When and How to Use Proximal Occlusion Devices During Carotid Artery Angioplasty and Stenting

Abstract

Methods of cerebral embolic protection during carotid artery stenting focusing on the newest method, proximal occlusion, are discussed in this manuscript. Proximal occlusion devices achieve cerebral protection through flow reversal from the internal carotid artery into the arterial guide sheath that is the conduit for the deployment of devices across the carotid bifurcation. We review the literature and draw upon our experience-based opinion.

VASCULAR DISEASE MANAGEMENT 2012;9(1):E5–E12

Key words: carotid artery angioplasty and stenting, cerebral protection, embolic protection devices

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Introduction

Carotid artery angioplasty and stenting (CAS) is an alternative to carotid endarterectomy (CEA) for stroke prevention in symptomatic and asymptomatic carotid stenosis. The concerns of procedure-related embolic complications led to the emergence and acceptance of cerebral protection devices. There are 3 main types of devices for cerebral protection: distal occlusion balloons, distal filters, and proximal occlusion devices with and without flow reversal. Each approach possesses unique strengths and weaknesses.¹

Distal occlusion balloon systems (PercuSurge GuardWire system, Medtronic,² and TriActiv System, Kensey Nash) include a compliant, radiopaque-labeled, inflatable balloon that is advanced over a wire to the internal carotid artery (ICA), distal to the lesion. An export aspiration catheter is placed over the shaft of the guardwire to remove debris generated by the procedure. Advantages of this type of system include greater ease of traversing critically stenotic or tortuous lesions because of the low crossing profile of the balloon, along with flexibility and increased trackability.³ Known associated complications include vasospasm or dissection of the ICA, bradycardia, asystole, and hypotension due to stretch on the carotid sinus.³ Approximately 5% of the patients do not tolerate initial balloon occlusion,⁴ raising concerns that patients with severe contralateral carotid artery disease might not neurologically tolerate ICA occlusion. This type of device is cumbersome and complex to deploy and compromises visualization of the distal ICA owing to flow occlusion by comparison with filter-type devices.³

Filter-type distal embolic protection devices, which do not involve occlusion or reversal of cerebral blood flow, eventually replaced distal occlusion balloons as a means to provide cerebral protection. There are several different filter designs that each allow continuous antegrade flow and filtration of debris. Five filter-type devices have received FDA approval for use in the U.S., which are the Accunet with the Acculink stent (Abbott Vascular), the EmboShield with the Xact stent (Abbott), the Spider (ev3 Endovascular), the FilterWire EZ with the NexStent (Boston Scientific), and the AngioGuard with the Precise stent (Cordis).

Distal filters consist of a supporting wire made of nitinol and a basket composed of a polyurethane membrane that has 80 mm-130 mm pores. The diameter of the filter ranges from 3.0 mm-7.0 mm. The filter is connected to the distal end of a 0.014-inch wire with a floppy tip, which is used as a guidewire during the interventional procedure. The closed filter is advanced through the lesion and opened in the ICA, distal to the lesion. At the end of the procedure, a retrieval sheath is advanced and the filter is closed and removed from the artery.⁵ Filter protection devices generally have larger crossing profiles than occlusion balloon protection devices with an abrupt change in diameter between the floppy-tipped distal wire and the filter basket that can adversely affect trackability.³

Proximal occlusion devices work by stopping or reversing flow in the ICA. These types of devices include a long sheath catheter with a central working lumen that is connected to 2 balloons inflated to occlude the external carotid artery (ECA) and the common carotid artery (CCA), thereby allowing the entire procedure to be performed under complete cerebral protection. To achieve this type of protection (ie, to prevent embolization), there is a need to create complete cessation or reversal of flow in the ICA.¹ The MO.MA device (Invatec) and the Gore flow reversal system (W. L. Gore & Associates) are FDA-approved proximal occlusion devices.

By understanding the mechanisms of procedure-related thromboembolism and the advantages and disadvantages of the protection devices, operators can tailor the choice of device to patient-specific anatomy and lesion characteristics in order to reduce the risk of periprocedural stroke and improve the safety of CAS.

