Can Transcranial Doppler Be Useful in the ED? A Review of the Literature

Transcranial Doppler offers exciting potential - but in what settings? This literature review evaluates its use in trauma, ICU, pediatric, rural, and international settings.

Transcranial Doppler (TCD) ultrasound was first utilized in 1982 as a noninvasive way to evaluate cranial blood flow, particularly of the middle, anterior, and posterior cerebral arteries (MCA, ACA, and PCA, respectively).¹ Since then, it has gained momentum as a tool to evaluate intracranial emergencies including post-aneurysmal hemorrhage, middle cerebral artery (MCA) vasospasm, cerebral ischemia/arterial occlusion, and midline shift.^2,3 While TCD is at a disadvantage to CT in definitive diagnostic ability, it boasts distinct advantages that CT cannot replicate. Neurologic emergencies rely heavily on speed of provider recognition and appropriate escalation of care. TCD has the ability to provide dynamic, real-time information on intracranial processes in a way that a single snapshot in time, as obtained by CT, cannot. While it can be difficult to obtain a good acoustic window in 5-20% of patients,⁴ TCD allows us an opportunity to diagnose and treat intracranial pathologies when other modes of brain imaging are not available. A comparison of the advantages and disadvantages can be seen in Table 1.

Table 1. Advantages and Disadvantages of TCD in the ED

In addition to the aforementioned clinical uses, cranial ultrasound has been shown to have emerging roles in the evaluation of pediatric skull fracture and other cranial pathologies.⁵ In the scope of emergency medicine practice, four promising applications for TCD have been increasingly recognized in the literature for their potential to aid providers in the care of neurologic emergencies. Here, we will review the relevant anatomy in TCD, followed by a review of the literature on four promising applications of emergency TCD: rule-in vasospasm, MCA occlusion, midline shift, and pediatrics. While we will focus on these four aspects, there are many more indications for TCD as detailed in Table 2.

Table 2. Indications for Transcranial Doppler

I: TCD Basics
Understanding the current and future applications of TCD in the emergency department first requires a review of basic concepts. In this section, we will review the anatomy, probe locations, as well as basic concepts and definitions.

Anatomy
The Circle of Willis is of central importance in TCD (see Figure 1).³

  
^{Figure 1.  A) A review of the major intracranial arteries in the Circle of Willis. Also shown is flow direction of various arteries and depths of insonation (in mm) for an average human skull. OA: ophthalmic artery, ICA: internal carotid artery, MCA: middle cerebral artery, BA: basilar artery, VA: vertebral artery, PCA: posterior cerebral artery, PCOM: posterior communicating artery, ACA: anterior cerebral artery.3 B) View of Circle of Willis and right MCA from transtemporal positioning of 2Mhz probe.}

Probe Placement
Now that we have reviewed the relevant anatomy, let’s review the transducer placement, which is key to obtaining a good acoustic window. There are four primary probe locations in TCD: transtemporal, suboccipital, orbital, and submandibular, as detailed in Table 3.

Table 3: Transducer windows and clinical application³

 Figure 2. Ultrasound Transducer Windows

^{Figure 2. Four sonographic TCD windows depicted using 2MHz ultrasound probe. A) transtemporal window B) submandibular window C) suboccipital window D) transorbital window3}

Terminology
Table 4 defines common terminology and formulas used in evaluation of TCD³

Figure 3. TCD Velocities and Measurements

^{Figure 3. Common measurements taken in TCD evaluation using transtemporal window [3]: PSV (peak systolic velocity in cm/s). EDV(end diastolic velocity in cm/s). Note how EDV is 38% (25.3/65.9) of PSV. MFV (mean flow velocity in cm/s). Note pulsatility index (PI) of 1.05 and depth and angle of insonation (3.5cm).}

