Preventing Stroke

Noninvasive label-free detection of circulating white and red blood clots in deep vessels with a focused photoacoustic probe

Blood clotting is a serious clinical complication of many medical procedures and disorders including surgery, catheterization, transplantation, extracorporeal circuits, infections, and cancer. This complication leads to high patient morbidity and mortality due to clot-induced pulmonary embolism, stroke, and in some cases heart attack. Despite the clear medical significance, little progress has been made in developing the methods for detection of circulating blood clots (CBCs), also called emboli. We recently demonstrated the application of in vivo photoacoustic (PA) flow cytometry (PAFC) with unfocused ultrasound transducers for detection of CBCs in small vessels in a mouse model. In the current study, we extend applicability of PAFC for detection of CBCs in relatively large (1.5-2 mm) and deep (up to 5-6 mm) blood vessels in rat and rabbit models using a high pulse rate 1064 nm laser and focused ultrasound transducer with a central hole for an optic fiber. Employing phantoms and chemical activation of clotting, we demonstrated PA identification of white, red, and mixed CBCs producing negative, positive, and mixed PA contrast in blood background, respectively. We confirmed that PAFC can detect both red and white CBCs induced by microsurgical procedures, such as a needle or catheter insertion, as well as stroke modeled by injection of artificial clots. Our results show great potential for a PAFC diagnostic platform with a wearable PA fiber probe for diagnosis of thrombosis and embolism in vivo that is impossible with existing techniques.

Computational modelling of emboli travel trajectories in cerebral arteries: influence of microembolic particle size and density

Ischaemic stroke is responsible for up to 80 % of stroke cases. Prevention of the reoccurrence of ischaemic attack or stroke for patients who survived the first symptoms is the major treatment target. Accurate diagnosis of the emboli source for a specific infarction lesion is very important for a better treatment for the patient. However, due to the complex blood flow patterns in the cerebral arterial network, little is known so far of the embolic particle flow trajectory and its behaviour in such a complex flow field. The present study aims to study the trajectories of embolic particles released from carotid arteries and basilar artery in a cerebral arterial network and the influence of particle size, mass and release location to the particle distributions, by computational modelling. The cerebral arterial network model, which includes major arteries in the circle of Willis and several generations of branches from them, was generated from MRI images. Particles with diameters of 200, 500 and 800 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upmu \hbox {m}$$\end{document}μm and densities of 800, 1,030 and 1,300 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {kg/m}^{3}$$\end{document}kg/m3 were released in the vessel’s central and near-wall regions. A fully coupled scheme of particle and blood flow in a computational fluid dynamics software ANASYS CFX 13 was used in the simulations. The results show that heavy particles (density large than blood or a diameter larger than 500 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upmu \hbox {m}$$\end{document}μm) normally have small travel speeds in arteries; larger or lighter embolic particles are more likely to travel to large branches in cerebral arteries. In certain cases, all large particles go to the middle cerebral arteries; large particles with higher travel speeds in large arteries are likely to travel at more complex and tortuous trajectories; emboli raised from the basilar artery will only exit the model from branches of basilar artery and posterior cerebral arteries. A modified Circle of Willis configuration can have significant influence on particle distributions. The local branch patterns of internal carotid artery to middle cerebral artery and anterior communicating artery can have large impact on such distributions.

Efficacy of Dual Antiplatelet Therapy in Cerebrovascular Disease as Demonstrated by a Decline in Microembolic Signals

BACKGROUND

The presence of microembolic signals (MES) may indicate an increased risk of recurrent ischemic events in patients with stroke. The optimal management of such patients is uncertain. We report the effect of clopidogrel in addition to aspirin on the number of MES in a series of patients with ischemic stroke and transient ischemic attack (TIA) due to large-vessel disease.

METHODS

8 patients with either extracranial or intracranial artery stenosis were identified in 30-min MES studies by transcranial Doppler sonography as having MES. All patients were on antiplatelet therapy prior to baseline transcranial Doppler monitoring. The patients were subsequently treated with clopidogrel in addition to aspirin. Repeat MES studies were performed between day 3 and 7 with aspirin and clopidogrel.

RESULTS

All patients were Chinese. The median interval time from symptom onset to initial MES study was 7 days (range of 2-30). MES donor sites included 4 severely stenosed or occluded internal carotid arteries and 4 stenosed middle cerebral arteries. The median MES number at baseline was 8 (range 3-51). Repeat MES studies showed a significant decrease in MES (p = 0.012, Wilcoxon signed ranks test). 4 patients had complete cessation of MES and all patients showed a decline in MES. No patient had recurrent strokes or bleeding complications.

CONCLUSION

The rapid and significant decline of MES in our stroke and TIA patients suggests the possible efficacy of dual antiplatelet therapy with aspirin and clopidogrel in patients with MES and symptomatic large-artery occlusive disease. Randomized controlled trials should be conducted to confirm this preliminary observation.

