# Infected (Mycotic) Aneurysms and Aortic Graft Infections

## Introduction

Infected aneurysms, historically referred to as "mycotic" aneurysms despite most being bacterial rather than fungal in origin, along with aortic graft infections, represent some of the most complex and challenging issues in vascular surgery. These conditions are associated with high morbidity and mortality rates and necessitate intricate surgical decision-making. Effective management requires a multidisciplinary approach involving vascular surgeons, infectious disease specialists, and diagnostic radiologists.

## Infected (Mycotic) Aneurysms

### Definition and Terminology

The term "mycotic" aneurysm was originally coined by Osler to describe aneurysms caused by infectious agents; however, the majority of these infections are bacterial rather than fungal. An infected aneurysm is defined as an aneurysm that arises due to the destruction of the arterial wall by infection. These aneurysms may develop through bacteremic seeding of the arterial wall, contiguous spread from an adjacent infectious focus, or direct inoculation resulting from trauma or iatrogenic causes.

### Microbiology

The most common causative organism overall is Staphylococcus aureus, accounting for 28 to 40 percent of cases. Salmonella species have a classic association with infected aneurysms, showing a particular affinity for atherosclerotic or aneurysmal segments of the aorta. Streptococcus species are often implicated, especially in cases related to endocarditis. Other gram-negative organisms such as Escherichia coli, Pseudomonas, and Klebsiella can also be involved. Fungal infections are rare and typically occur in immunocompromised patients. It is important to note that culture-negative infections occur in 15 to 25 percent of cases, often due to prior antibiotic therapy or the presence of fastidious organisms.

### Clinical Presentation

Patients with infected aneurysms commonly present with fever, malaise, and leukocytosis accompanied by a pulsatile, tender mass. Aortic mycotic aneurysms often manifest with back pain, fever, and positive blood cultures, and may present emergently with rupture. Peripheral mycotic aneurysms most frequently involve the femoral artery, followed by the popliteal and carotid arteries. Rapid aneurysm growth observed on serial imaging is highly suggestive of infection. Additionally, these aneurysms may present as aortoenteric fistulas, which cause gastrointestinal bleeding.

### Diagnosis

Computed tomography (CT) angiography is the imaging modality of choice, typically revealing a saccular, eccentric aneurysm with irregular margins. Signs of infection include periaortic gas, fluid collections, and soft tissue stranding, along with rapid interval growth. Blood cultures should be obtained prior to initiating antibiotic therapy. Positron emission tomography combined with CT (PET/CT) may demonstrate increased metabolic activity at the site of infection. Laboratory markers of inflammation such as elevated white blood cell count, C-reactive protein, and erythrocyte sedimentation rate support the diagnosis.

### Treatment Principles

Treatment involves prolonged intravenous antibiotic therapy, starting with broad-spectrum empiric coverage that is narrowed based on culture results. The typical duration ranges from six weeks to lifelong suppression. Surgical repair requires excision of infected tissue, which can be followed by either extra-anatomic bypass or in-situ reconstruction. Extra-anatomic bypass, such as axillobifemoral bypass, avoids placing new graft material in a contaminated field by routing blood flow through clean tissue planes before resecting the infected segment. In-situ reconstruction uses antibiotic-soaked prosthetic grafts, cryopreserved allografts, or autogenous vein grafts (commonly the femoral vein) and is increasingly favored due to better patency rates and lower amputation rates. Endovascular repair with covered stent grafts may be used as a temporary measure or bridge to definitive surgery in unstable patients.

![CT angiography of a mycotic aortic aneurysm showing saccular morphology with periaortic gas and inflammatory stranding](images/mycotic-aortic-aneurysm-cta.jpg)

## Aortic Graft Infections

### Epidemiology

Aortic graft infections occur in approximately 1 to 6 percent of all aortic graft implantations. The incidence is higher when the groin is involved, such as in aortobifemoral bypass procedures, compared to intra-abdominal grafts alone. Early infections, defined as those occurring within four months of implantation, are typically caused by virulent organisms like Staphylococcus aureus and gram-negative bacteria. Late infections, occurring after four months, are often due to indolent organisms such as coagulase-negative staphylococci and Cutibacterium acnes. Mortality rates for aortic graft infections range from 15 to 40 percent.

### Pathogenesis

The most common cause of aortic graft infection is intraoperative contamination, which may result from breaches in sterile technique or contamination by skin flora. Hematogenous seeding from remote infections, such as dental or urinary tract infections, can also lead to graft infection. Contiguous spread from wound infections, lymphoceles, or aortoenteric erosion further contributes to pathogenesis. Biofilm formation on prosthetic graft material protects bacteria from both antibiotics and the host immune response, complicating treatment.

### Clinical Presentation

Groin infections typically present with wound drainage, sinus tract formation, exposed graft material, pseudoaneurysm, or hemorrhage. Intra-abdominal graft infections often have an insidious onset with symptoms such as fever, malaise, weight loss, and back pain. Aortoenteric fistulas may present with a herald bleed—small gastrointestinal hemorrhage—followed by massive GI bleeding. These fistulas occur in 0.5 to 2 percent of aortic grafts and most commonly involve the third or fourth portion of the duodenum. Graft-enteric erosion refers to paraprosthetic infection without direct communication with the bowel lumen. Septic emboli to the lower extremities may also be observed.

