Intern Ultrasound of the Month: Iliac Aneurysm Mimicking Pelvic Structures

The Case

A 90-year-old male with past medical history significant for chronic urinary retention requiring an indwelling Foley catheter complicated by urinary tract infections (resulting in a hospitalization for septic shock approximately one month prior to this), benign prostatic hyperplasia, pulmonary embolus (on apixaban), and severe aortic stenosis presented to the emergency department for urinary retention. The patient stated that after he recovered from urosepsis during his last admission, he was discharged from the hospital with a Foley catheter, which was most recently changed five days ago at his nursing facility. According to the patient and his wife, since the new foley was placed, it had not been draining, with little to no urine output. He denied any additional symptoms, including fevers, chills, nausea, abdominal pain, diarrhea, nausea, vomiting.

On exam, there was approximately 5 cc of straw colored urine in the foley bag. He had suprapubic fullness with moderate tenderness to palpation. There were no other masses, organomegaly, or overlying skin changes present. The rest of his physical exam was normal, including +2 dorsalis pedis pulses bilaterally, and no costovertebral angle (CVA) tenderness. 

To evaluate placement of his foley and to estimate bladder volume, point-of-care ultrasound was performed. 

Sagittal view of the bladder: urinary retention is present with part of the foley poorly visualized at the edge of the bladder (screen right)

Sagittal view with color doppler showing pulsatile flow within the hypoechoic structure deep to the bladder

Transverse view of the bladder: urinary retention and part of his catheter again noted

On closer inspection, the Foley balloon is faintly visualized within the bladder

Transverse view with color doppler: pulsatility again noted

Diameter of the pulsatile structure measured to be 5.1 cm

POCUS Findings: 

There is urinary retention with a bladder volume of approximately 450 ml of urine with the foley balloon faintly visualized. Deep to the bladder is an incidental well-circumscribed hypoechoic structure measuring just over 5cm; when color doppler is applied, there is pulsatile flow, raising concern for an aneurysm.

Iliac aneurysms

Aneurysm of the abdominal aorta is a relatively common disease in patients over fifty years of age, and rupture of an abdominal aortic aneurysm (AAA) has high mortality (1). As well-described in a previous blog post by Dr. Thai, AAA can be diagnosed when the AP diameter of the aorta exceeds 3.0 cm. The risk of rupture increases dramatically when the diameter of the aorta exceeds 5.0 cm (1). Clinicians with less ultrasound experience may focus solely on the aorta and end their exam once they determine it is less than 3cm… But what about the iliac arteries?

A complete aortic ultrasound exam includes obtaining a longitudinal axis view and a short axis/transverse view of the proximal, mid, and distal portions of the aorta, as well as extending down to the aortic bifurcation. This requires continuing to slide your probe caudally until you visualize the aorta bifurcating into the left and right common iliac arteries, which is usually around the level of the umbilicus (2-3).

Iliac artery aneurysms are present in up to 20% of AAAs, but are found in the absence of AAA in 2-7% of cases. The normal diameter of the iliac arteries is approximately 1 cm. A diameter that exceeds 1.5 cm is considered an aneurysm, and elective repair is generally considered once it reaches 3.5 cm (4).

 

Figure 1. Aorta Anatomy (3)

 

Figure 2. Aorta and surrounding anatomy visualized sonographically (2)

Figure 3. Bifurcation into the left and right common iliac arteries (2)

Figure 4. Iliac aneurysm (3)

Recap of the components for a complete aortic ultrasound:

  1. Transverse view of the proximal aorta

  2. Transverse view of the mid aorta

  3. Transverse view of the distal aorta

  4. Transverse view of the aortic bifurcation into the right and left common iliac arteries

  5. Longitudinal view of the aorta

…with the focused clinical questions:

  1. Is the abdominal aorta > 3 cm in diameter?

  2. Are the iliac arteries > 1.5 cm in diameter?

**Measure outer wall to outer wall to ensure an accurate determination of size (2). This is especially important when an intraluminal thrombus is present, as the diameter of only the patent part of the lumen may underestimate the true aortic size.

