ReviewClinical application of transcranial Doppler monitoring for embolic signals
Introduction
Transcranial Doppler (TCD) is a rapid, non-invasive, reproducible and dynamic examination of the intracranial circulation. It has been used widely in recent years to predict the pathophysiology and prognosis of stroke. A unique advantage of TCD is the ability to detect microembolic signals (MES) during long-term monitoring.
In 1964, Austen et al.1 observed transient increases in Doppler signal intensity during ultrasound examination of carotid arteries in a patient undergoing open-heart surgery. Similar signals were reported in decompression sickness and during angiography.[2], [3] In 1990, Spencer et al. described high-intensity transient signal (HITS) during carotid endarterectomy.4 Other authors have detected HITS in other clinical settings, for example patients with carotid stenosis and heart valve lesions, but they were rarely detected in age-matched controls.[5], [6] Experimental studies have also demonstrated the ability of Doppler ultrasound to detect MES in animal models7 and bench-top perfusion models.8
Investigation of the occurrence of embolic signals in comparable patient populations has revealed variable findings between different centres. Reasons for this discrepancy include differences in signal identification criteria and instrumentation, because signal characteristics depend on equipment settings at the time of recording and ultrasonic carrier frequencies. To address this problem, the Consensus Committee of the 9th International Cerebral Haemodynamic Symposium9 determined the following criteria for MES: unidirectional, high intensity (⩾ 3 dB above background), short duration (< 300 ms) and accompanied by a specific audible signal. The use of a higher decibel threshold, ⩾ 6–7 dB above background, results in a marked improvement in reproducibility and interobserver agreement for the detection of MES.10
There is increasing evidence that MES have clinical significance. Potential applications of MES detection in the management of cerebrovascular disorders include determining the pathophysiology of cerebral ischaemia, identifying patients at increased risk for stroke who may benefit from surgical and pharmacological intervention, assessing the effectiveness of novel antiplatelet therapies and perioperative monitoring to prevent intra- and postoperative stroke.11
The present article summarizes the potential clinical applications of TCD detection of MES, including extracranial carotid stenosis, carotid dissection, aortic arch atherosclerosis, cardiac disorders and intracranial atherosclerosis. The importance of MES in acute ischaemic stroke and subarachnoid haemorrhage (SAH) is discussed. In addition, we present data concerning the relationship between MES and the evolution of cognitive impairment, fat embolism syndrome and vasculitis, and we review the literature regarding intra-operative monitoring for MES and investigation of the therapeutic efficacy of medical treatments. Finally, we discuss some of the technical developments underway for MES detection to become a widely applicable clinical tool.
Section snippets
Carotid stenosis
Symptomatic carotid stenosis is a significant predictor of recurrent brain infarction and other vascular events. Carotid endarterectomy (CEA) lowers the risk of ischaemic stroke in patients with recently symptomatic internal carotid artery stenosis. However, based on a risk-modelling analysis, many patients do not benefit from CEA.12 The Asymptomatic Carotid Surgery Trial (ACST)13 showed that, in asymptomatic patients with carotid stenosis of 60% or more on ultrasound, CEA halved the 5-year
Ischaemic stroke
Microembolic signals are frequently detected during TCD monitoring in unselected patients with acute ischaemic stroke despite antiplatelet or anticoagulant treatment. The prevalence of MES in acute stroke varies from 9.3% to 71%,[49], [50], [51], [52], [53], [54], [55], [56] probably due to different casemix, antithrombotic agents, criteria for MES detection, timing after stroke and number of times monitored. Del Sette et al.51 and Sliwka et al.54 evaluated MES in patients with acute ischaemic
Cerebral MES and cognitive impairment
Although MES are not associated with immediate focal neurological symptoms, there is growing evidence that ongoing and generalized occurrence of MES may lead to deterioration of cognitive function.
