Advances in Open Microsurgery for Cerebral Aneurysms

Davies, Jason M. MD, PhD; Lawton, Michael T. MD

Section Editor(s): Bendok, Bernard R. MD; Levy, Elad I. MD

doi: 10.1227/NEU.0000000000000193
Intracranial Aneurysms

BACKGROUND: Endovascular techniques introduced strong extrinsic forces that provoked reactive changes in aneurysm surgery. Microsurgery has become less invasive, more appealing to patients, lower risk, and efficacious for complex aneurysms, particularly those unfavorable for or failing endovascular therapy.

OBJECTIVE: To review specific advances in open microsurgery for aneurysms.

METHODS: A university-based, single-surgeon practice was examined for the use of minimally invasive craniotomies, surgical management of recurrence after coiling, the use of intracranial-intracranial bypass techniques, and cerebrovascular volume-outcome relationships.

RESULTS: The mini-pterional, lateral supraorbital, and orbital-pterional craniotomies are minimally invasive alternatives to standard craniotomies. Mini-pterional and lateral supraorbital craniotomies were used in one-fourth of unruptured patients, increasing from 22% to 28%, whereas 15% of patients underwent orbital-pterional craniotomies and trended upward from 11% to 20%. Seventy-four patients were treated for coil recurrences (2.3% of all aneurysms) with direct clip occlusion (77%), clip occlusion after coil extraction (7%), or parent artery occlusion with bypass (16%). Intracranial-intracranial bypass (in situ bypass, reimplantation, reanastomosis, and intracranial grafts) transformed the management of giant aneurysms and made the surgical treatment of posterior inferior cerebellar artery aneurysms competitive with endovascular therapy. Centralization maximized the volume-outcome relationships observed with clipping.

CONCLUSION: Aneurysm microsurgery has embraced minimalism, tailoring the exposure to the patient’s anatomy with the smallest possible craniotomy that provides adequate exposure. The development of intracranial-intracranial bypasses is an important advancement that makes microsurgery a competitive option for complex and recurrent aneurysms. Trends toward centralizing aneurysm surgery in tertiary centers optimize results achievable with open microsurgery.

ABBREVIATIONS: BRAT, Barrow Ruptured Aneurysm Trial

EC-IC, extracranial-intracranial

IC-IC, intracranial-to-intracranial

ISAT, International Subarachnoid Aneurysm Trial

MPt, mini-pterional craniotomy

OPt, orbital-pterional

PICA, posterior inferior cerebellar artery

Pt, pterional craniotomy

Author Information

Department of Neurological Surgery, University of California, San Francisco, San Francisco, California

Correspondence: Michael T. Lawton, MD, Department of Neurological Surgery, University of California, San Francisco, 505 Parnassus Ave, M780, Box 0112, San Francisco, CA 94143-0112. E-mail:

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Received June 09, 2013

Accepted September 09, 2013

Article Outline

Aneurysm surgery evolved dramatically after the introduction of the operating microscope. Approaches to specific aneurysms were established, with the pterional craniotomy (Pt) for most aneurysms around the circle of Willis, the orbitozygomatic-Pt for aneurysms at the basilar apex, the bifrontal craniotomy for anterior cerebral artery aneurysms, and the far lateral craniotomy for posterior inferior cerebellar artery (PICA) aneurysms. Clip configurations (straight, curved, angled, fenestrated, angled fenestrated, etc) and clipping techniques (simple, tandem, right-angle tandem, overlapping, fenestration tube, etc) were also established. Microsurgical instruments remain simple (microscissors, bipolar forceps, and microdissectors), and microsurgical techniques are well described for subarachnoid dissection, aneurysm exposure, proximal and distal control, and neck dissection. Today, after all of these technical refinements, the intrinsic forces driving advances in aneurysm surgery are weak. However, the advent of endovascular techniques, their popularization with results of clinical trials like the International Subarachnoid Aneurysm Trial (ISAT) and the Barrow Ruptured Aneurysm Trial (BRAT), and the relentless progression of endovascular technology have introduced strong extrinsic forces that have provoked reactive changes in aneurysm surgery. It is no longer good enough for open aneurysm surgeons to achieve excellent results; instead, they must achieve results that are competitive. Open aneurysm surgery has advanced to make it less invasive, more appealing to patients, lower risk, and efficacious for complex aneurysms unfavorable for or failing with endovascular therapy. This review examines these refinements in open microsurgery for aneurysms.

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Perhaps the most significant advance in open aneurysm surgery is the embrace of minimalism. The difference between femoral artery catheter access and the craniotomy is the starkest contrast between endovascular therapy and open surgery, and minimally invasive surgical approaches are a direct response to this critical difference. This change is driven by patient perceptions that opening the head is dangerous and potentially harmful and that smaller openings are safer. There are no surgical data demonstrating improved results with smaller craniotomies. In fact, the opposite may be true; miniaturized craniotomies may compromise the exposure, make aneurysm surgery more difficult, and lead to increased complications and worse results. Still, the drive to increase the patient appeal of open surgery has led to a re-examination of the elements of aneurysm exposure and a paring down to those elements that are truly necessary for aneurysm clipping and no more.

A decade ago, aneurysms around the circle of Willis were accessed with either a standard Pt or, for more complex or deeper aneurysms, an orbitozygomatic-Pt. Today, the menu of craniotomies for these aneurysms has expanded to 5 choices with the addition of the mini-Pt (MPt), lateral supraorbital craniotomy, and the orbital-Pt (OPt; Figure 1). Opting for a minimally invasive approach requires careful selection of a craniotomy that tailors the surgical exposure to the patient’s unique anatomy.

The standard Pt, as introduced by Yaşargil in the 1970s,1 remains the workhorse for aneurysm surgery1-5 because it accesses the anterior and posterior circulations, sellar and parasellar regions, superior orbital fissure and sphenoid wing, cavernous sinus and orbit, and temporal and frontal lobes.1,2,4-7 However, much of this exposure is unnecessary for simple, unruptured aneurysms. With most of the dissection occurring down into the sylvian fissure and carotid cistern, the outer limits of the craniotomy (to the middle fossa floor inferiorly, beyond the supraorbital notch medially, and above the pterion superiorly) add little to exposure, except in unusual circumstances. The MPt craniotomy eliminates these areas from the craniotomy and hones in on the central surgical corridor. The craniotomy extends to the temporal side of the sphenoid ridge to expose the sylvian fissure and to the medial side of the superior temporal line to expose the pterion and orbital roof, shrinking the diameter of the flap to about 3 cm. The exposure is adequate to allow an aggressive flattening of the pterion and sphenoid wing with the drill and to access the proximal sylvian fissure directly. The MPt approach is ideal for simple aneurysms that require a full sylvian fissure split: middle cerebral artery, internal carotid artery bifurcation, and ophthalmic artery aneurysms.8 The MPt craniotomy is small and contained beneath the temporalis muscle flap, which reduces exposure time, protects the brain, and improves cosmesis. Limitations include restricted access to the distal sylvian fissure, a tighter fissure in cases with brain swelling or hemorrhage, and closer edges of the craniotomy, which can limit maneuverability of instruments and deepen the surgical corridor.9

The lateral supraorbital craniotomy differs from the MPt craniotomy in its lateral extent, stopping at the pterion/lateral sphenoid ridge rather than crossing to the temporal side. Consequently, only the frontal lobe is exposed, and the sylvian fissure lies at the edge of the exposure. Dissection into the carotid cistern and proximal sylvian fissure drains cerebrospinal fluid and relaxes the brain, which drops the sylvian fissure into better view. Lamina terminalis can be opened to release cerebrospinal fluid from the ventricles as well. Nonetheless, access to the sylvian fissure is not as wide as with the MPt. The craniotomy measures <3 cm in diameter and is best suited to internal carotid artery aneurysms (posterior communicating artery and anterior choroidal artery) and simple anterior cerebral artery aneurysms (A1, anterior cerebral artery, and anterior communicating artery). The lateral supraorbital craniotomy limits access to the distal sylvian fissure, does not allow retraction of the temporal lobe, and may be tight in cases with brain swelling or hemorrhage. As with the MPt craniotomy, closer edges of the craniotomy limit maneuverability of instruments and deepen the surgical corridor.

