Ultrasound
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altThe pain was described as “squeezing” and was associated with nausea, vomiting, and decreased oral intake. The patient denied any chest pain, shortness of breath, or back pain. Initial vital signs were stable and the abdominal exam revealed generalized abdominal tenderness that was worst in the left lower quadrant. The initial differential diagnosis was focused on diverticulitis, pancreatitis, and abdominal aortic aneurysm. Because of the consideration for aortic aneurysm a rapid bedside abdominal ultrasound was performed by the emergency physician (Figures 1 and 2). No aneurysmal dilation of the abdominal aorta was seen; however, an apparent intimal flap suspicious for aortic dissection was detected. A CT scan was urgently performed of the chest, abdomen, and pelvis.
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To view the CTs and read the diagnosis and discussion, continue NEXT
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(1) Bedside abdominal ultrasound at the level of the upper abdominal aorta – transverse view. The aorta can be visualized on the bottom center of the image below the liver (thick arrow) next to the inferior vena cava (thin arrow). An intimal flap is seen within the aorta (asterisk). (2) Bedside abdominal ultrasound at the level of the upper abdominal aorta – longitudinal view (arrow). An intimal flap is seen within the aorta (asterisk). [Note: this image was taken from a patient other than the one presented here.]
 
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(3) CT scan at the level of the upper abdominal aorta (center of image) demonstrates an intimal flap contained within the aorta. (4) CT scan sagittal reconstruction of the aorta shows dissection flap within the aorta resulting in the creation of a false lumen.

A CT scan was urgently performed of the chest, abdomen, and pelvis (Figures 3 and 4) demonstrating a Stanford Type A dissection originating just distal to the aortic root extending through the thoracic and abdominal aorta into the left common iliac artery. It was also noted that the dissection extended into the superior mesenteric artery.

The cardiothoracic surgery service was contacted and the patient was emergently taken to the operating room for repair of his aortic arch and was discharged home on post-operative day eleven. At six-week follow-up, the patient was doing well without any complaints.

Discussion
An aortic dissection occurs due to a violation in the intimal layer of the aorta and allows blood to enter between this layer and the outer adventitial layer. This causes an intimal flap to develop creating a false-lumen. The estimated incidence of acute aortic dissection based on a review of several population-based studies is approximately three cases per 100,000 people per year1. The Stanford classification system categorizes dissections involving the ascending aorta as Type A, which typically require aggressive medical treatment and emergency surgery, and those limited to the descending aorta as Type B, which are usually managed without surgery unless certain complications develop. Various studies can be performed to aid in the diagnosis of aortic dissection including chest X-ray, CT and MR angiography1. Chest X-ray has both poor sensitivity and poor specificity for dissection. The sensitivity has been quoted as up to 80% when viewed retrospectively, but real-time sensitivity is probably much less. Findings of a widened mediastinum or abnormal aortic contour are probably the most useful, especially if there is a change from prior imaging, in suggesting the diagnosis. It has been previously reported that bedside trans-thoracic echocardiography (Trans-esophageal echo will be mentioned later) can be used to detect an intimal flap to aid in the diagnosis of an aortic dissection in the emergency room2-4. However, emergency physicians tend to be more familiar with the sonographic windows utilized in the focused assessment with sonography for trauma (FAST) exam and with the examination of abdominal aorta. One study describes the use of the subxiphoid cardiac view, as opposed to ultrasound windows more familiar to cardiologists, to detect a hyperechoic flap in the proximal aorta as an indication of dissection3. In this case, the intimal flap was visualized during the ultrasound examination of the abdominal aorta done to rule out an abdominal aortic aneurysm.

Interestingly, our patient complained only of abdominal pain, possibly due to the noted extension of the dissection into the superior mesenteric artery, and lacked the classic symptoms of chest pain radiating towards the back. A recent review of the literature notes that approximately 10% of patients with acute aortic dissection lack the typical findings of chest pain and pulse deficit1. Additionally, approximately 6% of patients with aortic dissection present without any symptoms of pain at all and are associated with increased mortality. Clinicians should be aware that aortic dissections may have many atypical presentations. In addition, emergency physicians should consider using bedside ultrasound to expedite the diagnosis for aortic catastrophes such as aortic aneurysm and aortic dissection, when every minute counts.

