Extracranial carotid artery occlusive disease is associated with the risk of stroke, disability, and death. The surgical treatment of carotid disease has undergone significant evolution from early attempts at carotid ligation to the first carotid endarterectomy by Michael Debakey in 1953. Over the following decades, carotid intervention was mostly limited for symptomatic patients. There was considerable research into the treatments of carotid disease and multiple randomized prospective trials were conducted, which demonstrated a definitive benefit to carotid endarterectomy in symptomatic patients with moderate to severe disease and asymptomatic patients with severe disease. Carotid endarterectomy became the treatment of choice for disease of the carotid bifurcation.
With the expansion of endovascular interventions in the 1990s, stenting for extracranial carotid disease with cerebral protection was explored as an alternative to endarterectomy in numerous trials. Several cerebral protection devices that were based upon distal cerebral occlusion balloons versus distal embolism filters were trialed, utilizing both open and closed cell stents. Interventions were performed both with and without devices to protect against embolization during interventions. Embolic protection included distal occlusive balloons and distal embolic filters. Multiple multicentered prospective randomized and nonrandom trials further explored transfemoral carotid stenting and established it as a viable treatment option for extracranial disease. These rigorous trials have also shed light on the nuances of this technique.
Based upon the results of these trials, the U.S. Food and Drug Administration granted approval for carotid artery stenting (CAS) with certain limitations. CAS from a transfemoral approach has been shown to have a significant increased risk of stroke compared with carotid endarterectomy. Factors that increase the risk include navigating tortuous anatomy, atheroma within the aortic arch, and crossing proximal common carotid lesions. The use of distal embolic protection devices improves outcome; however, these devices must be brought across the lesions in order to be deployed, which in itself can provoke embolization. Diffusion-weighted MRI studies of transfemoral CAS patients have shown an incidence of silent event as high as 40%. Many of these events were contralateral to the treated side, suggesting that navigation of the aortic arch may be a frequent cause of embolization. Transcarotid artery stenting (TCAR) with flow reversal was designed to mitigate many of these risks and to prevent stroke. Accessing the common carotid artery at the base of the neck allows the operator to avoid the complexities of challenging arch anatomy. Dynamic flow reversal is designed to prevent distal embolization beyond what is possible with distal filters.
The ENROUTE Transcarotid Neuroprotection System (Silk Road Medical, Sunnydale, California) is a flow reversal circuit that connects two 8-French sheaths through a flow modulator. One sheath is placed in the common carotid artery via arterial cutdown and the other is placed percutaneously into the femoral vein. The two sheaths are connected through a flow modulator, which can regulate the rate of flow reversal between high and low. The flow modulator can also temporarily stop flow reversal. When the common carotid artery is clamped proximal to the arterial sheath, the arterial–venous pressure gradient leads to flow reversal within the external and internal carotid arteries.
The ROADSTER trial was the pivotal trial for the ENROUTE system. It was a prospective, single arm, multicenter trial to assess the safety and efficacy of the flow reversal system in carotid artery stenting. The trial enrolled 141 patients, of whom 36 were symptomatic with stenoses >50%. The asymptomatic patients all had >70% stenosis. The enrolled patients were considered at high risk of endarterectomy based upon anatomic and physiologic criteria. The primary endpoint was major adverse events of death, stroke, and myocardial infarction. There were five major adverse events in the trial (two cerebrovascular accident, two deaths, and one myocardial infarction). There were also minor adverse events including eight arterial dissections, five hematomas, and one cranial nerve injury.
Procedure
The patient is positioned supine as for carotid endarterectomy. The groin is prepared for placing the venous sheath. The procedure may be performed under general or local anesthesia. Patients in the ROADSTER trial were split 53% to 47% between local and general anesthesia, respectively. The common carotid artery is exposed at the base of the neck through a longitudinal or transverse incision. The sternal and clavicular heads of the sternocleidomastoid act as anatomic landmarks. The artery is found by splitting the two muscle heads and mobilizing the internal jugular vein medially. Occasionally, the vagus nerve courses anteriorly and must be avoided. Approximately 3 cm of artery should be exposed to allow proximal occlusion and sheath placement. The artery should be freed circumferentially to allow proximal control with a Rummel tourniquet, silastic vessel loops, or atraumatic vascular clamp. Proximal common carotid occlusion is necessary to establish flow reversal. A silastic vessel loop or umbilical tape around the artery can facilitate control during artery access and sheath placement.
