Aortic Disease: What Are The Causes Of Infected (Mycotic) Aneurysms?

An infected (also known as mycotic) aneurysm was first described near the end of the 19^th century in a patient who died from mycotic endocarditis (an inflammation of the aortic valve caused by fungi). However, the name “mycotic” is a misnomer in the modern world, because it has been proven that bacteria are responsible for this type of aneurysm far more often than fungi. Another, more rarely used, term is infective vasculitis.

Thanks to antibiotics, infected aneurysms of the aorta are extremely rare these days, causing less than one percent of all diagnosed aneurysms. Historically, however, they were frequently caused by syphilis in its later stages (luetic aneurysm). Other than the aorta, infected aneurysms can nowadays still be diagnosed on some arteries branching off the aorta, most commonly the superior mesenteric artery, which supplies a portion of the digestive tract, as well as on arteries that supply the legs and the brain.

What causes infected aneurysms?

The first step in infected aneurysm formation is, of course, the infection of the aorta. This happens when bacteria find their way into the bloodstream, most commonly due to the following medical conditions:

• Infective endocarditis – infections of the heart valves or the inner lining of the heart chambers
• Pneumonia – lung infection
• Osteomyelitis – infection of the bone
• Cellulitis – infection of the skin

Besides diseases, surgical and dental procedures and trauma may also lead to the contamination and infection of the aorta. The bacteria can reach the aortic wall either by directly spreading from the initial focus, or as a mass traveling through the bloodstream, called a septic embolus. This type of spreading increases the risk of embolism, meaning that the mass can act as a plug, therefore stopping circulation.

Prosthetic grafts used in aortic repair surgery may also be infected, which is a rare, but serious complication of this procedure. Blunt trauma can also cause infected aneurysms, due to potential detaching of bacterial masses found on the endocardium. Other risk factors include intravenous drug use, reduced activity of the immune system, as well as existing aneurysms and other aortic diseases.

Bacteria that most commonly cause these types of aneurysms are:

• Salmonella species, which seems to really like this type of tissue
• Staphylococcus aureus
• Streptococcus species
• E. coli

How are infected aneurysms diagnosed?

The symptoms usually manifest as any other systemic infection, and mainly consist of fever, abdominal pain, and a pulsatile abdominal mass.

A detailed medical history, especially information about previous infections, may help doctors identify the underlying cause, which is ultimately confirmed by analyzing the patient’s blood sample. Other symptoms may include fatigue, weakness, nausea, and weight loss, although all of these are non-specific to aneurysms alone.

Are infected aneurysms treatable?

Reparation of an infected aneurysm caused by syphilis is the same as repairing a regular aneurysm, since the organism causing syphilis (Treponema pallidum) doesn’t have the capability to infect the graft. However, infected aneurysms caused by other bacteria, as well as infections affecting an existing graft, require removing all of the infected tissue before performing aortic repair. This procedure has shown to have a satisfyingly high survival rate.

Sometimes the graft replacement can’t be achieved due to the possible spread of infection of the new graft, which is the exact complication the surgeon aims to dodge. In these cases, a special kind of bypass surgery (extra-anatomic bypass) is performed, where the usual pathway of the blood is avoided. One example of this procedure is the axillobifemoral bypass, where the blood flow is redirected, and instead through the aorta, the blood passes through a graft placed between the axillar artery (found just under your collar bone), and the femoral arteries (in your groin).

Since these aneurysms are caused by bacteria, antibiotic therapy is necessary both before and after the surgery, sometimes even for the rest of the patient’s life. As with most aortic diseases, early diagnosis and treatment, significantly increases chances of a better outcome for the patient.