Need for a Protective Device

An ex vivo experimental model of human carotid artery specimens showed that embolic particles are evenly distributed throughout each step of the carotid angioplasty procedure (ie, when the stenosis is crossed with the guidewire or the balloon and after balloon angioplasty).⁶ In a clinical study, transcranial Doppler (TCD) monitoring was used to compare the frequency of microembolic signals during CAS without protection versus CAS using a distal occlusion balloon (PercuSurge GuardWire, Medtronic).⁷ In the protected stenting group, a significant reduction of microembolic signal counts (68 ± 83 vs. 164 ± 108 in the non-protected group; P=0.002) was observed during predilation, stent deployment, and postdilation. Kastrup et al8 reviewed the literature published between January 1990 and June 2002 and compared 896 CAS procedures performed with distal protection devices (temporary distal balloon occlusion with subsequent aspiration and intravascular filter devices) and 2,537 CAS procedures performed without protection devices. The protected stenting group showed a statistically significant reduction in the rates of combined stroke and death (1.8% vs. 5.5%; P<0.001), minor stroke (0.5% vs. 3.7%; P<0.001), and major stroke (0.3% vs. 1.1%; P<0.05) at 30 days. The results of the Endarterectomy versus Angioplasty in Patients with Symptomatic Severe Carotid Stenosis trial support the use of cerebral protection devices.⁹ The investigators found that the 30-day risk of stroke or death was significantly lower in patients treated with a distal embolic protection device (7.9% vs. 25% for unprotected cases, P=0.03). In the absence of randomized studies comparing CAS with and without embolic protection devices, these studies created a consensus for the use of these devices to lessen the risk of perioperative stroke.

Ultrasound Lesion Characteristics

A clear indication for proximal versus distal protection has not been established. Logically, intraluminal thrombus, soft plaque, severe symptomatic stenosis, and poor distal landing zone (tortuous poststenotic vessel) would be indications for proximal protection.

A Dutch single-center prospective analysis of microembolic signals on procedural TCD ultrasonography for unprotected CAS and filter-protected CAS reported that the use of filters may be associated with an increase in the number of distal microemboli.¹⁰ However, a comprehensive statistical comparison could not be made between the patient groups because of the infrequent occurrence of cerebral sequelae. Explanations for these findings in conjunction with distal embolic protection include the following: a macroembolus may be propelled into the filter and subsequently disintegrated into smaller particles that can pass through the micropores; thrombus formation on the distal filter surface or on the tip; or movement of the filter causing microtrauma to the vessel wall, which in turn might lead to increased embolic load.¹¹

A study that evaluated the frequency of periprocedural events during CAS showed fewer microembolic signals on TCD ultrasonography and fewer hyperintense signals on diffusion-weighted MRI with proximal protection compared with distal protection,¹² which implies proximal protection results in fewer periprocedural events. The results of the European Imaging in Carotid Angioplasty and Risk of Stroke (ICAROS) prospective registry showed that gray-scale median (GSM) scores ≤25 (representing echogenic plaque) are associated with higher embolic potential.¹³ Eleven of 155 (7.1%) patients with preprocedural GSM scores ≤25 had strokes after stenting versus 4 of 263 (1.5%) patients with GSM >25 (P=0.005). The ICAROS investigators showed that patients with GSM >25 benefit from the use of distal embolic protection (P=0.01). However, for patients with GSM of <25, distal embolic protection did not seem to maintain their efficacy; in these cases, CAS performed with proximal embolic protection devices or CEA may prove safer.

Disadvantages of Distal Protection Devices

With distal protection devices, you need to cross the stenotic lesion with the device and cerebral microembolization may occur before cerebral protection is achieved. Filters may provide incomplete protection, which allows the passage of particles smaller than their pore sizes (ie, particles <60 µm) to the brain.¹⁴ A filter can become filled with debris and need to be removed or aspirated to avoid spilling of its contents during retrieval.¹⁵

When the ICA distal to the lesion is too tortuous or when there is little space between the lesion site and the cerebrum, distal protection systems cannot be used because of the lack of a landing zone for the device. Improper filter-vessel apposition can lower the efficacy of emboli capture and may cause vessel damage. The diameter of the filter may preclude passage of the filter across tight lesions. Tortuous vessels occasionally make the device difficult to retrieve, especially after deploying an open-cell stent, which may then become caught on a stent tine.

