Mycotic aneurysms are aneurysms arising from infection of the arterial wall, usually bacterial. It is a complication of the hematogenous spread of bacterial infection, classically from the heart. 

Epidemiology

Mycotic aneurysms are thought to represent only a minority of (0.65-2.6%) of all aortic aneurysms ^10-13.

The epidemiology of mycotic aneurysms mirrors that of identifiable risk factors: 

• infective endocarditis (common)
• intravenous drug use
• immunosuppression
• iatrogenic arterial trauma
• pre-existing atherosclerotic plaque or a native aneurysm
• prosthetic arterial devices (e.g. stents and grafts)
• direct arterial involvement from adjacent infection

Pathology

The vessel wall becomes infected with bacteria, is digested, and false aneurysm forms, which is unstable and highly prone to rupture. The most common organisms are Staphylococcus aureus and Salmonella spp. Mechanisms of infections include:

• septicemia
• septic emboli
• contiguous spread from adjacent infection

Location

Frequently found in atypical locations. The most common sites are:

• thoracic and abdominal aorta
• abdominal visceral arteries
• lower extremity arteries (femoral artery is the most frequently involved and often associated with intravenous drug use ⁶)
• intracranial arteries: typically more peripheral than berry aneurysms (see intracranial mycotic aneurysm for detailed discussion)

Radiographic features

CT

Described features include the following (some are however non-specific):

• saccular
• centric aneurysmal sac in an odd location for atheromatous disease
• often wild, multilobulated appearance
• interruption of arterial wall calcification
• adjacent soft tissue stranding
• adjacent collection +/- gas
• adjacent reactive lymphadenopathy
• hazy aortic wall with rupture ⁴
• retroperitoneal para-aortic fluid collection and vertebral erosion
• thrombus formation within a false lumen after aneurysmal rupture

Treatment and prognosis

Mycotic aneurysms carry a very high mortality.

Complications

• aneurysmal rupture (high risk) with hemorrhage
• an ongoing source of sepsis
• embolic infarction

History and etymology

The term "mycotic" (meaning fungus) was coined in 1885 by Canadian physician William Osler (1849-1919) on describing the gross pathological appearance of two small saccular aortic aneurysms, not the underlying pathological organism ⁹. 

Differential diagnosis

• atherosclerotic aneurysm
• inflammatory aortic aneurysm