Comparison of Stent-Retriever Devices versus the Merci Retriever for Endovascular Treatment of Acute Stroke

Study Population

All patients with stroke who were admitted to our stroke unit with large cerebral artery occlusion were registered prospectively in a nationwide data base. One hundred twenty-two patients treated with either the Merci retriever or with a stent retriever between September 2006 and March 2011 were analyzed for this study.

Our data base includes demographic characteristics, time of onset of symptoms, and baseline and follow-up stroke severity, measured by NIHSS information on functional outcome at discharge (7–14 days after stroke onset) and at 90 days by the mRS. Good clinical outcome was defined as mRS 0–2,¹⁴ and death was recorded as mRS 6. Clinical progress was documented by a stroke neurologist.

Classification of Recanalization

EMR of the primary large cerebral artery occlusion was assessed by the TICI score.^15,16 Success was defined as a score of TICI 2b or 3.

Radiologic Investigations

All patients were assessed by using CT, including a CTA on admission, or MR imaging with perfusion-weighted sequences and DWI and MRA. Large cerebral artery occlusion, defined as complete obstruction of the flow in the ICA, MCA, or BA, was confirmed in all cases.

The first follow-up CT was performed within 24 hours after thrombectomy or at the time of clinical deterioration. Complications, including ICH and the need for decompressive surgery due to space-occupying infarctions, were documented. An ICH was defined as severe when an increase of ≥4 points in the NIHSS score within 24 hours in concordance with the evidence of any blood on the 24-hour head CT/MR imaging was documented. Intracerebral hemorrhagic transformation in the infarction area without clinical deterioration was defined as mild or moderate.

Treatment

Patients admitted to our stroke unit were treated according to stroke guidelines.² Patients without contraindication received IVT as a bridging concept based on the European Cooperative Acute Stroke Study III.^2,3,17,18 In our internal guidelines, the time window was extended up to 12 hours in patients with BA occlusion if unconsciousness was <4 hours. If indicated, the patient was transferred immediately to the angiography suite under bridging therapy. Patients with undefined symptom onset beyond 4.5 hours were treated with EMR only if a mismatch in MR imaging was diagnosed. Treatment time was recorded and defined from arterial access to the end of the procedure.

Device Selection

The selection of the respective clot retriever was determined by the chronologic order of availability of each device. It was not dependent on the patient's symptoms and history or doctor's preference. Because the Merci retrieval system was primarily obtainable, all patients were consecutively treated with this device. In 2009, the Solitaire stent became the next available thrombectomy device. From that time on, all patients were consecutively treated with the Solitaire device. Since 2011, the Trevo retriever was used routinely in our institution. Since then, we have treated all patients included in this study consecutively with the Trevo device.

Description of Clot-Retriever Devices

The Merci retriever system is a nitinol wire with an imprinted shape-memory effect covered with a platinum coil to improve the visibility under fluoroscopy. It is inserted through a braided microcatheter to a position distal to the thrombus. After release from the microcatheter, it assumes a shape similar to a corkscrew and has to be slowly pulled back under continuous aspiration via the guiding catheter.¹⁹

The main component of stent-retriever devices (Solitaire and Trevo retrievers) is a stent-like mesh of wires, attached to a core wire and delivered through a microcatheter. Trevo and Solitaire can create a temporary endovascular bypass after expansion into the clot. Additionally, Trevo has a stent cell geometry specifically designed to integrate the clot into the stent structure for clot removal.^1,13

Endovascular Procedure

DSA was performed on a biplane high-resolution angiography system (Artis zee; Siemens, Erlangen, Germany) via a femoral approach. All procedures were performed exclusively by 1 neurointerventionalist (M.K.-O.) with the patient under general anesthesia.

