Carotid Revascularization

Zinovy M. Katz, Martin G. Radvany and Mark H. Wholey

Stroke represents the third leading cause of death in the United States, with an incidence of 1.5 deaths per thousand people. Of the more than half-million strokes occurring annually, occlusive disease of the extracranial circulation is responsible for approximately 30%. The traditional standard of care in treating cervical carotid artery occlusive disease has been carotid endarterectomy (CEA), a procedure initially performed in the 1950s and described by Scott, DeBakey, and Cooley. In 1988, the landmark North American Symptomatic Carotid Endarterectomy Trial (NASCET) demonstrated a reduction in stroke and death rates from 26% at 2 years to 9% after endarterectomy.¹ Since that time, additional studies have further validated this approach and demonstrated the benefit of intervening in patients who are asymptomatic from their disease.^2–4

From a historical standpoint, Mathias, Theron, and Kachel pioneered angioplasty for cervical carotid artery occlusive disease treatment in the early 1980s. With the advent of stent technology, interventional management of carotid artery disease began to develop as a practical technique, as shown by the early work of Diethrich, Roubin, Wholey, and Mathias. Stents provided significant improvements over conventional angioplasty and helped reduce restenosis rates, prevent elastic recoil, and treat dissections. During the pioneering stage of carotid stent placement from 1995 to 1999, especially in the United States, there were primarily only two peripheral stent systems available: the balloon-mounted Palmaz stent (Cordis Corp., Miami Lakes, Fla.) and the Wallstent (Boston Scientific, Natick, Mass.). When nitinol stents became available in 1999 in the United States, many operators had changed or were in the process of changing from balloon-mounted stents to either self-expanding Wallstents or the newer nitinol stents. With rapid technologic advancements in both the stents themselves and distal protection devices, indications for this procedure and its overall application have grown exponentially worldwide.

Indications

On May 6, 2011, the U.S. Food and Drug Administration (FDA) approved an expanded indication for use of the stent to include all patients with carotid artery stenosis who are at risk for stroke, not just those who are not good candidates for surgery. This decision comes based on the results of the Carotid Revascularization Endarterectomy versus Stenting Trial (CREST). Previously, the Centers for Medicare and Medicaid Services (CMS) decision limited carotid stenting to the “high-risk” surgical subset with 70% stenosis or greater as a service-covered admission with reimbursement for the Medicare population. All other subsets and categories had been excluded from reimbursement unless they had been enrolled in a category B FDA-approved investigational device exemption (IDE) study. This essentially excluded all asymptomatic patients with high-grade preocclusive lesions involving the internal carotid artery (ICA) who did not have appropriate entry criteria for a trial (typically 80% or greater is required for study entry). The ruling also excluded as a service-covered admission stenotic lesions involving the petrosal portion of the internal carotid, base of skull, and cavernous sinus segment. The caveat to the revised criteria is that a postapproval study be conducted that will:

• Follow patients for at least 3 years after treatment.

• Assess how patients older than 80 years of age respond to treatment.

• Evaluate whether treatment success is impacted by operate experience.

• Examine whether asymptomatic and symptomatic patients have different results.

Currently there are no absolute contraindications for carotid artery stenting (CAS). Relative contraindications to the use of CAS can be divided into anatomic and patient-specific factors. Anatomic factors are physician and trial specific, but general contraindications are similar to those outlined in the CREST trial ⁵ (asymptomatic lesions < 70% by ultrasound or < 60% by angiography and symptomatic lesions < 70% by ultrasound or < 50% by angiography). Lesion characteristics are also increasingly recognized as being pertinent to the risk of stroke. Lengthy lesions, those with appearance of extensive thrombus (either by conventional angiography or by intravascular ultrasonography), those with globular calcifications, and those with type III and type IV arches are at increased risk and must be approached with caution. Inability to pass the protective filter, and a tortuous vessel in which the angiographer believes he or she is unlikely to pass the stent must be warning flags and must cause the performing physician to have a low threshold for abandoning the procedure. The aortic arch is the “Achilles heel” for carotid stenting, and if the angiographer has difficulty accessing the vessel of interest, with extensive manipulation of the aorta and multiple catheter exchanges, it is best to abandon the procedure and consider alternative methods of treatment.

Other contraindications involve general contraindications to angiography, such as patients with borderline renal function (although this can usually be handled with prehydration, N-acetylcysteine, nonionic contrast, and small contrast volumes), inability to access the femoral arteries (although the brachials or a direct carotid puncture can also be performed), and patients with extensive cervical hardware that will make precise visualization of stent placement difficult. Additional contraindications include:

• Evolving stroke

• Intolerance/resistance to aspirin and/or clopidogrel

• Patient with active bleeding diathesis or absolute contraindication to anticoagulation

• Severe dementia

• Uncontrolled hypertension

Aspirin and clopidogrel are the most common dual platelet therapy in use today. Resistance to antiplatelet agents is not uncommon and can result in in-stent restenosis or thrombosis. Resistance can be assessed by platelet aggregometry. Regarding clopidogrel, some studies have shown that greater than 50%⁶ of patients undergoing cerebrovascular stent placement might be low responders, and 0% to 44%⁷ may be resistors, which can be related to genetic polymorphisms of cytochrome P450 3A4 and the P2Y12 receptor. Alternatives are available for low responders/resistors and include increasing the clopidogrel dose, ticlopidine, prasugrel, or ticagrelor. Resistance to aspirin is far less common, occurring in approximately 5%⁶ of patients undergoing cerebrovascular stent placement.