The orbitozygomatic-Pt craniotomy remains the preferred approach to aneurysms at the basilar apex (basilar bifurcation, posterior cerebral artery, and superior cerebellar artery) because it optimizes exposure, illumination, and maneuverability with difficult pathology located in tight quarters. However, many of the advantages of the orbitozygomatic-Pt craniotomy can be gained with the OPt craniotomy, a simple modification that leaves the zygoma behind at the expense of temporal lobe exposure. The OPt craniotomy does not require dissection of the temporalis fascia to expose the zygoma, which shortens the opening times. Only a small portion of the lateral orbit is removed, which improves cosmesis. Retention of the zygoma eliminates the risk of complications from the temporomandibular joint, extensive temporalis muscle dissection, and dissection in the lateral orbit that might injure the abducens nerve. Removal of the superior orbit flattens the surgical corridor beyond that of the Pt or MPt craniotomies, which opens the view to the anterior communicating artery region, which can reduce the need for gyrus rectus resection or frontal lobe retraction with anterior communicating artery aneurysms. However, the temporal lobe cannot be mobilized and retracted as easily with the OPt craniotomy as with the orbitozygomatic-Pt craniotomy, which can compromise exposure of more complex aneurysms. Therefore, the OPt may be adequate for simple superior cerebellar artery and low-riding basilar apex aneurysms.

The challenge of a minimally invasive approach is to select the smallest possible craniotomy that still provides adequate exposure of the aneurysm.10 An advantage of minimally invasive approaches is a smaller skin incision. The standard pterional skin incision can be reduced to a shorter hairline incision, an eyebrow incision, or an eyelid incision. Patient preferences should dictate the choice of skin incision. In our experience, although eyebrow and eyelid incisions are less than half the length of the hairline incisions and scars are barely noticeable after the hair has regrown, patients seem to be more satisfied with hairline incisions that leave no trace of surgery on the face. Therefore, our preference is for a hairline incision that is shorter in length (beginning 2 cm above the zygoma and ending 2 cm lateral to the midline) and more anteriorly located than the traditional pterional incision.

Review of our practices over the last 2 years demonstrated an increasing trend in the use of minimally invasive aneurysm craniotomies (Table 1). Mini-craniotomies (MPt and lateral supraorbital craniotomy) were used with unruptured aneurysms, not with ruptured aneurysms, and accounted for one-quarter of our approaches. Their use increased from 22% to 28% over this review period, and the increase is expected to continue as our familiarity and comfort increase. The OPt craniotomy is not limited to use with unruptured aneurysms. It was used in 56 patients during the 2-year period, of which 29 (52%) were ruptured, accounting for 15% of all craniotomies for aneurysms. Use of the OPt craniotomy trended upward from 11% to 20%.11 Use of lighted instruments and retractorless technique has also increased.

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Although ISAT and BRAT demonstrated a lower risk of death and dependence at 1 year in patients treated with endovascular coiling compared with surgical clipping,12,13 they also demonstrated lower rates of complete aneurysm occlusion and higher rates of aneurysm recurrence during long-term follow-up, resulting in increased rates of rebleeding and increased rates of retreatment.14 Despite these issues related to the durability of coiling, endovascular therapy is increasingly popular among patients and increasingly practiced in community hospitals. In these settings, coil embolization has become the de facto first-line treatment for both ruptured and unruptured aneurysms, and the ranks of neurointerventionalists have been growing. Consequently, aneurysm care is decentralizing away from tertiary centers and is being performed by neurointerventionalists with low-volume practices, which runs counter to established volume-outcome relationships in aneurysm treatment. These trends, in combination with the inherent recurrent risk associated with coiling, have led to a new surgical entity: the recurrent coiled aneurysm.

Our surgical experience with previously coiled aneurysms stands at 74 patients over 15 years, making them an uncommon problem affecting 2.3% of our overall aneurysm cases.15,16 To put it in perspective, this frequency is half that of giant aneurysms in our experience. It can be difficult to know when to treat a recurrent aneurysm with repeat coiling or with open surgery. Gurian et al16a recommended exhausting all endovascular options before changing to a surgical option because of difficulties in clipping aneurysms with an intraluminal coil mass, dangers associated with coil extraction, and impaired visualization around an immobile aneurysm. These authors identified the important factors in this decision as aneurysm location, degree of neck occlusion, chronicity since treatment, age, and clinical condition. Dorfer et al16b based these treatment decisions on the reason for aneurysm recurrence, urging additional endovascular treatment for aneurysms that recur as a result of coil compaction with a resulting decrease in the volume of coils and urging surgical treatment for aneurysms that grow with a resulting increase in the volume of the sac. We have a low threshold for surgical management in patients with recurrent aneurysms unless the recurrence is minimal and the surgical risks are significant.