Advantages of Proximal Embolic Protection

The application of a proximal occlusion device achieves cerebral protection before crossing the lesion and reduces the risk of cerebral emboli and stroke, which is especially valuable when plaque is associated with a tight stenosis. Proximal embolic protection is similar to arterial clamping during CEA in that it affords cerebral protection during all steps of the procedure and allows all sizes of particles to be captured. This advantage was shown in a study comparing procedural microembolic signals by TCD in which use of the MO.MA proximal protection system led to significantly lower counts of microembolic signals during CAS, compared with a filter device (FilterWire, Boston Scientific).¹⁶

A proximal protection device is a valuable tool, in the authors’ opinion, in cases of complex lesions, such as severe carotid stenosis, or cases of mobile thrombus associated with ulcerative lesions in the ICA, where crossing the lesion is a challenge with a high risk of distal embolization.^17-18 Another advantage is that the operator can choose the guidewire that will best cross the lesion.

In patients with acute ischemia who have occlusion or thrombus in a carotid artery,^17,20 the operator can use the proximal protection device to penetrate the plaque or clot, open the carotid artery, aspirate the debris, and then proceed with the recanalization procedure intracranially under flow arrest to prevent distal embolization.

Patients with symptomatic carotid artery disease may also benefit from proximal protection. There is a higher risk of a lesion having a large plaque burden that may be fractured during the CAS procedure,¹⁹ which poses a greater risk for overflow, spilling, and distal embolization of the filter contents.

In general, cases with a complex or tight lesion, distal tortuosity, or a symptomatic or ulcerated plaque are better suited for a flow reversal protection system.¹⁹ If the system can be navigated through a difficult arch or proximal vessel tortuosity, the balloon sheath may act in favor by “stabilizing” the system.²¹ In our practice, proximal protection devices are thought to be most useful in symptomatic, elderly patients, as they represent the group with the highest risk of microembolization during CAS (Table 1).

Disadvantages of Proximal Embolic Protection

As with other devices, complication rates associated with cerebral protection during CAS depend on operator experience with the technique and devices. Proximal protection devices are relatively bulky and technically complex to navigate around a difficult arch. For example, placement of the ECA balloon may be challenging in the presence of an external stenosis or a very proximal superior thyroidal artery. These devices should not be used when the lesion involves the CCA and is near the carotid bifurcation because the ECA balloon will traverse the lesion before proximal protection is achieved, which poses a risk for microembolization.

Approximately 5%-10% of patients are intolerant of balloon occlusion,¹¹ but with patient selection aimed at avoiding proximal occlusion in those who clearly had an isolated territory (ie, a cerebral hemisphere with little or no collateral perfusion and fed in its entirety by the ipsilateral carotid artery, as demonstrated by angiography), only 2.4% of patients in the Embolic Protection with Reverse Flow (EMPiRE) trial were intolerant.^19,22 Occlusion intolerance is infrequent, fairly simple to resolve, and does not appear to have any clinical sequelae. In cases like this, after performing aspiration of blood from the ICA, flow should be re-established. After a few minutes, fluids are administered to the patient, and blood pressure is allowed to rise slightly. Another attempt can then be made to occlude the ICA. The patient usually tolerates this. With flow reversal, the stent and the balloon should be prepared before occlusion is performed to cut occlusion time down to <5 minutes.

Patients with occlusion or stenosis of the contralateral ICA or any stenosis of the ipsilateral ECA might be considered by some to be unsuitable candidates for proximal occlusion devices.^1,23 In cases of chronic severe stenosis of the ICA, collateral filling of intracranial vessels may be through the ECA and ophthalmic collaterals. In these cases, ECA balloon occlusion may cause symptomatic cerebral ischemia.

Proximal embolic protection devices have large introducer sheaths (8- or 9 Fr, versus the 6- to 8 Fr sheaths needed with distal protective devices) and are relatively cumbersome devices potentially associated with a higher rate of bleeding complications although FDA registries did not show a significant increase in access site complications. Access difficulties can be encountered in patients with extensive calcification or stents in the iliac arteries. If femoral artery access is contraindicated (Figure 1), the use of a proximal protection device is less desirable, although there is a report of 3 cases of transradial CAS procedures in which the 8 Fr MO.MA device was used in patients with severe ICA stenosis.²⁴

Specific Proximal Cerebral Protection Devices

MO.MA device

The MO.MA device has a catheter system that is compatible with an 8- or 9 Fr introducer sheath; the working channel is 6 Fr, which is compatible with stent placement. Two inflatable low-pressure compliant balloons allow independent occlusion of both the CCA (5 mm-13 mm) and the ECA (3 mm-6 mm) to allow antegrade and retrograde flow blockage, respectively (Figure 2).

