Saturday, November 10, 2012

The Bugs Burst Out -- Mycotic Aneurysm

Aneurysms arise when the arterial wall is weakened and the arterial pressure overcomes the tensile strength of the wall (see "Laplace's theorem" post on 11/7/12). This wall weakness and the formation of an aneurysm is most often a combination of atherosclerosis, aging (cystic medial necrosis), and hypertension...

...but this isn't the only way to weaken the arterial wall. A mycotic aneurysm can also arise if an infectious process begins to weaken the wall. Although this a much less common event than a "bland" aneurysm, its prognosis is different and its treatment can be more tricky.

There are four main ways that an infectious aneurysm can form:
1.  Septic emboli of cardiac origin ("Mycotic aneurysm") lodging in the lumen or vasa vasorum of peripheral arteries. These "mycotic" aneurysms (more likely bacterial... not fungal) have been recorded as occuring in virtually every artery, although they more commonly occur in the aorta, intracranial, superior mesenteric, and femoral arteries. The continued refinement of antibacterial therapy and the ability to replace infected cardiac valves have lead to a decrease in this etiology.  Because the etiology is partly embolic, these aneurysms tend to form at arterial branch points and occlusions.

2. Microbial arteritis with aneurysm formation: With the decline of the prevalence of intracardiac vegetations, this etiology -- although rare -- has been becoming relatively more common. If the intima is interrupted (e.g. atherosclerosis), then bacteria have the capacity to penetrate into the arterial wall. If the infection takes hold, a pseudoaneurysm can result. Compatible with atherosclerosis, the aorta is the most common site for this process. Immunsuppresion, hemodialysis, and radiation arteritis are considered risk factors. It has been noted that the diseased aorta is more vulnerable to Salmonella spp.

3. Infection of a pre-existing aneurysm: Although colonization of aortic aneurysm walls has been shown to not be rare (~15%), whether infection of a pre-existing aneurysm is a significant mechanism for development of an infectious/inflammatory aneurysm is still controversial.

4. Post-traumatic infected pseudoaneurysm:  These are reported as being more common in IVDUs and an increasing incidence of this etiology may also be partly due to increased numbers of percutaneous interventional therapies.

Signs of an infected central arterial aneurysm can be subtle. Elevated WBCs, ESR are sensitive, but not specific. Positive blood cultures in a patient with an aneurysm increase the specificity, but are not as sensitive (50%), so negative blood cultures alone are not enough to rule out the diagnosis.
With the lack of a definitive lab test, imaging becomes vital.

Although U/S is the usual screening tool for abdominal aortic aneurym, it is unable to differentiate between an infectious or a bland aneurysm.

CTA, however, provides the information necessary to help with a diagnosis.

Findings on a CTA that raise suspicion for an infectious aneurysm are:
1) saccular aneurysms as atypical locations,
2) an aneurysm in an otherwise normal aorta,
3) lack of calcification,
4) soft tissue mass or rind around the aorta,
5) multilobulated aneurysm,
6) an eccentric aneurysm with a narrow neck (pseudoaneurysm),
7) rapid change/enlargement on short interval follow-up,
8) multifocal aneurysms.

MRI is being used with increased frequency for diagnosis.  In-111 has also been used in problem-solving.

Treatment involves antibiotics, but whether (and when) to pursue an extra-anatomic reconstruction or an in-situ reconstruction of the aorta is more controversial. Endovascular therapy is limited since drainage of sepsis and wide debridement are not possible.  A role for endovascular therapy as a bridge to definitive therapy or as palliative therapy is still being established.

Quick and dirty Terrerecon movie for entertainment purposes.

1. "Rutherford's Vascular Surgery" Cronenwett and Johnston. 7th ed. (2010).
2. Lee WK, Mossop PJ, Little AF, et al. "Infected (Mycotic) Aneurysms: Spectrum of Imaging Appearance and Management" RadioGraphics 2008; 28:1853–1868