Mechanical Thrombectomy for Acute Ischemic Stroke

EXECUTIVE SUMMARY

• Large vessel occlusions account for 24% to 46% of acute ischemic strokes.
• The most sensitive clinical screening tools for large vessel occlusion in acute ischemic stroke involve the detection of both motor deficits (face or limb weakness) and cortical deficits (neglect, aphasia, gaze deviation, and hemianopia).
• The three clinical scoring tools with the best sensitivity to detect large vessel occlusion in acute ischemic stroke are the National Institutes of Health Stroke Scale (NIHSS), the Field Assessment Stroke Triage for Emergency Destination (FAST-ED) and the Rapid Arterial Occlusion Evaluation (RACE).
• The imaging protocol for acute stroke should include an initial noncontrast computed tomography (CT) (to detect hemorrhage), and if no hemorrhage is found, followed by CT or magnetic resonance (MR) angiography (to locate the arterial occlusion).
• Mechanical thrombectomy can be done safely after intravenous thrombolytic therapy.
• Mechanical thrombectomy has proven benefit in patients with strokes that involve the large vessels of the anterior circulation (internal carotid artery or proximal middle cerebral artery) performed within six hours.

We all know the drill; emergency medical services calls in with a patient having weakness and trouble speaking — possible stroke. Stroke protocol activated, computed tomography scanner held open, patient taken directly for scan, no hemorrhage seen, then evaluated for thrombolytic therapy. Now, we add mechanical thrombectomy to the mix. How we screen for potential candidates and determine eligibility is the subject of this issue.

— Joseph Stephan Stapczynski, MD, Editor

Introduction and Overview of Ischemic Stroke

Stroke is defined as a disease process that results in decreased blood flow to the brain. Annually in the United States, approximately 795,000 people experience strokes (ischemic and hemorrhagic); approximately 76% of these are first-time events.¹

Acute ischemic stroke (AIS) accounts for 87% of all strokes and is subdivided further into three major categories: thrombotic, embolic, or hypoperfusion related.^1,2 Thrombotic AIS refers to local obstruction of an artery due to disease of the arterial wall, such as dysplasia, arteriosclerosis, or dissection. Embolic AIS is secondary to debris from elsewhere in the body that then travels through the circulation and occludes arteries in the brain. AIS secondary to hypoperfusion manifests as neurological deficits secondary to inadequate cerebral blood flow in the setting of systemic illness associated with circulatory compromise, such as septic shock. AIS secondary to thrombotic or embolic processes presenting with permanently debilitating symptoms or symptoms that may diminish a patient’s quality of life requires prompt recognition and treatment with reperfusion strategies.²

Overview of Reperfusion Strategies

After stabilization in the emergency department, the gold standard for reperfusion treatment of AIS in the United States has been intravenous (IV) thrombolytic therapy.³ After the U.S. Food and Drug Administration approved alteplase in 1996 following National Institutes of Health (NIH)/National Institute of Neurological Disorders and Stroke (NINDS) data showed decreased functional disability in AIS patients treated with IV alteplase, IV thrombolytics have become widely available for patients presenting to stroke centers with symptoms concerning for AIS.^3,4 As the landscape of stroke treatment changes with advancements in technology and imaging, mechanical thrombectomy is an evolving treatment for patients presenting with stroke.

Mechanical thrombectomy is indicated for patients who are experiencing AIS due to large vessel occlusion (LVO).² While many centers limit their eligibility criteria for mechanical thrombectomy only to LVO involving the anterior circulation, some centers will perform thrombectomy on patients involving both anterior and posterior circulation. Anterior circulation strokes include the anterior cerebral artery (ACA), middle cerebral artery (MCA), and the internal carotid artery (ICA); posterior circulation strokes include the posterior cerebral artery (PCA), the basilar artery, and the vertebral artery. Mechanical thrombectomy is appropriate for selected patients presenting with stroke due to LVO within 24 hours of the time last known to be well (at their neurological baseline) and can be performed regardless of IV thrombolytic administration.²