The patient is systemically anticoagulated. An adventitial U-stitch placed prior to access facilitates closure of the arteriotomy at the end of the procedure. The artery is accessed with a micro puncture needle and exchanged over a wire for a microcatheter. A stiffer wire is placed into the common carotid artery to allow placement of the 8-French flow-reversal sheath. The arterial sheath is placed over the wire up to the 2.5 cm marker. The sheath is secured to the patient externally to prevent accidental dislodging during the procedure. The venous sheath is placed in the contralateral superficial femoral vein and the arteriovenous circuit is flushed and connected. Flow reversal is established following clamping of the common carotid artery proximal to the sheath entry site.
Flow reversal should be demonstrated by arteriogram through the arterial sheath after first confirming by clearing the line with saline injection and allowing reversal of flow to be established once more. The ENROUTE Neuroprotection System allows temporary cessation of flow reversal during arteriograms for imaging purposes.
Once flow reversal has been established and the lesion is identified, it is crossed with a wire and the stent is deployed in the usual fashion. If the stenosis is particularly tight, predilation with an undersized balloon can be performed. Some surgeons advocate predilatation of all lesions. Postdilation can be performed as needed. A completion cerebral angiogram is performed with cranial imaging to evaluate cerebral emboli and adequacy of stenting.
Antegrade cerebral flow is reestablished by unclamping the proximal common carotid artery. The wires and sheaths are removed and the arteriotomy is then closed with the previously placed suture.
Complications
The supraclavicular incision is well tolerated in patients. Ultrasound facilitates identifying an exposure site where the artery will be the most superficial and not diseased. In the pivotal trial, the incidence of cranial nerve injury was 0.7% (1/141). The incidence of hematoma formation was 3.5% and the incidence of hematoma requiring surgical evacuation/exploration was less than 1%. The vast majority of carotid stent patients receive dual-antiplatelet therapy and are anticoagulated during the procedure. Care must be taken during common carotid artery cutdown and mobilization to prevent postprocedure bleeding. During closure, particular attention should be paid to the sternocleidomastoid to ensure there is no muscle bleeding provoked by mobilization and periprocedural anticoagulation.
Proximal common carotid artery dissections at the sheath entry site were seen in 2.1% of patients in the ROADSTER trial. The arterial sheath has since been modified to have more flexibility and is curved to accommodate the entry angle. To avoid carotid dissection, which probably occurs as a result of injury to the back wall, adequate sheath, dilator, and stiff wire access must be maintained. Care must be taken during arterial access and sheath placement. Proximal control of the artery during needle access, without undue tension, can help ensure single-wall access. Passage of the microcatheter and the sheath is performed over a wire to decrease the risk of dissection. Blindly passing the sheath can also lead to embolization if the plaque is engaged. An initial arteriogram through a microcatheter prior to sheath placement allows the operator to define the lesion and bifurcation more accurately. If the sheath cannot be placed over a stiff wire within the common carotid artery, the wire is advanced into the external carotid artery for additional stability. In the event of a dissection, stent placement or open surgical repair are both options for repair.
Trial results have shown that flow reversal is generally well tolerated by patients. There may be a small subset of patients who will be intolerant to flow reversal. This effect can be mitigated in several ways. Some patients who do not tolerate prolonged high flow reversal tolerate low flow reversal, which occurred in one patient in the ROADSTER trial. Systemic blood pressure can be raised to increase cross-cerebral perfusion. If flow reversal must be abandoned, carotid stenting can still be achieved with placement of distal embolic protection through the carotid sheath. Alternatively, the procedure can be converted to endarterectomy.
Meticulous care must be taken to ensure there are no air bubbles within the contrast. A slow controlled injection of contrast allows adequate visualization without embolization.
Each exchange through the sheath has the potential to introduce air emboli. Methodical visual attention during exchanges help to ensure that no air is advanced through the sheath. If air is detected, the sheath is back bled. All balloons and stents must be carefully flushed to be free of air.
Any commercially available stent can be used with the ENROUTE Neuroprotection System. Silk Road Medical has developed a stent on a shorter (57 cm) deployment catheter to facilitate deployment from the transcarotid approach.
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