Recurrences can be managed surgically 3 ways: direct clip occlusion without coil manipulation; direct clip occlusion after coil extraction and thrombectomy; and parent artery occlusion with bypass (Table 2). Direct clip occlusion without coil manipulation is possible in the majority of cases (77%), as long as there is sufficient coil compaction to create a soft aneurysm neck (compaction ratio < 2.5 or wedge angle < 90°).15 The neck is occluded optimally with a clip below the coils across the neck completely, but a clip across some loops of coil or against a portion of the coil mass has proven effective. Only rarely is coil manipulation necessary. Temporary parent artery trapping enables the aneurysm to be opened and coils or thrombus to be extracted (Figure 2) or mobilized away from the neck. Coils in the setting of acute recurrence are easily extracted, but coils in the setting of chronic recurrence are adherent, and coil extraction can tear the neck or parent artery. Furthermore, coil extraction can injure surrounding arteries or cranial nerves outside the aneurysm sac. We rely on coil extraction in <10% of cases, when it is clear that clips will not close otherwise or will kink the parent artery. Aneurysms treated in the distant past with densely scarred coils or with complex necks or branch artery anatomy may not be amenable to direct clipping, and bypass plus indirect aneurysm occlusion is performed. Bypass/trapping and bypass/proximal occlusion are good options for high-risk aneurysms like giant aneurysms and posterior circulation aneurysms that have recurred. We routinely use indocyanine green videoangiography on all aneurysm cases and especially these to confirm patency of parent and branch arteries, complete occlusion of the aneurysm, and bypass patency. Techniques like flash fluorescence are particularly helpful in selecting the appropriate recipient artery.17 Adenosine-induced cardiac arrest may facilitate clip application in some cases, but temporary clipping typically offers a longer time window of flow arrest to extract or remodel coils. We do not use endoscopy in aneurysm surgery, preferring direct visualization with the operating microscope instead.

The recurrent aneurysm is an intriguing entity. Its occurrence depends on the interplay between hemodynamic stresses, aneurysm anatomy, and packing density of coils. Its incidence depends on how it is measured and, from the neurosurgeons’ perspective, how often the patients are referred for surgery. Because of the risk of rehemorrhage, recurrent aneurysms must be prevented with aggressive initial treatment, identified with diligent surveillance angiography, and treated when found. Despite the fact that half of our surgically explored recurrent aneurysms have extruded coils and aneurysm walls appear degraded, the recurrence problem remains less than expected. We have not detected any significant increases in coil recurrences despite the growth in endovascular market share. Surgical management of previously coiled aneurysms is a new and evolving neurosurgical challenge that will continue to change as novel technologies are introduced into clinical practice. Most recurrent aneurysms are amenable to surgical therapy but require advanced microsurgical expertise.

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Despite the negative results of the Carotid Occlusion Surgery Study, a prospective, randomized, controlled trial that failed to show a benefit of superficial temporal artery-middle cerebral artery bypass compared with medical management in patients carefully selected on the basis of positron emission tomography scans and oxygen extraction, bypass surgery has been widely embraced as an important treatment for complex aneurysms. As discussed with recurrent aneurysms after coiling, bypass techniques combined with parent artery trapping or occlusion are important in the treatment of complex aneurysms, particularly those without a favorable endovascular option. Despite all of its advances, endovascular technology has yet to introduce an endovascular bypass that can challenge open microsurgical bypass.