Currently, only select stroke centers have sufficient resources to perform mechanical thrombectomy, creating a best practice gap in emergency stroke treatment. As per a Joint Commission survey, 60% of hospitals certified as stroke centers perform endovascular therapy, such as mechanical thrombectomy.⁵ The remaining hospitals perform a “drip and ship” approach to patients meeting requirements for mechanical thrombectomy, wherein patients are started on IV thrombolytic and transferred by emergency medical services (EMS) to a receiving center with thrombectomy capabilities. With the American Heart Association (AHA)/American Stroke Association (ASA) recommending all eligible AIS patients receive consideration for adjunctive endovascular therapy, rapid recognition of candidates who would benefit from additional interventional therapy, such as mechanical thrombectomy, must be recognized promptly.^5,6

Identifying Potential Candidates for Thrombectomy

Screening tools differ between both hospital and prehospital providers. Because of the time-sensitive nature of reperfusion therapy, a reliable system for recognition of LVO is required for both hospital and prehospital providers. LVO typically is associated with motor and cortical deficits; therefore, screening tools that evaluate both of these areas would be expected to be most sensitive. The predominant acute assessment stroke scale used within hospital systems in the United States is the National Institutes of Health Stroke Scale (NIHSS) because of its reliable and reproducible assessment in patients for LVO.^7,8 Referred to as the gold standard stroke scale by the AHA/ASA, the NIHSS assesses for neurological impairment on an 11-item scale, with a sensitivity for LVO ranging from 53% to 99% depending on score.^7,9,10 Using a threshold value of 10 or greater, the NIHSS score has a sensitivity of 96% and a specificity of 63% for LVO.¹¹ (See Table 1.) The major limitation to the NIHSS is its inability to recognize posterior circulation strokes because of its lack of assessment for unsteady gait, dizziness, or diplopia.⁹

Emergency medical technicians and paramedics use several different tools in their evaluation of prehospital patients presenting with stroke-like symptoms. The Cincinnati Prehospital Stroke Scale (CPSS), the Los Angeles Prehospital Stroke Screen (LAPSS), and the Melbourne Ambulance Stroke Screen (MASS) are the three most common screening tools used by EMS personnel in the United States.^{2, 12-14} (See Tables 2-4.) A multicenter study revealed only three prehospital stroke scales to have a diagnostic accuracy > 95% for AIS: CPSS, Face Arm Speech Time (FAST), and Medical Prehospital Assessment for Code Stroke (Med PACS).¹⁵ Expanding CPSS with a detailed-severity index (d-CPSS scale) significantly improved detection of AIS secondary to LVO within the prehospital environment while maintaining the ease of use of the stroke scale itself.¹⁶ (See Table 5.) As the landscape of LVO treatment rapidly evolves and EMS crews may divert to endovascular-capable facilities to provide standard of care, a stroke scale optimized for detecting LVO-specific diagnostic features will need to be developed and used in conjunction with general stroke assessment tools.¹⁷ (See Table 6.)

In a literature review of the 20 most common stroke assessment tools among both prehospital and hospital clinicians, the most sensitive indicators among the reviewed diagnostic tools for LVO are a combination of motor deficit and cortical signs (gaze deviation, aphasia, and neglect).⁹ The three tools that demonstrated the highest diagnostic accuracy for LVO were the NIHSS, Field Assessment Stroke Triage for Emergency Destination (FAST-ED), and Rapid Arterial oCclusion Evaluation (RACE).^9,18,19 (See Tables 1, 7, and 8.) In addition to RACE, the prehospital-specific stroke scales Gaze, Face, Arm, Speech, Time (G-FAST) and Conveniently-Grasped Field Assessment Stroke Triage (CG-FAST) have been shown to have high diagnostic accuracy in LVO detection.^20-22 (See Tables 9-10.) Despite multiple different prehospital stroke screening tools with high sensitivity for anterior circulation LVO, they remain significantly less sensitive for detection of more distal occlusions beyond the M2 segment.²³

Supplementing existing stroke scales with specialist consultation or combined scoring systems has been shown to increase diagnostic accuracy and sensitivity for LVO detection. The addition of a neurologist available for phone consultation in conjunction with an in-person advanced life support EMS crew increased the sensitivity, positive predictive value (PPV), and negative predictive value (NPV) of the FAST-ED stroke scale in LVO detection.²³ Although phone consultation increases on-scene time and subsequently time to definitive care by EMS, instituting a FAST-ED threshold value for neurological phone consultation could eliminate those concerns.^24-25 Combined scales also can increase the diagnostic accuracy for LVO for EMS teams; the use of a positive CPSS, defined as at least one positive screening item to trigger a secondary screen with FAST-ED, resulted in increased detection of LVO in the prehospital environment.²⁶

As clinical practice moves toward mechanical thrombectomy as the gold standard for LVO AIS, use of stroke assessment tools with a high efficacy in detecting LVO could affect EMS decisions to divert patients to centers that can perform mechanical thrombectomy in addition to IV thrombolytics and standard stroke care.

