Thoracic aortic dissection occurs when the intima of the aorta becomes compromised and “tears” or “dissects” a new layer that fills with blood between the intima and the media. This “false lumen” can start in different locations and has variable propagation. The most common predisposing factor for aortic dissection is hypertension. While ascending aortic dissection is a surgical emergency, a descending aortic dissection can be treated medically in most cases.
There are two classification systems used to describe thoracic aortic dissections:
The DeBakey classification:
Type I: Dissection originating in the ascending aorta extending a variable length frequently into the aortic arch and/or descending thoracic aorta
Type II: Dissection originating in the ascending aorta remaining confined to this area.
Type III: Dissection originating in the descending thoracic aorta. This is further classified as IIIa which is confined to the thoracic aorta and IIIb which extends to the abdominal aorta.
The Stanford classification:
Type A: Dissection involving any part of the ascending aorta
Type B: Dissection remaining confined to the descending thoracic aorta
When the intima becomes compromised, a tear can occur leading to dissection, however it is well known that pathology of the aortic media is involved as well and is a prerequisite in most non-traumatic aortic dissections. Connective tissue disorders such as Marfan’s syndrome, Turner’s syndrome, Ehlers-Danlos syndrome and bicuspid aortic valve have a significantly increased risk of aortic dissection.
The onset of symptoms in aortic dissection are usually sudden without warning. While pain of an ascending aortic dissection frequently is in the center of the chest radiating to the mid upper-thoracic area, the pain from descending dissection is usually localized to the back sometimes radiating to the abdomen. The pain is frequently described as “sharp” or “tearing” in nature and is quite severe in intensity. Uncontrolled hypertension is frequently present at the time of aortic dissection. Syncope from sudden hypotension can occur. When the carotid arteries are involved stroke symptoms may occur. The right coronary artery can also be involved causing an inferior myocardial infarction. Rupture of the ascending aortic dissection into the pericardium can cause a rapid pericardial effusion resulting in cardiac tamponade which is frequently fatal. Acute aortic regurgitation can cause acute heart failure, pulmonary edema and even cardiogenic shock if severe.
The physical examination is non-specific for ascending thoracic aortic dissection, but a radial pulse deficit can be seen if the subclavian artery is involved on the left or brachiocephalic artery on the right. Frequently, the radial pulse is absent on the left and present on the right. The blood pressure may be elevated. A murmur of aortic regurgitation can occur if the aortic root is dilated from the dissection. Neurologic deficits can be seen if the carotid artery is involved or if spinal ischemia is present.
Since rapid diagnosis is crucial in ascending aortic dissections and the presentation of descending dissection are similar, the clinician must always have a high index of suspicion which is based on the patient’s clinical presentation and symptoms.
Chest x-ray can show mediastinal widening and/or an abnormal aortic contour. Computed tomography (CT) is the definitive diagnostic modality and has a sensitivity approaching 100%. Transesophageal echocardiography and MRI can be useful as well.
Ascending aortic dissections are a surgical emergency. Without surgery, mortality rates within 2 weeks approach 80%. The longer the delay to surgical repair, the higher the mortality.
Medical therapy is minimal initially since immediate surgical intervention is required. This is aimed at reducing blood pressure and the shear force on the aorta. Beta-blockers with some alpha blocking properties (carvedilol, labetalol) are commonly used with a goal systolic blood pressure < 120 mmHg. Calcium channel blockers and nitrates can also be helpful. Caution is advised when lowering blood pressure during acute ascending aortic dissection since shock can occur from the many possible complications.
Aortic regurgitation: Acute aortic regurgitation can result from dilation of the aortic root. This may cause acute left heart failure with hypotension and pulmonary edema. Respiratory failure can ensue and again, surgical repair/replacement is required urgently.
Inferior myocardial infarction: When the ascending aortic dissection involves the ostium of the right coronary artery, an inferior myocardial infarction can occur. This is diagnosed on the 12-lead ECG where ST elevation is seen in leads II, III and aVF with reciprocal ST depression in the high lateral leads I and aVL. Treatment is emergent coronary bypass surgery.
Carotid artery dissection: When the carotid artery is involved in the ascending aortic dissection, symptoms of carotid artery dissection may occur which include headache, neck pain and Horner’s syndrome (ptosis - drooping eyelid, miosis - constriction of the pupil, and hemianhidrosis - lack of sweating on one side of the face), tinnitus and focal neurologic deficits.
Cardiac tamponade: An acute pericardial effusion can occur causing cardiac tamponade if the proximal portion of the ascending aortic dissection ruptures into the pericardium. In this situation physical exam findings include:
Elevated jugular venous pressure
Pulsus paradoxus (see below)
Pericardial friction rub (from pericarditis if present)
Distant heart sounds (from heart sound muffling related to the pericardial effusion)
Kussmaul’s sign (rarely) - increase in jugular venous pressure during inspiration
"Pulsus paradoxus" which is present in cardiac tamponade reflects a decrease in systolic blood pressure with inspiration of more than 12 mmHg. Pulsus paradoxus also occurs in severe asthma or COPD exacerbations.
Double barrel: A “double barrel” aorta can occur when a dissection progresses forward, then spontaneously reconnects to the aorta at the distal part of the dissection.