‘Mycotic’ is a term which was introduced in 1885 by Osler to describe aortic aneurysms caused by weakening of the arterial wall by infected emboli arising from bacterial endocarditis. This is now an uncommon cause of mycotic aneurysm and the term has come to be used to describe all infected aneurysms. Mycotic aneurysms may be ‘true’ (an arterial swelling lined by intima) or ‘false’ (an artery related swelling arising from a leak) and the point at which an infected artery or an infected anastomosis becomes a mycotic aneurysm is debatable, making an estimate of the incidence of the condition meaningless. In the mind of the vascular surgeon the term ‘mycotic’ is often associated with arteries, for, while infected venous aneurysms do occur and may even be described as mycotic, they are usually treated by simple excision and there is little risk of vascular insufficiency developing.

The most useful classification is based on the putative status of the artery before infection and the source of infection. Arteries may be normal, atheromatous, aneurysmal, or may be associated with a prosthetic graft. The source of infection may be intravascular (septic emboli or bacteraemia) or extravascular (perivascular lymphatics, local abscess, trauma). Normal arteries are not thought to be affected by simple bacteraemia, while diseased arteries may be. An extravascular source of infection is often associated with more diffuse inflammation of the surrounding tissues than an intravascular source. This is especially so in cases of trauma where arterial disruption, haematoma and infection may eventually lead to a mycotic false aneurysm, the most common form of the condition, which is now frequently associated with the use of contaminated syringes by drug addicts.

The type of organism recovered from a mycotic aneurysm depends on the source of infection and the location of the affected artery (Table 1) 182. In the classical form of mycotic aneurysm due to infected emboli from bacterial endocarditis, Pneumococcus, Streptococcus, Haemophilus, and Enterococcus will usually be found. In non-endocarditic peripheral mycotic aneurysms Staphylococcus and Enterobacter are commonly present, but the list of other organisms isolated from such lesions is a lengthy one.

Salmonella is by far the most common infecting organism in mycotic aortic aneurysms. Nevertheless, numerous other bacterial species have been cultured from these lesions, including Staphylococcus epidermidis (of uncertain significance) and normal gut flora, the presence of which suggests that a bacteraemia arising from the gut may lead to organisms lodging in an already diseased aortic wall. A variety of other bacteria and fungi have been described in aortic lesions, including Treponema pallidum and Mycobacterium tuberculosis, which both have a prediliction for the aorta rather than other vessels.

Forty per cent of mycotic aneurysms involve the femoral artery and 30 per cent affect the aorta; these two vessels are the most common sites of mycotic aneurysms. The most common cause is trauma (in about 40 per cent of cases) followed by bacteraemia affecting an already diseased artery. It should be remembered, however, that virtually any artery can be the site of a mycotic aneurysm and that intravenous drug abuse and percutaneous catheterization procedures account for an increasing incidence of the problem in the brachial and femoral vessels. Other iatrogenic causes include aortic lesions in neonates monitored with umbilical catheters, and upper limb arterial lesions from infected needling of vessels or grafts in the vicinity of an arteriovenous fistula used for haemodialysis access. As a result the incidence of peripheral mycotic aneurysms is greater than that of aortic mycotic aneurysms.

This depends on many factors, not the least of which is the site of the aneurysm. The vast majority of peripheral aneurysms can be palpated, in contrast to most central aneurysms (Fig. 1) 285. Typically, the patient has a fever, perhaps of ‘unknown’ origin and a history of bacterial endocarditis or operative or other trauma to the affected vessel. A mycotic aneurysm tends to enlarge rapidly and, if palpable, may appear as a tender, warm pulsatile mass with an overlying systolic murmur. Depending on the nature of the infective source, lesions may be multiple, and the natural history is of continued growth followed by rupture which, depending on the site and containment or otherwise by surrounding structures, may be rapidly fatal. A persistent fever of unrecognized origin is likely to be associated with slow and insidious aneurysmal growth, particularly if antibiotics have been administered at an early stage. Even if an aneurysm is recognized, the diagnosis may be elusive, especially if antibiotic therapy obscures the infective nature of the lesion.

There is usually a fever and the presence of petechiae is of great significance. Leucocytosis is very common unless antibiotics have been used. Blood cultures are useful in both diagnosis and management, and if venous samples fail to detect an organism, blood from an artery distal to the aneurysm should be tested.

Angiograms, although not in themselves diagnostic of infection, confirm the presence of an aneurysm and are essential for planned treatment. Non-calcified aneurysms in otherwise normal looking vessels should raise the question of a mycotic aetiology (Fig. 2) 286. Ultrasonography, and particularly computed tomography, may be useful, particularly if fluid collections are visualized around prosthetic grafts: such a collection may be aspirated for bacteriological culture. Scanning using leucocytes labelled with a radioisotope of indium has proved useful in some centres.

Prophylaxis with broad spectrum antibiotics such as cefuroxime is important in patients with a known septic focus and atheromatous occlusive disease or aneurysm or a vascular prosthesis. When confronted with an intact mycotic aneurysm, however, all infected tissue, including a thrombosed vascular prosthesis if present, must be excised. Open drainage may be both feasible and desirable in some cases. Alternatively, irrigation with antibiotic solution through a catheter may be used, along with systemic chemotherapy directed by appropriate culture.

Arteries should be ligated or oversewn, and anastomoses for bypass operations placed only in uninfected vessels using monofilament sutures. Ligation and excision of infected tissue without reconstruction is preferable, since any reconstruction is liable to further infection and this may lead to early catastrophic haemorrhage. If immediate reconstruction is deemed necessary to save life or limb it can often be performed away from the infected area, and it may even be done before excision of the infected lesion; for instance an axillofemoral bypass may be inserted before excising a mycotic abdominal aortic aneurysm. If reconstruction must be done at the same time and a graft must be placed near or in the area of excised infection, autogenous vein rather than grafts of foreign material should be used if at all possible. Even so, persisting infection with early anastomotic bleeding is a profound danger which may occur at any time from a few hours to several months after operation.

Although a mycotic aneurysm is a major operative challenge, conservative treatment is not appropriate. Antibiotic therapy alone will not prevent eventual rupture, and chemotherapy, guided by sensitivity reports from the bacteriology laboratory, is best used in conjunction with surgery. Antibiotics should be given systemically during the perioperative period and continued orally thereafter for several months.

So much depends on the location and aetiology of mycotic aneurysms that overall results are meaningless. Lesions arising from a bacterial endocarditis are generally treated successfully, with a 75 per cent early survival rate, while secondary infection of pre-existing, usually abdominal, aortic aneurysms has in the past been associated with a survival rate of only 10 to 20 per cent. Peripherally located lesions are generally associated with survival, although limb loss is not uncommon. In the future a better appreciation of the principles of treatment and the prolonged use of appropriate antibiotics may have a beneficial impact on this most difficult of vascular surgical problems.

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