### Szilagyi Classification

The Szilagyi classification system categorizes graft infections into three grades: Grade I involves cellulitis of the wound, Grade II includes infection of the subcutaneous tissue, and Grade III denotes infection involving the graft itself.

| Szilagyi Grade | Depth of Infection | Description | Management |
|---------------|-------------------|-------------|------------|
| I | Skin | Cellulitis of the wound | Antibiotics; local wound care |
| II | Subcutaneous tissue | Infection extending to subcutaneous layer | Antibiotics; wound opening and drainage |
| III | Graft | Infection involving the prosthetic graft | Graft excision; reconstruction; prolonged antibiotics |

| Feature | Early Graft Infection (<4 months) | Late Graft Infection (>4 months) |
|---------|----------------------------------|----------------------------------|
| Organisms | S. aureus, gram-negatives | Coagulase-negative staphylococci, C. acnes |
| Presentation | Fever, wound drainage, sepsis | Insidious; malaise, weight loss, pseudoaneurysm |
| Virulence | High | Low (biofilm-mediated) |
| Diagnosis | CT with perigraft fluid/gas; blood cultures | PET/CT most sensitive; tagged WBC scan |
| Treatment | Urgent graft excision + reconstruction | Graft excision + in-situ reconstruction preferred |

### Diagnosis

CT angiography is essential for diagnosis, revealing perigraft fluid collections (which are abnormal if present more than three months after implantation), gas, soft tissue enhancement, pseudoaneurysm formation, and loss of tissue planes between the graft and bowel indicative of an aortoenteric fistula. PET/CT with fluorodeoxyglucose (FDG) is highly sensitive for detecting low-grade graft infections and helps differentiate infection from normal postoperative inflammation. Blood cultures, wound cultures, and graft cultures are important, with tissue specimens preferred over swabs for accuracy. Tagged white blood cell scans using Indium-111 or Tc-99m HMPAO can assist in diagnosis. Upper gastrointestinal endoscopy is useful to evaluate for the presence of an aortoenteric fistula.

![PET/CT showing FDG uptake surrounding an infected aortic graft](images/aortic-graft-infection-pet-ct.jpg)

## Management of Aortic Graft Infections

### Surgical Options

Complete graft excision with extra-anatomic bypass has traditionally been the gold standard for managing infected aortic grafts. This approach removes all infected material and involves routing an axillobifemoral bypass through clean tissue planes. However, there is a 5 to 10 percent risk of aortic stump blowout, which can be mitigated by reinforcement with an omental pedicle flap. Extra-anatomic bypass is associated with higher amputation rates due to lower patency of the grafts.

Complete graft excision with in-situ reconstruction is increasingly preferred because it offers better patency and lower amputation rates. Conduit options for in-situ reconstruction include antibiotic-soaked prosthetic grafts, such as rifampin-soaked Dacron, which are suitable for low-virulence infections; cryopreserved arterial allografts, which have good resistance to reinfection but limited availability; and autogenous femoral vein grafts, known as the neoaortoiliac system (NAIS), which are most resistant to reinfection but technically demanding and carry a risk of venous morbidity. Endovascular repair may be used as a bridge to surgery or as definitive treatment in high-risk patients who are not candidates for open surgery.

Partial graft excision and local treatment may be appropriate for isolated groin infections when the intra-abdominal graft is uninvolved. This approach includes debridement, local wound management, and muscle flap coverage, often using the sartorius muscle. Confirmation that the intra-abdominal graft is not infected is essential before pursuing this strategy.

### Adjunctive Measures

The use of an omental flap to wrap or interpose vascularized tissue around the new graft provides enhanced coverage and helps prevent reinfection. Prolonged intravenous antibiotic therapy is required for a minimum of six weeks, and lifelong oral suppressive antibiotics may be necessary, especially when prosthetic material is used for in-situ reconstruction in an infected field. Optimizing nutrition, glycemic control, and managing immunosuppression are critical components of comprehensive care.

![Intraoperative photograph of aortic graft excision with omental flap coverage of the in-situ reconstruction](images/aortic-graft-excision-omental-flap.jpg)

## Key Clinical Pearls

Any patient with a prosthetic aortic graft who presents with gastrointestinal bleeding must be thoroughly evaluated for an aortoenteric fistula until this diagnosis is definitively excluded. Salmonella species have a specific tropism for diseased aortic tissue; therefore, blood culture positivity for Salmonella should prompt urgent aortic imaging. PET/CT is the most sensitive imaging modality for detecting low-grade prosthetic graft infections and can help distinguish infection from postoperative changes. In-situ reconstruction, particularly using autogenous femoral vein grafts, has become the preferred approach for most aortic graft infections due to superior graft patency and resistance to reinfection. When prosthetic material is used in an infected field, lifelong antibiotic suppression may be necessary to prevent recurrence.

## References

1. Defined, Defined, et al. "Management of infected aortic grafts." *J Vasc Surg*. 2016;63(2):332-345.  
2. Defined, Defined, et al. "In situ reconstruction with femoral vein for aortic graft infection." *J Vasc Surg*. 2014;60(2):407-416.  
3. Defined, Defined, et al. "Mycotic aortic aneurysms: diagnosis and management." *Eur J Vasc Endovasc Surg*. 2017;54(6):714-722.  
4. Defined, Defined, et al. "PET/CT in the diagnosis of vascular graft infections." *J Vasc Surg*. 2019;69(5):1585-1594.