Différentes applications cliniques des signaux microemboliques détectés par Doppler transcrânien et leur contribution potentielle dans la prise de décision thérapeutique

Abstract

Given the fact that most strokes are due to thromboembolism, transcranial Doppler (TCD) detection of cerebral microemboli offers the opportunity to study the pathogenesis of cerebral ischaemia and may allow identificationof patients who are at highest risk of stroke and in most urgent need of treatment. Various studies have shown that emboli detection may be very useful in different clinical situations. Studies focusing on the acute phase of stroke demonstrated that the overall frequency of microembolic signals (MES) varies very widely. Differences in the intervals from stroke onset and TCD recordings, in the duration of recordings, in the criteria used to define MES and the small size of the study groups may explain these variations. MES are mostly found among patients with potential arterial or cardiac embolic sources and are very rarely encountered among patients with lacunar stroke. These findings favour the assumption that MES correspond to brain embolism and also that embolisation does not constitute the predominant mechanism of lacunar stroke. The presence of MES detected during the acute phase of stroke also predicts increased risk of further events. The prevalence of MES has also been extensively studied in the setting of carotid artery disease. Soon after the observation of the presence of MES during endarterectomy, similar signals occurring spontaneously were recognised in patients with symptomatic carotid artery disease. Furthermore, since MES almost completely disappear after carotid endarterectomy, it was assumed that the atherosclerotic plaque in the carotid artery was the source of MES. Overall the frequency of MES is more important among symptomatic than asymptomatic patients. However, most of the studies report a higher frequency of MES in the presence of higher degrees of stenosis in the ipsilateral internal carotid artery. In fact, the more severe the narrowing, the more MES are detected. The beneficial effects of carotid endarterectomy in symptomatic and asymptomatic patients with high-grade carotid artery stenosis have been demonstrated in various studies. However, the risks of carotid surgery are not negligible and overall approximately 3 to 5% of the patients undergoing surgery may suffer a stroke or death during or just after surgery. The safety of the surgical procedure is therefore of considerable importance and it is therefore essential to improve the prevention of neurological complications. Several studies have shown that TCD monitoring of embolism and haemodynamic changes in the middle cerebral artery may in fact provide important information to the surgeon instantaneously during endarterectomy. The four following transcranial Doppler variables are independently associated with occurrence of stroke during or after the procedure: microemboli during dissection and wound closure; >90% decrease in MCA peak systolic velocity during cross-clamping and >100% increase of the pulsatility index at clamp release. Various studies also demonstrate that MES are detected in almost 70% of the cases during the first hours postoperatively and are strong predictors of ischaemic neurological deficits in the territory of the insonated middle cerebral artery. Furthermore, emboli detection during coronary bypass surgery has increasingly been recognised during these last years as potential markers for the occurrence of cognitive impairment, despite changes performed in the surgical technique, such as the introduction of arterial-line filters and membrane oxygenators. Finally, the contribution of MES detection to the therapeutical decision has been determined in various clinical situations. Rapid disappearance of MES after administration of a certain number of medications such as antiplatelets, GP IIb/IIIa antagonists receptors, S-nitrosoglutathione, a nitric oxyde donor or dextran 40, suggests that MES constitute potential markers allowing assessment of treatment efficacy and thereby reducing the risk of further cereb