1. Golledge J, Eagle KA. Acute aortic dissection. Lancet. 2008;372:55-66.
2. Divakaran V, Mungee S, Salciccioli L, et al. Bedside ultrasound diagnosis of aortic dissection in a cocaine abuser. Eur J Intern Med. 2007;18:595-6.
3. Blaivas M, Sierzenski P. Dissection of the proximal thoracic aorta: A new ultrasonographic sign in the subxiphoid view. Am J Emerg Med. 2002;20:344-8.
4. Shirakabe A, Hata N, Yokoyama S, et al. Diagnostic score to differentiate acute aortic dissection in the emergency room. Circ J. 2008;72:986-90.
5. Park SW, Hutchison S, Mehta RH, et al. Association of painless acute aortic dissection with increased mortality. Mayo Clin Proc. 2004;79:1252-7.


Soundings is editing by Brady Pregerson, MD, who runs the QE Emergency Medicine Ultrasound Course and has a free Ultrasound Image Library online (www.ERPocketBooks.com)
 
Continue NEXT for Pearls and Pitfalls of Imaging the Aorta for Dissection
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Pearls & Pitfalls for Imaging the Aorta for Dissection
 
>>The gold-standard diagnostic tests for aortic dissection are CT-angiogram, trans-esophageal echocardiogram (TEE) and MRI.
>>Due to its speed and widespread availability, CT is the preferred test when renal function is normal or when suspicion is high and the risks of delay outweigh the risks of contrast.
>>TEE is the study of choice when IV contrast is contraindicated or when renal function is decreased but suspicion is low to intermediate. Unfortunately it has limited availability. MRI is very sensitive and specific for dissection, but it is used primarily when other tests are inconclusive, contraindicated or unavailable. The main downside of MRI is the delay usually involved.
>>Trans-Thoracic Echocardiogram (TTE) is only about 80% sensitive and 80% specific for aortic dissection, but it’s use can be advantageous when it is immediately available to the emergency physician at the bedside. Suspicious findings will heighten the level of concern and help expedite definitive testing and treatment.
>>Approximately 90% of aortic dissections will progress into the abdominal aorta. Therefore, start your scan where the anatomy is most familiar and simple. Obtain a cross-sectional view of the abdominal aorta and look for slight enlargement in the diameter. Anything over 2.0cm should raise your suspicion. More importantly, look for a hyperechoic flap.
>>An intimal flap within the aorta will usually, but not always, move with each pulse and should not extend beyond the inner wall of the aorta. If it does, it may be an artifact.
>>Check the subxiphoid and parasternal views for evidence of pericardial effusion and tamponade.
>>Next, examine the descending thoracic aorta behind the heart. On the parasternal long axis view it will be seen in cross-section as a circle, which normally should be less than 42mm in diameter. Rotate into the plane of the aorta to get a long axis view as you continue to look for an intimal flap.
>>Finally, check the aortic arch with the suprasternal view. Have the patient turn his or her head to the right and orient your probe in the plane of the aortic arch. A normal arch may be hard to find in the far field. However, an enlarged arch will come closer to your probe, and thus, be more visible. Look again for a flap or an enlarged diameter greater than 42mm.
>>Consider a quick bedside echo in patients presenting with chest pain where there is some suspicion for dissection. Neither pain radiating to the back nor asymmetric bilateral blood pressures is anywhere close to being specific for dissection. However, if you do see a dissection or a pericardial effusion in such a patient, the heightened urgency it gives you may mean the difference between a good outcome and a bad one.

>>Finally, if there is concern for CNS involvement, you can also look for evidence of the dissection progressing up into one or both carotid arteries.
 
PITFALLS FOR IMAGING THE AORTA FOR DISSECTION
>>Artifact within the aorta can mimic dissection. Artifact should be non-pulsatile and should extend beyond the borders of the aorta.
>>Don’t delay transfer to the CT scanner. Most surgeons are not going to rush to the OR based on your study. However, in the unstable patient a positive ultrasound may get them to drop what they are doing and get immediately involved. Be careful. A trip to CT may result in a bad outcome for the unstable patient. When in doubt, call the surgeon!

 

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