To position the MO.MA device, a 0.038-inch glide wire is navigated into one of the ECA branches (Figure 3). A diagnostic catheter is then advanced into the ECA. The 0.038-inch glide wire is replaced with a stiff wire (Supra Core [Abbott] or Amplatz wire [Cook]) and the diagnostic catheter is taken out. The MO.MA device is then advanced over the stiff wire.

After inflation of the balloons, under flow arrest, the lesion is crossed with a wire and stented. Post-stent angioplasty is performed. The working channel allows particulate debris to be removed by syringe aspiration prior to restoration of blood flow (Figure 4). After stenting, approximately 60 cc of blood is aspirated. The ECA balloon is deflated first so that additional debris will clear to the ECA; the CCA balloon is then deflated.

The nonrandomized proximal protection with the MO.MA device during carotid stenting (ARMOUR) trial¹¹ evaluated the safety and effectiveness of proximal protection with the MO.MA device during CAS in patients at risk for surgical complications during CEA. The primary end point of the study was the occurrence of any major adverse cardiac or cerebrovascular events (ie, myocardial infarction [MI], stroke, death) within 30 days of CAS using the MO.MA device. The 30-day major adverse events rate was 2.7% (95% CI [1.0%-5.8%]) with a 30-day stroke rate of 2.3%, including a major stroke rate of 0.9%. Overall, the MO.MA device was found to provide a safe and effective means of cerebral protection.

A previous prospective study utilizing the MO.MA device demonstrated that 7.6% of patients were intolerant to cessation of blood flow.²⁵ An advantage of the MO.MA is that if the patient is intolerant to blockage of flow or in patients in whom ECA branches cannot be occluded due to stenosis or tortuosity, a distal protection device can be used in conjunction with the MO.MA system to perform the CAS procedure.

A large single-center registry consisting of an unselected patient population treated with the MO.MA device showed that anatomical and/or clinical conditions of high surgical risk were not associated with an increased rate of adverse events.²³ A potential limitation of the MO.MA is that patients with large ECAs can not be candidates, according to the ARMOUR trial.¹¹ Potentially, the ECA or CCA diameter may be too large for complete blood flow occlusion by the MO.MA device. On the other hand, in a single-center registry of MO.MA device utilization, a critical stenosis of the ipsilateral ECA was present in 7.2% of the cases, but this did not preclude procedural success.²³ In the presence of significant stenosis of the ECA, the balloon was inflated distal to the lesion. Although the 9 Fr femoral introducer sheath is larger than needed for most distal protection devices, a previous study did not find a higher incidence of femoral artery access site complications related to the size of this sheath.²⁵

Gore Flow Reversal System

With the Gore device, flow reversal is achieved at the treatment site by selectively occluding CCA and ECA blood flow (Figure 5). By establishing a shunt between the carotid artery and femoral vein, blood from the collateral vessels is redirected to the lower pressure venous return. Redirected blood is filtered outside the body before being reintroduced into the venous system.

The EMPiRE study²² compared the 30-day safety and efficacy of the Gore flow reversal system, when used during CAS with any FDA-approved carotid stent, to an objective performance criterion derived from distal embolic protection studies. The study population included symptomatic patients with ≥50% carotid stenosis or asymptomatic patients with ≥80% carotid stenosis who were considered high-risk for complications from CEA because of anatomic characteristics or comorbid conditions. The results of the pivotal EMPiRE clinical study were based on the enrollment of 245 patients at 28 clinical sites. The 30-day stroke, death, and MI rate was 4.5%; and the rate of death and stroke was low, at 2.9%, as compared with other embolic protection trials. Importantly, the study also showed encouraging results in some of the most challenging patient populations with low death, stroke, and MI rates of 2.6% for octogenarians and 3.8% for symptomatic patients. As mentioned, flow-reversal intolerance was reported in 2.4% after selecting patients who had an isolated territory.

