissection, while blunt trauma injury and sudden deceleration in a motor vehicle accident is a major cause of aortic dissection.
Differentiating Aortic Dissection from other Diseases
Aortic dissection is a life threatening entity that must be distinguished from other life threatening entities such as cardiac tamponade, cardiogenic shock, myocardial infarction, and pulmonary embolism. An aortic aneurysm is not synonymous with aortic dissection. Aneurysms are defined as a localized permanent dilation of the aorta to a diameter > 50% of normal.
Epidemiology and Demographics
The incidence of aortic dissection is approximately 6 per 100,000 individuals worldwide.The incidence of aortic dissection increases with age, with a mean age of 63 years. Men are more commonly affected by aortic dissection than women. However, the prognosis tends to be worse in women due to unusual presentations. There is no racial predilection to aortic dissection. However, non-white race is associated with worse prognosis. The 30-days mortality rate of aortic dissection type A and B is approximately 47% and 13%, respectively.
Common risk factors in the development of aortic dissection include aging, atherosclerosis, diabetes, hypertension and trauma. Less common risk factors include bicuspid aortic valve, cocaine abuse, coarctation of the aorta, cystic medial necrosis, a history of collagen vascular disoders, giant cell arteritis, heart surgery, pseudoxanthoma elasticum, Turner's syndrome, tertiary syphilis and the third trimester of pregnancy.
Screening guidelines state that an EKG should be obtained for all patients who present with symptoms suspicious for aortic dissection. A chest x-ray should be obtained for patients determined to be in low-risk, and moderate-risk categories, and for patients in high-risk categories, TEE, CT Angiography or MRI Angiography should be obtained.
Natural History, Complications and Prognosis
The symptoms of aortic dissection usually develop in the fifth decade of life and start with symptoms such as sudden onset chest/back pain. If left untreated, patients with aortic dissection may progress to develop aortic regurgitation, myocardial ischemia, and cardiac tamponade. The complications of aortic dissection include but not limited to aortic rupture, pericardial tamponade, myocardial ischemia, compression of nearby organs and etc. Aortic dissection carries a poor prognosis. Mortality rate differs based on the type of dissection and is higher in type A compared to type B (25% versus 12%).
Diagnostic Study of Choice
The diagnostic work-up of aortic dissection is highly dependent on the pretest probability of the patients as well as their hemodynamic status. In hemodynamically unstable patients, transesophageal echocardiography has the greatest value. CT scan, MRI, and transesophageal echocardiography have been observed to have similar reliability in the diagnosis of aortic dissection. However, CT and MRI have been preferred modality for assessing the extension and branch involvement in aortic dissection and diagnosing other types of acute aortic syndrome other than dissection as well as traumatic aortic lesions.
History and Symptoms
History of patients with aortic dissection may be positive for factors such as connective tissue disease, known aortic valve disease, recent heart surgery, Known thoracic aortic aneurysm, and family history of the aortic disease. Sudden onset chest/back pain is the most common symptom of aortic dissection. Pain may be of sharp, ripping, tearing, and knife-like quality.
Aortic dissection is commonly associated with varying blood pressure (pseudohypotension or hypertension or hypotension), wide pulse pressure (if the aortic root is involved causing aortic insufficiency), tachycardia, pulsus paradoxus, swollen face due to superior vena cava compression (superior vena cava syndrome). Radial-radial delay or radio-femoral delay (deficit) in pulse can be seen in patients depending on the location of dissection. Pulse deficit is commonly seen in females. In proximal dissections involving aortic root, aortic insufficiency is a complication, and on physical examination an early diastolic decrescendo murmur, which is best heard in the right second intercostal space is noted.
There is no particular laboratory findings for the diagnosis of aortic dissection. Possible laboratory findings associated with aortic dissection may include troponin elevation, CK-MB elevation, hematuria, and etc. There has been reports of using a smooth muscle myosin heavy chain immunoassay to help diagnose aortic dissection.
