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Mechanical Thrombectomy (Endovascular Therapy)


Learning objectives

  • What is Mechanical thrombectomy
  • What is the evidence base
  • Indications
  • Contraindications

Introduction

Mechanical thrombectomy(Endovascular Therapy) is highly effective. The HERMES collaboration which is pooled data from 5 trials has shown that the proportion of patients achieving a good (independent) functional outcome (mRS 0–2 at 90 days) were 46.0% (mechanical thrombectomy) vs 26.5% (best medical treatment). Most of the thrombectomy patients also received Alteplase. Patients given both Thrombectomy and Alteplase did not appear to have any affect on mortality or the risk of ICH at 90 days over Alteplase alone. The benefits extended to those over the age of 80. Benefits were most favourable in those with an Early CT score (ASPECTS score), there was clear benefit only for those with an ASPECTS >5. A good clinical outcome is a measure of neurologic functional with a score of 0–2 on the modified Rankin scale assessed 90 days after treatment. This does not exclude clinically significant benefit in patients in whom an mRS score of 2 is not achieved

Stroke thrombolysis given IV will lead to recanalisation of proximal arterial occlusions in only 33% of patients. Mechanical thrombectomy provides a much higher rates of recanalisation with an NNT of less than 3 for improved functional outcome. This level of benefit is usually unheard of. If the patient is also eligible for intravenous TPA, this drug should be administered as a ‘‘bridging’’ strategy in parallel without delaying thrombectomy and there should be no delay in waiting to see if TPA is effective

Large vessel occlusions

  • Anterior circulation (including ICA, M1, M2 segment of MCA)
  • Posterior circulation basilar occlusions

Clot locations

  • Evidence exists for internal carotid artery terminus and M1. Carotid ‘‘T-lesions’’ have the poorest outcomes among anterior-circulation strokes. Proximal M1 occlusions have worse outcomes than distal M1 occlusions as a result of occlusion of lenticulostriate arteries with basal ganglia infarction and increased risk of reperfusion haemorrhage.
  • Evidence is less clear for distal M2 occlusions though classification between distal M1 and proximal M2 can be difficult. Some case-control studies have suggested that benefit persists in M2 occlusions
  • Basilar artery occlusion was not included in the recent trials and has a dismal prognosis if untreated and the clear improvement associated with recanalisation as sufficient grounds to treat.
  • No evidence for anterior cerebral artery, M3/4 segments, and posterior cerebral artery yet exists

Solitaire stent retriever showing extracted clot. Solitaire FR stent retriever device has become the benchmark for mechanical thrombectomy

Different methods of removing thrombus.

Criteria for endovascular therapy with a stent retriever

  • (a) prestroke mRS score 0-1 (some now pushing this out to 2-3)
  • (b) intravenous r-tPA within 4.5 hours of onset
  • (c) occlusion of the internal carotid artery or proximal MCA (M1)
  • (d) age ≥18 years
  • (e) NIHSS score of > 5
  • (f) ASPECTS of > 5
  • (g) groin puncture within < 6 hours of symptom onset

TICI Assessment

The thrombolysis in cerebral infarction (TICI) grading system was described in 2003 by Higashida et al 1 as a tool for determining the response of thrombolytic therapy for ischaemic stroke. In neurointerventional radiology it is commonly used for patients post endovascular revascularisation. Like most therapy response grading systems, it predicts prognosis. The original description 1 was based on the angiographic appearances of the treated occluded vessel and the distal branches. In 2013 Fugate et al reported marked variability in its definitions and application. A consensus paper from three collaborative groups published in Stroke in 2013 3 recommended a modified scale, and a change of name from Thrombolysis in Cerebral Infarction to modified Treatment in Cerebral Infarction (mTICI), to better reflect the increased use of endovascular therapies. The goal is that at least 70% of patients should have mTICI grade 2b/3 (ie, > 50%) reperfusion for all clot locations.

modified TICI Classification
  • Grade 0: No perfusion, complete obstruction; no flow past occlusion of ‘‘major’’ vessel
  • Grade 1: Perfusion past initial obstruction but limited distal branch filling with little/slow distal perfusion
  • Grade 2A: Partial perfusion: < 50% of ‘‘major’’ vascular territory perfused (eg, filling and complete perfusion through one M2 division)
  • Grade 2B: Partial perfusion: 50% of major vascular territory is filled, but there is not complete and normal perfusion of entire territory
  • Grade 3: Complete or full perfusion with filling of all distal branches

The Procedure

Mechanical thrombectomy (MT) is a procedure performed in the setting of an acute ischaemic stroke. The operator gains access to the cerebral circulation usually via the femoral artery in the groin and gains access to the cerebral circulation. Using specialised kit the operator can remove an occlusive thrombus from the proximal MCA. The clot is aspirated into the device and removed. This allows flow to be recommenced in the obstructed artery. The goal of thrombectomy is to achieve a complete recanalisation (TICI grade, 3). It may also be used on other large vessel occlusions such as Basilar artery thrombi. Mechanical treatment approaches for acute ischaemic stroke treatment aim for fast and efficient reperfusion with short procedure times and high recanalisation rates, thus extending the treatment window. MT may be used with or instead of IV Alteplase.

The patient is brought into the angiography suite and checklists are run through to ensure right patient and procedure. It is not clear if the procedure should be done under sedation or under general anaesthesia and both have their indications and contraindications. Many patients tolerate the procedure well. For some who are agitated general anaesthesia can be instigated. Access to the femoral artery is obtained and the INR (Interventional neuroradiologist) gains access to the internal carotid and circle of Willis. A microguide wire is passed into the clot. Aspiration is often tried to retrieve a thrombus under negative pressure. If needed a stent retriever can be slid over the microcatheter to emerge inside the clot and used to extract the clot. Difficulties and challenges arise when there are tandem occlusions of the internal carotid artery and intracranial vessels or a fixed intracranial stenosis, which can limit endovascular access. The procedure often takes an hour. However I have seen clots taken out within 5-10 mins of the procedure starting when in expert hands. Over recanaliation rates can be up to 80%.

National systems of Thrombectomy delivery

The powerful evidence for thrombectomy came out in early 2015 and was ground breaking and unexpected after several neutral trials. The implications for stroke services in the UK is profound and the NHS infrastructure contrasts with systems elsewhere. The UK has a significant lower number of doctors per head of population and even moreso when it comes to neurologists and other doctors involved in stroke. Stroke has major workforce issues and Brexit is not an incentive for highly trained European stroke and interventional neuroradiology specialists to want to work in the UK. There are different models of care for example the Finnish Helskinki model where there is a neurologist and trainee in the hospital at all times and their door to needle times are excellent due to a well planned and through out service. Their patients come from surrounding suburbs but others from 2-3 hrs away. The Irish model contrasts and patients come from across Ireland to the Beaumont Hospital in Dublin and then are returned to their local DGH. Only those with complications and other issues remain at the thrombectomy hospital. The UK model will likely be based around developing approximately 50 neuroscience centres. However we are well short of the number of interventional neuroradiologists needed to support such an expansion.

References

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