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Anthony J. Dean, M.D. and Bon S. Ku, M.D.

I.  Introduction and Indications

A ruptured abdominal aortic aneurysm (AAA) is a vascular catastrophe responsible for 1-3% of deaths in men from the age 65-85 in developed countries. (1)  AAA is the 13th leading cause of death in the United States, causing about 15,000, or 0.8% of all deaths annually. (2,3)  Rupture from an AAA is the 10th leading cause of death in males over 50 and the incidence of AAA continues to increase.(1)  Currently, the U.S. Preventative Services Task Force recommends that men from the age of 65-75 years who have ever smoked be screened once for an AAA by sonography. (4)
The lethality of a ruptured AAA is devastating; the mortality rate after rupture approaches 90%. (3)  Therefore, it is essential for the emergency medicine physician to recognize the presence of an AAA in an expeditious manner.  A patient who presents with a ruptured AAA will usually not have the classic triad of hypotension, back pain, and a pulsatile abdominal mass.  Wrong diagnoses such as gastrointestinal bleed, nephrolithiasis, and diverticulitis are often made.  Delayed diagnosis can obviously prove detrimental to the patient with a symptomatic AAA.
The physical exam is often unreliable in detecting the presence of an AAA and should never be used to rule it out.  Abdominal palpation has been shown to have a moderate sensitivity of 68% in diagnosing an AAA. (5)  Other modalities for detecting an AAA include conventional radiography, ultrasonography, computed tomography (CT), magnetic resonance imaging (MRI), and arteriography.  Ultrasound, utilized since the 1980’s to inspect the abdominal aorta, is an ideal method for detecting AAA’s due to its accuracy combined with low cost, rapid deployment, non-invasiveness, and avoidance of contrast material. (6)
Multiple studies have demonstrated that physicians from varied disciplines outside of radiology can be highly accurate in finding an AAA using ultrasonography. (7,8,9)   Kuhn et al. showed that emergency physicians can identify an AAA with 100% accuracy with only a brief 3 day training course. (7)
The current indications by the American College of Emergency Physicians (ACEP) for obtaining an emergency medicine based ultrasound to detect an AAA include “the presence of syncope, shock, hypotension, abdominal pain, abdominal mass, flank pain, or back pain especially in the older population. (10)

II.  Anatomy

The aorta passes through the diaphragm at the level of the 12th thoracic vertebral body.  It lies slightly to the left of the midline and bifurcates at the level of the 4th lumbar vertebral body.  The surface anatomy landmarks corresponding to these two points are the xiphoid process and the umbilicus.  The length of the abdominal aorta is about 13 cm (6 inches) which is less than the length of iliac arteries from the bifurcation to the inguinal ligament.  Most scanning of the aorta will therefore take place in the short distance between the sternum and the umbilicus. 
Immediately below the diaphragm, the celiac trunk is the first major vessel to arise from the aorta in the midline anteriorly.  This short (usually less than 1 cm) vessel can often be seen sonographically in the transverse plane, dividing in a “wide Y”.  The fork on the patient’s right is the common hepatic artery, heading to the porta hepatis; the fork on the patient’s left, is the splenic artery.  This sonographic view is known as the “seagull sign”.  About 1 cm inferior to the celiac trunk, again in the midline, arises the superior mesenteric artery (SMA), which often runs in a caudal direction immediately anterior and parallel to the aorta.  Measurements of the proximal aorta to use as a comparison with distal measurements are made at this level.  One centimeter below the SMA, the renal arteries arise on either side.  Although these cannot be seen on a sagittal view of the aorta, they can sometimes be identified with careful transverse scanning.  Thus, these three major vessels occur within about 3 centimeters of the diaphragm.  90% of all AAA’s will occur distal to this point.  With experience, it is easy to distinguish the aorta from the IVC, but initially they can be confused.  Distinguishing features are listed in Table 1. 

Illustration 1:  Overview of the main branches of the abdominal aorta
(CT= celiac trunk, SMA = superior mesenteric artery, IMA = inferior mesenteric artery).

 

IVC	Aorta
On patient’s right	On patient’s left
Compressible	Non-compressible
Thinner walls	Thick walls
Usually respiratory variation	No respiratory variation
Usually larger (can depend on hydration status)	Usually smaller (unless AAA)
Not pulsatile (or displays ‘double-pulsatile swing’ = transmitting the cardiac atrial and ventricular pressure wave)	Pulsatile (simple)

Table 1:  Sonographic features of the aorta vs. IVC.

Figure 1: A transverse image of the aorta shows a classic example of the seagull sign.   The celiac trunk branches into the hepatic (H) and splenic (S) arteries.  The inferior vena cava (IVC) is seen to the left of the aorta. 

III.  Scanning Technique, Normal Findings and Common Variants

Sonographic Technique 
A 3.5 MHz transducer is adequate for most abdominal scanning, including imaging of the aorta.  A lower frequency may be needed in large patients, and a higher frequency will give more detail in thin ones.  The aorta and iliac arteries are measured from outer wall to outer wall.  The steps for aorta scanning are outlined in Table 2. 