Bypass surgery for aneurysms has gone through an evolution from extracranial-to-intracranial (EC-IC) bypass to intracranial-to-intracranial (IC-IC) bypass. The superficial temporal artery-middle cerebral artery bypass as championed by Yaşargil18 is the prototypical bypass the redirects extracranial flow to the intracranial circulation with a single end-to-side anastomosis on the brain surface that is relatively simple and technically feasible for many neurosurgeons. In response to flow limitations of this bypass, second-generation bypasses also rely on EC-IC redirection of flow but use high-flow interposition grafts (ie, saphenous vein or radial artery grafts) connected to donor sites in the patient’s neck.19-34 EC-IC bypasses are susceptible to problems associated with harvesting donor vessels, inadequate donor caliber, mismatch between donor and recipient, neck incisions, graft length, and vulnerability of the bypass to external trauma, all of which are solved with a third generation of bypasses that are entirely intracranial. Reconstructive bypasses revascularize intracranial arteries with other intracranial arteries, without contribution from extracranial donor arteries. These IC-IC bypasses are simple, elegant, and more anatomic than their EC-IC counterparts.35-38 IC-IC bypasses require no harvest of extracranial donors, spare patients a neck incision, shorten any interposition grafts, are protected within the cranium, and use caliber-matched donor and recipient arteries. The development of an array of IC-IC bypasses represents an important advance in bypass surgery for brain aneurysms.

These bypasses are categorized into 4 types of intracranial arterial reconstruction: in situ bypass, reimplantation, reanastomosis, and intracranial bypass grafts (Figure 3). In situ bypass brings donor and recipient arteries together with a side-to-side anastomosis along their parallel course without interposition graft (eg, PICA-PICA). Reimplantation reattaches an aneurysm branch artery to normal parent or branch artery end to side after occluding the aneurysm, often by trapping (eg, PICA reimplantation onto the proximal vertebral artery). After trapping and excising an aneurysm, reanastomosis brings the normal arterials ends together with an end-to-end anastomosis. Intracranial bypass grafts use interposition grafts to revascularize a branch artery from a distant donor site and require 2 anastomoses. These bypasses are performed in 4 anatomic sites: sylvian fissure for middle cerebral artery aneurysms, callosal cistern for anterior cerebral artery aneurysms, cisterna magna for PICA aneurysms, and the carotid and crural cisterns for basilar apex aneurysms. IC-IC bypass is particularly useful for unanticipated bypasses because they do not require the preparation of the extracranial donor vessel or a cervical donor site.

Despite many advantages, these bypasses have their own issues. They are technically more difficult that EC-IC bypasses, often in deep, constrained fields. Whereas EC-IC bypass requires temporary occlusion of a single recipient artery during anastomosis, IC-IC bypass requires temporary occlusion of 2 vessels, putting 2 vascular territories at risk of ischemia and postoperative occlusion. Nonocclusive bypass techniques like excimer laser-assisted nonocclusive anastomosis eliminate this temporary occlusion time, but occlusion time is well tolerated, usually causes mild ischemia, and is not associated with complications. A comparison of EC-IC and IC-IC bypasses demonstrated equivalent results, with aneurysm obliteration rates of 97% and 98%, bypass patency rates of 89% and 94%, and good neurological outcomes (Glasgow Outcome Scale score, 4-5 at late follow-up) of 89% and 91%, respectively. With results like these, IC-IC bypasses are increasingly popular and, in our aneurysm practice, account for nearly half of our bypass cases (65 bypasses, 47%; Table 3).

The availability of a wide array of bypasses has advanced contemporary aneurysm practice. Although bypasses have a role in the management of dolichoectactic, dissecting, thrombotic, calcified, and recurrent aneurysms, their role in the management of giant aneurysms demonstrates their transformative impact. In the past, deep hypothermic circulatory arrest was used to convert an unclippable aneurysm into a clippable one by collapsing it, eliminating risk of intraoperative rupture, and permitting aggressive manipulation, even entrance into the aneurysm to remove thrombus and to create a supple neck.27,39-43 However, hypothermic circulatory arrest incurs significant operative morbidity. Cannulation of the femoral vessels can cause dissections, occlusions, and compromise of distal circulation; prolonged circulatory arrest is associated with cerebral ischemic injury and poor neurological outcomes; and heparinization, slowing of the coagulation cascade by hypothermia, and trauma to red blood cells and platelets by the cardiopulmonary bypass pump contribute to postoperative bleeding complications. These risks have resulted in declining use of hypothermic circulatory arrest.44 Instead, indirect aneurysm occlusion with a bypass has become a more acceptable alternative, with indirect occlusion consisting of proximal occlusion, distal occlusion, or trapping. This strategy has important advantages. The bypass can be performed with predictable ischemia times, cerebroprotection, and relatively low complication rates. The dangers of direct attack are avoided, including perforator dissection and preservation around clips. Clips are applied to afferent and efferent arteries, avoiding pathological tissues at the neck that can cause clip slippage, intraoperative rupture, and branch artery occlusions. Endovascular techniques can be used for indirect aneurysm occlusion, allowing the adequacy of a surgical bypass to be tested before permanent occlusion and allowing staged aneurysm occlusion with safe use of heparinization or optimization of hemodynamics in the intensive care unit. A heavy reliance on bypass techniques for giant aneurysms (38% in our experience with 141 giant aneurysms) resulted in good outcomes in 81% and represents an important advance in microsurgery for aneurysms.44