Point-of-care ultrasound (POCUS) can help quickly aid in the diagnosis of an aneurysm with high sensitivity and specificity (5-6). Ultrasound has been found to be comparable to CT in identifying aneurysms, especially when the size exceeds 5.5 cm (7). While POCUS cannot rule out rupture, the presence of the following findings raise concern for rupture: aneurysmal deformation, intraluminal thrombus +/- interruption, retroperitoneal hematoma, and hemoperitoneum (8). Optimal definitive management is largely dependent on several factors including the aneurysm size, presence of rupture, and patient comorbidities/risk factors (9).

Evaluation of foley placement

In patients with indwelling urinary catheters who have flank or abdominal pain, POCUS is useful for evaluating proper placement and functioning of the catheter. A functioning catheter will appear as a spherical object (the saline filled bulb) and a decompressed bladder (10), as shown in Figure 4.

 

Figure 4. Foley balloon visualized within the bladder appropriately draining urine

 

In an obstructed foley, which can occur from a variety of reasons, the bladder will be distended since the outflow tract is impeded, as seen in Figure 5. Common causes of catheter malfunction include stagnant clots that occlude the tip of the catheter’s intake as well as tip occlusion when pressed against bladder wall (10), illustrated in Figures 6 and 7, respectively.

Figure 5. Examples of a malfunctioning foley -- the balloon is appropriately placed within the bladder; however, the urinary retention indicates an issue with the foley.

Figure 6. Foley catheter with clot within the bladder resulting in urinary obstruction

Figure 7. Foley catheter tip lodged against wall resulting in urinary obstruction

Another potential reason for foley malfunction is inappropriate placement. In our patient’s case, there was initial concern that the foley balloon was inflated outside of the bladder wall. There is a case report from 2022 where a foley catheter was placed through a bladder diverticulum in an outpatient setting 5 days prior, see Figure 8; the chief complaint on presentation was lower abdominal pain (11). Another common location for foley misplacement is within the prostatic urethra, see Figure 9 (12). This, too, is often painful.

Figure 8. Foley balloon visualized within a bladder diverticulum, outside of the bladder wall (red = foley balloon; green = bladder) (11)

Figure 9. Foley balloon within the prostatic urethra (12).

Bladder volumes

There is much utility in estimating bladder volume with POCUS, with or without the presence of a foley. From this quick exam, you can directly visualize the amount of urine within the bladder rather than relying on a bladder scanner to correctly distinguish the bladder from other fluid. This can indicate if there is clinical utility in decompressing a full bladder while also potentially identifying causes of post-renal obstruction. For simplicity, the shape of the bladder in calculating volume is approximated to an ellipsoid or cylinder. There are many calculations to use and varying machines will often provide their own calculations with measurement inputs. If stuck with nothing but a probe and a calculator, the quickest and easiest calculation is 0.75 x width x length x height (2). For these measurements, you will need to obtain a transverse, longitudinal, and depth measurement, which will look like a “Plus” sign on your transverse view, and a “minus” sign on your sagittal view, see Figure 10. Post void residual volume of <50 ml is considered normal for most people, but <100 ml may be acceptable for people over 65 years old (13).

 

Figure 10. Estimating bladder volume on ultrasound.

 

Clinical application / discussion

As with everything we do in emergency medicine, ambiguous ultrasound (or other imaging) findings require clinical reasoning. Here, we had an anechoic spherical structure outside of the bladder wall following recent replacement of a foley catheter. The differential of this finding included foley catheter misplacement, free fluid in the pelvis, fluid filled bowel, abscess, and iliac aneurysm. Due to the defined circular structure of the object, this made free fluid in the pelvis unlikely. The patient was lying supine, and free fluid would most likely appear as anechoic fluid in the rectosigmoid space without its own distinct shape. A vascularized structure was distinguished from fluid-filled bowel, a foley bulb, or abscess/cyst by applying color doppler, which showed the structure to be pulsatile. This quickly narrowed the possibilities of our finding to an aneurysm versus pseudoaneurysm; since the entire vessel could be visualized without a focal outpouching or jet of color flow, we suspected this was more likely a true iliac aneurysm. Additionally, the foley balloon was deflated under ultrasound visualization and this hypoechoic structure persisted, making the foley balloon an unlikely explanation for this finding.

With these simple steps and using our clinical knowledge, our undifferentiated incidental ultrasound finding was identified in a matter of seconds.

Case Conclusion:

Following the identification of this patient’s iliac aneurysm, urgent CT angiography was obtained. Further chart investigation found that the patient had been diagnosed with this in the past but was lost to follow up with vascular surgery; at the time, they felt that he was not a good surgical candidate due to his multiple comorbidities. When comparing our POCUS images to comprehensive studies years prior to this, the aneurysm appeared to be slightly increased in size. Vascular surgery was consulted during this visit, and based on overall stability of the aneurysm on CT, they felt that outpatient follow up with frequent monitoring was appropriate. The patient’s foley catheter was exchanged with subsequent good return of urine and relief of the obstruction. He was discharged home in stable condition.

AUTHORED BY: JAKE PERINO, MD (PGY1)

FACULTY EDITING BY: LAUREN MCCAFFERTY, MD

References

  1. US Preventive Services Task Force. Screening for Abdominal Aortic Aneurysm: US Preventive Services Task Force Recommendation Statement. JAMA. 2019;322(22):2211–2218.

  2. Noble V, Nelson B. Manual of Emergency Medicine and Critical Care Ultrasound, 2nd ed. Cambridge: Cambridge UP, 2011.

  3. Risler Z, Dean AJ, Ku BS. Abdominal aortic aneurysm (AAA). American College of Emergency Physicians Sonoguide. August 2020. Accessed January 2024. https://www.acep.org/sonoguide/basic/aorta.

  4. Lewiss RE, Egan DJ, Shreves A. Vascular abdominal emergencies. Emerg Med Clin North Am. 2011;29(2):253-72, viii.

  5. Costantino TG, Bruno EC, Handly N, Dean AJ. Accuracy of emergency medicine ultrasound in the evaluation of abdominal aortic aneurysm. J Emerg Med. 2005;29(4):455-60.

  6. 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;20(2):128-38.

  7. Foo FJ, Hammond CJ, Goldstone AR, et al. Agreement between Computed Tomography and Ultrasound on Abdominal Aortic Aneurysm and Implications on Clinical Decisions. Eur J Vasc Endovasc Surg. 2011; 42, 608-614.

  8. Catalano O, Siani A. Ruptured abdominal aortic aneurysm. Categorization of sonographic findings and report of 3 new signs. J Ultrasound Med. 2005;24(8):1077–1083.

  9. Chaikof EL, Dalman RL, Eskandari MK, et al. The Society for Vascular Surgery practice guidelines on the care of patients with an abdominal aortic aneurysm. J Vasc Surg. 2018;67(1):2-77.e2.

  10. Koratala A, Bhattacharya D, Kazory A. Point of care renal ultrasonography for the busy nephrologist: A pictorial review. World J Nephrol. 2019;28;8(3):44-5

  11. Pettet D, Smilios C, Nelson M. Where did that catheter go? A case report of Foley catheter placement in bladder diverticulum with point-of-care ultrasound. JEM Reports. 2022 Dec 1;1:100001.

  12. Misplaced Foley in the prostatic urethra. Nephro POCUS. Available from: https://nephropocus.com/2020/02/07/misplaced-foley-in-the-prostatic-urethra/

  13. Woods A. POCUS for Bladder Assessment and Volume: Technique, Tips, and Role in System-Wide Use [Internet]. [cited 2024 Jul 28]. Available from: https://www.acep.org/emultrasound/newsroom/may-2022/pocus-for-bladder-assessment-and-volume-technique-tips-and-role-in-system-wide-use

UltrasoundLauren McCaffertypocus, ultrasound, pelvis, aneurysm, iliac