Stroke and the NeurosurgeonPeer-Review ReportMechanical Thrombectomy for Acute Ischemic Stroke Using the MERCI Retriever and Penumbra Aspiration Systems
Introduction
Stroke is the leading cause of disability and the third-leading cause of death in the United States. The natural history of intracranial large vessel occlusive disease is particularly poor, with documented mortality rates as high as 90%, depending on the affected vessel (8, 16, 17, 31, 40). Intravenous tissue plasminogen activator (IV tPA) has been shown to be efficacious in the recanalization of occluded intracranial vessels and to improve clinical outcome in patients experiencing ischemic stroke. The authors of a meta-analysis of 53 studies that 2066 patients with acute stroke demonstrated a 46.2% overall recanalization rate with IV fibrinolysis (34). Other investigators have shown, however, that the frequency of recanalization is lower for large vessel occlusions (10, 26, 34, 35, 49). Reported rates range from 28.9% to 54.7% in the middle cerebral artery (MCA), 30% to 80% in the vertebrobasilar (VB) system, and from 5.9% to 13.9% in the internal carotid artery (ICA) (34, 35, 49).
Many acute stroke patients are not candidates for IV tPA. Eligibility is restricted by medical contraindications and stringent exclusion criteria. A narrow time window and inadequate public awareness of stroke symptomatology further limit the number of patients who may benefit from this therapy. It is estimated that only 3%-10% of those patients eligible for IV tPA actually receive the drug (2, 5, 6, 9, 19, 44). Some patients are considered ineligible for treatment because of large areas of ischemia on magnetic resonance imaging (diffusion-weighted imaging sequences). However, several investigators have shown that diffusion-weight imaging changes, once thought to represent infarcted core, can be reversible in the both the anterior and posterior circulation (12, 22, 42, 48, 50).
Endovascular techniques allow for selective treatment via either thrombolytic delivery or mechanical clot extraction in patients who have failed, or are not eligible for, IV tPA. Intra-arterial (IA) tPA administration decreases the undesired effects of systemic infusion and allows for direct delivery to the region of maximal clot burden. Because of the risk of hemorrhage into infarcted tissue, treatment with intra-arterial thrombolytics is limited to a strict six-hour window. Mechanical revascularization establishes vessel patency via clot extraction, providing an alternative treatment for those who are not eligible for, or do not respond to, thrombolytics. Rha and Saver's (34) meta-analysis demonstrated a 63.2% overall recanalization rate for IA thrombolysis and an 83.6% revascularization rate for mechanical therapy. The authors were able to evaluate rates for specific vessels in 1054 patients for IA versus IV therapy. Recanalization was successful in the anterior cerebral artery (ACA) or MCA in 78.4% of patients (vs 54.7% IV), in the VB system in 100% of patients (vs 80% IV), and in the ICA for 77.8% of patients (vs 13.9% IV). Revascularization was significantly associated with good functional outcome, defined as a modified Rankin Score (mRS) (47) ≤2 (58.1% mRS ≤2 for recanalized patients vs 24.8% with no recanalization) and lower mortality (14.4% vs 41.6%) (34). Mechanical revascularization can establish vessel patency without the use of thrombolytic agents. This technique therefore extends the time window of and increases the number of patients eligible for acute stroke treatment. This article will focus on conventional endovascular mechanical recanalization techniques. At this time, there are two devices approved by the Food and Drug Administration (FDA) for endovascular treatment of acute ischemic stroke. These are the Merci Retrieval System (Concentric Medical, Mountain View, California, USA), and the Penumbra System (Penumbra, Alameda, California, USA).
Section snippets
Merci Retrieval System
The Merci device was initially approved by the FDA in 2004 as a Humanitarian-Use Device. A Humanitarian Device Exemption is granted for products used to treat a disease or condition that affects 4000 patients or fewer per year in the United States. The newest version of the device is approved via a 510k clearance, meaning that it has been shown to be equivalent in safety and efficacy to a device that has already been approved for a particular indication. The retriever is also approved for the
Penumbra System
The Penumbra system was approved for use in the United States in 2008, also under a 510k clearance from the FDA. It is currently approved for use in ischemic stroke in Europe, Australia, and parts of Asia. In the United States, its intended use is for “the revascularization of patients with acute ischemic stroke secondary to intracranial large vessel occlusive disease (in the internal carotid, middle cerebral – M1 and M2 segments, basilar, and vertebral arteries) within 8 hours of symptom
Future Directions
At this time, the Penumbra device is indicated for use in patients with symptoms for eight hours or less. Although the Merci retriever was approved for use without a time window, many physicians use eight hours as the maximum time limit for treatment on the basis of previous mechanical thrombectomy studies. However, the authors of several recent papers have challenged the traditional dogma that eligibility for endovascular stroke therapy should be limited by strict temporal criteria. These
Conclusions
The data from mechanical thrombectomy studies indicate that in carefully selected patients, recanalization of large vessel occlusions can be achieved safely. Successful revascularization results in more favorable clinical outcomes and decreased mortality when compared with persistent vessel occlusion. These findings are critical in light of the poor natural history and lack of treatment options for the majority of stroke patients. Each of these devices had increased risk of sICH and mortality
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Conflict of interest statement: The authors declare that the article content was composed in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.