Aneurysm, Abdominal Aortic (AAA), Imaging

Article Author:
Ryan Gibbons
Article Editor:
Steve Bhimji
Updated:
1/8/2018 7:43:47 AM
PubMed Link:
Aneurysm, Abdominal Aortic (AAA), Imaging

Introduction

Abdominal Aortic Aneurysms (AAAs) are a relatively common pathology with a prevalence of 1.3% in patients over 50 years and an incidence in elderly men over 12%. Ruptured aneurysms have an exceptionally high mortality rate ranging 50% to 95%. In fact, mortality increases by 1% with each subsequent minute, mandating prompt diagnosis and intervention. Nearly 30% of ruptured AAAs are misdiagnosed on initial presentation, especially since physical exam has an exceedingly poor sensitivity less than 65%. Moreover, less than 25% of patients present with the characteristic triad of hypotension, abdominal pain, and a pulsatile abdominal mass.

Prompt diagnosis by utilizing point of care ultrasound has demonstrated sensitivities of 94% to 99%. Costantino et al. confirmed that bedside ultrasound is accurate within 4 millimeters of CT measurements with respect to AAA. Bedside ultrasound is a safe and effective diagnostic imaging modality, which can be performed in under 5 minutes. More significantly, its use has decreased mortality by 20% to 60%.

Anatomy

The retroperitoneal abdominal aorta enters the abdomen via the aortic hiatus caudal to the xiphoid process. It rests anterior to the vertebral body and parallel to the inferior vena cava.  Extending about 1 to 2 centimeters below the umbilicus, the aorta divides into the common iliac arteries at the level of L4. The aorta diminishes in size as it descends through the abdominal cavity, moving more superficially as well. As it moves caudally, the aorta has consecutive arterial branches: the celiac, superior mesenteric, renal then gonadal, and ultimately the inferior mesenteric. 

An aneurysm is classified as a focal dilatation greater than 50% of a vessel’s normal diameter. A diameter greater than 3 centimeters demarcates an AAA. Two categories of AAAs exist: fusiform and saccular. The majority are fusiform. Fusiform aneurysms expand circumferentially. Whereas saccular aneurysms are localized outpouchings, often secondary to an infectious etiology.

Approximately 90% of AAAs occur infrarenal, although the renal vessels are often difficult to image with a point of care ultrasound. Scan to where the aorta bifurcates to confirm visualization of the aorta in its entirety.

Indications

Who to Scan?

It is important to remember that less than 25% of individuals present with the classic triad of hypotension, abdominal pain, and a pulsatile abdominal mass.

Consider sonographic assessment of the abdominal aorta in the following instances:

  • Greater than 50 years old with one of the following: chest, abdominal, flank, groin, or back pain; renal colic; hematuria; or hydronephrosis 
  • Cardiac Arrest
  • Hypotension
  • Syncope
  • Thromboembolic events to the lower extremities
  • Neurologic deficit of the lower extremities

Expert consensus regarding sonographic screening for AAAs in asymptomatic individuals includes the following.

The United States Preventative Services Task Force (USPSTF) & the American Academy of Family Physicians (AAFP) recommendations:

  • Men greater than 65 years who ever smoked

Society for Vascular Surgery recommendations:

  • All men age greater than 65
  • Men greater than 55 with a family history of AAA
  • Women greater than 65 with family history of AAA or who have ever smoked

Risk Factors

  • Greater than 50 years old
  • Family history of AAA
  • Male
  • Hypertension
  • Smoking
  • Coronary artery disease
  • Diabetes mellitus
  • Hyperlipidemia
  • Peripheral arterial disease

Contraindications

There are no known contraindications for the sonographic assessment of the abdominal aorta.

Equipment

Sonographic assessment of the aorta is performed in real time B-mode imaging programmed with the abdominal settings. The curvilinear transducer is ideal given its lower frequency and deeper penetration. However, the phased array transducer can be employed as well.

Preparation

Similar to any sonographic examination, the appropriate configuration is necessary to acquire detailed images of the abdominal aorta. Place the patient in the supine position with the bed adjusted to the level of the provider’s waist. Darken the room and apply sufficient gel.

Initially, maximize the depth to visualize the most important landmark, the vertebral body. It should be hyperechoic with posterior shadowing. Once identified, adjust the depth accordingly to enhance your image.  Particularly, as you scan inferiorly towards the umbilicus, the aorta moves to a more superficial position, and less depth is needed.

Continuous, firm pressure will displace impeding bowel gas; and adjustments to the gain will reduce artifact. Modify the probe angle or move slightly off midline. Then angle or rock back medially to improve image acquisition. Or slide caudal to hindering bowel and then tilt cephalad to enhance sonographic visualization. The opposite technique works similarly.  Finally, turn the patient into the left lateral decubitus position. This may improve your images as well. Nevertheless, the aorta will not be visible in 5% of individuals. In these instances, advanced diagnostic imaging modalities, such as CT or MRA, will supplement the evaluation.

Technique

To begin, place the transducer below the xiphoid process in the transverse alignment. Perform a complete scan through the bifurcation of the aorta around the level of the umbilicus. Several consecutive videos may be necessary to image the entire aorta. Measure the abdominal aorta at its maximal diameter to include each outer wall. Dimensions are most precise with the probe positioned completely perpendicular to the aorta. Perform a similar scan in the sagittal view with the transducer position towards the patient’s head.

Be sure to include any visible thrombus. Thrombus will appear as an echogenic substance within the aorta. However, it is easily overlooked. Generally, it is best visualized along the anterolateral wall and may create a false lumen that underestimates the actual extent of an aortic aneurysm. 

In unstable patients, providers should routinely complete a right upper quadrant ultrasound to evaluate for pathologic fluid. The majority of AAAs rupture in the retroperitoneum (70% to 90%), where ultrasound cannot assess routinely. Nevertheless, one study established a sensitivity of 97% for identifying ruptured AAAs when the point of care ultrasound was combined with clinical gestalt.

Lastly, sonographic assessment of the aorta includes assessing for the occurrence of an intimal flap, the pathognomonic finding that is 100% specific for aortic dissections. Strict blood pressure management and emergent surgical consultation are mandated.

Complications

There are no known complications from the sonographic assessment of the abdominal aorta. Limitations include body habitus, bowel gas, and operator experience. Instead of a limited evaluation of the abdominal aorta, consider advanced imaging based on the patient's hemodynamic status.

Clinical Significance

More than 90% of abdominal aortic aneurysms occur below the renal arteries. By scanning to the level of the bifurcation, providers ensure complete visualization of the aorta.

Do not overlook an intraluminal thrombus. Include the thrombus to measure the diameter accurately.

Identify intimal flaps, which are pathognomonic for aortic dissections.

It can be difficult to differentiate the IVC from the aorta. Generally, the pulsatile aorta is thick walled, non-compressible, and positioned medial to the IVC. The IVC is thin-walled and compressible. It may appear to pulsate in lieu of its proximity to the aorta and secondary to the normal respiratory cycle. Use pulsed-wave Doppler to delineate the aorta’s pulsatile flow from the venous flow of the inferior vena cava, which should display mild respirophasic variation only.

Although the majority are retroperitoneal, when concerned for a ruptured AAA, perform a right upper quadrant ultrasound to assess for pathologic fluid.