Clinical application of transcranial Doppler monitoring for embolic signals

Abstract

A major advantage of transcranial ultrasound is its suitability for continuous monitoring. Microembolic signals (MES) are brief, high-intensity transients that occur when particulate microemboli or gaseous microbubbles pass through the ultrasound beam. These MES have been detected in several clinical scenarios, but rarely in age-matched controls. 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. The present article summarizes the significance of MES in different clinical settings and outlines some of the problems to be resolved so that transcranial ultrasound can be applied in clinical practice.

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.