A small study evaluated the number of embolic signals detected by TCD monitoring during CAS with either the FilterWire EZ (Boston Scientific) or the Gore flow reversal protection system.²⁶ Both types of embolic protection devices significantly reduced but did not eliminate the number of microemboli reaching the brain during CAS. Embolic signals occurred throughout those procedures in which a filter device was in place, but there were none when the proximal flow reversal system was in use. However, embolic signals were detected during insertion and removal of the proximal protection device. A single-center, prospective, nonrandomized study that compared diffusion-weighted imaging lesions found no statistical difference in patients undergoing CAS with flow reversal (12%) and a control group of patients undergoing cerebral diagnostic angiography (18%).²⁷

The Gore device eliminates blood loss by venous shunting and maintains active flow reversal through the procedure. Despite this advantage, the requirement of additional venous access, along with the complexity associated with the use of this device, has discouraged some interventionists from widespread application of the device. Experience with the system seems to overcome the size disadvantages with both MO.MA and Gore devices as they are both very flexible, even with difficult arch anatomy. However, the Gore device has the advantage of a separate balloon wire so that patients with severe ECA disease can still be treated. The Gore device may also be preferred when the superior thyroidal artery is quite proximal and cannot be completely occluded with the distal balloon because the flow is reversed.

Combined Distal and Proximal Protection

Symptomatic occlusion or near-occlusion of the ICA or mobile intraluminal thrombus in the ICA may pose a high risk for patients with major strokes. According to a population-based study, the risk of cerebral infarction is very high in the acute phase and decreases with time (7% of cerebral infarctions occurred within the first week after diagnosis of the occlusion; the accumulative risk was 8% at 30 days, 10% at 1 year, and 14% at 5 years).²⁸ There is no optimal surgical option for the treatment of symptomatic ICA near-occlusion or intraluminal thrombus; moreover, a high risk of embolization is associated with surgical or medical treatment.^18,29 In the setting of symptomatic, acute complete occlusion, proximal protection accomplished with the use of the Concentric balloon guide catheter (Concentric Medical) during carotid revascularization with extracranial and intracranial stenting is feasible.³⁰ The Concentric balloon guide catheter was introduced for use with the Merci clot retrieval system (Concentric) for the treatment of acute stroke. This catheter can be introduced through an 8- or 9 Fr sheath and advanced into the CCA. After performing a cervical angiogram, the balloon guide is inflated to achieve flow arrest; the aspiration of blood removes embolic particles; and a microwire and microcatheter are passed through the guide catheter and used to cross the lesion. A microcatheter run is then performed to ensure that the true lumen was selected. A predilatation angioplasty is performed if needed, and a distal protection filter may be navigated distal to the lesion. A carotid stent is advanced over the filter wire across the lesion and deployed with reversal of flow maintained by aspiration. Post-stent angioplasty is performed, and the filter is re-sheathed and removed. The balloon is deflated; and, after obtaining a final craniocervical angiogram, the catheter is removed.

In our experience, recanalization of symptomatic extracranial ICA near-occlusion is better approached with a combination of proximal and distal emboli protection devices.³⁰ In a small series of patients described in the literature, this approach was feasible and resulted in successful recanalization, with no procedure-related complications or deaths.³¹

Case Reports

Case 1. CCA stenosis with challenging access. A 76-year-old man presented with a history of bilateral CEA, CCA origin stenting (Figure 6), peripheral vascular disease, pacemaker insertion for sick sinus syndrome, and coronary artery bypass grafting. An asymptomatic progressive left CCA stenosis was found on follow-up Doppler imaging, and the patient underwent diagnostic cerebral angiography.

Case 2. Acute stroke intervention and ICA intraluminal thrombus. An 83-year-old man woke up with left hemiplegia. The National Institutes of Health Stroke Scale score was 9. His medical history showed coronary artery disease status post-coronary artery bypass grafting, severe chronic obstructive pulmonary disease, dyslipidemia, hypertension, and atrial fibrillation (not treated with warfarin). A computed tomographic (CT) angiogram (Figure 9) showed right ICA thrombus with pseudo-occlusion and occlusion of the M1 segment of the right middle cerebral artery (MCA). A CT perfusion imaging study showed a potentially salvageable penumbra.

Due to thrombus in the cervical carotid artery (Figure 10) and MCA occlusion, we decided to use the MO.MA device (Figure 11). Through the MO.MA device, a Wallstent (Boston Scientific) was deployed in the cervical ICA (Figure 12). A cranial view showed the right M1-MCA segment occlusion (Figure 13), which was crossed, as a multiaxial system, with a Synchro-2 exchange microwire (Boston Scientific) inside a 0.32-inch catheter (Penumbra). A 0.54-inch catheter (Penumbra) was then advanced over these devices, and suction was activated with the 0.54-inch separator (Penumbra). TIMI grade 3 recanalization was achieved (Figure 14).

