31 Giant Aneurysms of the Anterior Circulation
Abstract
Giant aneurysms of the anterior circulation comprise a complex subcategory of intracranial aneurysms with an unfavorable natural history and technically demanding treatment options. In this chapter, the authors review the natural history, presentation, anatomical considerations, diagnostic workup, differential diagnosis, treatment considerations (as well as controversies surrounding various forms of treatment), and clinical outcomes of patients with these challenging vascular lesions.
Introduction
Giant intracranial aneurysms (GIAs) are typified by a fundus diameter ≥25 mm. While this size designation was defined somewhat arbitrarily, substantial evidence suggests that GIAs are associated with their own distinct biological, anatomical, and clinicopathological entity. GIAs are morphologically and angiographically classified as saccular (most common), fusiform, or serpentine. In large published series, GIAs comprise approximately 5% of all intracranial aneurysms. GIAs typically come to clinical attention during the middle decades of life, are more prevalent in women, and are located predominately in the anterior circulation. Presenting signs and symptoms may be caused by aneurysmal rupture, but may also be due to mass effect from the large size and/or thromboembolic phenomena.
Following diagnosis, the natural history of GIAs is poor with a high risk of spontaneous rupture with substantial accompanying morbidity and mortality. While recent advances in vascular neuroimaging, neuroanesthesia, neurocritical care, and microsurgical/endovascular techniques have ushered in a dramatic improvement in clinical outcomes, the large size, complex angioarchitecture, and frequent presence of atherosclerosis, calcification, and/or thrombus within GIAs render them a formidable treatment challenge. Successful treatment of GIAs is aimed at excluding the aneurysm from the intracerebral circulation to prevent risk of rupture, preservation of parent and perforating arteries, and decompressing surrounding neural elements. In general, this is best achieved by open microsurgery, which provides the best chance for a definitive, durable cure. Endovascular therapy may be considered for patients who are not ideal candidates for a craniotomy, and may be the preferred modality for GIAs in certain anatomical locations. However, endovascular techniques are not considered the standard treatment given the low aneurysm obliteration rates, high aneurysm rebleeding rates, need for multiple retreatments, and unsatisfactory relief of mass effect.
Major controversies in decision making addressed in this chapter include:
Whether or not treatment is indicated.
Microsurgical versus endovascular treatment for ruptured and unruptured GIAs.
Management of GIAs that present with hematomas requiring urgent evacuation.
When should adjuvant microsurgical techniques (e.g., bypass, aneurysmorrhaphy, local or systemic circulatory arrest) be utilized?
Whether to Treat
Our understanding of the natural history of GIAs remains incomplete; the preponderance of evidence suggests that these lesions carry a grave prognosis. Observational studies demonstrate that, after initial rupture, the cumulative rate of rebleeding for GIAs is 18.4% within the first 14 days. The mortality of GIAs is greater than 60% within 2 years. Overall, 80% of patients with symptomatic, untreated GIAs are dead or incapacitated within 5 years of initial diagnosis from hemorrhage or cerebral ischemia ( 1 in algorithm ). In the International Study of Unruptured Intracranial Aneurysms (ISUIA) trial, the investigators determined that GIAs had a rupture rate of 6% in the first year and a relative risk of rupture (as compared to an unruptured aneurysm ≤10 mm in size) of 59.0. This study suggested that GIAs located on the cavernous internal carotid artery (ICA) had a 5-year cumulative rupture rate of 6.4%, while GIAs on the anterior communicating artery (ACoA), middle cerebral artery (MCA), and ICA had a 5-year rupture rate of 40%. GIAs residing on the posterior communicating artery (PCoA) had an even higher cumulative rupture rate of 50% in 5 years. Taken together, these data suggest that the morbidity and mortality of untreated, symptomatic, intradural GIAs justifies their aggressive treatment ( 1 in algorithm ). This observation is juxtaposed to the natural history of extradural (e.g., petrocavernous) GIAs, which appear to have a more benign natural history and may warrant a more conservative approach.
Factors influencing the decision to treat GIAs of the anterior circulation include variables related to both the patient and aneurysm. Patient factors include age, medical comorbidities, clinical presentation, family history, smoking history, life expectancy, and preference for a specific treatment. Aneurysm factors include size, location, morphology, neck width, atherosclerosis, calcification, thrombus, location of parent arteries and perforators, rupture status, and collateral circulation (e.g., adequacy of circle of Willis). Overall, GIAs of the anterior circulation are high-risk lesions with a significant rate of spontaneous rupture. As such, most ruptured and unruptured GIAs should be treated. Microsurgical clip ligation is the preferred treatment modality, when possible. However, some patient and aneurysm characteristics favor endovascular treatment ( 1 in algorithm ). In addition, combined endovascular/microsurgical approaches have been increasingly utilized with success in this patient population.
