An 88-year-old man who had been hospitalized 3 months prior for Staphylococcus aureus bacteremia and presumed urosepsis presented to the emergency department with acute abdominal pain and acute worsening of chronic back pain. The patient underwent CT of the abdomen and pelvis, followed by a CT angiogram (CTA) (Figure 1). A CT performed during a prior hospitalization showed a normal caliber abdominal aorta (Figure 2). MRI of the lumbar spine during the hospitalization did not show any evidence of discitis or osteomyelitis (Figure 3).
Worsening back pain
New saccular abdominal aneurysm with periaortic fat stranding
Infectious aneurysm (mycotic aneurysm)
Penetrating atherosclerotic ulcer
Abdominal aortic aneurysms are defined by a > 50% focal dilation of the abdominal aorta or when the abdominal aortic diameter is > 3 cm.1-3 Aneurysms can be further classified into the more common fusiform subcategory (accounting for 80% of cases), or the rarer saccular type.3 Saccular aneurysms are focal and have a more lobular configuration with a narrower neck. If a saccular aneurysm is identified on imaging, the primary differential diagnoses include infected aneurysm, inflammatory aneurysm, and penetrating atherosclerotic ulcer, with or without contained rupture. The role of imaging is for diagnosis and treatment planning. Differentiating a saccular aneurysm from its fusiform counterpart is imperative since repair is recommended for all saccular aneurysms regardless of size or symptomology.1 Additionally, certain imaging features such as rapid growth, periaortic soft-tissue stranding, and periaortic fluid can suggest impending rupture.
Infected aneurysms most commonly involve the aorta, with the most common causative organisms being Staphylococcus, Salmonella, and Streptococcus species.4 An infected aneurysm is defined as infectious disruption in the wall of an artery with development of a saccular protrusion that is contiguous with the lumen of the artery. Development may be secondary to infection of a pre-existing intimal defect, hematogenous spread via the vasa vasorum in a normal or aneurysmal artery, direct spread from trauma or intervention, or contiguous spread from adjacent abscess. Clinical presentation can be widely variable, ranging from life-threatening hemorrhage to being clinically occult. Most patients usually present febrile or septic and with abdominal and back pain.3
CTA is the imaging modality of choice for evaluating a suspected infected aneurysm, which manifests as a saccular aneurysm with lobular contours and periaortic soft-tissue enhancement, edema, and stranding. Periaortic gas and adjacent abscess may also be present. The appearance of an infected aneurysm on MRI is similar to that on CT, with periaortic high-signal intensity on T2-weighted imaging (T2WI) and periaortic enhancement after administration of gadolinium-based contrast. Osseous involvement may be better delineated with MRI. Ultrasound is not reliable for diagnosis of infected aneurysms of the abdominal aorta but is utilized for evaluation of peripheral arteries.
Inflammatory abdominal aortic aneurysms are a variant of typical atherosclerotic aneurysm and are usually fusiform. They are characterized by extensive thickening of the aneurysm wall, perianeurysmal fibrosis or soft-tissue thickening, and adherence to adjacent retroperitoneal structures.5 Inflammatory aortic aneurysms occur in a younger population of patients than atherosclerotic aneurysms. Patients often present with abdominal or back pain (80% of patients), weight loss, and an elevated erythrocyte sedimentation rate.2
On CT, an inflammatory aneurysm typically presents as a fusiform aneurysm with soft tissue surrounding the aorta. The perianeursymal soft tissue enhances with administration of intravenous contrast, and the degree of enhancement correlates with stage of disease and treatment response. Early and active disease will demonstrate avid enhancement and later stage and chronic disease will have minimal enhancement. In later stages of the disease, there is often inflammatory fibrotic retraction of adjacent structures, including the ureters, which can cause hydronephrosis. On MRI, T2WI will exhibit characteristic high-signal intensity in early/active disease and low-signal intensity in late/chronic disease. In a similar fashion, postcontrast T1-weighted imaging (T1WI) will demonstrate increased enhancement in early/active disease and relatively low enhancement in late/chronic disease.
Penetrating atherosclerotic ulcers (PAUs) occur from an ulcerated atherosclerotic plaque that penetrates the internal elastic lamina.6 They are often associated with varying degrees of intramural hemorrhage and can progress to intramural hematoma, dissection, and aortic rupture. PAUs most often develop in the thoracic aorta in patients with advanced atherosclerotic disease.3
On CTA, PAUs appear as focal outpouching of contrast beyond the expected margin of the aortic wall in the region of an atherosclerotic plaque. The plaque will often be inwardly displaced and subintimal hematoma may be present.6 On MRI, mural indentation is surrounded by signal hyperintensity on T1WI and T2WI, owing to the presence of blood products.
Infected (mycotic) aneurysm
Identifying and differentiating a saccular aneurysm is of great importance due to the high morbidity and mortality associated with the differential diagnoses when not treated appropriately. Imaging, particularly CTA, is essential for diagnosis and treatment planning for patients with a new or enlarging saccular aneurysm. Imaging findings in conjunction with clinical presentation and laboratory values are important for the differential diagnoses: infected aneurysm, penetrating atherosclerotic ulcer, and inflammatory aneurysm. Thus, an understanding of the patient presentation, disease process, and imaging features is vital for the radiologist.
Pavidapha A, Kotecha H. New Saccular Abdominal Aortic Aneurysm. J Am Osteopath Coll Radiol. 2020;9(1):21-23.