Some authors have reported a decline in cognitive performance after open-heart surgery using the extracorporeal circulation technique. However, although consistent postoperative test score deterioration was infrequent, neuropsychological dysfunction was associated with substantial increases in
Cerebral MES in other conditions
Detection of embolic signals in patients with cerebral vasculitis and after long-bone fractures (fat embolism syndrome) offers new possibilities in risk estimation, therapeutic stratification and understanding pathophysiology. Some of the studies in these fields are summarized in Table 2.
Carotid endarterectomy and stenting
Transcranial Doppler monitoring during CEA was first reported in the late 1980s. Initially, the main interest was in the haemodynamic effects of cross-clamping. Spencer83 reported MES during CEA and hypothesized that these were the principal cause of cerebrovascular complications from CEA.
Transcranial Doppler monitoring during successive stages of CEA has shown an association between MES and peri-operative stroke and between MES and surgical technique. Van Zuilen et al.21 reported MES in 54% of
Cerebral MES and assessing therapeutic efficacy
There is a large gulf between the demonstration of antithrombotic effects of drugs in vitro or ex vivo experiments and therapeutic effects in large, expensive clinical trials. Because of the much greater frequency of MES compared with stroke, in some clinical settings, using MES as a surrogate end-point may allow the determination of efficacy in a smaller patient sample.105 Several authors have investigated the clinical predictors and potential therapeutic implications of MES in cerebrovascular
Technical problems and advances
The high variability in the detection of MES in different studies is a problem. Difficulty in finding transcranial ultrasound windows, differentiating real MES from artefacts and differentiating gaseous from solid MES have limited the clinical applications of TCD embolus detection.
Artefacts from subtle probe movement result in high-intensity signals that may be difficult to distinguish from MES. Discrimination of MES from spontaneous speckling in the background signal and artefacts is of
Conclusion
A major advantage of TCD is the ability to monitor blood flow in real time, allowing detection of cerebral emboli. Microembolic signals have been detected in several clinical scenarios. The clinical importance of MES and the possible therapeutic implications are still a matter of debate.
The detection of MES provides important pathophysiological information in different cerebrovascular and autoimmune disorders. The presence of MES is associated with increased risk of early ischaemic recurrence
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Classification of transcranial Doppler signals using individual and ensemble recurrent neural networks
2017, NeurocomputingCitation Excerpt :During an ultrasound examination of the carotid arteries, Austen and Howry [3] discovered transient increased in the Doppler signal intensity. Similar signals were recorded in cases covering decompression sickness and angiography [4]. In [5], Doppler techniques were deployed for measuring the blood flow velocity in diagnosing vascular diseases.
Microembolic signals in patients with cryptogenic stroke with or without patent foramen ovale
2012, Journal of Stroke and Cerebrovascular DiseasesMicroembolic signals in subarachnoid hemorrhage
2009, Journal of Clinical NeuroscienceCitation Excerpt :Although MES have been reported in SAH, it is uncertain where they originate or whether they contribute to brain ischemia.5 The detection of MES provides important pathophysiological information in a variety of disorders, but their clinical importance and possible therapeutic implications are still under debate.7 There are few data on MES frequency in aneurysmal SAH and no published data on MES frequency in non-aneurysmal SAH.
Identifying the Carotid 'High Risk' Plaque: Is it Still a Riddle Wrapped up in an Enigma?
2008, European Journal of Vascular and Endovascular SurgeryCitation Excerpt :Further studies are required to assess whether this information will be useful in identifying high risk asymptomatic stenoses most likely to benefit from treatment with surgery or angioplasty. Transcranial Doppler (TCD) is the only method capable of diagnosing micro-embolic signals (MES) in the intracranial arteries33 and the frequency of MES detected in the ipsilateral middle cerebral artery has been associated with an increased risk of cerebral events in patients with symptomatic carotid stenoses.34 The detection of MES is also a potentially useful measure of the efficacy of interventions such as medication or surgery.35
Point-of-Care Ultrasound in Neurology-Report of the EAN SPN/ESNCH/ERcNsono Neuro-POCUS Working Group
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