A mycotic aneurysm is a bulge in an artery wall caused by infection. Despite the name, most mycotic aneurysms are bacterial, not fungal. The term dates back to the 1800s, when the irregular shape of these aneurysms reminded physicians of mushroom-like growths. They’re rare but serious: a ruptured mycotic aneurysm carries an in-hospital mortality rate of about 44%, compared with roughly 22% for those caught before they burst.
How Infection Weakens the Artery
Mycotic aneurysms form when bacteria reach the artery wall and trigger an inflammatory response. The most common route is through the bloodstream. Infected clots, called septic emboli, break off from a source of infection (often a damaged heart valve) and lodge in the tiny blood vessels that supply the artery wall itself. Once there, the bacteria multiply and attract immune cells, which release enzymes that break down collagen and elastin, the structural proteins that give arteries their strength.
As these proteins degrade, the artery wall weakens at that spot and begins to balloon outward. What forms is usually a pseudoaneurysm, meaning the bulge isn’t contained by all the normal artery layers. Instead, it’s a thin-walled pocket of blood held in place by surrounding tissue. This makes mycotic aneurysms especially prone to rupture compared with other types of aneurysms.
Causes and Risk Factors
Infective endocarditis, an infection of the heart valves, is one of the most common underlying causes. When bacteria colonize a heart valve, small clumps of infected material can break free and travel to arteries anywhere in the body. People at highest risk include those with abnormal or prosthetic heart valves, a history of intravenous drug use, diabetes, or conditions that suppress the immune system such as HIV.
The most frequently identified bacteria vary by geography. In Western countries, Staphylococcus aureus accounts for about 28% of cases, followed by Salmonella species at 15% and Pseudomonas aeruginosa at 10%. In most Asian countries, Salmonella is the leading cause. Rarer pathogens include the bacterium responsible for syphilis, tuberculosis-causing bacteria, Listeria, and Klebsiella.
Where Mycotic Aneurysms Develop
Mycotic aneurysms can form in almost any artery, but certain locations are more common. The aorta, the body’s largest artery, is a frequent site. Among thoracic (chest) aortic mycotic aneurysms, the descending portion accounts for about 76% of cases, while the ascending aorta is involved in less than 1%. Mycotic aneurysms also develop in the brain’s arteries, in the arteries supplying the gut and kidneys (visceral arteries), and in the arteries of the arms and legs.
The location matters because it shapes the symptoms, the risks of rupture, and the treatment approach. A mycotic aneurysm in the brain presents very differently from one in the abdomen.
Symptoms to Recognize
Fever is the most consistent symptom, present in the vast majority of cases regardless of location. Beyond that, symptoms depend on which artery is affected. An aortic mycotic aneurysm often causes back pain. One involving a peripheral artery may produce a painful, pulsatile lump near the skin’s surface, sometimes with visible redness and warmth. About half of patients with peripheral involvement have a tender mass and an audible whooshing sound (bruit) that a doctor can hear with a stethoscope.
When a mycotic aneurysm forms in the brain, it typically announces itself through stroke symptoms, bleeding into the brain, or sudden severe headache from bleeding around the brain’s surface. If the aortic aneurysm grows large enough to press on nearby structures, it can cause difficulty swallowing or hoarseness by compressing the nerve that controls the voice box. An aneurysm near the spine may erode into the vertebrae, causing bone infection. Involvement of the pulmonary artery can lead to coughing up blood.
How It’s Diagnosed
CT angiography, a specialized scan that combines contrast dye with computed tomography, is the primary imaging tool. It’s the most sensitive method for detecting two hallmark signs: gas bubbles in or around the aneurysm, which strongly suggest infection, and calcification in the artery wall. On imaging, a mycotic aneurysm typically appears as a lobulated, saccular (one-sided) bulge with an irregular, indistinct artery wall. Surrounding the aneurysm, doctors look for swelling, a rim of inflammatory soft tissue that lights up with contrast dye, and sometimes visible gas pockets.
One particularly telling sign is rapid growth. An aneurysm that enlarges by more than 5 mm in two weeks is highly suspicious for an infectious cause. Blood cultures are drawn to identify the specific bacterium involved, which guides antibiotic selection. The combination of fever, positive blood cultures, and characteristic imaging findings usually clinches the diagnosis.
Treatment: Surgery and Antibiotics
Treatment involves two components: controlling the infection with antibiotics and repairing the damaged artery. Antibiotics are started intravenously as soon as the diagnosis is suspected and are continued for an extended course, often weeks, to clear the infection from the artery wall and surrounding tissue.
For the artery itself, there are two main surgical approaches. Open surgery involves removing the infected segment of artery and either replacing it with a graft or rerouting blood flow through a bypass. Endovascular repair is less invasive: a stent graft is threaded through a small incision in the groin and positioned inside the artery to seal off the aneurysm from within.
Open surgery remains the more established approach and appears to offer longer survival. In one retrospective study of 38 patients, those who underwent open repair had a median follow-up of about 4.2 years, compared with 2.5 years for the endovascular group. Mortality was 50% in the open surgery group versus 75% in the endovascular group over the study period, though this difference did not reach statistical significance, likely because the endovascular patients tended to be sicker or less able to tolerate open surgery in the first place. Deaths specifically from aneurysm rupture were similar between the two groups, at roughly 20 to 25%.
Endovascular repair does carry a specific long-term risk: because it seals the aneurysm with a synthetic device placed into infected tissue, there’s a meaningful chance of persistent or recurrent infection. The five-year rate of infection-related complications after endovascular repair of abdominal mycotic aneurysms is about 26%. Still, for patients who are too frail for open surgery, endovascular repair can be a lifesaving alternative.
Prognosis and Rupture Risk
Outcomes depend heavily on whether the aneurysm ruptures. In a large national study of patients with infective endocarditis who developed mycotic aneurysms, in-hospital mortality was 22% for unruptured aneurysms and 44% for ruptured ones. At one year, nearly half of patients with ruptured mycotic aneurysms had died, while the one-year mortality for unruptured cases remained at about 22%.
Even aneurysms that are initially stable can rupture during follow-up, and when they do, outcomes are poor. In the same study, one-third of patients whose aneurysm ruptured during the monitoring period died from causes directly related to the aneurysm. Among those who survived a delayed rupture, every single one was left with lasting complications. By contrast, only 25% of survivors with aneurysms that never ruptured experienced long-term sequelae.
These numbers underscore why early detection matters. Patients being treated for infective endocarditis are typically monitored with imaging to catch mycotic aneurysms before they rupture, when the odds of a good outcome are significantly better.