Does the Merci Retriever Work?
In a decade after approval of IV rt-PA for acute stroke, treatment leads to good outcomes in less than half of patients, and hemorrhage rates have not been reduced. Recanalization improves outcome1 but is often delayed and incomplete. Devices offer the promise of rapid recanalization with a greater proportion of good outcomes and fewer hemorrhages. The Merci retrieval system was introduced with hopes of fulfilling this promise. The FDA recently approved this device for removal of thrombus from intracranial arteries in patients with stroke. Despite this approval, the question of whether the device works remains unsettled.
The recently published Mechanical Embolus Removal in Cerebral Ischemia (MERCI) trial2 using the Merci retriever claims efficacy based on a recanalization rate that is greater than the spontaneous recanalization rate observed in the Prolyse in Acute Cerebral Thromboembolism (PROACT) II trial control group.3 Recanalization may result in a variety of outcomes depending on timing, depth and duration of ischemia.4 These include improvement, no clinical change, reperfusion hemorrhage or massive cerebral edema resulting in neurological worsening or death. Thus, the most important measure of efficacy for any acute stroke treatment is not recanalization, but clinical outcome. On this score, evidence that the Merci Retriever works is lacking. In the MERCI trial, only 28% of patients reached a mRS score of ≤2 at 90 days. This is only slightly greater than the 25% of control patients reaching this outcome in the PROACT II study and does not compare favorably with the 40% of patients achieving mRS ≤2 in the treatment group. In the MERCI trial, treatment was associated with a 44% mortality. Mortality in PROACT II was considerably lower at 27%. Although inclusion of distal internal carotid artery and basilar occlusions possibly biases the MERCI study toward worse outcomes, when considering only the patients with middle cerebral artery occlusion (similar to PROACT II) only 29% had good outcomes and mortality was 39%. Overall, these results certainly do not provide any signal suggesting the device improves outcome.
Does the device actually improve recanalization? Perhaps it does compared with spontaneous recanalization rates but not in comparison to other stroke treatments. In the PROACT II study, intra-arterial prourokinase recanalized 66% of arteries. Recanalization was achieved in 56% of patients treated with combined intravenous and intra-arterial rt-PA in the Interventional Management of Stroke (IMS) study.5 In the MERCI study, recanalization was achieved in 46% of all patients and 45% of middle cerebral arteries. Given the small sample size of these studies the differences may not be significant. However, there is no indication that the MERCI device is superior to these other interventional approaches which, unlike the Merci Retriever, are not FDA approved. Another potential advantage of a clot removal device is more rapid recanalization compared with the 2-hour infusion in PROACT II and IMS. Unfortunately, the report of the MERCI trial gives no information about time to recanalization. It is also unfortunate that recanalization was assessed by unblinded local investigators rather than by central blinded adjudication because this might have resulted in overestimation of recanalization.
The MERCI trial investigators point out that in patients with recanalization, good outcomes were greater (36%) with lower mortality (32%). However, these results remain unimpressive compared with outcomes with other acute stroke therapies and do not consider the possibility that patients may have been harmed by the device if recanalization did not occur.6
Another way the device could work is by reducing complications. By this criteria, the Merci Retriever again fails. Symptomatic hemorrhage occurred in 7.8% of patients, not different from the 10% rate in PROACT II and 6.3% in IMS. In addition, other complications such as perforation, dissection, embolization, groin hematoma and device fractures occurred in 13% of patients, and 7.1% were considered clinically significant. When added to the symptomatic hemorrhages, 14.9% of patients experienced clinically significant complications, a sobering figure for a treatment with unproven efficacy.
In summary, the MERCI study does not provide any evidence of improved outcomes or greater recanalization rates compared with other acute stroke therapies. In addition, both clinically significant complications and mortality are higher than the results of other interventional trials. Even acknowledging the hazards of comparing disparate studies, the results do not support the proposal that the Merci retrieval device works by any definition.
- Received January 31, 2006.
- Accepted February 3, 2006.
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