Indications for Thrombectomy in Patients with LVO

Once patients have been identified as having persistent disabling neurological deficits, multiple conditions must be fulfilled to qualify for mechanical thrombectomy. The primary qualifications to continue with thrombectomy are neuroimaging, either noncontrast computed tomography (CT) or magnetic resonance imaging (MRI) brain, showing a small infarct core with exclusion of hemorrhage, as well as dedicated vascular imaging (CT or MRI angiography) demonstrating an anterior circulation large arterial occlusion.^2,26-27 The criteria for thrombectomy are broken down further by categorizing patients presenting within six hours and those presenting within six to 24 hours of last known well.^2,28-29

For patients presenting within six hours, criteria for qualification of thrombectomy are based on the MR CLEAN trial and the ASPECTS method.^26,27 The MR CLEAN trial was a prospective, multicenter, clinical trial that evaluated 500 patients with AIS with a proximal arterial occlusion who then were randomized to receive either usual care (IV thrombolytics) or intra-arterial care (delivery of thrombolytic, mechanical thrombectomy, or both) plus usual care. The primary endpoint of the study was modified Rankin Scale (mRS) score at 90 days. (See Table 11.) Patients who received intra-arterial therapy in conjunction with usual care had more independence and statistically significant mRS of 0-2, with a number needed to treat (NNT) of 7. The MR CLEAN trial was the first multicenter prospective trial to show that intra-arterial therapy in conjunction with IV thrombolytic therapy is safe and effective for patients with AIS secondary to proximal occlusion of anterior circulation.²⁶

The ASPECTS method is a set of neuroimaging criteria that was created to identify patients with MCA infarcts who would be unlikely to recover with medical therapy alone but would benefit from the addition of thrombectomy. The MCA territory is divided into 10 regions of interest, with each assigned a point value: three points for subcortical structures (caudate, lentiform nucleus, and internal capsule) and seven points for the MCA cortex. One point is subtracted for areas that show ischemic changes, such as swelling or hypoattenuation.²⁷ A normal CT scan has an ASPECTS score of 10 points, whereas a diffusely ischemic MCA on CT presents a score of 0.

Some centers use more selective criteria for mechanical thrombectomy under six hours, following guidance from the AHA/ASA. The AHA/ASA recommend mechanical thrombectomy for patients with no significant pre-stroke disability characterized by an mRS score of ≤ 1 and an occlusion of the ICA or M1 segment of the MCA.⁶ These criteria notably are stricter than the inclusion criteria in MR CLEAN, likely because of the unclear benefit of thrombectomy in patients with greater prestroke mRS scores, NIHSS < 6, or ASPECTS score < 6 (i.e., a larger infarct core). Advocates for stricter thrombectomy criteria also point to the ESCAPE and EXTEND-IA trials, both of which studied intra-arterial therapy but restricted eligibility to patients who had independent function prior to the acute stroke and required CT evidence of collateral circulation (ESCAPE) or salvageable brain tissue (EXTEND-IA).^30,31

For patients presenting within six to 24 hours, centers use either the DAWN or DEFUSE3 trial criteria to determine eligibility for mechanical thrombectomy. The original data from the MR CLEAN trial showed decreased efficacy of intervention as the time from onset exceeded six hours; however, the DAWN and DEFUSE3 trials were conducted and showed a benefit of thrombectomy for certain patients presenting within six to 24 hours of LVO AIS onset, allowing for criteria for intervention in these patients.³² (See Table 12.)