EKG findings in aortic dissection are usually non-specific. Possible findings include non-specific ST-segment changes, atrioventricular block (second degree), left ventricular hypertrophy, and sinus bradycardia. It worth mentioning that ECG may lead to a delay in the diagnosis. Since clinicians usually follow the diagnosis of coronary artery disease due to its higher prevalence.
Imaging in Acute Aortic Dissection
There are a wide variety of imaging studies that can be used to diagnose aortic dissection, but in general, transesophageal imaging is the imaging modality of choice in the acutely ill patient and CT Angiography, MRI is the imaging modality of choice in the assessment of longstanding aortic disease in a patient who has chronic chest pain who is hemodynamically stable or for the evaluation of a chronic dissection.
Chest x-ray may be helpful in the diagnosis of aortic dissection. Findings suggestive of aortic dissection on x-ray include widening of mediastinum, wide aortic contour, tracheal deviation, aortic kinking, and displacement of previous aortic calcification. Chest x-ray has limited sensitivity (64%) and specificity (86%) in the diagnosis of aortic diseases. It also worth mentioning that a normal chest x-ray may not rule out the diagnosis of aortic dissection.
A CT scan can be used to diagnose aortic dissection if neither a TEE nor MRI is available in a timely fashion, or if there is a contraindication to their performance. An example would be after hours in an emergency room setting. If the results of the CT scan are non-diagnostic, then TEE or MRI should be performed to confirm the diagnosis.
MRI is the imaging modality of choice in the assessment of longstanding aortic disease in a patient who has chronic chest pain who is hemodynamically stable or for the evaluation of a chronic dissection.
The echocardiographic changes diagnostic of aortic dissection include Intimal flaps in the aorta obstruction of a false lumen, intimal calcification displacement toward the center of the lumen, separation of intimal layers from the thrombus, and shearing of different wall layers during aortic pulsation. The sensitivity and specificity of transthoracic echocardiography vary based on the type of dissection and are usually lower for the diagnosis of distally located aortic dissection. Echocardiography may also show severe pleural effusion, which is suggestive of the development of cardiac tamponade. Transesophageal echocardiography may be useful in the diagnosis of aortic dissection in patients in whom transthoracic echocardiography has limited efficacy. Prolapse of intimal flap through the aortic valve seen in transesophageal echocardiography is diagnostic of aortic dissection complicated by aortic regurgitation. Sensitivity is usually higher (99%). However, it has limited usage in the diagnosis of dissections involving the distal portion of ascending aorta.
Type A dissections of the proximal aorta are generally managed with the operative repair. The repair can be done via open surgery vs endovascular repair depending on the extent of dissection. Type B dissections of the descending aorta are generally managed medically. Even patients who are undergoing operative repair require optimal medical management. The two goals in the medical management of aortic dissection are to reduce blood pressure and to reduce the oscillatory shear on the wall of the aorta (the shear-force dP/dt or force of ejection of blood from the left ventricle). The target blood pressure should be a mean arterial pressure (MAP) of 60 to 75 mmHg. Aortic dissection is the only hypertensive emergency where target BP is less than 120/80 within the first four hours of presentation. Ideally, the target is to control the heart rate to less than 60 per minute. The first line of agents are beta-blockers, esmolol continuous infusion is used preferably as it is short-acting and works for blood pressure as well as heart rate. IV Labetalol or PO propranolol or metoprolol can also be used. The addition of other IV continuous nitroprusside can also be used.Cyanide toxicity is a concern when nitroprusside is used for more than 24 hours.
Any dissection that involves the ascending aorta is considered a surgical emergency, and urgent surgical consultation is recommended. There is a 90% 3-month mortality among patients with a proximal aortic dissection who do not undergo surgery. These patients can rapidly develop acute aortic insufficiency (AI), tamponade or myocardial infarction (MI).
Proper treatment and control of hardening of the arteries (atherosclerosis) and high blood pressure may reduce risk of aortic dissection. It is very important for patients at risk for dissection to tightly control their blood pressure. Taking safety precautions to prevent injuries can help prevent dissections. Many cases of aortic dissection cannot be prevented. If diagnosed with Marfan or Ehlers-Danlos syndrome, regular follow-up is advisable.