The clinical picture of infectious aneurysms is heterogeneous. It may be dominated by signs of systemic infection, functional impairment of the affected vessel or potentially lethal hemorrhages, but it may also be asymptomatic.

Fever is among the most common symptoms [1]. Infected aortic aneurysms may provoke chest and intrascapular pain, while abdominal and back pain may be manifested if other vessels are lesioned. Septic patients may additionally present tachykardia, tachypnoe, hypotension or even shock [12].

Other, frequently observed symptoms are vascular disturbances such as thromboembolism and thrombophlebitis, inflammatory reactions in adjacent tissues, abscess formation and compromise of neuronal structures. In some cases, a pulsatile mass can be palpated.

Infected aortic aneurysms are seldom asymptomatic. With regards to infected aneurysms of cerebral arteries, severe headaches, seizures and neurologic symptoms may be related to the presence of an aneurysm and rapidly worsen upon rupture. Infected aneurysms of peripheral vessels, e.g. of the superior mesenteric artery and infrarenal arteries, are often asymptomatic until neighboring organs are compromised. The, gastrointestinal hemorrhages, hypertension and hematuria may be experienced [13].

Due to symptoms being very little specific, diagnosis of infectious aneurysms can pose a real challenge. Most patients, more than 70%, present with general symptoms of an infection, with fever and hemogram findings such as leukocytosis. If such results are obtained in a patient reporting dull pain and an unexpected mass can be palpated, ideally a pulsatile mass, this may prompt suspicion of infectious aneurysm.

On the one hand, any suspicion for infection should lead to identification of its source, microbial cultures of blood and possibly tissue samples, and initiation of antibiotic treatment. Of note, 18 to 50% of all patients show sterile blood cultures and therapy needs to be based on experience.

Simultaneously and because of the great risk associated with potential rupture, a thorough diagnostic workup should be carried out as soon as possible in any patient with a tentative diagnosis of infectious aneurysm. Usually, diagnostic imaging should be applied to confirm this diagnosis. Computed tomography is considered the gold standard and is most frequently used to screen thorax and abdomen, but may also be utilized for other areas of the body, depending on the localization of pain [14]. Irregular enhancement of the vessel's wall is the most common finding in computed tomography images, it corresponds to focal inflammation and possibly effusion. A detailed image of the extent of damage may be obtained in angiographic examinations [15].

Small, asymptomatic infectious aneurysms may be adequately treated with mid- to long-term antibiotic therapies [16]. Initially, antibiotics are usually administered intravenously. Such treatments should ideally be based on an antibiogram and be supplemented with regular follow-ups to detect any possible aggravation early. Sole drug therapy is a matter of debate in patients of poor overall condition.

While antibiotic treatment is also necessary for larger, symptomatic and even ruptured infectious aneurysms, these constitute an emergency and require immediate surgical intervention. Procedures of choice are aneurysmectomy and aneurysmorrhaphy, remaining ends need to be joined by anastomosis or closure with patches [17]. Sometimes grafts need to be employed or endovascular stents and these measures are frequently necessary to treat infectious aortic aneurysms. In rare cases, stents may become infected and serve as a new starting point for bacterial spread and/or aneurysms. Extensive debridement of adjacent tissues is necessary to minimize the risk for re-infection.

With regards to repair of infectious cerebral aneurysms, stent therapy is gaining importance. To this end, endovascular embolization is an alternative approach. Here, the infectious aneurysm or its parent artery is occluded to avoid rupture and potentially lethal hemorrhages. This method may depend on the blood supply to irrigated tissues being covered by other vessels. Otherwise, cerebral infarction may occur.

Prognosis of infectious aneurysm is unfavorable to poor. Prognosis worsens in patients presenting comorbidities such as atherosclerosis, hypertension and heart insufficiency, but these diseases are mutually dependent. Another risk group are immunodeficient patients who are more susceptible to septicemia but who generally do not present a good overall condition [11]. There is a high mortality associated with arterial aneurysms due to fatal hemorrhages and lethal damage to vital organs, especially the brain.