II: Rule-in Vasospasm
Cerebral vasospasm is a common phenomenon following aneurysmal subarachnoid hemorrhage that can lead to worsened and irreversible cerebral ischemia, with early detection proving critical to improving patient outcomes.²² Utilizing the temporal view described above, TCD and Transcranial Color Doppler (TCCD) sonography have proven to be useful as a screening tool in identifying middle cerebral artery (MCA) vasospasm.^3,23 Evidence for the usefulness of TCD/TCCD in detecting vasospasm in other cerebral arteries is lacking at this time. Vasospasm is identified by measuring an abnormally high mean flow velocity (MFV) in the affected vessel while surrounding vasculature remains in the low to normal velocity range.² Per a meta-analysis authored by Mastantuono et al.,²⁴ TCD and TCCD detection of MCA vasospasm had a pooled sensitivity of 66.7% and 81.5% with a pooled specificity of 89.5% and 96.6%, respectively, when compared to the “gold standard” of angiography. A meta-analysis by Lysakowski et al. showed a pooled sensitivity and specificity of 67% and 99%, respectively.²⁵ Given these data, TCD/TCCD has been validated as a useful screening tool to rule-in MCA vasospasm, not to rule it out. The advantages of this technique include lack of radiation and contrast agent, as well as real-time evaluation of intracranial flow dynamics.However, the use of TCD/TCCD relies heavily on operator  experience and skill-level due to the complexity and tortuous nature of cerebral vasculature as well as the numerous factors that affect cerebral blood flow.^3,23 The primary factors affecting cerebral blood flow are age, gender, hematocrit, viscosity, carbon dioxide, temperature, blood pressure, and mental or motor activity.²³

Of note, cerebral vasospasm typically occurs 3-14 days after SAH,²⁴ so it may be difficult to use TCD for vasospasm monitoring in the emergency setting. That being said, progressive increases in MFV early in the setting of SAH were shown to be predictive of vasospasm.²⁴

III: Change in Midline Shift
Midline shift (MLS) following a traumatic brain injury or cerebrovascular incident can lead to rapid deterioration of the patient’s condition and is indicative of poor prognosis.²⁶ TCD may be used to identify this midline shift, especially in cases when immediate CT may not be an appropriate or available option. According to Montrief et al.³ and Blanco et al., MLS can be most reliably calculated through the operator’s visualization of the transtemporal window to measure the distance from each temporal bone to the midline of the third ventricle (see Figure 4 below). Additionally, the operator should measure the distance between the temporal bones to ensure that it is equal to the sum of the 2 temporal bone-to-midline measurements; this checks the reliability of the measured distances.

The following equation is then utilized to calculate the midline shift:

Midline shift = (distance A - distance B)/2

Note: A=distance from ipsilateral temporal bone to third ventricle midline, B=distance from contralateral temporal bone to third ventricle midline

If the calculated MLS is positive, then MLS is away from the ipsilateral side toward the contralateral side, with the inverse being true in the case of a negative MLS. TCD identification of significant MLS using this calculation has been shown to have a high degree of repeatability and reasonable correlation with CT findings (r² ranged from 0.58 – 0.87).^3,27,28

^{Figure 4. TCD used to identify midline shift. A) insonation from right temporal bone (via transtemporal window) to third ventricle shows distance of 6.36 cm. B) Follow-up CT scan confirming shift observed with TCD.3}

If a CT scan is not immediately available, using TCD to identify patients with deteriorating neurological status and concerns for mass effect could save valuable minutes, especially in locations that would need to transfer the patient to a tertiary care center for definitive treatment.

IV: MCA Occlusion
TCD can be utilized to assess for occlusion in cerebral arteries resulting in acute ischemic stroke, with the best evidence for use in the case of MCA occlusion. Signs of occlusion include decreased or absent arterial signal using doppler and sonographic evidence of collateral flow. To check for signs of primary stenosis, look for increased MFV at the site of luminal narrowing. Secondary stenosis can be observed as abnormal flow or decreased flow velocity distal to the lesion or increased pulsatility proximal to the lesion.²⁹ MCA occlusion identification with TCD has reported sensitivities and specificities of, 93-100% and 92-98%, respectively.^30,31,32 A limited number of studies have additionally shown utility in ICA occlusions.^3,30

