Patient Selection

22.1.1 Diagnosis

Clinical suspicion for an infectious intracranial aneurysm should be raised in a patient with bacterial endocarditis and acute onset of neurological symptoms. ^{1,​ 2} The presumed pathophysiology of infectious aneurysm formation is hematogenous spread from bacteremia and/or septic emboli derived from valvular vegetations in the setting of bacterial endocarditis. Risk factors for developing endocarditis-related valvular vegetations include intravenous drug use, immunosuppression, and prosthetic arterial devices such as stents, grafts, and valves. ³ Contiguous spread from adjacent infection including orbital cellulitis with cavernous sinus thrombophlebitis, sinusitis, or meningitis may also lead to infectious aneurysm formation. There is a slight male predilection. Infectious aneurysms typically present in a ruptured state, with any combination of subarachnoid hemorrhage (SAH), intracerebral hemorrhage (ICH), intraventricular hemorrhage, and, rarely, subdural hemorrhage. The most frequent organisms isolated from blood cultures include Streptococcus species (including viridans), Staphylococcus species, and Enterococcus species. Fungal species are rare, and approximately one-third of cultures are negative. Unlike typical saccular aneurysms occurring at arterial bifurcation points around the circle of Willis, infectious aneurysms are frequently found in the distal middle cerebral artery (MCA) distribution and have a fusiform morphology.

A noncontrast computed tomography (CT) of the head will determine if there is a hemorrhage. Infectious aneurysms are typically diagnosed on a CT angiography (CTA). Magnetic resonance angiography (MRA) may also be used, especially in patients with poor renal function. However, time-of-flight MRA sequences may miss smaller, more distal aneurysms. Digital subtraction angiography (DSA) is the gold standard for detecting infectious aneurysms, especially very small distal aneurysms.

The differential diagnosis of a solitary spontaneous intraparenchymal hematoma should include a ruptured infectious aneurysm, particularly in the setting of endocarditis. Designating an intracranial aneurysm as an infectious aneurysm on imaging is based on the morphology (e.g., fusiform) and location of the aneurysm (e.g., distal MCA) in the appropriate clinical context (e.g., endocarditis or some other infection).

22.1.2 Indications for Surgery

The key to determining whether an infectious aneurysm requires surgical or endovascular treatment depends on the rupture status ( ▶ Fig. 22.1). Ruptured infectious aneurysms are thought to have a high risk of rebleeding and therefore tend to be treated aggressively. Unruptured infectious aneurysms may be followed with serial imaging during medical treatment of the infection, with surgery considered if the aneurysm grows, persists, or, obviously, ruptures. Patients with a life-threatening intraparenchymal hemorrhage are taken to surgery for evacuation of the hematoma and, if feasible, clip reconstruction or aneurysm trapping with or without bypass. Otherwise, endovascular therapy is often the first option since many of these patients are medically ill with severe cardiac disease and may be on antithrombotic drugs. Endovascular options include both direct coiling and embolization with liquid embolic agents (e.g., n-butyl-2-cyanoacrylate [NBCA] or ethylene vinyl alcohol copolymer dissolved in dimethyl sulfoxide mixed with tantalum powder [Onyx, ev3 Inc.]). Surgery is considered if the endovascular option is unfeasible or unsafe (e.g., unable to selectively catheterize the feeding artery or the parent artery supplies a potentially eloquent region). The goals of surgery are to prevent rupture or rebleeding.

22.1.3 Contraindications for Surgery

Surgery is not recommended in patients who are hemodynamically unstable or patients who are in a moribund state. Also, patients must have any coagulopathy corrected prior to surgery. Repair should be undertaken expeditiously in ruptured cases.

22.1.4 Timing of Surgery

Infectious aneurysms are typically pseudoaneurysms. As a result, the aneurysm wall may be quite fragile. Therefore, if endovascular options are not feasible, some surgeons recommend waiting several days to a week with the patient on antibiotics after aneurysmal rupture to allow for fibrosis, resulting in a less-friable aneurysm. The infectious aneurysm wall can be very delicate with a consistency similar to “wet tissue paper,” which increases the risk of intraoperative rupture.

22.1.5 Alternatives to Surgery

The natural history is not well defined for infectious aneurysms due to the relative rarity of the disease and intervention with surgery or endovascular procedures in modern case series. Unruptured infectious aneurysms seem to have a relatively low risk of rupture. Of note, infectious aneurysms were excluded from the International Study of Unruptured Intracranial Aneurysms (ISUIA). Because infectious aneurysms can regress with antibiotic therapy alone, it is reasonable to consider a trial of conservative management for unruptured infectious aneurysms. Conservative management could be considered if a patient is in a moribund state.

Management strategies for infectious aneurysms include both surgery and endovascular procedures, with the latter being first-line treatment in most clinical scenarios. Endovascular procedures can include direct coiling of the aneurysm (with or without balloon/stent assist) or occlusion of the parent artery with coils or liquid embolic agents (NBCA or Onyx). Compared with NBCA, Onyx can be injected more proximally in the parent artery and can be injected multiple times. Stents are generally avoided due to a number of reasons, including the need for dual antiplatelet therapy, the fragile nature of the vessel wall, the hypercoagulable state resulting in increased embolic risk, and the creation of a potential nidus for ongoing infection. If the patient is a poor general anesthetic candidate due to cardiac concerns or other comorbidities, endovascular treatment could be considered under conscious sedation. However, treatment may be more challenging with issues of motion artifact, especially when attempting to navigate a microcatheter into a very distal parent artery. Endovascular treatment also minimizes direct manipulation of the infectious aneurysm, which may decrease the risk of intraoperative rupture. Treatment may potentially be initiated earlier with endovascular procedures compared with surgery, as there is theoretically less need to wait for aneurysm fibrosis. Endovascular treatment also potentially allows earlier heart valve repair/replacement, which requires systemic anticoagulation.

Endovascular Case Example ( ▶ Fig. 22.1)

A 54-year-old, right-hand dominant, man with known hypertension, and alcohol and cocaine abuse, presented to a peripheral hospital with confusion. Noncontrast CT head demonstrated a left posterior temporal ICH. His hospital course was marked by a non–ST-segment elevation myocardial infarction, alcohol withdrawal seizures, acute kidney injury requiring intermittent hemodialysis, and bacterial endocarditis. Echocardiogram revealed severe mitral valve regurgitation with a 2-cm vegetation. Blood cultures grew streptococcus viridans. He was treated with broad-spectrum antibiotics and later switched to vancomycin after culture sensitivities returned. The patient was then transferred to our institution. CTA was not performed because of the acute kidney injury. MRA revealed no aneurysm. DSA demonstrated a distal left M3/M4 aneurysm, 2 mm in size. This infectious aneurysm was treated with endovascular embolization with Onyx under general anesthetic approximately 3 weeks after the initial ICH. It was felt that the infectious aneurysm was on a quite distal artery suppling a noneloquent territory. The embolization procedure involved a right femoral artery puncture, followed by systemic heparinization and placement of a shuttle catheter in the left common carotid artery. A microcatheter was maneuvered distally from the inferior M2 branch, with the microcatheter tip just proximal to the aneurysm. Onyx was injected with subsequent occlusion of the aneurysm and mild proximal reflux requiring catheter tip detachment. There were no complications from the procedure and patient was transferred back to the peripheral hospital 9 days later with no focal deficits.

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