Pathogens may reach any blood vessel during septicemia or with septic thrombi that may then arrest in smaller vessels. Infection of the vessel wall is greatly facilitated if such thrombi attach to pre-existing lesions of the former or if they are able to enter vasa vasorum. Drug abuse and implantation of stents or grafts may pose another risk for infectious aneurysm and may carry microorganisms into the intravascular space. Additionally, direct infiltration from adjacent processes of infections are conceivable and may, for instance, be observed in venous aneurysms related to varicose veins. Direct infection of vessels may also occur after trauma.

As has been stated above, bacterial infections are the most common triggers of infectious aneurysms. Similar to triggers of bacterial endocarditis, Staphylococcus and Streptococcus strains account for the majority of infectious aneurysms [4]. Methicillin-resistant Staphylococcus aureus has been described as a causative agent of this type of aneurysm and such cases are generally related to intravenous administration of illegal drugs. With regards to gram-negative bacteria, distinct species of Enterobacteriaceae, e.g., Escherichia coli, Klebsiella, Pseudomonas and Salmonella have been detected in infectious aneurysms. Moreover, infectious aneurysms may sometimes result from infection with mycobacteria or distinct kinds of fungi, notably Candida albicans and different species of the genus Aspergillus. Recently, infectious aneurysms have been related to infection with the human immunodeficiency virus. Infections with more than one species have been reported, but are rare and also seem to relate to drug abuse [5].

The overall prevalence of infectious aneurysms can hardly be estimated due to an unpredictable number of as of yet asymptomatic cases. While the number of patients suffering from infectious aneurysms due to bacterial endocarditis is continuously decreasing due to availability of more effective therapies, incidence rates for immunocompromising conditions such as AIDS but also diabetes mellitus or other chronical illnesses increase. Also, a lot more patients undergo invasive intravascular procedures. Thus, infectious aneurysm prevalence may be diminishing on one side, but augmenting on another [6].

Aneurysms due to atherosclerotic degeneration of the vessel wall are much more common than infectious aneurysms. In this context, infected aortic aneurysms account for less than 1% of all surgically treated aortic aneurysms, although the aorta is the most frequently affected vessel. In general, any blood vessel, may be affected [7]. Infectious aneurysms are also often observed in cerebral arteries - typically observed in younger patients and amounting to up to 4% of all aneurysms - and visceral arteries, most notably the superior mesenteric artery. With regards to peripheral arteries, the femoral artery may become infected after use of unsterile needles to administer illegal drugs. Infrarenal vessels are more often affected in the elderly.

About one of three infectious aneurysm patients presents aneurysms in more than one vessel [8].

Certain causative agents seem to be more frequently isolated in certain geographical regions. This is the case for infected aortic aneurysms and Salmonella in East Asia.

The vast majority of infectious aneurysms occurs upon bacterial infection of vulnerable vessel walls, notably those initiating in the intravascular space [9]. If pathogens reach vessels during septicemia or with septic thrombi, they may easily adhere to intima lesions resulting from atherosclerotic plaques, pre-existing thrombi or aneurysms. In contrast, tissue damage occurs first to adventitia and media if microorganisms spread from adjacent infections.

In any case, pathogens may directly contribute to tissue damage, but also provoke an inflammatory reaction that, in turn, results in a release of a variety of cytokines and recruitment of further inflammatory cells. Matrix metalloproteinases are excreted by macrophages, neutrophils and lymphocytes, among others, and accelerate matrix degeneration and tissue remodelling. Hematoma may develop inside the vessel wall and form pseudoaneurysms. In this stage, aneurysms are particularly instable and may easily rupture. This whole process may be rather slow, but may be very rapid in other cases.

If the vessel wall still maintains blood pressure, fibrous tissue starts to form and delimits the aneurysm [10]. However, existing aneurysms not only maintain a mechanically weak point, particularly in patients suffering from hypertension, they also facilitate adherence of new pathogens, of new thrombi and vessel dissection may eventually occur in the subsequent cycle.

No preventive measures can be recommended.

An aneurysm in general is a focal, permanent dilation of a blood vessel, most frequently of an artery. Partially due to lower blood pressure inside venous vessels, the latter are less commonly affected by an aneurysm. Aneurysms may be congenital, but most often result from degenerative alterations of the arterial wall, particularly from atherosclerosis [1]. Other causes for aneurysms may be trauma, infection and/or inflammation.