\This case illustrates the application of a proximal protection device in an intraluminal thrombus that has a high risk of distal embolization. These devices can also be used in acute stroke cases and have the benefit of a stable large working channel to use with intracranial retrieval devices.

Conclusion

Although distal filtration devices protect the brain from the embolization of larger particles that would otherwise cause a major stroke, embolic events have been documented during their use. Proximal occlusion technique and flow-reversal devices offer a different mechanism for protection that allows the operator to tailor the procedure to patient-specific anatomy and pathophysiology.

References

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Acknowledgements

The authors thank Paul H. Dressel, BFA, for preparation of the illustrations and Debra J. Zimmer, AAS CMA-A, for editorial assistance.

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From the ¹Department of Neurosurgery and Toshiba Stroke Research Center; ²Department of Radiology, School of Medicine and Biomedical Sciences, University at Buffalo, State University of New York, Buffalo, New York; and ³Department of Neurosurgery, Millard Fillmore Gates Hospital, Kaleida Health, Buffalo, New York.

Disclosure: The authors have completed and returned the ICMJE Form for Disclosure of Potential Conflicts of Interest. Dr. Hopkins receives grant/research support from Toshiba; serves as a consultant to Abbott, Boston Scientific,* Cordis, Micrus, and W.L. Gore; holds a financial interest in AccessClosure, Augmenix, Boston Scientific,* Claret Medical Inc., Micrus, and Valor Medical; has a board/trustee/officer position with AccessClosure, Claret Medical Inc., and Micrus (until September, 2010); belongs to the Abbott Vascular speakers’ bureau; and receives honoraria from Bard, Boston Scientific,* Cordis, Memorial Healthcare System, Complete Conference Management, SCAI, and Cleveland Clinic. Dr. Levy receives research grant support (principal investigator: Stent-Assisted Recanalization in acute Ischemic Stroke, SARIS), other research support (devices), and honoraria from Boston Scientific* and research support from Codman & Shurtleff, Inc. and ev3/Covidien Vascular Therapies; has ownership interests in Intratech Medical Ltd. and Mynx/Access Closure; serves as a consultant on the board of Scientific Advisors to Codman & Shurtleff, Inc.; serves as a consultant per project and/or per hour for Codman & Shurtleff, Inc., ev3/Covidien Vascular Therapies, and TheraSyn Sensors, Inc.; and receives fees for carotid stent training from Abbott Vascular and ev3/Covidien Vascular Therapies. Dr. Levy receives no consulting salary arrangements. All consulting is per project and/or per hour. Dr. Siddiqui has received research grants from the National Institutes of Health (co-investigator: NINDS 1R01NS064592-01A1, Hemodynamic induction of pathologic remodeling leading to intracranial aneurysms) and the University at Buffalo (Research Development Award); holds financial interests in Hotspur, Intratech Medical, StimSox, and Valor Medical; serves as a consultant to Codman & Shurtleff, Inc., Concentric Medical, ev3/Covidien Vascular Therapies, GuidePoint Global Consulting, and Penumbra; belongs to the speakers’ bureaus of Codman & Shurtleff, Inc. and Genentech; serves on an advisory board for Codman & Shurtleff; and has received honoraria from American Association of Neurological Surgeons’ courses, an Emergency Medicine Conference, Genentech, Neocure Group LLC, Annual Peripheral Angioplasty and All That Jazz Course, and from Abbott Vascular and Codman & Shurtleff, Inc. for training other neurointerventionists in carotid stenting and for training physicians in endovascular stenting for aneurysms. Dr. Siddiqui receives no consulting salary arrangements. All consulting is per project and/or per hour. Dr Orion reports no financial relationships/disclosures.
*Boston Scientific’s neurovascular business has been acquired by Stryker.

Manuscript submitted August 4, 2011, provisional acceptance given September 12, 2011, final version accepted October 28, 2011.
Corresponding author: Dr. L. Nelson Hopkins, MD, University at Buffalo Neurosurgery, 3 Gates Circle, Buffalo, New York 14209. Email: lnhbuffns@aol.com