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Omar Choudhri and Michael T. Lawton

The middle tentorial incisural space, located lateral to the midbrain and medial to the temporal lobe, contains the ambient cistern through which courses the third, fourth, and fifth cranial nerves, posterior cerebral artery (PCA), superior cerebellar artery, and the choroidal arteries. Arteriovenous malformations (AVMs) in this compartment are supplied by the thalamogeniculate and posterior temporal branches of the PCA, and drain into tributaries of the basal vein of Rosenthal. We present a case of an AVM in this middle tentorial incisural space that persisted after embolization and radiosurgery, and was microsurgically resected through a subtemporal approach. This case demonstrates the anatomy of the middle incisural space and technical aspects in microsurgical resection of these rare AVMs.

The video can be found here: https://youtu.be/V-dIWh8ys3E.

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Michael T. Lawton and Michael J. Lang

Despite the erosion of microsurgical case volume because of advances in endovascular and radiosurgical therapies, indications remain for open resection of pathology and highly technical vascular repairs. Treatment risk, efficacy, and durability make open microsurgery a preferred option for cerebral cavernous malformations, arteriovenous malformations (AVMs), and many aneurysms. In this paper, a 21-year experience with 7348 cases was reviewed to identify trends in microsurgical management. Brainstem cavernous malformations (227 cases), once considered inoperable and managed conservatively, are now resected in increasing numbers through elegant skull base approaches and newly defined safe entry zones, demonstrating that microsurgical techniques can be applied in ways that generate entirely new areas of practice. Despite excellent results with microsurgery for low-grade AVMs, brain AVM management (836 cases) is being challenged by endovascular embolization and radiosurgery, as well as by randomized trials that show superior results with medical management. Reviews of ARUBA-eligible AVM patients treated at high-volume centers have demonstrated that open microsurgery with AVM resection is still better than many new techniques and less invasive approaches that are occlusive or obliterative. Although the volume of open aneurysm surgery is declining (4479 cases), complex aneurysms still require open microsurgery, often with bypass techniques. Intracranial arterial reconstructions with reimplantations, reanastomoses, in situ bypasses, and intracranial interpositional bypasses (third-generation bypasses) augment conventional extracranial-intracranial techniques (first- and second-generation bypasses) and generate innovative bypasses in deep locations, such as for anterior inferior cerebellar artery aneurysms. When conventional combinations of anastomoses and suturing techniques are reshuffled, a fourth generation of bypasses results, with eight new types of bypasses. Type 4A bypasses use in situ suturing techniques within the conventional anastomosis, whereas type 4B bypasses maintain the basic construct of reimplantations or reanastomoses but use an unconventional anastomosis. Bypass surgery (605 cases) demonstrates that open microsurgery will continue to evolve. The best neurosurgeons will be needed to tackle the complex lesions that cannot be managed with other modalities. Becoming an open vascular neurosurgeon will be intensely competitive. The microvascular practice of the future will require subspecialization, collaborative team effort, an academic medical center, regional prominence, and a large catchment population, as well as a health system that funnels patients from hospital networks outside the region. Dexterity and meticulous application of microsurgical technique will remain the fundamental skills of the open vascular neurosurgeon.

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Ana Rodríguez-Hernández and Michael T. Lawton

Object

Surgical routes to posterior inferior cerebellar artery (PICA) aneurysms are opened between the vagus (cranial nerve [CN] X), accessory (CN XI), and hypoglossal (CN XII) nerves for safe clipping, but these routes have not been systematically defined. The authors describe 3 anatomical triangles and their relationships with PICA aneurysms, routes for surgical clipping, outcomes, and angiographically demonstrated anatomy.

Methods

The vagoaccesory triangle is defined by CN X superiorly, CN XI laterally, and the medulla medially. It is divided by CN XII into the suprahypoglossal triangle (above CN XII) and the infrahypoglossal triangle (below CN XII). From a consecutive surgical series of 71 PICA aneurysms in 70 patients, 51 aneurysms were analyzed using intraoperative photographs.

