Inflammatory abdominal aortic aneurysm

Inflammatory abdominal aortic aneurysm (IAAA) is a variant of abdominal aortic aneurysm (AAA) characterized by inflammatory thickening of the aneurysm wall, perianeurysmal fibrosis, and adherence to surrounding structures.

Epidemiology

They account for  ~5 to 10% of all AAAs.

Clinical presentation

• patients younger (on average 10 years younger than patients with typical AAA)
• risk of rupture is less than for typical AAA.
• unruptured inflammatory AAAs are usually symptomatic: 
  • abdominal or back pain (70 to 80%)
  • abdominal tenderness
  • fever
  • weight loss
• elevated ESR (90%)

Pathology

Histopathological findings:

• intima: atherosclerosis, much like typical AAA
• media: atrophy and loss of elastic tissue
• adventitia: marked inflammatory thickening with abundant lymphocytes, plasma cells, and macrophages

Inflammation and fibrosis extend into the periaortic tissue and may entrap adjacent retroperitoneal structures such as the ureters or duodenum. There is some overlap with retroperitoneal fibrosis.

The circumference of the aneurysm wall is involved with inflammation, but for unknown reasons, the periaortic fibrosis is most prominent around the anterior and lateral walls of the aneurysm with relative sparing of the posterior wall.

The etiology of the inflammatory changes is poorly understood and many theories exist. Some believe it is an abnormally severe inflammatory response to the atherosclerotic process in the aorta. Some believe it is an autoimmune disease which is supported by the elevated ESR found in most patients; frequent constitutional symptoms; increased frequency of autoantibodies and other autoimmune diseases; and the not-uncommon family history of inflammatory AAA. Approximately 30% of IgG4-related aortitis involves abdominal aorta ⁴. Others postulate an infectious etiology.

Risk factors

• male gender
• atherosclerotic risk factors
• family history

Radiographic features

CT and MRI are best for imaging assessment.

Ultrasound

• will identify aneurysmal dilatation of the aorta
• may show thickened aortic wall but this is difficult to distinguish from thrombus in the aneurysm
• not good at detecting perianeurysmal fibrosis
• hydronephrosis if the ureters are entrapped
• may show a sonolucent halo around aorta

CT

• aneurysmal dilatation of aorta
• thickened aortic wall (>2 mm) ⁴
• cuff of perianeurysmal soft tissue and inflammatory changes, sparing the posterior wall, which enhances with contrast
• good at detecting entrapment of retroperitoneal structures

MRI

• aneurysmal dilatation of aorta
• thickened aortic wall
• cuff of perianeurysmal soft tissue and inflammatory changes, sparing the posterior wall, which is 
  • T1: hypointense
  • T2: hyperintense 
  • T1 C+ (Gd): shows enhancement
• will detect entrapment of retroperitoneal structures

Treatment and prognosis

Corticosteroids and other immunosuppressive drugs have been found to decrease symptoms and the degree of periaortic inflammation and fibrosis.

Definitive treatment of the aneurysm is performed via EVAR or open surgical repair. Repairing the aneurysm causes varying degrees of regression of the periaortic fibrosis in many patients. Surgery may be necessary if there is entrapment of retroperitoneal structures.

Differential diagnosis

Consider

• infective (mycotic) abdodominal aortic aneurysms
  • usually more saccular with nodularity, irregular configuration and may contain air in the aortic wall ⁵