The DAWN trial was a multicenter, prospective, randomized trial of 206 patients with AIS who had evidence of occluded intracranial ICA or proximal MCA who presented within six to 24 hours of symptom onset with disproportionately severe clinical deficits relative to the infarct volume seen on imaging. Patients were randomized to standard of care vs. standard of care plus thrombectomy. Standard of care for patients presenting greater than six hours from last known well included admission to a specialty stroke unit, antiplatelet therapy, antihypertensive management, venous thromboembolism (VTE) prophylaxis, and stroke rehabilitation. Patients who received thrombectomy had significantly better mRS and functional independence at 90 days.²⁸

The DEFUSE3 trial was an open-label, randomized, multicenter trial that evaluated 182 patients presenting within six to 16 hours after last known well with evidence of intracranial ICA or MCA occlusion with salvageable ischemic brain tissue. Patients were randomized to medical therapy alone (aspirin) or medical therapy plus thrombectomy; the primary outcome was patient function as determined by mRS at 90 days. There was a statistically significant decrease in mRS at 90 days in the thrombectomy group. Secondary outcomes showed significantly increased functional independence at 90 days.²⁹

Patients experiencing posterior circulation stroke may benefit from mechanical thrombectomy. Although a majority of mechanical thrombectomy trials published to date have excluded patients with posterior circulation infarcts, mechanical thrombectomy to the posterior circulation has been performed at centers with appropriate expertise. The BEST trial was a randomized trial that compared standard medical therapy to mechanical thrombectomy in patients with acute vertebrobasilar occlusion. Patients who received mechanical thrombectomy had similar 90-day mortality and similar favorable outcome compared to those who received standard medical treatment (i.e., IV thrombolytics).³³ The BASICS trial evaluated patients with basilar artery occlusion and found no significant difference in outcomes for standard medical therapy vs. mechanical thrombectomy.³⁴ Both the BEST and BASICS studies did show a non-statistically significant but favorable data trend of benefit to mechanical thrombectomy, demonstrating that more research is needed to determine the safety and efficacy of thrombectomy in posterior circulation stroke secondary to LVO.

Procedure of Mechanical Thrombectomy

Mechanical thrombectomy is performed while patients are under either general anesthesia or procedural sedation. The femoral artery is punctured, followed by insertion of a catheter into the artery, which is guided to the ICA, then past the intracranial site of LVO.³⁵ After the procedure is performed, patients are monitored in an intensive care unit until deemed clinically stable. There are two general styles of devices used for thrombectomy — stent retrievers and catheter aspiration devices — with utilization depending on provider preference.³⁶ Stent retrievers coil through, trap, then remove the clot from the vessel.³⁵ Several commercial stent retrievers have been used, with increased reperfusion rates and better patient outcomes seen with newer, second-generation stent retrievers, such as the Solitaire and Trevo neurothrombectomy devices.^37,38 Catheter aspiration devices work by aspiration of the obstruction, with stent retrieval used as a backup if the aspiration is unsuccessful.³⁹

The multicenter, open-label COMPASS trial assessed catheter aspiration vs. stent retrieval as first-line therapy in 270 patients with LVO presenting within six hours of symptom onset. At 90 days, functional disability as measured by mRS of 0-2 was similar among both groups, demonstrating aspiration as first-line therapy as non-inferior to stent retrieval.⁴⁰ Additionally, other studies have shown catheter aspiration to have similar rates of revascularization and functional outcome when compared to second-generation stent retrieval.^39-42

Benefits and Efficacy of Mechanical Thrombectomy

There have been five major studies that have demonstrated early mechanical thrombectomy (within six hours of last known well) for large artery occlusion in the proximal anterior circulation is safe, effective, and has a significant benefit on mRS as a measure of functional independence and decreased functional disability as compared to standard stroke care alone. Based on the data of these trials, the AHA/ASA announced in 2015 a Class IA recommendation that patients receive mechanical thrombectomy if they meet certain criteria.⁴³ (See Table 13.) The NNT for functional independence regarding mechanical thrombectomy ranges from 3 to 7.5 depending on the randomized controlled trial referenced.^{26,30,31,44-46} A meta-analysis of multicenter, randomized controlled mechanical thrombectomy trials evaluated 1,287 patients and found that patients had higher rates of functional independence (as measured on a mRS of 0-2 after 90 days), significantly reduced disability measured by improvement of ≥ 1 on mRS at 90 days, and showed benefit across several subgroups, including those presenting with high stroke severity and age ≥ 80 years.⁴⁷ The data for mechanical thrombectomy occurring after six hours have only shown proven effect in those with AIS due to occlusion of intracranial ICA or proximal MCA within six to 24 hours, defined as functional independence and reduced disability on the mRS.^28,29 In light of this evidence, the AHA/ASA has recommended application of strict inclusion or exclusion criteria from DAWN or DEFUSE3 to determine eligibility for mechanical thrombectomy after six hours from last known well.⁶