Even in an era with excellent endovascular options, bypass techniques can make open microsurgery a better therapeutic option, as demonstrated by PICA aneurysms. A prevailing attitude among the endovascular community is that the PICA is expendable with minimal morbidity. PICA occlusion can be a significant complication, and although cerebellar infarcts may be tolerable in some cases, more often, they cause secondary deterioration from edema and brainstem compression and require urgent decompressive craniectomy.45-51 Bypass techniques offer an alternative to this deconstructive approach, enabling a constructive approach that preserves the PICA with direct clipping or replaces flow with bypass when sacrificed.52,53 Reconstructive options for PICA revascularization include PICA-PICA bypass, PICA reimplantation, PICA reanastomosis, and interposition grafts that use the vertebral artery as a donor. The cisterna magna is a favorable working space to perform these bypasses, and our results have demonstrated high patency rates and low stroke rates. Distal aneurysms are particularly amenable to a constructive approach, and in our experience with 29 distal PICA aneurysms, we performed bypasses in 7 patients. Endovascular experiences use little or no bypass surgery, and cerebellar infarctions suggest that angiographic assessments of collateral circulation are unreliable or that bypasses were not performed aggressively enough. PICA aneurysms are difficult to treat endovascularly because of unusual morphology, aberrant branching, distal location, small caliber of the parent artery, tortuosity, and angulations that preclude coiling. The ability to perform a bypass should make microsurgery the primary treatment option, or at least the next option when endovascular coiling fails, rather than deliberate PICA occlusion.

Flow diverters introduce an endovascular option for selected giant and complex aneurysms, with excellent results reported for those along the carotid artery in the cavernous and paraclinoid segments. However, flow diversion is not the easy answer in all giant and complex aneurysms because of the following limitations. First, results with aneurysms in the middle cerebral artery and posterior circulation have been poor, with high rates of perforator infarction and hemorrhage thought to be due to “jailed” perforators and inflow into aneurysms without outflow (“ball-valve” effect), respectively.54,55 Second, deployment is technically challenging and may not be possible in aneurysms associated with tortuous vasculature, diminutive parent arteries, and acute angles between afferent and efferent arteries. Third, flow diverters require the use of aspirin and Plavix and therefore are restricted to unruptured aneurysms. Finally, the high cost of these devices limits their widespread application. In contrast, open surgical bypass remains a technique that is applicable to most aneurysms, regardless of aberrant aneurysm or parent artery anatomy, can be performed in patients with ruptured and unruptured aneurysms alike, is inexpensive, and is part of a durable cure for complex and giant aneurysms that are trapped. Flow diverters are changing the management of these aneurysms and have diminished noticeably the indications for surgical bypass for petrocavernous and paraclinoid aneurysms. As technology evolves and expands the endovascular armamentarium, indications for bypass are expected to decrease but not vanish; therefore, bypass proficiencies should be maintained and surgical solutions practiced as part of multidisciplinary teams.

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In contrast to endovascular therapy for aneurysms that has decentralized out of tertiary centers and into community hospitals, microsurgical therapy has increasingly centralized in tertiary centers. This trend, driven mainly by the proliferation of endovascular technology and practitioners, is decreasing the number of open aneurysm surgeons and increasing the complexity of aneurysms referred for microsurgical management. Centralization of aneurysm microsurgery has produced important advances in aneurysm outcomes, and historically, high-volume surgeons innovate new techniques and tools. Tertiary centers are equipped to apply multimodality approaches and to use the most sophisticated resources. In addition, the evidence is clear that patient morbidity and mortality are lower when complex procedures are performed at high-volume centers and by high-volume neurosurgeons.