Results

Forty-three PICA aneurysms were located inside the vagoaccessory triangle and 8 were outside. Of the aneurysms inside the vagoaccessory triangle, 22 (51%) were exposed through the suprahypoglossal triangle and 19 (44%) through the infrahypoglossal triangle; 2 were between triangles. The lesions were evenly distributed between the anterior medullary (16 aneurysms), lateral medullary (19 aneurysms), and tonsillomedullary zones (16 aneurysms). Neurological and CN morbidity linked to aneurysms in the suprahypoglossal triangle was similar to that associated with aneurysms in the infrahypoglossal triangle, but no morbidity was associated with PICA aneurysms outside the vagoaccessory triangle. A distal PICA origin on angiography localized the aneurysm to the suprahypoglossal triangle in 71% of patients, and distal PICA aneurysms were localized to the infrahypoglossal triangle or outside the vagoaccessory triangle in 78% of patients.

Conclusions

The anatomical triangles and zones clarify the borders of operative corridors to PICA aneurysms and define the depth of dissection through the CNs. Deep dissection to aneurysms in the anterior medullary zone traverses CNs X, XI, and XII, whereas shallow dissection to aneurysms in the lateral medullary zone traverses CNs X and XI. Posterior inferior cerebellar artery aneurysms outside the vagoaccessory triangle are frequently distal and superficial to the lower CNs, and associated surgical morbidity is minimal. Angiography may preoperatively localize a PICA aneurysm's triangular anatomy based on the distal PICA origin or distal aneurysm location.

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Adib A. Abla and Michael T. Lawton

Object

The authors describe their experience with intracranial-to-intracranial (IC-IC) bypasses for complex anterior cerebral artery (ACA) aneurysms with giant size, dolichoectatic morphology, or intraluminal thrombus; they determine how others have addressed the limitations of ACA bypass; and they discuss clinical indications and microsurgical technique.

Methods

A consecutive, single-surgeon experience with ACA aneurysms and bypasses over a 16-year period was retrospectively reviewed. Bypasses for ACA aneurysms reported in the literature were also reviewed.

Results

Ten patients had aneurysms that were treated with ACA bypass as part of their surgical intervention. Four patients presented with subarachnoid hemorrhage and 3 patients with mass effect symptoms from giant aneurysms; 1 patient with bacterial endocarditis had a mycotic aneurysm, and 1 patient's meningioma resection was complicated by an iatrogenic pseudoaneurysm. One patient had his aneurysm discovered incidentally. There were 2 precommunicating aneurysms (A1 segment of the ACA), 5 communicating aneurysms (ACoA), and 3 postcommunicating (A2–A3 segments of the ACA). In situ bypasses were used in 4 patients (A3-A3 bypass), interposition bypasses in 4 patients, reimplantation in 1 patient (pericallosal artery-to-callosomarginal artery), and reanastomosis in 1 patient (pericallosal artery). Complete aneurysm obliteration was demonstrated in 8 patients, and bypass patency was demonstrated in 8 patients. One bypass thrombosed, but 4 years later. There were no operative deaths, and permanent neurological morbidity was observed in 2 patients. At last follow-up, 8 patients (80%) were improved or unchanged. In a review of the 29 relevant reports, the A3-A3 in situ bypass was used most commonly, extracranial (EC)–IC interpositional bypasses were the second most common, and reanastomosis and reimplantation were used the least.

Conclusions

Anterior cerebral artery aneurysms requiring bypass are rare and can be revascularized in a variety of ways. Anterior cerebral artery aneurysms, more than any other aneurysms, require a thorough survey of patient-specific anatomy and microsurgical options before deciding on an individualized management strategy. The authors' experience demonstrates a preference for IC-IC reconstruction, but EC-IC bypasses are reported frequently in the literature. The authors conclude that ACA bypass with indirect aneurysm occlusion is a good alternative to direct clip reconstruction for complex ACA aneurysms.

