The current evidence and advances in mechanical thrombectomy for acute ischaemic stroke

By Neha MathurAug 14 2023Reviewed by Lily Ramsey, LLM

In a recent narrative review published in BMJ Medicine, researchers summarized the current evidence on advancements in mechanical thrombectomy (MT), a ground-breaking treatment for acute ischaemic stroke involving removal of a thrombus by recanalization of an intracranial occlusion of a large vessel via an aspiration catheter, stent retriever, or both. 

Study: Advances in mechanical thrombectomy for acute ischaemic stroke. Image Credit: SewCreamStudio/Shutterstock.com

Advances in mechanical thrombectomy

The evidence base proving the efficacy and cost-effectiveness of MT revolutionized back in 2015 with the publication of five randomized controlled trials (RCTs).

The results of MR CLEAN, the first positive trial on MT to be published, were so compelling that the data safety monitoring boards of the remaining four trials, namely SWIFT-PRIME, EXTEND-IA, ESCAPE, and REVASCAT, had to conduct interim analyses. The HERMES meta-analysis combines data from all these five RCTs.

Its researchers concluded that MT was most beneficial when conducted under seven hours and 18 minutes. Rapid recanalization and reperfusion ensured that the patient attained functional independence. Note that these trials included over 1,200 patients with varying time criteria ranging from six to eight hours from symptom onset.

Nonetheless, the meta-analysis showed that MT led to markedly good functional outcomes, as indicated by a modified Rankin score of zero to two, in adult patients of all age groups and geographies and even those who did not receive intravenous thrombolysis.

A striking 46% of patients receiving MT attained the primary outcome, compared to 26.5% among recipients of the best medical treatment.

The HERMES meta-analysis findings led to updated clinical practice guidelines in the United States of America (USA), Canada, and Europe, including the United Kingdom (UK). Now, MT is the preferred method for patients with acute ischaemic stroke. Yet, access to this treatment remains limited in many parts of the world.

Accordingly, of the 10% of patients eligible for MT in the UK, only 3% receive it. There is an urgent need to expand the evidence base to allow better patient selection for ischaemic stroke management.

Regarding procedural considerations, a direct aspiration approach for clot retrieval had the added advantage of reduced expense and an 11-minute lesser time to recanalization, as assessed in the COMPASS study.

This study also showed that stent retriever and aspiration catheter yielded good functional outcomes, i.e., modified Rankin score between zero and two at 90 days (50% vs. 52%).

The HERMES trials showed improved successful reperfusion rates of 71% for second-generation thrombectomy devices. Another clinical predictor of good clinical outcome, the first pass effect, improved to 61.3% with MT. 

Recent studies showed that patients receiving MT under general anesthesia achieved better functional outcomes, but more work is needed to establish this accounting for patient characteristics. Another area that needs management for MT procedures is managing peri-procedural blood pressure.

According to the authors, adopting MT globally would also require rapid clinical and radiological assessment (in tandem) by stroke physicians and neuroradiologists.

Based on data from the HERMES trials, physicians should refer patients with a National Institutes of Health Stroke Scale (NIHSS) score ≥6 for MT. In addition, treating physicians should clarify their pre-morbid level of independence. 

Next, they should do computed tomography (CT) and CT angiography of brain and vascular tissues to identify a large vessel occlusion and the degree of ischaemic change. Patients with Alberta Stroke Protocol Early CT Score (ASPECTS) of <6 have extensive early ischaemic change and might not benefit from MT. Also, they should present within the early window of six hours from stroke symptom onset for MT.

The HERMES trial clearly outlined that patients were less likely to achieve functional independence post-MT if presented after seven hours and 20 minutes of symptom onset, reinstating the significance of the time clock. However, time is less relevant for patients who do not have large core infarctions, as shown by the DAWN and DEFUSE-3 trials. 

An additional 2.7% of acute ischaemic stroke patients could qualify for MT with the DAWN/DEFUSE-3 criteria. A lack of advanced imaging techniques hinders access to CT perfusion, a prerequisite for patients treated in the late window. Despite extensive criticism in many trials, the DAWN/DEFUSE-3 criteria have a 30% rate of achieving functional independence. 

Intravenous thrombolysis, the standard of care for acute ischaemic stroke patients, is associated with the risk of symptomatic intracranial hemorrhage. However, it aids recanalization by thrombus softening. Consequently, six RCTs

are underway to compare and bridge direct MT with intravenous thrombolysis.

The researchers also described two pre-hospital pathways to thrombectomy: the mothership model and the drip and ship model.

In addition, they discussed four RCTs investigating whether MT could be used in basilar artery occlusions, constituting 20% of stroke cases. Furthermore, tandem occlusions occur in 10-20% of presentations of large vessel occlusion. 

The current data suggest that acute extracranial internal carotid artery stenting in tandem occlusions resulted in favorable functional outcomes while subsiding other complications. Two RCTs are evaluating the management of tandem occlusions for more conclusive answers on whether MT works in such challenging cases. 

Furthermore, the researchers mentioned that SELECT-2, RESCUE-Japan LIMIT, and ANGEL-ASPECTS trials showed favorable functional outcomes of large core thrombectomy in selected patients, which has led to its incorporation into the latest Stroke National Clinical Guidelines.

Conclusions

The current review highlighted what difference MT is making in the lives of patients with acute ischaemic stroke. Post-the HERMES meta-analysis, researchers have greatly improved the initial selection criteria for MT.

With further advancements in this technology, the priority would be equitable access to MT to a wider patient population. 

Future research should explore answers to the remaining questions, such as the effectiveness of interventions for patients presenting with tandem occlusions.