With regards to infectious aneurysms, these may occur after systemic spread or direct infiltration of blood vessels by bacteria and less frequently by viruses or fungi. Infectious blood vessel alterations tend to appear like fungal infections and thus the term mycotic aneurysm (MA) has been coined for any type of infectious aneurysms, irrespective of its causative agent. It has first been used at the end of the 19th century [2]. To date, there is no clear consensus regarding this term and it may sometimes be applied to describe virtually any form of aneurysm or just infectious aneurysms resulting from bacterial endocarditis. Here, it will be used to describe infectious aneurysms - a term that should be used instead of MA to avoid confusion [3].

True aneurysms comprise all main layers of the vessel wall, i.e., intima, media and adventitia. In contrast, false aneurysms, sometimes also deemed pseudoaneurysms, merely describe a hematoma in close proximity to the vessel wall that may raise suspicion of a true aneurysms due to its location. However, any vessel wall damage provoked by pathogenic microorganisms is a serious risk to vessel integrity and may therefore be associated with morbidity and even mortality. Aneurysms may rupture and cause potentially lethal hemorrhages. This may be the case if major vessels are affected or if vessels supplying vital and sensitive organs such as the brain are damaged.

Antimicrobial treatment is required, but may not be sufficient to eliminate the risk of rupture of vessel wall lesions and may thus need to be supplemented with surgical intervention, debridement and vessel reconstruction. Different surgical approaches may be employed to this end.

An aneurysm is a pathological dilation of a blood vessel, mostly of an artery. It may occur due to developmental defects, degenerative alterations of the vessel wall as well as infection and subsequent inflammation. When aneurysms were first investigated at the end of the 19th century, a physician described aneurysms that occur due to infection as having the aspect of "fresh fungal vegetation" and the Latin name for anything related to fungi is mycotic. Thus, an infectious aneurysm was subsequently called mycotic aneurysm. Today, the term infectious aneurysm is preferred because only a very little share of mycotic aneurysms actually results from mycotic infection, i.e., from infection with fungi. The greatest danger poses a possible rupture of the affected vessel.

Causes

Indeed, most infectious aneurysms are of bacterial origin. Bacteria may reach any vessel of the human body during septicemia, which is a systemic distribution of pathogens via the cardiovascular system. Also, infectious processes in determined spots, in the heart, for instance, may pose a continuous risk for spread of pathogens. Such pathogens may most easily adhere to pre-lesioned vessel walls and damage may result from diseases as common as atherosclerosis. Immunocompromising conditions as seen in diabetes mellitus patients and those suffering from other chronical diseases may further promote microorganism spread and development of infectious aneurysm.

Symptoms

Symptoms vary widely. They may be dominated by infection with microorganisms and septicemia (fever, weakness, accelerated heart and breathing rates, eventually hypotension and shock), from functional impairment of the affected vessel (thromboembolism and consecutive limitation of blood supply), from hemorrhages (particularly dangerous when occurring in major vessels such as the aorta or in cerebral arteries) or from compromise of adjacent structures (compression of neighboring nerves, bloody urine or diarrhea). Some infectious aneurysms may even be asymptomatic.

Diagnosis

A symptom triad of signs of infection, pain and palpation of a pulsatile mass may prompt suspicion of infectious aneurysm. Imaging techniques such as computed tomography will be applied to locate and evaluate a possible arterial dilation. Enhancement with contrast agents is utilized in a special form of computed tomography called angiography and allows for a precise assessment of the extent of damage if an aneurysm has been confirmed during previous diagnostic measures. Simultaneously, the causative agent needs to be isolated, potentially from blood samples, to examine its sensibility towards antimicrobial drugs.

Treatment

Drug therapy with antibiotics is only sufficient for small, asymptomatic infectious aneurysm and even in these cases, the respective patient needs to be monitored to detect possibly aggravations early.

Most patients suffering from infectious aneurysms need to undergo surgery. Here, the dilated part of the vessel may be resected and the remaining ends need to be rejoined. In some cases, this may be achievable by suturing both sections, but if parts of larger vessels that need to resist considerable blood pressure are excised, grafts or stents may be implanted instead.

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