The Catch system (Balt, Montmorency, France; Fig. 1.4-23) was one of the first to receive certification in Europe in animal trials and to be introduced into everyday clinical practice. The self-expanding basket, which is opened distal to the thrombus, is available in various diameters.
The Phenox Clot Retriever (Phenox Ltd., Bochum, Germany) consists of a soft wire with outward-pointing microfilaments woven into it, which are intended to grasp the whole length of the thrombus like a brush or pipe cleaner and mobilize it. Two systems can be used simultaneously for thrombi in the area of the middle cerebral artery bifurcation. The withdrawal maneuver has to be carried out with flow arrest and aspiration through the balloon-equipped guide catheter. The Merci retriever (Concentric Medical Inc., Fremont, California, USA; Fig. 1.4–24) is a nitinol wire, which after being extended from the microcatheter is designed to open like a corkscrew in order to catch and grasp the thrombus. It is delivered with an 8F or 9F balloon guide catheter, which blocks blood flow while the retriever is being withdrawn. The retriever is available in various diameters and with or without microfilaments (type L), which are intended to additionally attach the thrombus to the system. In a prospective registry study, recanalization rates of 43% were achieved with the initial system, which were increased to 64% when it was combined with local thrombolysis using rt-PA. In the Multi Merci Trial, including 131 patients with severe stroke (NIHSS 19), a recanalization rate (TIMI 2 and 3) of just under 70% was achieved with simultaneous intra-arterial rt-PA administration. Recanalization was achieved using the new Merci retriever alone in 57% of the cases.
Fig. 1.4–23 The Catch retriever system.
In general, we regard these mechanical systems as providing an opportunity to force recanalization in rare, exceptional situations when the clinical and imaging findings show it to be necessary. Use of the systems requires experience not only with the instruments themselves, but also with the cerebral circulatory system, cerebral function, and neurophysiology. A careful review of the literature confirms the impression that the clinical improvement achieved cannot fully match the reported recanalization results, for several reasons:
Fig. 1.4–24a-c The Merci retriever. (a) Type A. (b) Type L with filaments. (c) Recanalization of an M1 occlusion using a Merci retriever, with the type X Merci in the M1 segment of the middle cerebral artery.
1.4.5.3 Percutaneous transluminal angioplasty (PTA) and stenting (Figs. 1.4-25, 1.4-26)
The principle of PTA and stent treatment involves pressing the thrombus into the wall and fixing it there with the stent if appropriate. It is known from animal experiments that the microcatheter does not penetrate the thrombus, but passes the occlusion site between the vascular wall and the thrombus. The thrombus is thus not attached to the vascular wall circumferentially, but attached to the part of the vessel opposite the point at which it has been passed with the wire or catheter. Attention therefore needs to be given to ensure that the microwire over which the stent will later be introduced passes the occluded basilar artery in the dorsal part of the vascular lumen, and this is only technically possible with the microcatheter when an enlarged lateral road map projection is used. Then, when the stent opens, the thrombus will be shifted ventrally and thus away from the branches to the pons that arise from the dorsal circumference of the basilar artery. Passage of an occluded M1 segment between the thrombus and the upper circumference of the middle cerebral artery is achieved analogously, so that the upward-directed origins of the lenticulostriate branches are kept open.
Animal studies and our own experience have confirmed that PTA can lead to rapid recanalization, but that the lumen closes again relatively rapidly if the thrombus is not fixed and compressed against the wall by a stent. The risk of fragmentation and embolization of the thrombus into distal branches is also greater with PTA alone. Rapid recanalization rates of up to 90% have been achieved with both balloon-mounted and self-expanding stents (Wingspan, Enterprise).
The disadvantage of stent recanalization is the relatively high rate of stent thrombosis, which requires a relatively aggressive form of management with double aggregation (i.v. aspirin and rapid titration of clopidogrel). The cause of these early stent occlusions is that the thrombus attached to the wall slowly expands back into the lumen through the mesh of the stent, and the lumen is then occluded again by thrombus. This observation has led to the development of a “removable stent”, also known as “stentrievers”.
Fig. 1.4–25a-e A 61-year-old man with NIHSS 15 due to a left-sided M1 occlusion. (a, b) The wire and microcatheter are passed between the upper wall and the thromboembolic blockage. (c) PTA thus compresses the thrombus caudally. (d) The lenticulostriate end arteries arising from the upper wall, which supply the basal ganglia, are thus kept free or reopened. (e) Remodeling of the M1 lumen by secondary placement of a self-expanding stent.
Fig. 1.4–26a-i A 62-year-old patient with mild motor aphasia. The diffusion-weighted image shows ischemic areas in the territory