Oxidative stress during the chronic phase after stroke

Stroke is a complex disease originating and developing on the background of genetic predisposition and interaction between different risk factors that chronically damage blood vessels. The search for an effective treatment of stroke patients is the main priority of basic and clinical sciences. The chronic phase of stroke provides possibilities for therapy directed toward stimulation of recovery processes as well as prophylaxis, which reduces the probability of subsequent cerebrovascular events. Oxidative stress is a potential contributor to the pathophysiological consequences of stroke. The aim of the present review is to summarize the current knowledge of the role of oxidative stress during the chronic phase after stroke and its contribution to the initiation of subsequent stroke. The relationship among inflammation, hemostatic abnormalities, and platelet activation in chronic stroke patients is discussed in the context of ongoing free radical processes and oxidative damage. Free radical-mediated effects of increased plasma level of homocysteine and its possible contribution to the processes leading to recurrent stroke are discussed as well. The status of the antioxidant defense system and the degree of oxidative damage in the circulation of stroke survivors are examined. The results are interpreted in view of the effects of the vascular risk factors for stroke that include additional activation of inflammatory and free radical mechanisms. Also, the possibilities for combined therapy including antioxidants in the acute and convalescent stages of stroke are considered. Future investigations are expected to elucidate the role of free radical processes in the chronic phase after stroke and to evaluate the prophylactic and therapeutic potential of anti-radical agents.

Solid or gaseous circulating brain emboli: are they separable by transcranial ultrasound?

High-intensity transient signals (HITS) detected by transcranial Doppler (TCD) ultrasound may correspond to artifacts or to microembolic signals, the latter being either solid or gaseous emboli. The goal of this study was to assess what can be achieved with an automatic signal processing system for artifact/microembolic signals and solid/gas differentiation in different clinical situations. The authors studied 3,428 HITS in vivo in a multicenter study, i.e., 1,608 artifacts in healthy subjects, 649 solid emboli in stroke patients with a carotid stenosis, and 1,171 gaseous emboli in stroke patients with patent foramen ovale. They worked with the dual-gate TCD combined to three types of statistical classifiers: binary decision trees (BDT), artificial neural networks (ANN), and support vector machines (SVM). The sensitivity and specificity to separate artifacts from microembolic signals by BDT reached was 94% and 97%, respectively. For the discrimination between solid and gaseous emboli, the classifier achieved a sensitivity and specificity of 81% and 81% for BDT, 84% and 84% for ANN, and 86% and 86% for SVM, respectively. The current results for artifact elimination and solid/gas differentiation are already useful to extract data for future prospective clinical studies.

Oxidative stress in the chronic phase after stroke

The spontaneous and the stimulated extracellular generation of reactive oxygen species (ROS) by peripheral phagocytes, the blood antioxidant capacity and the degree of oxidative damage were evaluated in patients with severe ischemic and hemorrhagic stroke in the chronic phase of disease. It was found in patients compared to the control group that: (i) the spontaneous phagocyte oxidative activity was enhanced independently of the type of stroke and the time elapsed after stroke onset; (ii) there was no difference in the extracellular ROS generation stimulated by opsonin-dependent and independent receptor mechanisms; (iii) there was no change in the indices of blood antioxidant capacity; (iv) the concentration of plasma lipid peroxides was enhanced regardless of the type of stroke, but it significantly increased over time; and (v) the concentration of blood thiobarbituric acid-reactive material was also enhanced. It was independent of the type of stroke and remained elevated during the whole period studied. We have demonstrated an enhanced spontaneous phagocyte oxidative activity and oxidative damage to lipids in patients in the chronic phase after stroke. The elimination of generated ROS and products of lipid peroxidation from the circulation could prevent the aggravation of chronic vascular injury in patients and could reduce the possibility of a subsequent stroke. This suggests the need for complex therapy, including antioxidant treatment directed to exclude the effects of free radicals, after the oxidative stress of stroke.

Antiplatelet therapy: in search of the 'magic bullet'

The central importance of platelets in the development of arterial thrombosis and cardiovascular disease is well established. No other single cell type is responsible for as much morbidity and mortality as the platelet and, as a consequence, it represents a major target for therapeutic intervention. The growing awareness of the importance of platelets is reflected in the increasing number of patients receiving antiplatelet therapy, a trend that is likely to continue in the future. There are, however, significant drawbacks with existing therapies, including issues related to limited efficacy and safety. The discovery of a 'magic bullet' that selectively targets pathological thrombus formation without undermining haemostasis remains elusive, although recent progress in unravelling the molecular events regulating thrombosis has provided promising new avenues to solve this long-standing problem.

Embolization in atherosclerosis

Much has been learned about microembolization in the last two decades. The promising blood markers for brain injury will further enhance our future understanding of microembolic events. New surgical techniques, drugs, and devices have substantially reduced microembolization during carotid angioplasty, CEA, and CABG.