Routinely, a 6F guiding catheter (Envoy; Cordis, Miami Lakes, Florida) was placed with the help of a guidewire (Radifocus Guide wire angled; MicroVention Terumo, Aliso Viejo, California) into the targeted artery. Using standard cerebral catheterization techniques, we guided the respective microcatheter (Merci or Trevo 18; or Rebar, ev3) into the occluded vessel and passed beyond it the thrombus. In cases with tortuous vessels, a long sheath (Strada; St. Jude, Minnetonka, Minnesota) was used because it gives better support during catheter manipulation and has the possibility of using a distal access catheter (DAC; Concentric Medical).

According to our preference, a DAC 044 or 057 was used as a coaxial system to increase the stability of the microcatheter. The microcatheter and DAC were navigated together and placed just proximal to the clot to apply suction directly at the proximal face of the clot instead of at the guiding catheter. A selective angiogram with 1–2 mL of contrast was obtained distal to the thrombus to evaluate the distal vasculature and verify the correct position of the microcatheter.

Procedure by Using the Merci Retriever

In cases using the Merci retriever, the retriever was advanced through the microcatheter and several of the helical loops were deployed distal to the thrombus. The Merci retriever was then retracted to the face of the thrombus, and the proximal loops were then deployed within the thrombus. The Merci retriever with the ensnared thrombus and the microcatheter were withdrawn together into the guide catheter lumen or DAC. Manual aspiration with a 50-mL syringe was performed through the hemostatic valve during retrieval to reverse the flow and to aspirate clot debris present in the lumen of the guide catheter or DAC, respectively.

Procedure by Using Stent Retrievers

In cases using a stent retriever, the device was placed in the same manner as the Merci retriever, distal to the clot. Then the microcatheter was retracted to a position where the distal marker was aligned with the proximal marker of the stent. On device deployment, an angiographic control was performed to evaluate the correct placement and expansion of the device. After the angiographic run, the device was maintained in place for a few minutes (1–4 minutes) to allow device expansion and clot integration. After this time, the fully deployed device and the delivery microcatheter were gently pulled back together and recovered through the guiding catheter or DAC. The integrity of the stent was consistently checked after every pass. Continuous aspiration was applied during the retrieval maneuver, as described above. If, after several passes (4–6) with either the Merci or stent-retriever devices, no obvious clot removal was realized, another clot retriever device was used for the next pass. In case of a proximal high-grade stenosis of the ICA, a carotid stent was placed, to gain access to the distal territory. At the end of the procedure, an Angio-Seal 6F (St. Jude Medical) vessel closure system was used.

Therapies Additional to Thrombectomy

If not contraindicated and if a 100% reperfusion was not achieved by the first pass, during the following passes, additional intra-arterial delivery of tPA (IAT) (Actilyse; 2–20 mg) or abciximab (ReoPro; 1–5 mg) was performed. In instances in which an underlying stenosis was noted at the occlusion site or a very hard thrombus could not be removed, a stent was implanted. In cases of acute stent placement, a bolus of abciximab (0.5 mg/kg) was given, half intra-arterial and half intravenous. Then, continuous intravenous application of 0.125 μg/kg/min of abciximab was given for 24 hours. The first day after the procedure, oral medication with acetylsalicylic acid, 100 mg, and clopidogrel (Plavix), 75 mg, was started if there was no contraindication seen in the 24-hour postprocedural CT scan.

Statistical Analysis

Descriptive statistics such as means, percentages, and ranges were computed for several variables. The Fisher 2-sided exact test was used to compare percentages between the Merci and Trevo/Solitaire group. A repeated-measures ANOVA with 1 fixed effect (Merci versus stent retriever) was used together with 2-sided unpaired Student t tests post hoc to compare both groups over time. A whisker-plot with 95% confidence intervals was used to illustrate the mean NIHSS scores between both groups with time. A P level < 5% indicated a statistically significant difference. All analyses were done by using Statistica 6.1 (2004) (StatSoft, Tulsa, Oklahoma) and Mathematica 7.0 (2008) (Wolfram Research, Champaign, Illinois).