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Signs in Imaging |
1 From the Department of Radiology, Thomas Jefferson University Hospital, 111 S 11th St, Philadelphia, PA 19107. Received February 17, 1998; revision requested March 30; revision received June 18; accepted October 26. Address reprint requests to the author.
Index terms: Aneurysm, abdominal, 981.731 • Aneurysm, aortic, 981.731 • Aorta, CT, 981.12911, 981.12912
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Histopathologic examination of the hemorrhage within the mural thrombus reveals "nonendothelialized channels" filled with blood scattered from the inner to the outer layers of the thrombus (3). At a later stage, blood penetrates the aortic wall and damages its muscle fibers; this predisposes the AAA to complete rupture. As described above, the hemorrhage within the mural thrombus, aortic wall, or both, is seen as a high-attenuating crescent on CT images.
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Because of the risk of rupture, AAAs are closely followed up for signs suggestive of AAA instability by using ultrasonography or CT. The size and rate of expansion of the AAA are important in determining the stability and likelihood of the aneurysm rupture. In ultrasonographic and CT studies, acceptable annual AAA expansion rates between 0.30 and 0.57 cm per year have been reported, with an average expansion rate of 0.4 cm per year (6). Depending on the size of the AAA, the risk of rupture varies. For a 4-cm aneurysm, the risk of rupture is approximately 2%; however, for aneurysms larger than 5 cm, the risk of rupture increases to 25%–41% over 5 years (5). Because the risk of AAA rupture increases substantially with larger aneurysms, most vascular surgeons will electively repair aneurysms larger than 5 cm in diameter unless there is a strong contraindication to surgery (6). For aneurysms smaller than 4.5 cm and those larger than 5.0 cm in patients who are poor surgical candidates, 6-month radiologic surveillance is recommended (6).
The classic clinical triad of abdominal pain, hypotension, and a palpable pulsatile aneurysm occurs in less than 50% of patients with aortic rupture (2). Because the clinical scenario may be ambiguous, patients are often referred for medical imaging to exclude a ruptured aortic aneurysm as a cause of abdominal pain or unexplained hypotension. A noncontained AAA rupture is usually a straightforward diagnosis to make by using CT. A large aneurysm is identified, and hyperattenuating periaortic blood may be seen extending into the perirenal space, pararenal space, or both. Furthermore, at the site of presumed rupture, the aortic wall may be indistinct, and extravasation of contrast material may be seen at contrast-enhanced CT imaging. However, CT findings of a contained or impending rupture may be subtle and difficult to diagnose. Therefore, it is important to begin the CT evaluation of a suspected AAA rupture without contrast material enhancement to prevent obscuration of the hyperattenuating crescent indicating an acute or impending AAA rupture. Following the evaluation of the aneurysm without contrast material, contrast material may be administered intravenously to complete the CT examination.
As stated above, the hyperattenuating crescent sign represents blood dissecting into the mural thrombus and/or aortic wall and thus weakening the support structure of the aneurysm and increasing the likelihood of complete aortic rupture. Therefore, this hyperattenuating crescent paralleling the aortic lumen should be recognized as a sign of an unstable AAA. Regardless of the patient's hemodynamic status or clinical symptomatology, this CT finding should prompt a surgical consultation. Failure to recognize an acute or impending AAA rupture may cause a delay in surgical treatment, with a resultant mortality rate approximating 100%.
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A trainee (resident or fellow) wishing to submit a manuscript for Signs in Imaging should first write to the Editor for approval of the sign to be prepared, to avoid duplicate preparation of the same sign.
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