By: Adam Weightman, MD, MBA

A 56 year-old male presents at 2 AM complaining of flank pain. He tells you the pain came on gradually over the past 8 hours and he describes it as a dull, aching sensation in the middle of his lower back. The pain is 9/10 on arrival. He also reports some nausea and chills over the same time course. The pain is constant and non-radiating. He denies fevers, chest pain, shortness of breath, vomiting, or dysuria. His past medical history is significant for HTN, HLD and kidney stones. It’s been a while since the kidney stone episode, but he thinks this feels pretty similar to the pain he experienced then. He denies any surgeries. He drinks 2-3 beers nightly and has smoked 1 pack per day for 40 years. He denies drug use.

On exam, he’s afebrile, BP 110/70, HR 105, RR 12, SpO2 94% on RA. He’s uncomfortable appearing, moving around as you try to examine him. Lungs are CTAB, no wheezes, rales, or rhonchi. Tachycardic, regular rhythm, no murmurs, rubs or gallops. Mild CVA tenderness over L > R flank. Abdomen soft, nondistended, mildly TTP without palpable masses.

 You’re feeling pretty confident at this point, so you send off a UA and search for your ultrasound machine. The UA comes back positive for blood, but, several patients are waiting to be seen, so you decide to send the patient for a CT to further define his obvious kidney stone. You give him some morphine for pain and trot back to the work station, certain you’ve got this case in the bag.

 The patient goes for CT and upon return, the nurse grabs you to tell you his pressures have been a little soft, he’s become slightly more tachycardic, and he’s complaining of numbness of his L leg. Repeat BP 93/67, HR 115. As you get up to evaluate him, the clerk yells out “Radiology, Line 1.” You pick up the phone, wondering how big this stone must be to warrant a call. The radiologist tells you, “Your patient has a 6 cm infrarenal aortic aneurysm with evidence of leak.”


 

If you’re feeling down about missing the diagnosis, you’re not alone: according to one study, abdominal aortic aneurysms are misdiagnosed upwards of 30% of the time, due to an often vague or asymptomatic presentation.[1] Wondering how anyone could possibly miss that hallmark “pulsatile abdominal mass?” One study found a sensitivity of merely 68% and specificity of only 75% for exam findings of a pulsatile mass, yielding a positive LR 2.7 and negative LR 0.43[2] — hardly something to hang a diagnosis on. To help yourself become part of the 70%, for patients over the age of 50 complaining of abdominal or back pain, particularly those with a history of HTN and smoking, bedside aortic ultrasound should be standard practice in the ED setting. The exam is quick, relatively easy, and reliable; in fact, a study in the Annals of Emergency Medicine found that EM physicians with minimal training had 100% sensitivity for aortic aneurysm with bedside ultrasound, allowing for earlier operative intervention and decreased radiation exposure.[3] In this post, we will review the essentials of this exam and some tips to optimize your images at the bedside.

First, it is important to recall the relevant anatomy of the aorta, much of which is visible by ultrasound:

aorta1
Source: https://commons.wikimedia.org/ (Mikael Häggström)

The abdominal aorta first branches into the celiac trunk, composed of the left gastric, common hepatic and splenic arteries. The next major branch is the superior mesenteric artery (SMA), then the paired renal arteries branch laterally prior to division into the right and left common iliac arteries.

To examine the abdominal aorta with bedside ultrasound, the probe should first be placed in a transverse plane, probe marker to the patient’s right, just below the xiphoid process. With firm, constant pressure (to push bowel gas out of the way), you should see the following image come into view:

aorta2
Source: https://commons.wikimedia.org (Nevit Dilmen), modified for this post

The hyperechoic vertebral body with posterior shadowing is often the easiest landmark to identify as you orient yourself. The thick-walled, pulsatile aorta lies to the left of and anterior to the vertebral body, running alongside the more compressible, thin-walled IVC. Remember that the IVC may have some pulsatility as well, thus it is important to ensure proper probe orientation and use landmarks to distinguish these two vessels.

Once you have the aorta in view, slide the probe caudally, keeping the probe perpendicular to the abdominal wall in order to obtain an accurate anterior-posterior (AP) measurement. A normal AP aortic diameter is 2.7-3.0 cm above the level of the celiac arteries and closer to 2.0-2.3 cm at the level of aortic bifurcation.[4] A complete aortic evaluation requires scanning (and taking measurements) through from the celiac trunk to the bifurcation, as most abdominal aortic aneurysms are infrarenal.[5] By convention, any measurement > 3.0 cm from outer wall to outer wall in the AP dimension while imaging in the transverse plane is considered abnormal and aneurysmal.[6] It is important to take measurements in this way to avoid being fooled by a false lumen or thrombosis.