Table 2 .  Technique for ultrasound scanning of the aorta Orientation.  Start in the transverse plane (pointer to “9 o’clock”), high in the epigastrium, using the liver as a sonic “window”.  Identify the vertebral body (a dark, rounded shape, with dense shadow).  Identify the aorta on the patient’s left, and the IVC (patient’s right) “above” the vertebral body on the ultrasound image.  (If the patient is hypovolemic, use light probe pressure in order to avoid effacement of the IVC).  In real time obtain transverse images of the aorta from the celiac to the bifurcation. If the gas-filled transverse colon obscures the aorta move the probe until you find a sonographic window between loops of bowel: rock the probe up and down without moving it across the patient’s skin to maximize the information attained through the “window”.    Frequently identifiable “sonic windows” in the upper abdomen include: high in the epigastrium.  Ask the patient to “take a deep breath and hold” to lower the liver margin. above or around the umbilicus.   Obtain views of the iliacs if possible.  Rotate the probe’s pointer clockwise from the "9 o' clock" to the “12 o’clock” position for sagittal views from the celiac to the bifurcation. Attempt to obtain: at least 3 transverse views, labeled, “high”, “middle”, “low”, with calipers.  One view should show the maximal aortic diameter.  Sagittal view(s) from the celiac to the bifurcation If an AAA is identified... Does probe pressure reproduce symptoms? Is there free fluid?

 

	Figure 2		Figure 3

Figure 2:  Transverse image of the normal proximal aorta (A) shown in its relationship to the vertebral body (arrow).  IVC = inferior vena cava; H = hepatic artery; L= liver.  Figure 3:  Transverse image of the normal mid to distal aorta (A) and inferior vena cava (IVC) before the bifurcation into the iliac arteries.  The vertebral body (arrow) causes a characteristic shadowing artifact. 

Figure 4:  Longitudinal view of the normal proximal aorta showing the branches of the celiac artery and SMA.  SMA = superior mesenteric artery; VB = vertebral body.

 

	Video clip 1		Video clip 2

Video clip 1:  Real time scanning of a normal abdominal aorta in the transverse view.  Note the gallstone at the left aspect of the screen in the beginning of the clip.  Video clip 2:  Real time scanning of a proximal or high normal aorta in the sagittal view.

Special Techniques for Ultrasound Evaluation 
If bowel gas makes it difficult to obtain images, some or all the following can help:

1. “Jiggle” the probe, while applying gentle pressure.  This sometimes allows the bowel to be gently moved aside. 
2. Reposition the patient. 
3. Obtain coronal views, using the liver as a window.  The probe is placed in the “12 o’clock” position in the mid axillary line at or below the costal margin, directed slightly anterior.  With practice, both the IVC and aorta can be seen running parallel in this view, with the aorta lying “deep” on the screen to the IVC. 
4. Try imaging from below the umbilicus with the probe directed cephalad. 
5. Try imaging the aortic bifurcation from an oblique angle with the probe placed lateral to the umbilicus (right or left) and pointing towards the spinal column (Figure 5).

 

Figure 5:  Longitudinal-oblique view of a normal lower aorta and bifurcation. (Image courtesy of B. Hoffmann, M.D.)

IV.  Pathology

An AAA is described as being a focal dilatation of the abdominal aorta of 150% of normal.(11,12,13)  Although there is no established definition of aneurysm size, conventionally, an AAA is diagnosed when the diameter exceeds 3.0 cm. (1)  It has been demonstrated that the risk of rupture for an AAA of 3.0 cm is less than 4% over 5 years; this risk, however, substantially increases for AAA’s with larger diameters. (14)
A true aneurysm by definition involves dilation of all three layers of the aorta: Intima, media, and adventitia.  The majority of aneurysms are fusiform, affecting the entire circumference of the vessel.  Saccular aneurysms are uncommon and affect only part of the aortic circumference. 
Treatment of AAA is entirely surgical.  The first resection was performed in Paris in 1951.  Prior to that, patients lived with their aneurysms until they ruptured, at which time they usually died.  Aneurysms sometimes reached enormous size.  In his description, Osler reports on AAAs in living patients, as well as pathological specimens where “the tumour mass… [was] as large as the adult head.” (15)

 

Figure 6:  Transverse image of a nearly 6 cm AAA with calipers measuring the aorta in the anterior-posterior direction. 

 

Figure 7:  Transverse image of an AAA with an intraluminal thrombus.  This figure demonstrates the importance of measuring the aorta from the outer walls. 

	Video clip 3		Video clip 4

Video clip 3: Real time transverse imaging of an AAA with an intramural thrombus.
Video clip 4: Real time transverse imaging of an AAA bifurcating into the iliacs.  Notice the presence of an iliac aneurysm.  

 

V.  Pearls and Pitfalls Obtain measurements of the aorta from outer wall to outer wall.  Since aneurysms will often contain a thrombus, one may accidentally mistake the inner rim of the thrombus for the aortic wall.  Doing this will lead a falsely decreased measurement of the true aortic diameter. Avoid oblique or angled cuts if possible, especially with a tortuous aorta, which will exaggerate the true aortic diameter. Since images are often obtained from paramedian probe positions, and the aorta may be tortuous, it is simpler (and more accurate) to speak of “longitudinal” rather than “sagittal” sections of the aorta. Although axial resolution is usually greater than lateral resolution, transverse views are needed because many AAAs have larger transverse than AP diameter. (16) A small aneurysm does not preclude rupture: (17,18) Any symptoms consistent with rupture in a patient with an aortic diameter greater than 3.0 cm should have this diagnosis (or alternative vascular catastrophes) ruled out. Scanning should be systematically performed in real-time from the diaphragmatic hiatus to the bifurcation in order to avoid missing small, localized saccular aneurysms.

 

VI.  References

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   Abdominal Aortic Aneurysm. Lancet.2005;365:1577-89.
   
   
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10. American College of Emergency Physicians. Policy Statement.
    2001:Emergency Ultrasound Guidelines.
    
    
11. Johnston KW, Rutherford RB, Tilson MD, Shah DM, Hollier L, Stanley JC.
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12. Sox HJ, Huber JM (eds.)
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    Current views on the pathogenesis of the abdominal aortic aneurysms. J Am Coll Surg.1995;181:371– 82.
    
    
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15. Osler W.
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17. Miller J, Grimes P, Miller J.
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