a Life Threatening Circumstance in Chiropractic Clinical Practice! ne of the highest risk factors a practicing chiropractor faces in the aging patient population is the presence of an abdominal aortic aneurysm (AAA). These patients may often present with occult back pain mimicking degenerative disc disease or facet syndrome. The symptoms are often vague and confusing. The early diagnosis and referral of the patient for appropriate medical treatment is imperative. The overall incidence of abdominal aortic aneurysm (AAA) is estimated at between 2-and-4%. Males are more frequently affected by a 5:1 ratio. Almost 40% exhibit hypertension and 30% have heart disease. At least 95% of the aneurysms begin l-to-2 cm below the renal arteries (infrarenal AAA); 5% are suprarenal. Atherosclerosis accounts for at least 90-to-95% of AAA, with the remainder predominantly the inflammatory type. The normal aorta is typically less than 30 mm, beyond which it is described as being aneurysmal. The dilatation is anterior and lateral in most cases and usually fusiform in configuration. One or both common iliac arteries are involved by direct extension of the AAA or as a separate aneurysm in 21-to-66%. Clinical Features Clinically, these patients are often asymptomatic. Intermittent abdominal and back pain is common. Other symptoms may include lower limb ischemia (claudication, thromboembolism), paralysis that is due to cord ischemia, and bloody diarrhea from ischemic bowel. Leakage is characterized by increasing abdominal and back pain and signs of hypovolemic shock. Frank rupture results in sudden death. The most common site for rupture is through the left posterior lateral wall. The imaging strategy for the detection of AAA depends on clinical presentation of the surgical risk factors for the patient. Clinical suspicion for an AAA, based on risk factors, palpation, and auscultation, should lead to confirmation with ultrasound. Accidental discovery is extremely common when plain radiographs, CT, or MR imaging are taken for evaluation of back pain. Once the aneurysm is identified, lesions over 5 cm typically go for elective surgery requiring pre- and postcontrast CT and, often, angiography. Those lesions under 5 cm are followed with ultrasound every 4-to-6 months and, if showing enlargement, will go on to elective resection and graft. The average rate of enlargement is 2-to-5 mm per year, though rates as high as 3 cm have been recorded. Elective surgery carries a 5% operative mortality, while emergency operations for rupture may have as high as 80% mortality. Radiological Features The commonly employed imaging modalities are plain films, CT, ultrasonography, MR imaging, and angiography. 1. Plain film. Frontal, lateral, and oblique radiographs may show the lesion. Most abdominal aortic aneurysms occur between the renal artery (L2) and iliac bifurcation (L4), and this is the key region to evaluate. On the frontal film, they are usually seen on the left side of the spine, with often only the left margin visible. A soft tissue density demarcated by a thin, curvilinear rim of continuous or discontinuous calcification is the most characteristic finding. Calcification is detectable in 55% to 85% of cases. Uncalcificd AAA only presents as a soft tissue density, difficult to identify in most cases. The lateral projection, while often the most demonstrative, may be misleadingly normal. Frequently, there is tight collima-tion anterior to the spine, which limits the field of view and may not identify the anterior margin of the lesion. In such cases, the only clue may be a horizontally oriented calcified plaque. Occasionally, the lesion may extend posteriorly and overlay the spine. Measurement for determining the size of the aorta is made between the most distant calcified borders; in the presence of ancurysm, the measurement exceeds 3.8 cm. The close proximity of the aorta to the anterior vertebral bodies in approximately 5% of AAA may precipitate extrinsic anterior body erosions (Oppcnheimer erosions), which can develop in less than 2 years. Erosions are relatively uncommon, since the aorta is not firmly adherent to the vertebral column and the expansion is predominantly anterior and lateral. They are more commonly found in inflammatory states, and contained leaking (sealed rupture) of an AAA, since this will produce adherence to the spine and allow for the transmission of pulsatile impulses to create the erosions. An additional factor is that saccular aneurysms produce more erosions than fusiform AAA; saccular aneurysms are most commonly associated with syphilis and arc a rare occurrence in recent times. These defects are smooth and concave, with the disc-endplate region being relatively unaffected. Other causes of anterior body erosions include lymphoma, lytic metastasis, and subligamentous tuberculosis. Signs of leakage on supine plain film include obliteration of the psoas margin, loss of the renal outline, lateral bulging of the psoas margin, soft tissue extension beyond the calcified margin, ileus, and loss of the properitoneal flank stripe. On erect films, blood may collect within the pelvis, creating a hazy density to the pelvic inlet, loss of the perivesical fat, and air-fluid levels of overlying ileus. Ultrasound. This is the diagnostic modality of choice for confirming the diagnosis, evaluating size, and monitoring progression or stability of the AAA. It is accurate to within 3 mm of actual size of the dilatation. It may show thrombus, periaortic abnormalities, dissections, cephalad and caudal ex tent, and complications. Its usefulness can be limited by obe sity and excessive bowel gas. Suprarenal extension cannot be directly determined, though extension above the superior me- senteric artery provides indirect evidence. CT. CT is capable of determining the extent of the AAA, the relationship to the renal arteries, degree of thrombosis, site of rupture, distribution and amount of leakage, presence of perianeurysmal fibrosis, and ureteral involvement. Previous sealed rupture can be determined by the presence of a retro- peritoneal hematoma below the renal artery behind the anterior renal fascia. Postinfusion scans delineate the true lumen of the AAA from thrombus. CT is the technique of choice for exam ining the aorta after surgical repair. MR imaging. Multiplanar imaging techniques can dem onstrate all of these features without the use of contrast, and can reduce the use of angiography. MR imaging allows accu rate measurement, isolates flow abnormalities, identifies clot, and can assess visceral branch involvement. Angiography. Placement of intravascular contrast to depict the AAA may alter the surgical approach in as many as 25% of cases, especially in the detection of renal artery anoma lies, suprarenal extension, significant lower limb disease (iliacs, femorals), patency of the mesenteric arteries, associated aneu- rysms elsewhere, aortic complications (occlusion, inferior vena caval lesions), and identification of a horseshoe kidney.| Dr. Terry R. Yoclmm is a second-generation chiropractor anil a cum laudc graduate of the National College of Chiropractic, where he subsequently completed his radiology special/}: He is currently Director of the Rnckv Mountain Chiropractic Radiological Center, in Denver. CO, an Adjunct Professor of Radiology at the Los Angeles College of Chiropractic, as well as Professor of Radiology at the Colorado College of Chiropractic at Marycrest University, Denver, CO. Dr. Yoclmm is, also, a consultant for Virtu-Rad. a digital imaging and health care manufacturing company that offers a Stored Energy system. For more information. Dr. Yoclmm can he reached at: (303) 940-9400 or by e-mail at Reference Yochum JR. Rowe I.J: The Essentials of Skeletal Radiology, 2n ed.. Baltimore. Williams & Wilkins. 1996