There are a few landmark signs to help orient you as you scan through the aorta. The first occurs at the level of the celiac trunk and is referred to as the “seagull sign.” The celiac artery is seen arising anteriorly from the aorta forming the body of the seagull, with the splenic artery branching to the patient’s left and the hepatic artery to the patient’s right, forming the wings. As noted above, the left gastric artery also branches from the celiac trunk, but is not visible on bedside ultrasound.

            Sources: https://commons.wikimedia.org/ (Nevit Dilmen), modified for this post and  https://pixabay.com (Ben_Kerckx)

Scanning caudally, the next landmark occurs at the level of the SMA and is referred to as the “mantle clock sign.” The splenic vein (SV) in longitudinal section will be seen coursing superficial to the SMA seen in transverse section. The left renal vein (LRV) in longitudinal section can also be seen deep to the SMA as it courses towards the IVC, together forming the likeness of a mantle clock. The aorta (AO) will be seen, again in transverse section, deep to the left renal vein, superficial to the vertebral body.

      Sources: Image obtained by Tuyen Kiet, MD; exported from QPath and www.maxpixel.freegreatpicture.com (author unknown)

Scanning further caudally still, you will finally encounter the aortic bifurcation into the right and left iliac arteries near the umbilicus (pro tip: fill the umbilicus with gel if your view becomes obscured as you scan over this region). The single circular aorta seen in transverse will become two as you scan through the bifurcation. The other branches off the aorta, such as the renal arteries, are typically not visible on bedside ultrasound.

After scanning in a transverse plane, the probe should again be moved superiorly to the level of the celiac bifurcation and rotated 90 degrees clockwise such that the probe marker now faces the patient’s head, to image the aorta in longitudinal view. In this view, the celiac trunk can be seen arising superiorly (mostly in transverse), with the SMA arising just inferiorly. The SMA will be imaged for much of its length in longitudinal view as it courses caudally.

aorta5
Source: https://commons.wikimedia.org/ (Nevit Dilmen), modified for this post

As a reminder, measurements should be taken in the transverse, not the longitudinal view. However, the longitudinal view may help to differentiate a saccular (asymmetric outpouching, image A) from a fusiform (entire circumference involved, image B) aneurysm.

AneurysmsAn example of an aneurysmal aorta can be seen below. Of note, it is important to remember that while aortic ultrasound is superbly sensitive for aortic aneurysm, it is not sensitive for aortic rupture, given the tendency for blood to accumulate in the retroperitoneal space, which is difficult to image via ultrasound.3

aorta7

Source: image obtained by Andy Mittleman, MD; exported from QPath

aorta8
Source: images obtained by Andy Mittleman, MD; exported from QPath

 

PEARLS:

  • Abdominal aortic aneurysm is frequently misdiagnosed due to vague or absent symptoms
  • Abdominal aortic US is quick, easy, and highly sensitive for aortic aneurysm (but not rupture) and should be considered in anyone >50 years-old presenting with abdominal or back pain
  • A complete exam images from the celiac trunk through the aortic bifurcation in both transverse and longitudinal planes
  • Measurements should be taken only in an anterior-posterior fashion in transverse view
  • Any AP diameter >3.0 cm is abnormal and considered aneurysmal and should prompt further evaluation

 

Additional Resources:


References

[1] Azhar B, Patel SR, Holt PJ, Hinchliffe RJ, Thompson MM, Karthikesalingam A. “Misdiagnosis of ruptured abdominal aortic aneurysm: systematic review and meta-analysis.”J Endovasc Ther. 2014 Aug;21(4):568-75.

[2] Fink HA, Lederle FA, Roth CS, Bowles CA, Nelson DB, Haas MA. “The accuracy of physical examination to detect abdominal aortic aneurysm.” Arch Intern Med. 2000;160(6):833.

[3] Kuhn, Marie et al. “Emergency Department Ultrasound Scanning for Abdominal Aortic Aneurysm: Accessible, Accurate, and Advantageous.” Annals of Emergency Medicine, Volume 36 , Issue 3 , 219 – 223.

[4] Wanhainen A, Themudo R, Ahlström H, Lind L, Johansson L. “Thoracic and abdominal aortic dimension in 70-year-old men and women–a population-based whole-body magnetic resonance imaging (MRI) study.” J Vasc Surg. 2008 Mar;47(3):504-12.

[5] Golledge J, Muller J, Daugherty A, et al. “Abdominal aortic aneurysms: pathogenesis and implications for management.” Arteriosclerosis, Thrombosis and Vascular Biology. 2006;26;2605.”

[6] Johnston KW, Rutherford RB, Tilson MD, Shah DM, Hollier L, Stanley JC. “Suggested standards for reporting on arterial aneurysms. Subcommittee on Reporting Standards for Arterial Aneurysms, Ad Hoc Committee on Reporting Standards, Society for Vascular Surgery and North American Chapter, International Society for Cardiovascular Surgery.” J Vasc Surg. 1991;13(3):452.


Resident Author: Adam Weightman, MD, MBA
Faculty Review: Faculty Review: Meera Muruganandan, MD