Aneurysms are abnormal bulges or dilations that form in the wall of a blood vessel. A mycotic aneurysm is a particularly dangerous type of aneurysm caused by an infection of the arterial wall itself. This condition is rare, accounting for a small percentage of all aneurysms, but it carries a high risk of rupture, hemorrhage, and death if not treated quickly. Because the artery wall is compromised by a microbial attack, the condition is far more unstable and aggressive than typical aneurysms. The term “mycotic” is misleading, as the infection is typically bacterial rather than fungal, but the name persists from its initial description.
Defining the Condition and Its Pathogenesis
The name “mycotic aneurysm” was coined in 1885 by physician William Osler, who observed a mushroom-like appearance in the arterial growths of a patient with infective endocarditis. Despite the term suggesting a fungal cause, the majority of these aneurysms are caused by bacteria, with common culprits being Staphylococcus aureus and Salmonella species. The infection leads to a condition that is more accurately described as an infected aneurysm or microbial arteritis.
The pathogenesis centers on the destruction of the artery’s structural layers. Arteries are composed of three main layers: the intima, media, and adventitia. The infectious process typically begins when microorganisms reach the arterial wall, often through the bloodstream (bacteremia), or via tiny blood vessels that nourish the artery wall (vasa vasorum).
Once the microbes establish themselves, they trigger an intense inflammatory response. Bacteria release enzymes that actively digest the structural proteins within the arterial wall, focusing particularly on the media layer, which is responsible for the vessel’s strength and elasticity. The destruction of the media layer causes the vessel to lose its structural integrity, leading to a rapid, localized expansion or dilation.
The resulting aneurysm is often a pseudoaneurysm, meaning the bulging sac is not contained by all three layers of the original arterial wall, making it highly unstable. The infected area frequently develops a saccular or eccentric shape, which is distinct from the fusiform shape typically seen in non-infected aneurysms. This fragility explains the high risk of rupture associated with mycotic aneurysms.
Primary Sources of Infection and Risk Factors
Mycotic aneurysms almost always arise as a complication of a pre-existing systemic infection, where bacteria travel through the bloodstream to seed the artery wall. The most frequent source is infective endocarditis, a bacterial infection of the heart’s inner lining or valves. Fragments of infected material, called septic emboli, can break off and travel to distant arteries, which is a common mechanism for intracranial mycotic aneurysms.
The infection can also originate from direct seeding during episodes of widespread bacterial infection, or sepsis, which allows bacteria to lodge in a pre-existing site of damage. These damaged sites can include atherosclerotic plaques or an existing, non-infected aneurysm. Infection may also spread directly from an adjacent localized infection, such as an abscess or osteomyelitis.
Certain patient populations face a higher susceptibility to developing this condition due to compromised defenses. Individuals with chronic conditions like diabetes mellitus or those who are immunocompromised (due to HIV, chemotherapy, or chronic steroid use) are at increased risk. Intravenous drug use is another significant risk factor, as repeated unsterile injections can directly introduce bacteria into the bloodstream, often leading to infections in peripheral arteries like the femoral artery.
Recognizing the Signs of a Mycotic Aneurysm
The symptoms of a mycotic aneurysm are often vague and non-specific, making the condition challenging to diagnose in its early stages. Since the condition is rooted in an infection, signs of systemic illness are commonly present, such as fever, chills, and fatigue. Some patients may initially be evaluated for a fever of unknown origin.
Localized pain is the most consistent symptom, though its location depends entirely on the affected artery. If the aneurysm is in the abdominal aorta, a person may experience severe back or abdominal pain. An aneurysm in a limb artery, such as the femoral artery, may present as swelling, tenderness, or a noticeable, pulsatile mass.
When the aneurysm develops in the brain’s arteries, symptoms can be neurological, including sudden, severe headaches, vision changes, or signs mimicking a stroke. The inflammatory nature of the condition means that even before rupture, the surrounding tissues may become inflamed, contributing to the localized pain. Diagnosis often requires a high degree of suspicion, especially in patients with known risk factors.
Confirmatory Diagnosis and Necessary Treatment Approaches
The diagnosis of a mycotic aneurysm requires laboratory tests to confirm the infection and advanced imaging to locate and characterize the aneurysm. Blood cultures are typically drawn immediately to isolate the causative microorganism and determine its susceptibility to antibiotics. Elevated markers of inflammation, such as C-reactive protein (CRP) and a high white blood cell count, also support the presence of an active systemic infection.
Imaging studies are essential for confirming the size, location, and shape of the vascular lesion. Computed Tomography (CT) angiography is the preferred method, as it provides detailed cross-sectional images of the blood vessels and surrounding soft tissues. A mycotic aneurysm typically appears as a saccular or eccentric bulge with signs of inflammation and fluid collection around the artery, which helps distinguish it from non-infected aneurysms.
Treatment for this condition is a dual-modality approach that focuses on eradicating the infection and structurally repairing the weakened artery wall.
Antibiotic Therapy
The first step involves the immediate initiation of prolonged, targeted intravenous antibiotic therapy. This is often continued for six to eight weeks, or even longer, to ensure the infection is completely cleared from the tissue. The specific antibiotic regimen is guided by the results of the blood cultures.
Structural Repair
Structural repair of the aneurysm is necessary to prevent a fatal rupture. Depending on the location and extent of the damage, this may involve open surgical repair, where the infected artery segment is removed and replaced with a graft, or a less invasive endovascular repair. Endovascular techniques, such as placing a covered stent graft, involve threading a device through the artery to seal off the aneurysm from the inside. However, the long-term success requires careful monitoring to ensure the infection does not persist on the foreign material. Delaying either the antibiotic treatment or the necessary surgical intervention significantly raises the risk of life-threatening complications.