Transcranial Doppler in the Evaluation of Cerebrovascular Disease

Santiago Herrera and Anthony J. Comerota

Transcranial Doppler (TCD) ultrasonography is the only noninvasive real-time examination that adds important physiologic information to anatomic imaging studies when evaluating blood flow in major intracerebral vessels. It is considered an extension of the clinical neurologic examination, which includes ultrasound of the extracranial vessels. Introduced by Aaslid and colleagues in 1982 as a technique using ultrasound technology to record velocity measurements of cerebral arteries through the temporal bone, its original application was limited to the detection of cerebral arterial vasospasm following subarachnoid hemorrhage. Since then, TCD has rapidly evolved to an important and relatively inexpensive imaging modality with a broad range of clinical applications established by the Clinical Practice committee of the American Society of Neuroimaging (Box 1).

BOX 1   Clinical Applications of Transcranial Doppler Ultrasonography

Evaluate need for blood transfusions in sickle cell disease (children)

Detect, localize, and quantify cerebral embolism

Determine stroke pathogenic mechanism

Determine presence of intracranial occlusive disease, embolization, shunting, and impaired vasomotor reactivity

Detect progression and regression of stenoses during follow-up

Monitor vasospasm in subarachnoid hemorrhage

Monitor surgery affecting the cerebrovascular circulation (CABG, repairs of ascending aorta)

Periprocedural or surgical monitoring during carotid endarterectomy or stenting

Monitor evolution of brain death

CABG, Coronary artery bypass graft.

TCD has the advantage of being the most convenient method to evaluate the intracranial vasculature at the patient’s bedside because it allows measurements in both the acute setting and for prolonged periods. However, TCD has its limits; it is highly operator dependent and requires in-depth training and an understanding of cerebrovascular physiology, anatomy, and pathology. Moreover, there is a 15% rate of inadequate imaging through temporal windows, which is commonly observed in Asian, African American, and elderly female populations.

Technique

A TCD examination is performed with the patient in the supine position. Carotid duplex imaging in most cases should be performed as part of the patient’s evaluation, because intracranial hemodynamic properties may be altered with extracranial carotid disease.

TCD is performed through four ultrasound windows: the transtemporal, which provides the most intracranial circulatory information of all windows; the transorbital, for evaluating the ophthalmic artery and the siphon of the internal carotid artery (ICA); the submandibular window, for imaging the proximal intracranial portion of the ICA; and the suboccipital window, for evaluating the basilar and vertebral arteries.

Clinical Indications

Cerebral Ischemia

Acute Cerebral Ischemia

TCD can evaluate up to 16 intracranial arterial segments for the detection of normal, stenotic, or occluded vessels. Because TCD provides real-time information regarding the direction and velocity of blood flow and the hemodynamic significance of intracranial or extracranial stenotic or occluded lesions, it has the ability to assist in defining the pathogenic mechanism of stroke and the collateral circulation (which often is dormant under normal circulatory conditions). This information is useful to correlate intracerebral hemodynamics with information obtained from other noninvasive tests and brain imaging studies such as computed tomography (CT) or magnetic resonance imaging (MRI).

Two recent studies have validated the diagnostic accuracy of TCD versus CT angiogram for evaluating arterial occlusive disease in the setting of acute cerebral ischemia. The yield of TCD is greatest the closer in time it is performed to the onset of stroke symptoms, and anatomically it has higher yield for the anterior (sensitivity, 70%–90%; specificity, 90%–95%) than for the posterior circulation (sensitivity, 50%–80%; specificity, 90%–96%).

As reported in the Stroke Outcomes and Neuroimaging of Intracranial Atherosclerosis (SONIA) trial, TCD (and magnetic resonance angiography) can reliably exclude intracranial stenosis (negative predictive value >80%). However, if abnormal findings are encountered, a confirmatory test is required to identify a stenotic lesion.

Assessment of Collateral Circulation

The development of collateral pathways helps to maintain cerebral perfusion when arterial obstruction or occlusion is present, and TCD can be used to identify these collaterals. Under normal conditions, intracranial collateral pathways remain dormant. When a pressure gradient develops between two arterial systems owing to the presence of a flow-limiting lesion, collateral channels open. Using TCD to evaluate collateral supply can provide prognostic value based on flow direction between arterial systems. Muller and coworkers assessed collateral flow through the basilar arterial system using TCD and reported a sensitivity of 86% and specificity of 92% when compared with angiography. In another study, Maltezos and colleagues suggested that preoperative collateral circulatory evaluation with TCD may be valuable in identifying patients at risk for developing hyperperfusion syndrome following carotid endarterectomy (CEA).

Vasomotor Reactivity

Vasomotor reactivity is characterized by the ability of the cerebral circulation to maintain adequate perfusion to the brain in spite of changes in pressure gradient, systemic blood pressure, or position of the body. TCD can assess the changes in cerebral blood flow in response to such stimuli. Carbon dioxide (CO₂) is the most common agent used to alter cerebral vasomotor tone. Cerebral autoregulation maintains constant cerebral blood flow during physiologic swings of blood pressure. Changes in CO₂ concentration induce a vasomotor response that alters cerebral blood flow parallel with the blood flow velocity changes. Impairment of CO₂ reactivity and pressure autoregulation are associated with poor outcomes after head injury.

Vasomotor reserve testing is used to determine if cerebral blood vessels can dilate and maintain enough blood perfusion in response to a decrease in systemic blood pressure. During the test, voluntary breath-holding for 30 seconds induces hypercapnia, during which the flow velocity of the middle carotid artery (MCA) is monitored. The breath-holding index (BHI), which was described by Markus and colleagues, can be calculated. This may be useful in identifying patients at risk for ischemic stroke in the setting of asymptomatic extracranial internal carotid stenoses or occlusion.

Characterizing the status of a patient’s intracranial vasomotor reactivity has important prognostic implications. Silvestrini’s group found that patients who had carotid artery occlusion and who had impaired vasomotor reactivity had an annual risk of ipsilateral stroke of 32%, compared with 0% with patients with normal vasomotor reactivity. Therefore, vasomotor reactivity testing as an index of intracranial collateral capacity could have value in predicting the natural history of carotid occlusive disease and stroke risk. Additionally, identification of disturbances in CO₂ reactivity and pressure autoregulation may be used for optimization of cerebral hemodynamics after head injury.

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