A mycotic aneurysm is a bulge in an artery wall caused by infection. Unlike the more common aneurysms that develop slowly from wear and tear on blood vessels, mycotic aneurysms form when bacteria (or rarely fungi) invade and weaken the artery from within, causing it to balloon outward. They are rare, dangerous, and can grow quickly. Despite the name, most mycotic aneurysms have nothing to do with fungus.
Why the Name Is Misleading
The term “mycotic” comes from the Greek word for fungus, but the name is a historical accident. Sir William Osler coined it in the 1800s because the infected, mushroom-shaped growths he observed on heart valves reminded him of fungi. The label stuck, even though bacteria cause the vast majority of these aneurysms. You may also see them called “infected aneurysms” or “infective aneurysms” in more modern medical writing, which is more accurate.
How Infection Destroys the Artery Wall
Arteries are tough, layered structures designed to handle constant blood pressure. A mycotic aneurysm forms when bacteria reach the artery wall and trigger a rapid inflammatory response. White blood cells flood into the vessel wall to fight the infection, but in doing so, they break down the structural tissue that keeps the artery intact. This destruction happens far faster than the gradual weakening seen in standard aneurysms.
Bacteria can reach the artery wall in a few ways. The most classic route is through infected clots that break off from a heart valve infection (infective endocarditis) and travel through the bloodstream until they lodge in a smaller vessel or at a branch point. Bacteria can also seed the tiny blood vessels that supply the artery wall itself, or invade directly from a nearby source of infection. Once established, the infection erodes the wall from the inside out, creating a weak spot that bulges under arterial pressure.
Which Bacteria Are Responsible
Staphylococcus species are the most commonly identified cause, accounting for about 26% of cases where an organism is found. Streptococcus species are close behind at roughly 24%. Salmonella is another well-known culprit, particularly in aneurysms involving the aorta. In about one-third of cases, no specific organism is identified, often because antibiotics were started before cultures could be taken.
True fungal infections can cause mycotic aneurysms, but they’re uncommon and tend to occur in people with severely weakened immune systems. A wide range of unusual bacteria have been reported in individual cases, including some that are rarely seen in other types of infection.
Where Mycotic Aneurysms Form
These aneurysms tend to appear at branch points in major arteries, where blood flow creates turbulence and infected material is more likely to lodge. The femoral artery in the upper thigh is the single most common site, followed by the abdominal aorta, the combined thoracoabdominal aorta, and the thoracic (chest) aorta.
Mycotic aneurysms can also form in the brain, though this is less common. When they do, they typically affect the arteries in the front part of the brain’s circulation. Between 20% and 33% of intracranial mycotic aneurysms arise on the circle of Willis, a ring of arteries at the base of the brain. These need to be distinguished from berry aneurysms, which are a completely different, non-infectious type. Aneurysms in the arteries supplying the abdominal organs are extremely rare.
Symptoms and Warning Signs
Symptoms depend heavily on where the aneurysm forms. A mycotic aneurysm in the aorta or a leg artery might cause localized pain, a pulsating mass you can feel, or fever that doesn’t respond to standard treatment. Because these patients often have an underlying bloodstream infection, they may already be quite sick with fevers, chills, and general malaise.
Brain mycotic aneurysms are particularly tricky. Most produce no symptoms at all before they rupture. When they do cause symptoms, headaches, seizures, or sudden neurological changes like weakness on one side of the body are the main warning signs. A sudden stroke-like event in someone with an active heart valve infection should raise immediate concern for a ruptured brain mycotic aneurysm.
One hallmark that distinguishes mycotic aneurysms from ordinary ones is their rapid growth. A standard aortic aneurysm might enlarge by a few millimeters per year. A mycotic aneurysm can expand noticeably over days to weeks, making early detection critical.
How They’re Diagnosed
CT scans with contrast dye (CT angiography) are the primary tool for spotting mycotic aneurysms. Doctors look for an abnormal bulge in the artery wall, but several features help distinguish an infected aneurysm from a non-infected one: swelling of the soft tissue around the vessel, fluid collections near the artery, and occasionally small pockets of gas in the surrounding tissue. These signs point toward active infection rather than simple age-related weakening.
One important difference from regular aneurysms: size alone does not predict how dangerous a mycotic aneurysm is. A standard aortic aneurysm that measures under 5 centimeters is generally considered low risk. With mycotic aneurysms, even a small one can rupture because the wall is actively being destroyed by infection. This means the usual “watch and wait” approach based on size doesn’t apply.
The Connection to Heart Valve Infections
Infective endocarditis is the single biggest risk factor for developing a mycotic aneurysm. When bacteria colonize a damaged or artificial heart valve, they form clumps of infected material that can break off and travel anywhere in the body. These infected clots can lodge in artery walls throughout the body, seeding new infections.
People with weakened immune systems, those who inject drugs, and patients with prosthetic heart valves face the highest risk. Sepsis from any source can also lead to mycotic aneurysm formation, even without a heart valve infection, though this is less common.
Treatment: Antibiotics and Surgery
Every mycotic aneurysm requires prolonged antibiotic therapy, but the duration varies significantly depending on the location and severity. For brain mycotic aneurysms, a minimum of 4 to 6 weeks of intravenous antibiotics is standard. For aneurysms in the aorta, treatment ranges from 6 weeks to 6 months, and some patients with resistant organisms or extensive infection require lifelong suppressive antibiotics. Peripheral artery aneurysms typically call for at least 6 weeks of treatment.
Many mycotic aneurysms also require surgical repair, either through traditional open surgery or a less invasive endovascular approach where a stent graft is threaded through the blood vessels to seal off the aneurysm. Both methods have trade-offs. A study from the Journal of Vascular Surgery found that 30-day mortality was 9% for open surgery versus 5% for endovascular repair, and one-year survival was similar (78% vs. 75%). However, at the five-year mark, open surgery showed a clear advantage: 69% survival compared to 41% for endovascular repair. The open surgery group also had fewer readmissions and fewer infection-related repeat procedures over three years.
The reason for this difference likely relates to infection control. Open surgery allows surgeons to physically remove infected tissue and wash out the area, while endovascular repair leaves the infected tissue in place behind the stent. For a condition driven by infection, that residual bacteria can cause ongoing problems.
Rupture Risk and Mortality
The most feared complication is rupture, which turns an already serious condition into a life-threatening emergency. In a large prospective study of patients with infective endocarditis, those whose mycotic aneurysms ruptured had an in-hospital mortality rate of 43.6%, compared to 21.7% for those with unruptured aneurysms. At one year, nearly half (48.7%) of patients with ruptured aneurysms had died, versus 21.7% of those whose aneurysms were caught and treated before rupture.
Patients with unruptured mycotic aneurysms who received treatment had no significantly higher one-year mortality compared to endocarditis patients without aneurysms at all. This underscores a critical point: mycotic aneurysms that are found and managed before they burst carry a far better prognosis than those that aren’t. The gap between ruptured and unruptured outcomes is one of the starkest in vascular medicine, making early detection the single most important factor in survival.