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Abdominal Aorta

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How to Scan the Aorta

AAA

One of the oldest indications for bedside US is its use to detect abdominal aortic aneurysm. US has a high sensitivity and specificity in diagnosing AAA even in asymptomatic patients (1). Bedside US is so important in this disease because of the varied presentations leading to misdiagnosis in 30-60% of patients when using clinical judgement alone (2). Always consider AAA in patients >50yo presenting with unexplained hypotension, renal colic, back pain...

AAA is >3cm and usually infra-renal

ANATOMY

An AAA is defined as an aortic diameter >3cm (3) and can occur anywhere along the abdominal aorta but occurs typically below the renal arteries (4). Most aneurysms are fusiform, however, a small proportion may be saccular. So always image the aorta in two planes so as not to miss a saccular aneurysm. 

SCANNING

Begin at the epigastric aorta and travel along the length of the vessel caudally until you reach the iliacs. Always keep the aorta in the middle of the screen, making sure you have the correct vessel by looking for the landmarks: the hyperechoic vertebrae and shadow posteriorly, a thick walled pulsatile vessel etc. If you see a dilatation, measure the largest AP diameter outer wall to outer wall in two dimensions. 

Aorta trans

Aorta long

Make a note of whether the aneurysm is above or below the SMA. Below the SMA means that it is likely infra-renal which is an important piece of information for the vascular team. 

Longitudinal AAA with coeliac trunk and SMA above the aneurysmal area: likely infrarenal (annotated below)

SIGNS OF RUPTURE

Detecting signs of rupture is definitely not part of bedside US. However, patients often present with a contained rupture and atypical symptoms. Seeing signs of rupture increases the patients risk profile and you are more likely to expedite CT, alert vascular and theatre and call for blood even in the normotensive patient. Most of these patients will still go to CT for further imaging, however, some advocate for immediate transfer to theatre if signs of rupture are detected on bedside US (5)

In a retrospective review Catalano et al (6) identified signs of rupture on US:

1. Aortic shape deformation (also seen in unruptured AAA)

2. Intramural thrombus (also seen in unruptured AAA)

3. Floating thrombus

4. Disruption to intraluminal thrombus with hypoechoic areas

5. Disruption to aortic wall

6. Retroperitoneal or intraperitoneal free fluid

7. Para-aortic hypoechoic area suggestive of thrombus

AAA trans showing several features of rupture: disrupted outer wall (black arrow), disrupted intraluminal (black writing) haematoma (white arrow) and paraaortic haematoma (white writing)

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Dissection

REFERENCES

1. Rubano E, Mehta N, Caputo W, Paladino L, Sinert R. Systematic review: emergency department bedside ultrasonography for diagnosing suspected abdominal aortic aneurysm. Acad Emerg Med. 2013 Feb;20(2):128-38.

2. Akkersdijk GJ, van Bockel JH. Ruptured abdominal aortic aneurysm: initial misdiagnosis and the effect on treatment. Eur J Surg. 1998 Jan;164(1):29-34

3.Sakalihasan N, Limet R, Defawe OD. Abdominal aortic aneurysm. Lancet. 2005 Apr 30-May 6;365(9470):1577-89

4. Prisant LM, Mondy JS 3rd. Abdominal aortic aneurysm. J Clin Hypertens (Greenwich). 2004 Feb;6(2):85-9

5. Bhatt S, Dogra VS. Catastrophes of abdominal aorta: sonographic evaluation. Ultrasound Clin. 2008;3(1):83–91.

6. Catalano O, Siani A. Ruptured abdominal aortic aneurysm: categorization of sonographic findings and report of 3 new signs. J Ultrasound Med. 2005 Aug;24(8):1077-83.