Mycotic (Infected) Aneurysm
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Direct answer
Osler's term is a historical misnomer: the mycotic aneurysm is an infected arterial wall aneurysm, and fungi are rarely the culprit. Septic emboli from infective endocarditis seed distal vessels such as the femoral or cerebral arteries, while direct spread and bacteraemia infect atherosclerotic aortas, often with Salmonella. The aneurysm is saccular, eccentric and non-contiguous, develops rapidly over days to weeks, and ruptures early. Blood cultures plus contrast CT establish the diagnosis; treatment is culture-directed antibiotics for at least six weeks combined with surgery — resection and debridement with revascularisation, because antibiotics alone almost never sterilise an aneurysm sac.
What you must remember
- Terminology: Osler coined "mycotic" in 1885 for endocarditis-related aneurysms; modern usage means any infected aneurysm, bacterial in over 90% of cases.
- Two routes, two organisms: haematogenous seeding from infective endocarditis (Staphylococcus aureus dominant, Streptococcus next) hits peripheral and cerebral vessels; Salmonella species particularly infect an atherosclerotic or pre-aneurysmal aorta through an intact but diseased endothelium.
- Morphology: saccular, eccentric, multilobulated, frequently multiple and non-contiguous, with periaortic stranding or rim-enhancing fluid on CT — unlike atherosclerotic fusiform aneurysms.
- Clinical triad to recall: fever with a rapidly expanding tender mass, negative past aneurysm history, and positive blood cultures in roughly half to two-thirds.
- Risk groups: intravenous drug users, endocarditis, immunosuppression, diabetes, prior vascular surgery — infection of a prosthetic graft produces the same disease at the anastomosis.
- Imaging and cultures: CT angiography first; obtain blood cultures before antibiotics (and pus or tissue cultures intraoperatively); FDG-PET helps in occult cases.
- Treatment rule: 4-6 weeks of targeted IV antibiotics plus operative excision of the infected segment with extra-anatomic bypass or in-situ reconstruction using autogenous vein or cryopreserved homograft; endovascular stenting is a bridge in unstable patients, not definitive care.
Choosing the operation
Consider a 58-year-old diabetic admitted with fever, thigh pain and a rapidly enlarging, tender groin swelling; blood cultures grow Salmonella. CT shows a 3 cm saccular femoral aneurysm with surrounding inflammatory stranding. The limb is viable, so antibiotics start immediately after cultures and the operating room is organised without waiting for the full antibiotic course — these aneurysms expand and rupture over days. At surgery the aneurysm is excised with all infected tissue, the field is debrided, and the question is how to restore flow. Through a clean extra-anatomic plane — an obturator bypass here — flow is restored away from the infected bed, or an autogenous vein graft may be placed in situ when the contamination is limited. Prosthetic material inside a pus-filled field risks graft infection, the feared complication that forces repeat surgery. In the aorta, the same logic governs: debride, culture the wall, and choose between extra-anatomic and in-situ homograft reconstruction depending on contamination and the patient's reserve.
High-yield viva angles
Examiners enjoy the etymology trap: "mycotic" does not mean fungal — the histology of Osler's era merely resembled fungal growth. Expect the Salmonella-aorta association, the saccular-versus-fusiform comparison with atherosclerotic aneurysms, and why blood cultures must precede antibiotics. Multiple, non-contiguous saccular aneurysms in different territories should immediately suggest endocarditis, prompting a cardiac evaluation — a common single-best-answer stem. Finally, if asked about endovascular stent grafts, frame the answer carefully: they temporise in haemodynamically unstable or prohibitive-risk patients, but leaving a foreign body in infected tissue carries a high risk of persistent infection, so open debridement remains the standard against which everything else is measured.
Frequently asked questions
Why is it called mycotic when bacteria cause it?
Osler introduced the term in 1885 describing endocarditis-related aneurysms; fungal infection is rare, and the name has survived as convention.
Which organism is typical on the aorta?
Salmonella species have a predilection for atherosclerotic aortic wall; Staphylococcus aureus dominates in peripheral vessels and endocarditis seeding.
How does it differ radiologically from an atherosclerotic aneurysm?
It is saccular, eccentric, lobulated and often multiple with periaortic inflammation, whereas atherosclerotic aneurysms are fusiform and contiguous.
Can antibiotics alone cure it?
No — the necrotic wall prevents sterilisation; antibiotics are mandatory adjuncts to surgical excision, with 4-6 weeks of targeted therapy after cultures.
What revascularisation options exist after excision?
Extra-anatomic bypass through clean tissue planes or in-situ reconstruction with autogenous vein or cryopreserved homograft, avoiding prosthetic material in pus.