Conservative Management
For some patients (e.g., advanced age, poor medical condition, medical comorbidities, short life expectancy, inability to tolerate general anesthesia, or patient preference), it is reasonable to consider conservative therapy ( 10 in algorithm ). However, these patients must be carefully counseled regarding the inadmissibly high risk of spontaneous rupture. These patients may be followed clinically for symptom progression and radiographically with noninvasive neuroimaging.
Anatomical Considerations
GIAs can be classified morphologically into three distinct entities: saccular, fusiform, and serpentine. Saccular GIAs, the most common type, result from progressive enlargement of a smaller, saccular aneurysm. These aneurysms occur at arterial bifurcations and are more frequent in the anterior circulation. Fusiform aneurysms are caused by atherosclerotic degeneration over a longer arterial segment. They lack a discrete aneurysm neck and are more common in the vertebrobasilar system. Serpentine aneurysms are rare. These lesions are partially thrombosed with tortuous vascular channels that display delayed antegrade flow on angiography. In this chapter, we will focus predominately on saccular GIAs of the anterior circulation.
According to location, the most common sites of occurrence of GIAs in the anterior circulation include the proximal intradural ICA, cavernous ICA, MCA, and anterior cerebral artery (ACA)/ACoA. As a large aneurysm progresses into a GIA, its neck widens to incorporate the efferent and perforating arteries. The parent artery may enter and exit the GIA from various angles and locations. The fundus and neck of many GIAs harbor regions of atherosclerosis, laminated thrombus in varying stages of evolution, and/or dense calcifications. These factors oftentimes render microsurgical clipping of the neck more challenging, if not impossible. Moreover, the bulky domes of GIAs are frequently embedded in deep, eloquent brain parenchyma, which makes their manipulation treacherous due to risk of neural injury. Finally, given their large size, proximal arterial control can be difficult given that the aneurysmal mass occupies the natural brain corridors and blocks access to proximal arterial segments.
Workup
Clinical Evaluation
Every patient warrants a complete history and neurological examination. The neurological examination should encompass the patient′s level of arousal, cranial nerve function, and motor/sensory capacity. Fundoscopic examination should be performed to assess for papilledema if obstructive hydrocephalus is suspected. For GIAs in the paraclinoidal ICA that present with visual symptoms, a neuro-ophthalmological analysis (including visual acuity and field testing) should be performed. Cranial nerve deficits affecting ocular motility can be assessed by a neuro-ophthalmologist for baseline function. Laboratory studies must be performed to assess for metabolic derangements and/or an occult coagulopathy. Endocrine function should be tested in patients with GIAs that involve the pituitary gland and/or stalk.
Imaging
Head computed tomography (CT): A noncontrast head CT is important to evaluate acute hemorrhage from a GIA. The degree of calcification of the aneurysm fundus and neck can also be seen. Thin sections through the skull base are important for surgical planning to assess the bony anatomy.
Magnetic resonance imaging (MRI): MRI is a superior modality for assessing GIA size (as angiography only shows luminal patency), mass effect, surrounding vasogenic edema, oxidative state of intraluminal blood products, and relationship to adjacent neural structures. Flow void signals on T2-weighted MRI are helpful in differentiating the GIA lumen and afferent/efferent arteries from the thrombosed fundus. Diffusion-weighted sequences may demonstrate acute cerebral ischemia related to thromboembolic events or compromise of perforating vessels.
Noninvasive angiography: These studies include CT and MR angiography. CT angiography is helpful for defining GIA anatomy and its relationship to the skull base. This study is ideal for unstable patients requiring urgent craniotomy and hematoma evacuation. MR angiography provides lower resolution imaging, but may be more suitable for patients with renal insufficiency, iodine contrast allergy, or who are pregnant.
Cerebral catheter angiography: Catheter-based angiography is the gold standard study for delineating GIA angioarchitecture. Select microcatheter injections may also be performed. Digital subtraction angiogram (DSA) allows for dynamic vascular imaging, which includes early, mid, and late arterial phases; capillary phase; and early, mid, and late venous phases. A complete study should include ample views of the anterior and posterior circulation. External carotid artery (ECA) injections are important to evaluate the size and course of the superficial temporal artery (STA) for STA–MCA bypass planning. Also, the extent of collateral flow and the adequacy of the circle of Willis are assessed.