An additional aspect of the benefits of mechanical thrombectomy are within cost effectiveness and quality-adjusted life years (QALY) added after treatment. Data from the United Kingdom have suggested that the net monetary benefit (NMB) to society for mechanical thrombectomy ranges from 95 to 174.9 million U.S. dollars (USD), with the willingness to pay for a QALY ranging from 33,000 USD to 49,500 USD. For comparison, IV thrombolysis alone had an NBM ranging from 72.5 to 135 million USD.⁴⁸ A model using pooled outcome data from the ESCAPE, MR CLEAN, EXTEND-IA, REVASCAT, and SWIFT PRIME trials found mechanical thrombectomy in addition to standard care to have led to gained QALY ranging from 0.47 to 2.12, with an incremental cost-effectiveness ratio of 3,110 USD/QALY.⁴⁹ The cost effectiveness extends to patients presenting with later last known well times — a model using data from DAWN and DEFUSE3 found late mechanical thrombectomy added to standard medical therapy to be cost-effective in all subgroups evaluated in both studies.⁵⁰

Adverse Effects of Mechanical Thrombectomy

A review of mechanical thrombectomy trials and related literature revealed rates of adverse effects across three broad procedurally related categories: access site complications (4%), new embolization (5%), and vascular complications, which include intracranial hemorrhage, extracranial bleeding, and arterial perforation (6%).⁵¹ The percentile range of complications reported across all studies was wide, given there were no uniform definitions of these complications and in some trials no reported adverse outcomes.

Access site problems include vessel injury, nerve injury, hematoma, and infection. Of the major trials, the definition of hematoma was not consistent, thus, rates ranged from 2% to 10.7%.⁵¹ ESCAPE reported two episodes of hematoma (one neck secondary to failed groin approach, one groin) and EXTEND-IA reported one patient who required transfusion secondary to a groin and retroperitoneal hematoma.^30,31,51

New embolization occurs during mechanical thrombectomy secondary to the ability of a dislodged clot to travel to a proximal, unaffected territory or distally within the target artery, resulting in new areas of infarct or occlusion. Occurring in 1% to 12.5% of cases, new embolization was associated with catheter technique and clot mechanics, with higher risk in more proximal clot retrievals.⁵² Balloon-guided catheters reported less embolization risk as compared to standard stent retrievers and have demonstrated a higher first-pass success rate and lower mortality rate compared to non-balloon-guided catheters.^53,54

Intracranial hemorrhage (ICH)occurs more commonly and is associated with more serious adverse events. In literature review, symptomatic ICH ranged from 3.6% to 9.3% from data collected via randomized controlled trial (RCT) and registry analyses; however, a pooled analysis of five trials showed the overall risk of symptomatic ICH to be 4.4% in patients who received both IV thrombolysis and mechanical thrombectomy compared to 4.3% in patients who received IV thrombolysis or standard care alone.^47,51 Mortality related to symptomatic ICH also had no significant difference in mechanical thrombectomy (15%) vs. IV thrombolysis or standard care alone (19%).⁴⁷ Stent detachment, although usually asymptomatic, was associated with higher rates of both ICH and arterial perforation, ranging from 0.66% to 3.9%.^51,55,56 Arterial perforation occurred in 0.7% to 4.9% of patients based on factors including type of image quality secondary to angiographic equipment available, patient movement during procedure, and operator experience.^51,57

To limit adverse events of mechanical thrombectomy procedures, multiple societies recommend that physicians trained specifically in intracranial endovascular procedures perform these techniques. In addition, it is recommended that physicians must undertake these procedures regularly to maintain skills and, thus, reduce procedurally related complications.^58-60

Summary

Stroke is a debilitating illness affecting millions of individuals across the globe. While effective and life-saving treatment modalities exist at a majority of stroke receiving centers in the form of IV thrombolytics, there is an emerging new standard in AIS with mechanical thrombectomy. A practice gap exists wherein some patients are not recognized as having a salvageable LVO based on older standards, imperfect screening tools, or insufficient resources to recognize and perform thrombectomy in patients who might otherwise benefit from procedural intervention. While mechanical thrombectomy comes with small risk, it has several significant benefits, including return to functional independence, quality of life years gained, and cost-savings for an overtaxed healthcare system.

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