The evolving role of transcranial doppler in stroke prevention and treatment.

Without focusing on established indications for transcranial Doppler (TCD) such as monitoring vasospasm after subarachnoid hemorrhage and detecting intracranial stenosis (sickle cell disease, stroke, etc.), we describe the role of TCD in carotid endarterectomy (CEA) and angioplasty, acute ischemic stroke, as well as competence and the practice of TCD. In addition to duplex ultrasound and angiography TCD can be used to select patients for CEA because it detects hemodynamically significant extracranial stenosis and tandem intracranial stenoses, and identifies asymptomatic patients at potentially high risk of stroke because of exhausted vasomotor reactivity or brain microembolization. TCD identifies in real time brain hypoperfusion, embolism, and hyperperfusion and thus may be helpful in reducing cerebrovascular complications of CEA/angioplasty. In acute ischemic stroke, TCD can reliably identify the patency of middle cerebral and basilar arteries, high resistance flow patterns due to increased intracranial pressure, and progression to cerebral circulatory arrest. TCD also can monitor spontaneous or induced arterial recanalization. Limitations include operator and interpreter dependency, absent temporal "windows" leading to unsuccessful insonation in 10% to 15% of patients older than 60 years, as well as difficulties with probe positioning and fixation for monitoring. However, the use of ultrasound contrast agents and improved probe fixation devices help avoid these factors. The key to the successful practice of TCD is training of technical personnel and education of the interpreting and referring physicians as to when to use TCD and what to expect from it. The advantages of TCD should be given particular consideration: portability, repeatability, long-term monitoring, emboli detection, and inexpensiveness. TCD machines and transducers need to be tuned to the target disorders; that is, larger sample volume, higher power, and so forth, and TCD technology should be implemented in phases I to II as well as in phase III trials of preventive interventions and stroke therapies.