TCD is also a useful tool in monitoring MCA occlusions post thrombolytic therapy. TCD has a high positive predictive value (91%) for determining complete reperfusion of the MCA via comparative Doppler pulse wave velocity measurements which utilize the non-occluded contralateral artery flow rate as a standard. Full restoration of flow rates in the MCA without any distal signals of continued occlusion correlated well with TIMI grade III flow on CT angiography (complete reperfusion).³³ Partial or no restoration of flow rates via TCD can be a good clinical indication for additional angiography and can aid in decision making for local thrombolytic therapy such as intra-arterial thrombolysis or mechanical thrombectomy.^2,3,9,33

There is some evidence that TCD monitoring post thrombolytic therapy may act synergistically with tissue plasminogen activator (tPA)to achieve an ultrasound-enhanced thrombolytic effect.³⁴ Through inducing mechanical pressure waves, ultrasound is believed to increase the surface area of thrombus exposed to intra-arterial tPA.³⁴ When tPA was combined with TCD insonation in treatment of CVA, recanalization rates were higher (37.2%) than those treated with tPA alone (17.2%).³⁴ However, recent publications have been inconsistent in demonstrating if there are true improvements in 90-day mortality and morbidity associated with TCD used adjunctively with intravenous thrombolytics.^13,35 More research is certainly indicated before this treatment modality becomes a standard of care.

V: Pediatrics
Cranial ultrasound is used frequently in pediatric practice to assess infants and neonates at risk for intracranial pathology. The use of cranial ultrasound in this population takes advantage of the relatively large acoustic windows formed by open fontanelles, while reducing exposure to ionizing radiation and need for sedation during imaging. Traditionally, this imaging is conducted by ultrasound technicians and the studies are interpreted by radiologists.³⁶

Recent cases and studies have shown an emerging role for cranial ultrasound in evaluating for pediatric skull fracture (PSFx), traumatic intracranial hemorrhage, and intraventricular hemorrhage using point-of-care ultrasound (POCUS) performed by emergency physicians. PSFx may be the most promising area of application for transcranial POCUS. While clinical decision tools such as PECARN have helped to reduce unnecessary imaging in children,³⁷ their comparative effectiveness has been questioned.³⁸ The addition of POCUS evaluation may augment the use of clinical decision tools, especially in situations with diagnostic uncertainty. Multiple studies report that POCUS in the ED for PSFx in children has sensitivities and specificities of 82-100% and 85.2-95%, respectively.^39,40,16 In addition to PSFx, a pediatric ED case study indicates the potential utility of POCUS in traumatic intracranial hemorrhage.⁵ Additionally, cranial ultrasound is currently a popular modality used for newborn perinatal hemorrhage screening in the NICU setting.⁴¹ As with all POCUS studies, there is inherent variability due to physician experience and training levels.

Table 5. Summary: 4 TCD indications discussed, their technique, and clinical applications

Conclusion
Whenever we encounter a new technology or diagnostic approach, we must weigh the benefits versus the costs. Due to the repeatability of TCD, many of these applications are useful in the ICU environment where serial monitoring is vital. TCD can be reliably used for pediatric patients with skull fractures, and increased evidence is pointing towards usage for pediatric ischemic and hemorrhagic strokes. At this point, the data indicates that TCD has a high degree of utility in ICUs, pediatric, rural, and international EDs. However, for many of us who practice in trauma centers with CT consistently and readily available, the evidence currently recommends continued usage of CT/MRI for intracranial pathologies in the acute setting.  

TCD is indeed an exciting field with a lot of potential and opportunities. As emergency physicians, we are trained to excel in providing timely, cost-effective treatment, and TCD has the potential to be widely beneficial. Regardless of the possibilities for this mode of diagnosis, it is imperative that we as an emergency medicine field decrease the variation in user ability. If we focus on increasing the level of experience and education for our medical students and physicians regarding POCUS, TCD is just one of many examples of the incredible potential that we could reach.

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