Case i -  Aortic Dissection

Evaluation of the aorta in CTA windows shows an intimal flap extending from the aortic root through to the bifurcation of the abdominal aorta.  You have seen this before on the schematic depicting aortic injuries.  This is the classic appearance of an aortic dissection.  The flap is the abnormal line crossing through the lumen of the vessel.

An aortic dissection is part of the spectrum of injuries that ranges from aortic intramural hematoma to aortic rupture.  Usually caused by hypertension, but also by trauma, a dissection occurs when there is a tear in the intimal layer of the endothelium.  Blood dissects into the actual wall of the aorta creating a false lumen between the intima and remainder of the vessel wall.

When any part of the dissection occurs proximal to the left subclavian artery the injury is classified as a type A dissection.  If the injury is limited to the aorta distal to the arch, it is called a type B dissection.  This is the Stanford classification system and is used instead of the older Debakey classification system. This distinction is important.  A Type A dissection requires urgent surgical attention; or at least the vascular and thoracic surgeons must be made aware.  A type B dissection is ususally managed with blood pressure control unless there is an ischemic organ.

The important things to determine in a dissection are

• Is it type A or type B?
• Which lumen is the false lumen and which one is the true lumen.
• Which branch arteries are involved?
• Are there ischemic endorgans?

We just discussed type A vs type B.  This flap inovolves the ascending and descending aorta. so is a type A aortic dissection.

There are 4 general ways to distinguish the true lumen from the false lumen.  First, the true lumen fills early.  If you did the scan using a timing prep you may see which lumen fills first.  Alternatively, one lumen may be brighter on the earliest arterial phase images.  I did not provide the smart prep images so this technique is not useful here.  Second, the false lumen is usually larger.  This is not always the case.  Third, the true lumen is always the one completely surrounded by endothelium.  Patients with aortic dissections will usually have calcium in the walls of the aorta, and this calcium is located in the endothelium. The lumen with circumferential calcifications is the true lumen.  Here are several axial images where you can identify the true lumen by the circumferential calcifications (1, 2, 3).  Lastly, in places the false lumen will look like it is peeling the intima off the wall of the vessel.  This appearance has been called the "claw sign."  You can see this on several of the axial images (1, 2, 3).  The false lumen looks like it is wrapping around the true lumen, like a claw. I find early enhancement and calcification are the best two indicators of the true lumen.  Size and the claw sign are less reliable.

By careful examination of the images, you can see that the dissection extends into the right subclavian artery, right carotid artery, and left subclavian artey.  In the abdominal aorta, the true lumen supplies the celiac trunk, the SMA, the left renal artery, the IMA and the left common iliac artery (verify this yourself).  The false lumen supplies the right renal artery.  The dissection extends into the right common iliac artery.  All the vessels are patent except for the distal left common iliac artery which contains a filling defect representing thrombus.  This thrombosis extends into the left internal and external iliac arteries.  This is all important information to relay to the surgeons.  If there is an ischemic end organ they may fenestrate the origin of  the affected vessel.  They therefore must know where the false and true lumen originate so that they get their catheters in the appropriate place.

How can you tell if there is end organ damage?  Look for perfusion abnormalities.  In this case both kidneys show normal cortical enhancement.  But always look on the delayed images for a delayed nephrogram or delayed contrast excretion.  Look for hypoenhancement of portions of the bowel, spleen, and liver.  In this case there were no areas of underperfusion.

Look now at the chest xray.  The patient is rotated accounting for the prominence of the right mediastinum.  The aortic contour is distinct.  On the CT images there is no mediastinal hemorrhage.  A normal chest xray does not exclude dissection because the abnormality is within the wall of the aorta!  In traumatic dissections, or severe dissections there can be chest xray findings.   However, do not use the radiograph as a screening exam.  It is not sensitive for dissection.