This article reviews current knowledge about intracranial aneurysms due to infectious and neoplastic causes. Direct mural injury or invasion of intracranial arteries by infectious organisms or neoplastic cells can produce aneurysmal dilatation, often of an irregular, fusiform, or ectatic shape. Infectious aneurysms, sometimes called mycotic or infective aneurysms, are most commonly due to bacterial pathogens, and neoplastic aneurysms are often due to a select group of neoplasms. Neoplastic aneurysms can occur with cardiac myxomas, choriocarcinoma, or occasionally with other neoplasms. These rare nonsaccular aneurysms frequently rupture and are life-threatening. Although treatment of these aneurysms must be individualized and can be challenging, surgical approaches and endovascular techniques, in addition to systemic antibiotic or cancer therapies, are sometimes successful in stabilizing or obliterating these aneurysms.
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• Infectious or neoplastic processes, through direct arterial mural injury or invasion, can produce intracranial aneurysms, often of an irregular, fusiform, or ectatic shape or in an unusual location as compared to more common saccular aneurysms.
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• Infectious aneurysms, also sometimes called mycotic or infective aneurysms, are more often due to bacterial than to fungal causes.
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• Neoplastic aneurysms most often occur from cardiac myxoma or from choriocarcinoma and can present with serious hemorrhages.
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• Infectious or neoplastic aneurysms often present with intracranial hemorrhage but can also be detected incidentally.
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• Management of these aneurysms is challenging, but individualized surgical and endovascular approaches can be effective in preventing growth or rupture.
Historical note and terminology
An aneurysm is a pathologic, localized blood vessel dilatation. Aneurysms are called “saccular” when the inflow and outflow points are in common and “nonsaccular” when arterial dilatation is greater than 1.5 times normal without a clearly defined neck (ie, the inflow and outflow points are longitudinally separate) (12). “Fusiform” refers to nonsaccular, spindle-shaped aneurysms with focal circumferential dilatation whereas “dolichoectatic” aneurysms are predominantly elongated and tortuous with a uniform enlarged circumference (dolichos=long; ectasia=distended). Saccular aneurysms typically arise through genetic and environmental factors in an idiopathic fashion, and nonsaccular aneurysms can commonly arise from acute arterial dissection or from chronic atherosclerosis or other arteriopathies. Rarely, however, distinctive spherical or nonsaccular aneurysms can be identified as resulting from direct injury or invasion of the arterial wall by infectious organisms or by neoplastic cells. This article focuses on these infectious and neoplastic aneurysms.
Infectious intracranial aneurysms were recognized as early as 1869 in the setting of infective endocarditis (27). The term "mycotic" is used widely today for aneurysms of infective cause. It is usually attributed to Sir William Osler, who did not use the term to describe a cerebral aneurysm of infective origin but, rather, as an example of endarteritis in an aortic aneurysm (31). It is unclear whether Osler meant to convey a fungal or bacterial etiology to the cardiac vegetations during his Gulstonian Lectures of 1885 on malignant endocarditis or to simply describe "fungating" excrescences on the cardiac valves, as others had done before him. The term “mycotic,” meaning fungal, is a misnomer because infective aneurysms are usually bacterial and are rarely due to fungi. Moreover, "mycotic aneurysm" has been used to describe noninfective aneurysms, such as those associated with nonbacterial thrombotic endocarditis because of "fungating" excrescences on the cardiac valves. Inflammatory noninfectious aneurysms due to vascular inflammation are also sometimes referred to as "mycotic." They occur in systemic inflammatory diseases, presumably on an immunologic basis.
For intracranial aneurysms arising due to infectious cause, Ishikawa and colleagues suggested the name “bacterial intracranial aneurysms,” reserving the term “mycotic” for those aneurysms due to fungi (18). Others argue that the aneurysm should be classified only after the infecting organism is discovered, which is impractical because in many cases no organism is ever identified, particularly if antibiotics have been administered. Although the term "infectious aneurysm" is used commonly today, the alternative terms "infective aneurysm" or “microbial aneurysm” are sometimes preferred (19).
“Oncotic,” “neoplastic,” or “metastatic” aneurysms are those that are formed due to metastatic tumor injury to the arterial wall, often by direct tumor cell invasion, which can weaken the vascular media, causing either saccular or fusiform aneurysmal dilatation, or both. The proximal cause of these very rare aneurysms is often metastatic atrial myxoma or choriocarcinoma, but they have also resulted from other tumors, such as bronchogenic carcinoma.