Transcranial Doppler (TCD) is a non-invasive ultrasonic technique that measures blood flow velocity and direction in intracranial arteries. Even though the concept was known for quite some time, the clinical utility has considerably increased recently with the advent of the power M mode TCD machines. Properly validated, the accuracy of TCD almost equals to the diagnostic cerebral angiography. TCD has great clinical utility in the diagnosis of cerebrovascular diseases as well as in its management. TCD is an extremely safe investigational modality requiring minimal patient cooperation.
In Transcranial Doppler, the intracranial vessels are insinuated through the cranium using a low frequency 2 MHZ probe. Routine TCD testing relies on three natural acoustic windows present in the skull-transtemporal, transforaminal and ophthalmic windows – to study the basal segments of the main cerebral arteries. Insonation through the temporal window detects MCA (middle cerebral artery) MI segment, M2 subdivisions, A1 (Anterior cerebral artery) segment and the PCA, P1 & P2 branches (Posterior cerebral artery). These are the main arteries supplying blood to the brain.
The occipital window takes advantage of the foramen magnum’s opening into the skull. Flow in the intracranial vertebral and basilar artery is detected. These arteries supply the posterior circulation of the brain. The ophthalmic artery and carotid siphon can be detected through the orbital window. The power output of the instrument must be decreased while insonating through the orbital window. Flow velocities change with age and differ among men and women.
Transcranial Doppler is non-invasive, portable, fast, inexpensive, and repeatable, and can be used at the bedside of the patient. TCD observes flow change in real time.
Acute ischemic stroke
Arterial recanalization indicates successful thrombolysis and often precedes clinical improvement in ischemic stroke. The timing of maximum completeness or recanalization on TCD correlates with clinical recovery. The speed of clot lysis can be measured through the duration of flow improvement assessed by the Thrombolysis in brain ischemia (TIBI) residual flow signals. The speed of recanalization in human stroke is associated with early improvement and long-term outcome. The speed, timing and amount of recanalization are several parakeets of the thrombolysis that can be determined with online TCD monitoring making TCD an extremely useful modality in stroke management.
Detection of intracranial stenotic lesions
Intracranial stenotic lesions are increasingly recognized as a significant risk factor for ischemic stroke with the genetic predisposition of certain ethnic groups. TCD enables non-invasive monitoring of these lesions. These lesions can be followed up with TCD to know whether they are progressing of regressing and accordingly appropriate intervention can be done.
Vasospasm detection in subarachnoid hemorrhage (SAH)
Vasoconstriction of intracranial arteries is the leading cause of delayed cerebral infarction or delayed ischemic neurological deficit (DIND) and mortality after aneurysmal SAH. Vasospasm is clinically detected 3 or 4 days after the SAH and usually resolves after 12 days. The detection of vasospasm is important because it may potentially be treated with medications, hemodynamic management, and endovascular interventions. TCD can be useful in detecting and monitoring the degree of vasospasm following SAH.
A characteristic pattern of changes can be detected in patients with increased intracranial pressure (ICP). When ICP equals the diastolic blood pressure flow stops during diastole, flow continues during systole and spiking systolic peaks are observed. A further increase in ICP is associated with reverberating flow pattern with the flow in systolic and retrograde flow in diastolic. This corresponds to compete arrest of flow as demonstrated by angiography. TCD is useful in detecting cerebral circulatory arrest. Brain death is a clinical diagnosis and TCD can be used as a confirmatory tool. This is especially useful if organ harvesting is under consideration.
Cerebrovascular reactivity tasing evaluates the presence of abnormal cerebral hemodynamic changes of potentially identify patients at increased risk of recurrent stroke. Both IV Acetazolamide and Co2 inhalation are used to assess cerebrovascular reactivity. In patients with ICA (Internal carotid artery) occlusion and impaired cerebrovascular reactivity determines by TCD, the annual rate of cerebral ischemic events is approximately 10 percent.
Sickle cell disease
An occlusive vasculopathy characterized by a fibrous proliferation of the intima of the blood vessels often involves the basal cerebral arteries of patients with sickle cell disease. Cerebral infarction (stroke) is a common complication. TCD identifies sickle cell disease patients at increased risk of stroke
Carotid endarterectomy (CEA) and carotid artery stenting (CAS) remain important interventions for hemodynamically significant carotid stenosis. TCD monitoring is often performed to identify and correct periprocedural events that can lead to cerebrovascular complications.
TCD monitoring during CEA shows a consistent pattern of low-velocity changes. TCD monitoring has the unique capability of detecting micro embolism as it occurs. This offers a considerable edge to TCD when compared with other monitoring techniques because the majority of preoperative infarcts are through to be secondary to Cerebral Embolism.
To assess the downstream effects of extracranial carotid stenosis
TCD helps in identifying the hemodynamic significance of an extracranial carotid lesion. TCD assessment of intracranial circulation helps in identifying the extracranial carotid lesion which has to be intervened either by CAS or CEA.
Patent foramen ovale (PFO) detection
TCD helps in identifying patent foramen ovale a course of the cryptogenic stroke. Patent Foremen Ovale is a communication in the heart between the left and right atrium. By doing a Bubble TCD using agitated saline, patent foramen oval can be detected on the bedside non-invasively with accuracy comparable to Trans Esophageal Echocardiography (TEE).
- TCD has given us an opportunity for real-time monitoring of material occlusion in Acute Ischemic Stroke.
- In a stroke, TCD can identify the residual flow signals to and around the clot, the beginning and speed of clot lysis and the timing and amount of recanalization.
- TCD has a sensitivity of 91 percent and specificity of 93 percent compared to angiography for MCA (middle cerebral artery) occlusion.
- Urgent evaluation of patients with Acute Ischemic Stroke offers a high frequency of abnormal TCD findings such as occlusion, Intracranial Clot dissolution, distal embolization, reocclusion, and stenosis.