In GIAs where parent artery ligation, trapping, and/or bypass is anticipated, a temporary balloon occlusion (TBO) trial is performed. Serial neurological evaluations are conducted during the trial (with or without a hypotensive challenge), followed by radionucleotide imaging to determine the patient′s tolerance for ICA sacrifice. If the patient is unable to clinically or radiographically tolerate a TBO, a bypass (e.g., low flow, high flow, or in situ) is usually required ( 7, 8, 12, 13 in algorithm ). For GIAs in the paraclinoidal region where a neck dissection may be required for proximal arterial control (e.g., ophthalmic artery [OphA], superior hypophyseal artery [SHA], and OphA segment variants), images of the cervical ICA are helpful to determine the level of the common carotid artery (CCA) bifurcation.
Differential Diagnosis
Diagnostic considerations (particularly for lesions in the sellar and suprasellar region) should include the following:
Intracerebral neoplasm.
Pituitary adenoma.
Craniopharyngioma.
Rathke′s cleft cyst.
Treatment
Choice of Treatment and the Influence of Intracerebral Hematoma
All GIAs of the anterior circulation recommended for treatment—both ruptured and unruptured—were treated traditionally via open microsurgery. In the early 1990s, the advent of endovascular coiling techniques introduced another treatment modality. Since then, however, the clinical and radiographic outcomes for patients with GIAs treated by primary coiling have been disappointing. Reasons for this include (1) wide aneurysm neck making complete coil obliteration difficult without parent artery compromise, (2) complex branching of afferent/efferent arteries, (3) coil compaction with need for retreatment, (4) coil migration into intraluminal thrombus with recanalization, (6) regrowth of residual aneurysm neck, (7) need for frequent angiographic surveillance, (8) higher rate of aneurysm bleeding and/or rebleeding, and (9) inability to eliminate mass effect.
The past decade has witnessed a renaissance in the field of endovascular neurosurgery with the development of novel devices and techniques. These advances include smaller, more flexible microcatheters, balloon- and stent-assisted coiling techniques, and parent artery reconstruction with flow diverters. Endovascular treatment options for GIAs now include the following: (1) embolization/sacrifice of parent artery, (2) primary coiling (with or without balloon and/or stent assistance), and (3) parent artery reconstruction with flow diversion (with or without coiling; 7, 8, 10, 12, 13 in algorithm ).
Endovascular treatment should be considered in the setting of poor patient neurological status, failed operative intervention, anticipated surgical difficulties (e.g., heavily calcified or atherosclerotic aneurysm neck), surgical inaccessibility, medical comorbidities precluding a craniotomy, short life expectancy, and patient preference ( 10 in algorithm ). Increasing evidence suggests that endovascular surgery is the preferred treatment for ruptured and unruptured aneurysms of the extradural petrocavernous ICA ( 7, 8, 12, 13 in algorithm ). Alternatively, flow diversion is an option for treating unruptured GIAs of the clinoidal, ophthalmic, and communicating ICA segments, as well as GIAs of the ICA terminus ( 12, 13 in algorithm ). For unruptured fusiform GIAs, endovascular flow diversion is also an option. In the setting of fusiform GIA rupture, however, the need for dual-antiplatelet therapy places these patients at an unacceptably high risk of hemorrhage following flow diverter placement with unclear protection status of the aneurysm.
Patients harboring GIAs that present with acute subarachnoid hemorrhage (SAH) and large intracerebral hematoma require urgent craniotomy/craniectomy for clot removal and clip ligation ( 2, 3, 5 in algorithm ). Depending on hematoma location, the GIA can often be secured prior to hematoma evacuation, thereby avoiding premature aneurysm rupture during clot evacuation. In these unstable patients, catheter-based angiography (with TBO) cannot be safely performed. Moreover, arterial bypass is technically more challenging given the edematous, hemorrhagic brain, and frequent arterial vasospasm. As such, the operative goal should be clip ligation/reconstruction without bypass. Some authors advocate urgent coil embolization, followed by craniotomy/craniectomy for clot evacuation, but this is not standard practice. On occasion, in the absence of a large hematoma, coiling of the aneurysm dome can be performed for temporary protection while leaving the neck unobstructed for later definitive microsurgical treatment when the patient is more stable.
For unruptured GIAs and ruptured GIAs in a stable patient, cerebral angiography with TBO testing should be performed in selected patients with ICA aneurysms. Microsurgical treatment options include (1) ligation of parent artery, (2) direct clipping of aneurysm neck (with or without thromboendarterectomy), (3) trapping (with our without bypass), and (4) wrapping ( 5, 7, 8, 12, 13 in algorithm ). Direct clipping of the aneurysm neck is preferred for aneurysm obliteration with preservation of the afferent/efferent arteries. In patients who fail TBO testing, a vascular bypass is required ( 8, 13 in algorithm ). The timing of surgery should be as early as reasonably possible.
For ruptured fusiform GIAs in the anterior circulation that cannot reasonably be clip reconstructed, the diseased arterial segment may be circumferentially wrapped with a Gore-Tex patch, followed by clip application to reconstruct the parent artery.
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