Equipment

Endovascular treatment of carotid arterial stenosis was originally performed with equipment that was not specifically designed for this purpose. This initially led to significant complications in the form of periprocedural strokes. The introduction of balloon and stenting systems specifically designed for carotid stenting, along with the introduction of embolic protection devices (EPDs) and greater operator experience, have resulted in a steady decrease in periprocedural stokes.

Endovascular stents come in two designs: balloon-expandable and self-expanding. Balloon-expandable stents, while appropriate for ostial lesions at the aortic arch, should not be used for treatment of cervical carotid arterial lesions, owing to their lack of flexibility or ability to spontaneously reexpand after external compression. Self-expanding stents have essentially two designs: open cell and closed cell. Each design has certain advantages and disadvantages. The open-cell design has more flexibility, which allows for better vessel wall apposition as well as better deliverability through tortuous anatomy. However, open-cell designs have lower radial force, which may result in a weaker scaffold and thus compromise stent expansion due to recoil, especially in heavily calcified lesions. Closed-cell stent designs have more radial force, but they are less flexible and can introduce kinks and stenosis in the blood vessel when used in tortuous anatomy. Data from the Stent-Protected Angioplasty versus Carotid Endarterectomy (SPACE) trial suggest that there is a higher rate of embolic complications with open-cell designs (11%) than with closed-cell carotid stents (5.6%, P = 0.029).⁸ Several studies have been performed to assess the outcome of incomplete stent apposition; many of these involve intracranial⁹ and coronary stenting.^10,11 With respect to carotid stenting, the Wallstent can at times have incomplete apposition at the edges of the stent when deployed at vascular turns.¹² Existence of a segment of incomplete stent apposition has no adverse morphologic or clinical effect.¹³

There are a number of stents available worldwide; those currently available in the United States are:

• Rx Acculink (Abbott Laboratories, Abbott Park, Ill.)

• Xact (Abbott)

• NexStent (Boston Scientific)

• Precise (Cordis)

• ADAPT (Stryker Corp., Kalamazoo, Mich.)

• Zilver (Cook Medical, Bloomington, Ind.)

There are two types of EPDs: (1) those that provide protection distal to the treatment site by blocking flow or filtering flow in the ICA distal to the treatment site, and (2) flow-reversal devices, which occlude inflow to the common carotid artery and external carotid artery causing reversal of flow in the ICA (Fig. 91-1).

FIGURE 91-1 A, Theron technique. Procedure is performed under flow interruption obtained by inflating a balloon within the internal carotid artery (ICA). After placement of stent (and post dilatation if performed), delivery guide is advanced through the stent and placed immediately below inflated balloon. Blood stagnating below balloon (containing potential plaque debris) is aspirated forcefully. After aspiration, space below balloon is flushed with a bolus of saline solution that chases reminder of dead space content into external carotid artery. B, EPD, filter-type. In this technique, filter is advanced through stenosis, using either its built-in wire tip or an over-the-wire technique with a wire selected by operator. Filter is deployed in the ICA and kept open for duration of procedure. At the end, a special recovery catheter is used to retrieve filter and its content. C, EPD, flow-reversal type: with this technique, blood flow through lesion is reversed during the procedure, carrying potential debris away from cerebral circulation. Flow reversal is created by inflating two balloons, one in the common carotid artery (CCA) and one in external carotid artery, while blood leaving through the device is reinjected in a femoral vein. D, Final result with stent deployed in the ICA, traversing the stenosis into the CCA. (© 2011 lydia Gregg.)

Devices that provide occlusion distal to the treatment site may consist of a balloon or, more commonly, a filter attached to a wire. The balloon occlusion concept originally pioneered by Theron has been refined.¹⁴ A balloon is attached to the working wire and advanced distal to the stenosis and inflated (PercuSurge [Medtronic, Minneapolis, Minn.]). After stenting and angioplasty are completed, a catheter is then advanced and used to aspirate debris prior to deflating and recovering the balloon. Disadvantages of the balloon system include occluding the entire flow during all or a majority of the procedure in patients who frequently have a compromised contralateral carotid artery and circle of Willis circulation. In addition, the angiographer cannot evaluate the lesion fluoroscopically while the balloon is inflated in the ICA. There is also a risk of flushing embolic debris retrograde into the common carotid and aorta or into collateral vessels off the external carotid artery, such as the middle meningeal and orbital branches.

With filter-type devices, the filter is attached to the working wire and deployed distal to the stenosis. After the procedure it is recovered, bringing the debris with filter. There are many different filter devices and they are paired with their individual stent platforms; they should not be mixed. The advantage to this type of design is that antegrade flow is maintained unless the filter is full of debris. The various companies in different stages of development and prototypes include: