Low back pain is just one symptom of abdominal aortic aneurysms. Diagnose and locate these with the right film and imaging modalities.

Aneurysms refer to the focal dilatation of a vessel commonly occurring in the aorta, but also occurring in other locations including the common iliac, popliteal, splenic arteries, and the Circle of Willis.¹ Inclusion of this topic is an important entity to consider in patients presenting to the chiropractor. Abdominal aortic aneurysms (AAAs) may be clinically silent or cause localized symptoms including low back pain. It is with this in mind that a discussion about AAAs is included in this edition.

The majority of AAAs occur those over 60 and are more common in males. The most common cause of AAA is atherosclerosis. Additional less common causes in the adult population include arteritis (for example, as a result of a systemic vascular disease like rheumatoid arthritis), poststenotic dilation, postinfection, and conditions such as diabetes, gout, and tuberculosis. Additional risk factors include tobacco use, hypertension, chronic obstructive airway disease, claudication, and an AAA in a first-degree relative.

However, it is also possible for the younger population to be at risk of developing AAAs, although this is much less common. Causes in the younger population include infection, iatrogenic trauma, vasculitis, connective tissue disorders such as Marfans syndrome, or tuberous sclerosis. A case of a primary congenital AAA has also been reported.²

The explanation for the development of an aneurysm formation is multi-factorial but is considered to be a combination of degeneration with the presence of atherosclerosis. The calcification and soft-tissue changes about the atheroma formation may compromise the media leading to progressive failure. The most common location for an AAA is in the infrarenal region, an area with a notable absence of medial vasa vasorum, the vascular supply for the vessel walls.

The aorta, a soft-tissue structure, is normally not visible on conventional radiography. In the presence of atherosclerosis, calcification of the wall of the aorta can occur and is visible.

It has been suggested that this leads to a further propensity for degeneration at this site with reduction in supply of oxygen and nutrients (including collagenase, elastase and anti-proteases).³ With the constant pulsatile flow, failure of the media leads to the gradual dilatation of the vessel, an aneurysm.

The site of predilection in the infrarenal area with possible extension into the common iliac arteries. The most common morphology for an AAA is that of a fusiform-shaped appearance with circumferential widening of the vessel.

It is also possible that due to the progressive failure of the vessel walls that blood may track between the media and adventitial layer, a dissecting aneurysm. With time, the progressive wall failure may also lead to initially gradual expansion in the smaller AAA and latterly rapid progression of the diameter of the vessel to an extent where rupture of the AAA occurs with a concurrent high mortality.

Imaging Modalities
Imaging. Abdominal aortic aneurysms may be detected on conventional radiography studies performed for a different reason—for example, a a 78-year-old hypertensive man presenting to the chiropractor with an insidious onset of low back pain. However, several imaging modalities may detect the presence of an aneurysm, including computed tomography (CT), MRI, abdominal ultrasound, and angiography.⁴

Assessment. AAAs may be detectable on conventional radiography. The aorta, a soft-tissue structure, is normally not visible on conventional radiography. In the presence of atherosclerosis, calcification of the wall of the aorta may occur which may be visible (see Figure 1). The normal appearance of the aorta using conventional radiography on the lateral view is less than 30 mm and in the presence of calcification, the walls of the vessel should be parallel. If the diameter of the walls of the aorta at the lower margin of L4 are measured at more than 38 mm or the walls of the aorta are noted to be divergent, strong evidence of an abdominal aortic aneurysm is present.

A caveat is that to visualize the anterior margin of the aorta in patients over age 45 years old, use of a 35 X 43-centimeter film is recommended. The reason is that should an aneurysm be present, it is more likely to be detected using a larger film that allows for the inclusion of the divergent anterior wall of the aorta. With a narrower film size, it is much more likely that such a finding would be missed.

The predominant feature of an AAA on a plain-film radiograph is that of deviation of the walls of the aorta. However, it is usually necessary for calcification of the walls of the aorta to be present before such deviation exists. As such, 50% of AAAs will have such calcification. An additional sign that is less easy to visualize is that of deviation of the psoas fascial plane on the posterior-anterior (or anterior-posterior) view.

It is important to evaluate the diameter of the abdominal aorta that is possible to perform initially on conventional radiography but more specifically on an abdominal ultrasound/B-mode ultrasound, CT, and MRI. The measurement is taken at the level of the lower limit of L4 from the posterior to anterior margin of the calcified wall of the aorta. An AAA is present if the measurement is more than 3.8 cm or if there is divergence of the walls of the vessel at this level.

To evaluate the AAA with more detail, abdominal ultrasound is recommended, not only to characterize the aneurysm in more detail but also to follow the progression of the size of the aneurysm with time. Ultrasound is a relatively comfortable procedure to endure and is both economical and time efficient.

Previously, CT was predominantly used as a preoperative imaging modality. However recent evidence points to discrepancies arising between the measurements on abdominal ultrasound and CT, with CT producing larger wall diameters compared with ultrasound.⁵ CT allows for the determination of the aortic diameter enlargement and may also demonstrate perianeurysmal fibrosis, which may lead to complications depending on the location of the fibrosis.

Treatment can include elective surgical resection of AAAs (prior to rupture), which offers a low operative mortality and good prognosis. Patients commonly have a midline abdominal or left flank retroperitoneal. After the aneurysm is exposed, a graft is placed from the unaffected proximal to distal portion of the aorta adjacent to the aneurysm. According to the size of an AAA, a propensity to rupture may be present especially in an abdominal aorta that is more than 5.5 cm (based on conventional radiography measurements). Rupture usually occurs in to the left retroperitoneum, gastrointestinal tract or may involve the inferior vena cava. Additional complications include peripheral embolisation, infection, and spontaneous occlusion.

A sensible approach for a patient with an AAA is to use more soft tissue techniques in the lumbar spine. In addition it is important to recall that since atherosclerosis is commonly involved in the development of an AAA that other arteries may be affected by a similar process such as the carotid artery. This should be considered when selecting techniques appropriate for these regions. CP

Michelle Wessely, BSc (Chiropractic), DC, DACBR, FCC (UK/Radiology), is the departmental head of Radiology at Institut Franco-European de Chiropratique, Paris. She completed her fellowship in musculoskeletal radiology at the Department of Osteoradiology, Veterans Affairs Healthcare Center, San Diego, under Donald Resnick, MD, professor of radiology. Wessely provides a film-reading service for chiropractors and lectures to both chiropractic undergraduates and postgraduates in Europe. She can be reached at: mwessely@ifec.net;  michelle_ wessely@yahoo.com;  (033) (0)1 45 15 89 10.

References
1) Yochum T, Rowe L. Essentials of Skeletal Radiology. Vol 2. Baltimore: Williams & Wilkins; 1996:1306–1318.
2) Bell P, Mantor C, Jacocks MA. Congenital abdominal aortic aneurysm: a case report. J Vasc Surg. 2003;38(1):190–193.
3) Garbarsch C, Matthiessen ME, Helin P, Lorenzen I. Arteriosclerosis and hypoxia. I. Gross and microscopic changes in rabbit aorta induced by systemic hypoxia. J Atherosclero Res. 1969;9(3):283–294.
4) Crawford CM, Hurtgen-Grace K, Talarico E, Marley J. Abdominal aortic aneurysm: an illustrated narrative review. J Manipulative Physiol Ther. 2003;26(3):184–195.
5) Sprous LR II, Meier GH III, Lesar CJ, et al. Comparison of abdominal aortic aneurysm diameter measurements obtained with ultrasound and computed tomography: Is there a difference? J Vasc Surg. 2003;38(3): 466–472.