Volume-outcome relationships have been demonstrated for aneurysms, with drops in mortality and morbidity seen in high-volume centers.56,57 Although large administrative data sets can be difficult to interpret because they lack detailed neurological outcome data, these large-cohort power analyses might not otherwise be feasible with single-provider or single-institution sources. In an analysis of pediatric aneurysms in the Nationwide Inpatient Sample, we found clear relationships between hospital-level aneurysm volume and outcomes of interest: discharge disposition, which is a surrogate for functional outcome and neurological status, and complication rates. Pediatric aneurysm patients have better discharge and complications profiles at higher-volume hospitals (Figure 4A and 4B). As demonstrated by these data, it is not uncommon to observe an initial decrement in outcomes going from the lowest quartile to the next, which is generally thought to be due to transfer practices that bias toward better outcomes. Data for both adults and children demonstrate that the odds of discharge to home without supportive services are improved at high-volume centers (Figure 4B). Interestingly, volume-outcome relationships are not universal for all procedures, as seen with pediatric endovascular interventions. These findings support the decentralization of endovascular therapy and the centralization of open microsurgical therapy for aneurysms.

Volume-outcome relationships have likewise been demonstrated for EC-IC bypass58 (J.M.D., manuscript in preparation), vascular malformations,59 and other neurosurgical procedures60,61 (J.M.D., manuscript in preparation) in large regional and national databases. These data suggest that volume effects are not confined to aneurysms but extend to other diseases within vascular neurosurgery. Hospitals that manage a high volume of aneurysms and achieve good outcomes also tend to manage a high volume of other cerebrovascular disorders with improved outcomes, which is not surprising given the shared skill set needed to treat diseases like moyamoya disease and arteriovenous malformations. The concentration of expertise within a single center of excellence benefits all vascular patients.

With aneurysm patients, it is appropriate to manage their high acuity, treat their complex lesions, and maximize their outcomes through specialization, experience, and expertise that are available at the hospital-wide level. For a variety of cerebrovascular procedures, not only do high-volume surgeons at high-volume centers have lower mortality rates and lower complication rates, but patients have shorter lengths of stay and hospitals gain important economic advantages. Hospital and surgeon volume-outcome effects are independent and the hospital-related effect is greater, implying that the ecosystem of care supersedes a surgeon’s technical expertise. Specialized nursing care, diagnostic services, and a team of providers likely account for improved outcomes at high-volume centers.

The trend toward centralized aneurysm surgery is favorable and should progress even further. Bypass proficiency provides a useful explanation. Sundt et al62 reported a threshold effect for bypass surgery, with approximately 20 bypass operations needed to achieve a patency rate >90%. With centralization, >75% of aneurysm centers in the United States perform bypass with sufficient volume to meet this threshold. However, some of these centers require more than a decade to reach the threshold. Therefore, centralization benefits patients and aneurysm surgeons by concentrating case volume and improving outcomes, and further centralization may be needed to maximize these volume-outcome effects. Large metropolitan areas might be better served to further reduce the number of centers of excellence, and regions might be better served to develop networks for transferring patients within the region.

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Endovascular techniques unleashed strong extrinsic forces that provoked reactive changes in aneurysm surgery. Open microsurgery must now compete with endovascular therapies, forcing surgery to become less invasive, more appealing to patients, lower risk, and efficacious for complex aneurysms, particularly those unfavorable for or failing with endovascular therapy. Open aneurysm surgery has advanced by embracing minimalism. Minimally invasive approaches tailor the surgical exposure to the patient’s unique anatomy, selecting the smallest possible craniotomy that still provides adequate exposure. The development of an array of IC-IC bypasses is another important advance that makes microsurgery a competitive option for complex and recurrent aneurysms. Trends centralizing aneurysm surgery in tertiary centers optimize results achievable with open microsurgery.

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The authors have no personal financial or institutional interest in any of the drugs, materials, or devices described in this article.

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Aneurysm; Clipping; Coiling; Cerebrovascular disease; Extracranial-intracranial bypass; Surgical outcomes

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