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Roberto C. Heros

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Adib A. Abla and Michael T. Lawton

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Jason M. Davies and Michael T. Lawton

OBJECTIVE

Treatment of cerebrovascular malformations has grown in complexity with the development of multimodal approaches, including microsurgery, endovascular treatments, and radiosurgery. In spite of this changing standard of care, the provision of care continues across a variety of settings. The authors sought to determine the risk of adverse outcome after treatment of patients with vascular malformations in the US. Patient, surgeon, and hospital characteristics, including volume, were tested as potential outcome predictors.

METHODS

The authors examined data collected between 2000 and 2009 in the Nationwide Inpatient Sample (NIS) database, assessing safety, quality, and cost-effectiveness. They performed multivariate analyses of trends in microsurgical, radiosurgical, and endovascular treatment by hospital and surgeon volume, using death, routine discharge percentage, length of stay (LOS), complications, and hospital charges as end points. They further computed the value of care, which was defined as the ratio of the functional outcome (routine discharge percentage) to cost of care to the payer (hospital charges).

RESULTS

The authors identified 8227 patients with vascular malformations who were treated at US hospitals. Hospitals and surgeons were classified by yearly case volume. Compared with low-volume hospitals (2 or fewer cases/year), high-volume hospitals (16 or more cases/year) had shorter LOS (3 vs 2 days, p = 0.005), higher total charges ($37,374 vs $19,986, p = 0.003), more frequent discharge to home (p < 0.001), and lower mortality rates (0.7% vs 1.16%, p = 0.010). High-volume surgeons (7 or more cases/year) likewise had superior outcomes compared with low-volume surgeons (1 or fewer cases/year), with shorter LOS (2 vs 3 days, p = 0.03), more frequent discharge to home (p < 0.001), and lower mortality rates (0.7% vs 1.10%, p = 0.005). Underlying these outcomes, the rates of intervention for surgery, angiography, embolization, and radiosurgery were likewise significantly different in high- versus low-volume practices.

Based on these results the authors modeled how outcomes might change if care were consolidated at designated centers of excellence (COEs), and found that on an annual basis, care at high-volume hospital COEs would result in 18.5 fewer deaths, 1252.1 fewer hospital days, 182.7 more discharges home without additional services, 48.5 fewer medical complications, and 117.4 fewer perioperative complications. Surgeon-level rates for high-volume COEs demonstrated an even larger benefit over current standards, with 27.4 fewer deaths, 10,713.7 fewer hospital days, a $51.6-million reduction in charges, 370.9 additional routine discharges, and reduced complications in all categories (27.8 fewer surgical, 198.0 fewer medical, and 32.1 fewer perioperative) compared with care at non-COEs.

CONCLUSIONS

For patients with vascular malformations who were treated in the US between 2000 and 2009, treatment performed at high-volume centers was associated with significantly lower morbidity and, for high-volume surgeons, with lower mortality rates. These data suggest that treatment by high-volume institutions and surgeons will yield superior outcomes and superior value. The authors therefore advocate the creation of care paradigms that triage patients to high-volume institutions and surgeons, which can serve as cerebrovascular COEs.

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Ana Rodríguez-Hernández, Albert L. Rhoton Jr. and Michael T. Lawton

Object

The conceptual division of intracranial arteries into segments provides a better understanding of their courses and a useful working vocabulary. Segmental anatomy of cerebral arteries is commonly cited by a numerical nomenclature, but an analogous nomenclature for cerebellar arteries has not been described. In this report, the microsurgical anatomy of the cerebellar arteries is reviewed, and a numbering system for cerebellar arteries is proposed.

Methods

Cerebellar arteries were designated by the first letter of the artery's name in lowercase letters, distinguishing them from cerebral arteries with the same first letter of the artery's name. Segmental anatomy was numbered in ascending order from proximal to distal segments.

