What is a mycotic aneurysm?

What is a Mycotic Aneurysm?

A mycotic aneurysm is an infected aneurysm of a blood vessel caused by microbial invasion of the arterial wall, most commonly by bacteria (despite the misleading term "mycotic" which historically referred to its mushroom-like appearance rather than fungal etiology). 1

Historical Context and Terminology

The term "mycotic aneurysm" was coined by William Osler in 1885 to describe the mushroom-like appearance of infected aneurysms associated with bacterial endocarditis, not because they were caused by fungi. 2, 3, 4 This creates ongoing confusion since "mycotic" by definition suggests fungal origin, yet the vast majority are bacterial. 3 True fungal aneurysms are exceedingly rare, with only seven cases reported between 1966-1999. 3

It is critical to distinguish mycotic aneurysms from syphilitic aneurysms, which develop through a completely different chronic inflammatory mechanism 10-25 years after initial spirochetal infection and predominantly affect the ascending thoracic aorta as fusiform aneurysms. 5, 6 Syphilitic aneurysms are not acute infectious processes like true mycotic aneurysms. 6

Epidemiology and Location

Mycotic aneurysms are rare, accounting for only 0.7% to 4.5% of all aortic aneurysms and approximately 0.6% of aneurysms overall in Western countries. 1, 2 The distribution by location is: 1

• Abdominal aorta: ~70% (predominantly suprarenal, unlike atherosclerotic aneurysms which are 85% infrarenal)
• Thoracic aorta: ~30%
• Visceral arteries: <1%
• Intracranial arteries: 0.7-6.5% of all intracranial aneurysms, most commonly in the middle cerebral artery (44% of cases) 5

Men predominate over women by 3:1, with average age >65 years, and strong associations with cigarette smoking and diabetes mellitus. 1

Pathophysiology: Five Key Mechanisms

The American Heart Association describes five distinct mechanisms for mycotic aneurysm development: 1

1. Infected atherosclerotic plaque: The normal aortic intima resists infection, but damaged endothelium from atherosclerotic plaques or ulcers can be colonized during bacteremia, leading to infected atherosclerotic aneurysm formation. 1

2. Vasa vasorum infection: Bacteremic or contiguous spread to the vasa vasorum proceeds inward toward the vessel wall, causing localized infection with wall thinning and aneurysm development. 1

3. Contiguous spread: Infection extends from gastrointestinal sources (especially Salmonella gastroenteritis) or vertebral osteomyelitis, creating primary abdominal aortic mycotic aneurysms or secondarily infecting preexisting atherosclerotic aneurysms. 1

4. Congenital abnormalities: Endothelial damage from cystic necrosis or coarctation makes vessels susceptible to infection from bacteremia or contiguous spread. 1

5. Cardiac surgery-related: Mycotic aneurysms in the sinus of Valsalva or aortic arch develop following reconstructive surgery, infective endocarditis, or prosthetic valve endocarditis. 1

Microbiology: Evolving Spectrum

The microbiological profile has shifted dramatically in recent years: 1

• Staphylococci (S. aureus and coagulase-negative): Now the most common cause, accounting for 50-60% of cases 1
• Gram-negative bacilli: Previously dominant, now 30-40% of cases 1
  • Salmonella species (especially nontyphoidal strains like S. enteritidis and S. choleraesuis): More common in Asia, with special predilection for vascular tissue; 25% of patients ≥50 years with Salmonella bacteremia have endovascular infection 1
  • Other gram-negatives including Arizona species, Pseudomonas aeruginosa 1, 2
• Less common pathogens: Listeria monocytogenes, Mycobacterium tuberculosis (via vertebral osteomyelitis erosion), Streptococcus pneumoniae, anaerobes 1, 2
• Fungi: Candida and Aspergillus species are rare but reported 1, 3

Clinical Presentation: Nonspecific but Critical Features

The classic triad of fever, pain, and pulsatile abdominal mass is actually uncommon. 1, 7 More realistic presentations include: 1

• Fever: Present in ≥70% of intra-abdominal cases (uncommon in uninfected atherosclerotic aneurysms) 1
• Back pain: 65-90% of cases (less common in uninfected aneurysms) 1
• Intrathoracic mycotic aneurysms: Fever, chest and interscapular pain, findings of infective endocarditis or prosthetic valve infection 1
• Visceral artery involvement:
  • Superior mesenteric: Acute abdominal pain, fever, rarely palpable epigastric mass, potentially catastrophic GI hemorrhage 1
  • Hepatic artery: Fever, colicky upper abdominal pain, hemobilia, jaundice, GI hemorrhage 1
  • Renal artery: Fever, hematuria, elevated blood pressure 1

Catastrophic Complication: Rupture

Complete or contained rupture occurs in 50-75% of patients, with rupture sites including: 1

• Retroperitoneal/peritoneal space (50%)
• Duodenum (12%)
• Pleural cavity (10%)
• Esophagus, mediastinum, or pericardium (3-5%)

Impending or contained rupture presents with severe pain, hemodynamic instability, or sudden catastrophic rupture with shock and high mortality. 1

Diagnostic Approach

CT angiography (CTA) is the initial imaging procedure of choice, allowing rapid examination in potentially unstable patients requiring urgent surgical intervention. 1 CTA defines: 1

• Precise location and vascular anatomy
• Detection of impending rupture
• Associated complications
• Serial monitoring capability

Key imaging characteristics distinguishing mycotic from atherosclerotic aneurysms: 5

• Saccular, lobular, or irregular morphology (vs. fusiform)
• Minimal to absent calcification
• Periaortic soft tissue stranding/inflammatory changes
• Rapid enlargement on serial imaging

Laboratory findings are nonspecific: 1

• Leukocytosis (65-85% of cases)
• Elevated inflammatory markers (ESR, CRP in 75-80%)
• Blood cultures positive in 50-90% of cases 5

Critical Clinical Pitfalls

1. Do not confuse with inflammatory abdominal aortic aneurysms (5-10% of all AAAs), which occur in older men with smoking/diabetes history but fever is less common. 1

2. Suspect aortoenteric fistula and mycotic aneurysm in patients with recent or remote AAA repair; up to 4% develop aortoenteric fistulae. 1

3. In renal transplant recipients, consider mycotic aneurysm related to perinephric infections, particularly in the external iliac artery. 1

4. Lumbar spine osteomyelitis is present in up to one-third of patients with Salmonella-caused aortic mycotic aneurysms. 1