A high resolution scanning electron microscopy analysis of intracranial thrombi embedded along the stent retrievers

“Endovascular treatment with stent retriever thrombectomy is a major advancement in the standard of care in acute ischemic stroke (AIS). The modalities through which thrombi embed along stent retriever following mechanical thrombectomy (MTB) have not yet been elucidated. Using scanning electron microscopy (SEM), we analyzed the appearance of thrombi retrieved by MTB from AIS patients, when embedded into the stent retriever. We observed that the organization and structural compactness vary for compositionally different thrombi. The modalities of attachment onto the stent vary according to thrombus composition and organization.”

Figures 1 and 2 are examples illustrating an RBCs rich thrombus incorporated into the stent. As thrombus spirals along the stent, its segments are anchored on single or multiple stent struts at a time—Fig. 1a,b. The RBCs rich thrombus segments display a compact core of polyhedrally shaped red blood cells and an outer layer formed of fibrin, characteristic for cerebral arterial thrombi23,24—Fig. 1c,d and Supplementary Fig. S1. The polyhedral shape of red blood cells is acquired due to compressive forces in vivo, during thrombus formation, and is considered a marker for intravital thrombus contraction25,26,27. The porosity of thrombus increases towards its periphery where platelets and white blood cells are also present along with fibrin. The segments of thrombus are interlinked by fibrin strings—Fig. 1e,f. The various modalities through which thrombus is incorporated into the stent are illustrated in Fig. 2. Protrusion of the stent through the thrombus (Fig. 2a,b; also Supplementary Fig. S2) occurs at sites with loose cellular packing, where the red blood cells are biconcave in shape. The thrombus can deform, wetting the stent surface—Fig. 2c,d. We also observed, between the double struts of the stent mesh, the existence of films, or bridges, of fibrin, with or without cellular content—Fig. 2e,f. It is unlikely that such fibrin bridges are native to the original thrombus that caused the stroke. Most probably, the bridges across the stent struts are formed during the retrieval process, and they potentially aid in securing the thrombus attachment.

Figure 1
figure 1

RBCs rich thrombus incorporated onto the stent retriever (Case 2 in Table 1). (a) Optical micrograph. (b) Collage of SEM micrographs. (c) Cross section of a thrombus segment, showing a compact core, porous periphery and fibrin outer layer. Remnants of vascular tissue are visible at the thrombus surface (arrows). (d) High magnification view of the compact core composed of polyhedrocites. (e) Fibrin strings are found in between thrombus segments. (f) White blood cells and platelets are attached to the fibrin strings (higher magnification view of (e), region marked with arrow).

Figure 2
figure 2

Modalities of RBCs rich thrombus attachment onto the stent. (a) Stent strut protruding through thrombus. (b) Higher magnification view from (a) (dashed rectangle), showing a platelets cap and biconcave RBCs. (c) Thrombus conforming with the stent strut. (d) Higher magnification view from c (region indicated by arrow), illustrating the thrombus contact area with the stent. (e) Bridges of fibrin between the adjacent stent struts. (f) High magnification view from (e) (region indicated by arrow).

Intermediate, compact thrombus

When embedded into the stent, intermediate thrombi are wrapped all around the struts and, in some cases, they clutch the struts. Illustrative examples are presented in Fig. 3 and in Supplementary Fig. S3. The compact organization of intermediate thrombi is illustrated in Fig. 4 and Supplementary Fig. S4. Intermediate thrombi can wet the surface of the stent, either at sites where peripheral fibrin is organized in parallel strings (Fig. 3d; Supplementary Fig. S3), with an overall flexibility and affinity for the stent surface, or when compact thrombus deforms and clutches the stent. Remnants of vascular tissue, found at the surface of retrieved thrombi, are illustrated in Supplementary Fig. S5. Supplementary Fig. S6 presents an intermediate thrombus trapped between the stent struts.

Figure 3
figure 3

Intermediate thrombus incorporated into the stent retriever (Case 4 in Table 1). (a) Optical micrograph. (bc) SEM view of thrombus on stent. Insert in (c) View of sectioned thrombus and the stent struts at the anchoring site. (d) Fibrin strings wetting the stent surface (indicated by arrows in ad). (e) Thrombus clutches onto the stent struts. (f) Section through the compact thrombus, showing the contact region with the stent strut (indicated by arrow). (g) Higher magnification view of the thrombus surface at the contact with the stent (indicated by arrow).

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