The mismatch is the area of reduced or absent cerebral blood flow—the perfusion lesion—that is not accompanied by restricted diffusion, indicating potentially salvageable brain if blood flow can be restored prior to infarction. Data from the EPITHET and DEFUSE trials suggest that patients with a DWI/PWI mismatch of sufficient volume might benefit from IV thrombolytics up to 6 hours from stroke onset. The use of neuroimaging to direct treatment beyond the 4.5-hour window, however, remains at the investigational stage and is not recommended as standard care.
Efforts to restore blood flow to the ischemic penumbra by mechanical disruption and/or retrieval of intra-arterial clot have taken various forms. Although encouraging results have been seen with some of these methods, none, as yet, are recommended as standard care; several trials are currently under way that may eventually justify their widespread use. Perhaps the most studied of these techniques is clot retrieval with the use of the MERCI Retriever (Figure 3). This device is a catheter-guided wire that can penetrate through thrombus. Once the wire is distal to the occlusion, a corkscrew-like apparatus is released and is used to snare and retrieve the clot. Regulatory approval for the use of this device for clot removal was obtained after the MERCI trial, published in 2005, demonstrated its ability to remove blood clots and restore blood flow within 8 hours of stroke symptom onset. The study, however, did not demonstrate improved neurological outcome and, therefore, currently there is insufficient evidence to establish its clear utility for the treatment of acute stroke. Two trials further evaluating the use of the MERCI Retriever in acute stroke, MR RESCUE and IMS III, are ongoing and may demonstrate improved outcomes.
Additional methods in the investigational stage directed at mechanical clot disruption and/or removal include ultrasound aided thrombolysis, angioplasty and stenting, and the Penumbra System—a catheter-guided device that disrupts and aspirates the intra-arterial clot. These methods are mostly practiced at advanced comprehensive stroke centers and are often used in conjunction with thrombolytic therapy.continued ...