Seminars in Cerebrovascular Diseases and Stroke最新文献

Spontaneous arterial recanalization occurs at a rate of 6% per hour, and it can be doubled with intravenous tissue plasminogen activator (TPA) therapy since early dramatic clinical improvement, a substitute for early thrombus break-up, occurs in TPA-treated patients. TPA activity can be enhanced with ultrasound including 2-MHz transcranial Doppler (TCD). TCD identifies residual blood flow signals around thrombi, and, by delivering mechanical pressure waves, exposes more thrombus surface to circulating TPA. In the CLOTBUST trial, the dramatic clinical recovery from stroke coupled with complete recanalization within 2 hours after TPA bolus occurred in 25% of patients treated with TPA+TCD compared with 8% who received TPA alone (P = 0.02). Complete clearance of a thrombus and dramatic recovery of brain function during treatment are feasible goals for ultrasound-enhanced thrombolysis that can lead to sustained recovery. An early boost in brain perfusion seen in the Target CLOTBUST group resulted in a trend of 13% more patients achieving favorable outcome at 3 months, thus providing the rationale for a pivotal trial. The ability of TPA to break up thrombi can be further enhanced with harmless diagnostic ultrasound contrast agents. Current ongoing clinical trials include phase II studies of 2-MHz TCD with ultrasound contrast agents, or microbubbles: TCD+TPA+Levovist; TCD+TPA+MRX nano-platform (C₃F₈ ImaRx). Intraarterial ultrasound-enhanced TPA delivery is tested in the Interventional Management of Stroke (IMS) clinical trial using 1.7- to 2.1-MHz pulsed-wave ultrasound catheter (EKOS). Dose escalation studies of microbubbles, ultrasound exposure, and the development of an operator-independent ultrasound device are currently underway.

High-resolution ultrasound images of the carotid artery wall can be used to identify early changes associated with atherosclerosis. The precision of the intima-media thickness (IMT) measurement is dependent on the sonographer, the imaging device and the analysis software. When done under tight quality control, IMT measurements can identify individuals at risk for future cerebrovascular events starting at an earlier age than what is possible with other diagnostic tests.

Ultrasound contrast agents (UCA) consist of shell-encapsulated air- or gas-filled microbubbles, which are capable of surviving the heart transit and are able to pass through the lung’s capillary bed when injected intravenously. They massively increase the backscattered ultrasound signals, which is clinically useful in all situations of slow and low flow as well as insufficient ultrasound penetration. Because UCA act as intravascular contrast agents, diagnosis of vessel occlusion can be made with high diagnostic confidence based on a lack of contrast enhancement. In the first section of this review, the acoustical properties, the intricate balance of forces, which act on UCA in the human circulation, and UCA-specific ultrasound imaging modes are summarized. In the second section, current clinical applications are summarized and the level of evidence regarding key statements is evaluated using the American Academy of Neurology rating system. The final section is devoted to yet experimental applications, where accumulating data suggest that use of UCA has the potential to be clinically useful and to become incorporated into routine clinical practice.

No methods routinely are used to detect brain injury during cardiothoracic and vascular surgery and no information exists on the combined time-profile and consequences of cerebral and systemic hemodynamic changes during surgery on postoperative complication and postoperative length of stay. At present, experience with neurophysiological techniques includes the ability to measure cerebral blood flow velocity/emboli and regional cerebral venous oxygen saturation by transcranial Doppler ultrasound and by Near-Infrared-Spectroscopy, respectively. Continuous monitoring of these variables along with systemic hemodynamics will provide a better understanding of mechanisms of brain and other organ injury during cardiothoracic and vascular surgery.

Ultrasound perfusion imaging of the cerebral microcirculation is a new semi-invasive bedside technique to evaluate human brain perfusion. Several approaches have been evaluated for the qualitative assessment of brain perfusion in healthy subjects and in patients suffering from acute ischemic stroke. The analysis of ultrasound contrast agent bolus kinetics yields various time-intensity curve parameters that qualitatively describe regional brain perfusion. In healthy subjects, there is a close correlation between the time to peak intensity measurements as performed by perfusion ultrasound and perfusion-weighted MRI. In the acute phase of ischemic stroke, the peak signal increase is the most useful curve parameter to predict the area of definite infarction. Ultrasound perfusion imaging performed in the early phase can predict the outcome of the individual stroke patient. Diminution and Replenishment kinetics are new modalities for the visualization of brain perfusion, the latter being more promising because of the fast imaging time resulting in a lower vulnerability to movement artifacts. The different approaches will have to be compared regarding their ability to provide valid thresholds for differentiation between normal and abnormal perfusion in the acute stroke situation. At present, the foremost limitations of transcranial ultrasound perfusion imaging are attenuation phenomena, caused by the temporal bone, which might be overcome by new imaging systems that are currently under development.

Noninvasive tests used to evaluate the stroke-prone patient need to be studied by rigorous methods. Clinicians need reliable data to avoid confusion over the wide reported ranges of diagnostic test performance, ranging from poor to perfect. Published studies of large groups of patients who are representative of the patients in whom the tests will ultimately be used are needed in order for physicians to adopt rigorous diagnostic algorithms effectively. Such algorithms reduce the undesirable clinical consequences of false-positive and false-negative test errors, and the expense of identifying diseases that cause stroke, such as intracranial atherosclerotic stenosis. This review focuses on the methodology used to evaluate the performance of transcranial Doppler, an ultrasound test used to identify intracranial atherosclerotic stenosis.