Mycotic Aortic Aneurysm

A mycotic aortic aneurysm is an infected aneurysm of the aortic wall, from bacteria in most cases and fungi or mycobacteria less often. The term is a misnomer, since most are not fungal.

It is rare, accounting for about 1 to 3% of aortic aneurysm repairs. It is a surgical emergency, because the aneurysm grows quickly and ruptures early. Untreated mortality approaches 100%, and with treatment it is about 20 to 40%.

Organisms: Staphylococcus aureus (about 30%), Salmonella (15 to 25%, especially in the descending and abdominal aorta), streptococci including Streptococcus pneumoniae, Gram negative bacilli (Escherichia coli, Klebsiella, Pseudomonas), Mycobacterium tuberculosis, Treponema pallidum, Brucella, Coxiella burnetii and Candida.

Risk factors are age over 60 years, atherosclerosis, diabetes, immunosuppression, HIV infection, alcohol use disorder, injecting drug use, an endovascular graft, endocarditis, bacteraemia, and spondylodiscitis. In sub-Saharan Africa and Asia tuberculosis and Salmonella (with HIV or sickle cell disease) are important causes.

Pathophysiology. Bacteria seed the wall through the vasa vasorum, from a contiguous focus, or at a diseased intima. Suppuration destroys the media, and a saccular or multilobed aneurysm develops and enlarges within days to weeks. Contained rupture and fistulas into the bowel, airway or vena cava follow.

• Fever (about 70%), rigors, malaise, weight loss and night sweats.
• Pain in the chest, back or abdomen, often severe, and a tender pulsatile mass.
• Sepsis. Haemoptysis, haematemesis or gastrointestinal bleeding (fistula).
• Embolic phenomena, vertebral tenderness, and signs of a primary source.
• Leucocytosis and raised CRP. Blood cultures are positive in about 50 to 85%.

Red flags.

• Fever with an aortic aneurysm or an endovascular graft
• Rapid growth, saccular shape or periaortic gas on imaging
• Haemoptysis, haematemesis or gastrointestinal bleeding
• Sepsis or haemodynamic instability
• Immunosuppression or HIV infection
• Persistent bacteraemia

• Blood cultures (3 sets) before antibiotics.
• CT angiography. Saccular or lobulated aneurysm, eccentric, with periaortic stranding, fluid or gas, soft tissue mass, vertebral erosion, and rapid growth on serial studies. Gas within the wall is highly suggestive.
• FDG PET-CT for activity and extent, and the source. MRI for vertebral and spinal involvement.
• Tissue. Culture and 16S rRNA PCR of the aortic wall and periaortic tissue at surgery, with fungal and mycobacterial culture and histology.
• Cause workup. Transoesophageal echocardiography for endocarditis, stool culture for Salmonella, tuberculosis tests (interferon gamma release assay, sputum Xpert MTB/RIF Ultra), syphilis serology, Brucella and Q fever serology, HIV test, ESR, CRP, procalcitonin and glucose.

Differential diagnosis.

• Infected pseudoaneurysm
• Inflammatory aortic aneurysm and aortitis
• Atherosclerotic aneurysm with a periaortic haematoma
• Penetrating atherosclerotic ulcer
• Retroperitoneal or mediastinal abscess and spondylodiscitis
• Aortic graft infection and aortoenteric fistula
• Malignant mass, lymphoma or sarcoma

Combined prolonged antibiotics and surgical debridement. Start antibiotics at once.

Empirical antibiotics intravenously, after blood cultures.

• Vancomycin 25 to 30 mg/kg as a loading dose (maximum 3 g), then 15 to 20 mg/kg every 8 to 12 hours (usual maximum 2 g for each dose), adjusted to a trough of 15 to 20 mg/L or an AUC of 400 to 600 mg·h/L, lengthening the interval and using levels in renal impairment.
• Plus piperacillin-tazobactam 4.5 g every 6 hours (4.5 g every 8 hours at creatinine clearance 20 to 40 mL/min, every 12 hours under 20 mL/min), or meropenem 1 g every 8 hours (1 g every 12 hours at creatinine clearance 26 to 50 mL/min, 500 mg every 12 hours at 10 to 25 mL/min, 500 mg every 24 hours under 10 mL/min). Ceftriaxone 2 g every 24 hours when Salmonella is likely.
• Candida risk: caspofungin 70 mg as a loading dose, then 50 mg once daily (35 mg in moderate liver impairment).
• Tuberculosis: isoniazid 5 mg/kg (maximum 300 mg), rifampicin 10 mg/kg (maximum 600 mg), pyrazinamide 25 mg/kg (maximum 2 g) and ethambutol 15 mg/kg orally once daily for 2 months, then isoniazid and rifampicin, usually extended to 9 to 12 months by the specialist team, with pyridoxine 25 mg once daily. Check liver enzymes at baseline, 2 weeks and monthly.

Directed therapy. Flucloxacillin 2 g every 4 hours (maximum 12 g daily) or cefazolin 2 g every 8 hours for methicillin susceptible staphylococci. Vancomycin continued for methicillin resistant staphylococci. Ceftriaxone 2 g every 24 hours for Salmonella and susceptible streptococci.

Duration. At least 6 weeks intravenously from effective debridement. Many patients need oral suppression for 6 to 12 months, and lifelong suppression when a graft or endograft remains, with the drug chosen by susceptibility. Fluoroquinolones are avoided in aneurysmal disease where alternatives exist.

Surgery.

• Open resection and debridement with in situ reconstruction (rifampicin soaked or silver coated graft, cryopreserved allograft, autologous femoral vein, or bovine pericardium), or extra-anatomic bypass with closure of the aortic stump. Omental or muscle flap coverage.
• Endovascular repair as a bridge for rupture or instability, or as definitive treatment when open surgery is prohibitive, with lifelong antibiotics.
• Blood pressure control while waiting: esmolol 500 microgram/kg intravenously over 1 minute, then infusion 50 microgram/kg/min, repeating the bolus and increasing the infusion by 50 microgram/kg/min every 4 to 5 minutes, maximum 200 microgram/kg/min, to a heart rate near 60/min and systolic pressure of 100 to 120 mm Hg.

Special groups. HIV infection: start or continue antiretroviral therapy with interaction checks, particularly rifampicin. Renal impairment: adjust antibiotics and use levels.

Monitoring. CRP and white count weekly. Vancomycin levels and creatinine twice weekly. CT at 3, 6 and 12 months, then yearly, and PET-CT when relapse is suspected. Blood cultures if fever returns.

Referral and escalation. Admit to a centre with a vascular or cardiothoracic aortic team and infectious diseases. Emergency for rupture, bleeding, sepsis or enlarging aneurysm.