Results

The superior cerebellar artery was divided into 4 segments: s1, anterior pontomesencephalic segment; s2, lateral pontomesencephalic segment; s3, cerebellomesencephalic segment; and s4, cortical segment. The anterior inferior cerebellar artery was divided into 4 segments: a1, anterior pontine segment; a2, lateral pontine segment; a3, flocculopeduncular segment; and a4, cortical segment. The posterior inferior cerebellar artery was divided into 5 segments: p1, anterior medullary segment; p2, lateral medullary segment; p3, tonsillomedullary segment; p4, telovelotonsillar segment; and p5, cortical segment.

Conclusions

The proposed nomenclature for segmental anatomy of cerebellar artery complements established nomenclature for segmental anatomy of cerebral arteries. This nomenclature is simple, easy to learn, and practical. The nomenclature localizes distal cerebellar artery aneurysms and also localizes an anastomosis or describes a graft's connections to donor and recipient arteries. These applications of the proposed nomenclature with cerebellar arteries mimic the applications of the established nomenclature with cerebral arteries.

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Adib A. Abla, Cameron M. McDougall, Jonathan D. Breshears and Michael T. Lawton

OBJECT

Intracranial-to-intracranial (IC-IC) bypasses are alternatives to traditional extracranial-to-intracranial (EC-IC) bypasses to reanastomose parent arteries, reimplant efferent branches, revascularize branches with in situ donor arteries, and reconstruct bifurcations with interposition grafts that are entirely intracranial. These bypasses represent an evolution in bypass surgery from using scalp arteries and remote donor sites toward a more local and reconstructive approach. IC-IC bypass can be utilized preferentially when revascularization is needed in the management of complex aneurysms. Experiences using IC-IC bypass, as applied to posterior inferior cerebellar artery (PICA) aneurysms in 35 patients, were reviewed.

METHODS

Patients with PICA aneurysms and vertebral artery (VA) aneurysms involving the PICA’s origin were identified from a prospectively maintained database of the Vascular Neurosurgery Service, and patients who underwent bypass procedures for PICA revascularization were included.

RESULTS

During a 17-year period in which 129 PICA aneurysms in 125 patients were treated microsurgically, 35 IC-IC bypasses were performed as part of PICA aneurysm management, including in situ p3-p3 PICA-PICA bypass in 11 patients (31%), PICA reimplantation in 9 patients (26%), reanastomosis in 14 patients (40%), and 1 V3 VA-to-PICA bypass with an interposition graft (3%). All aneurysms were completely or nearly completely obliterated, 94% of bypasses were patent, 77% of patients were improved or unchanged after treatment, and good outcomes (modified Rankin Scale ≤ 2) were observed in 76% of patients. Two patients died expectantly. Ischemic complications were limited to 2 patients in whom the bypasses occluded, and permanent lower cranial nerve morbidity was limited to 3 patients and did not compromise independent function in any of the patients.

CONCLUSIONS

PICA aneurysms receive the application of IC-IC bypass better than any other aneurysm, with nearly one-quarter of all PICA aneurysms treated microsurgically at our center requiring bypass without a single EC-IC bypass. The selection of PICA bypass is almost algorithmic: trapped aneurysms at the PICA origin or p1 segment are revascularized with a PICA-PICA bypass, with PICA reimplantation as an alternative; trapped p2 segment aneurysms are reanastomosed, bypassed in situ, or reimplanted; distal p3 segment aneurysms are reanastomosed or revascularized with a PICA-PICA bypass; and aneurysms of the p4 segment that are too distal for PICA-PICA bypass are reanastomosed. Interposition grafts are reserved for when these 3 primary options are unsuitable. A constructive approach that preserves the PICA with direct clipping or replaces flow with a bypass when sacrificed should remain an alternative to deconstructive PICA occlusion and endovascular coiling when complete aneurysm occlusion is unlikely.