Brain echography includes transcranial Doppler (TCD) and B-mode transcranial color-coded duplex (TCCD) ultrasonography. TCD is able to identify the cerebral arteries “blindly” on the basis of the spectral display and standard criteria, whereas TCCD combines color-coded Doppler vessel representation with bidimensional pulsed-wave Doppler ultrasound imaging and enables direct visualization and improved identification of cerebral arteries.
Brain echography is a noninvasive, safe, repeatable, and low-cost bedside technique that has become a standard of care in multiple neurocritical care settings, including traumatic brain injury, aneurysmal subarachnoid hemorrhage, intracerebral hemorrhage, acute ischemic stroke, and for the diagnosis of brain death.1 Although there is evidence for the application of brain echography in clinical settings outside of neurocritical care,1 its utilization in these settings is currently limited. Increasing experience with ultrasound, especially point-of-care ultrasound, has generated interest in brain echography in intraoperative monitoring, general and cardiological intensive care, and in the emergency department.1
Here, we briefly outline the potential applications of brain echography outside the neurocritical care unit, including its use for the early diagnosis of intracranial injuries and midline shift (Fig. 1), for the measurement of optic nerve sheath diameter (ONSD) and to assess cerebral perfusion (Fig. 2), and for screening patients at risk of elevated intracranial pressure or reduced brain compliance (Fig. 3). We also provide future perspectives for the wider implementation of brain echography.
Sonographic measurement of the ONSD is a useful technique for ruling-in or ruling-out intracranial hypertension,2 and can be performed quickly and safely at the patient’s bedside (Fig. 2A). The duplex Doppler machines required for ONSD measurement are widely available and the learning curve for this technique is favorable, allowing easy application in routine practice.2 Moreover, the findings of a systematic review and meta-analysis support the measurement of ONSD in patients with cardiac arrest for accurate prediction of neurologic outcomes.3 There appears to be renewed interest in brain echography for the assessment of patients in the emergency department for the early diagnosis of brain injuries, such as intracerebral hemorrhage (Figs. 1A, B) and hydrocephalus, as well as for screening patients at risk of developing intracranial hypertension (Figs. 3A–C). In this context, brain echography could potentially be included in the Focused Assessment with Sonography for Trauma (FAST) protocol, allowing rapid diagnosis and decision making in patients with intracranial emergencies.
A pulsatility index >1.25 and diastolic blood flow velocity <25 cm/s in the middle cerebral arteries in mild traumatic brain injury patients predicts neurologic deterioration with 80% sensitivity (95% confidence interval, 56%-94%) and 79% specificity (95% confidence interval, 74%-83%),4 and can be useful for assessment of patients in the emergency department. TCCD has also been shown to have a favorable learning curve, and there is emerging interest in possible point-of-care applications of TCCD.1 Potential indications for TCCD and TCD in the emergency department include identifying elevated intracranial pressure (Figs. 3A–C), assessing midline shift (Figs. 1C, D), assessing brain compliance (Figs. 3D, E), diagnosing cerebral vasospasm, and evaluation of thrombolysis efficacy in acute ischemic stroke.1
GENERAL INTENSIVE CARE
Cerebral edema is common after acute liver failure, but invasive intracranial pressure monitoring is often not possible due to the associated coagulopathy. Brain echography using TCD is a logistically feasible option. Measurement of the resistance index in the middle cerebral artery can also accurately diagnose minimal hepatic encephalopathy in patients with cirrhosis; clinical improvement in cognition in patients undergoing liver transplantation is associated with perioperative cerebral hemodynamic changes on TCD and changes in systemic inflammation.1 Similarly, TCD can provide useful information about cerebral autoregulation and cerebral perfusion pressure in patients with sepsis.5 Mechanical ventilation strategies used in acute respiratory distress syndrome, such as positive end-expiratory pressure, prone ventilation, and lung recruitment maneuvers, are potentially associated with an increased risk of intracranial hypertension.1 In this context, brain echography can facilitate monitoring of possible cerebral complications of various ventilation strategies. Cerebrovascular complications are also common in patients treated with extracorporeal membrane oxygenation, consequent to solid or gaseous microemboli due to thrombosis within the circuit or cannulae. Brain echography is the screening technique of choice to detect these complications (Fig. 2B) that can lead to adverse consequences such as parenchymal hemorrhages, infarcts, and diffuse edema; it can also be used as an ancillary test to identify irreversible brain injury.1
Finally, in pregnant women, brain echography can be used to assess cerebrovascular changes associated with eclamptic encephalopathy.1 The ONSD is significantly greater in pre-eclamptic women compared with healthy pregnant women at delivery,6 and autoregulation determined by TCD is significantly impaired in pre-eclamptic compared with normotensive women.1
Brain echography is a useful neuromonitoring tool during procedures with a risk of neurological complications. For example, perioperative neurological complications following carotid endarterectomy are attributable to cerebral ischemia, microemboli or hyperperfusion syndrome, and TCD is the only monitoring technique that can reliably identify all of these, thereby allowing timely interventions.1 Similarly, TCD is also useful during carotid artery stenting procedures. Cardiac surgery presents particular risks related to cardiopulmonary bypass, including cerebral microemboli and cerebral hypoperfusion, both of which can be reliably detected using TCD. Brain monitoring with TCD is of particular value when deviations from established surgical or anesthetic techniques may place the brain at risk for cerebral hyperperfusion or hypoperfusion, embolization, or their combined effects.7
The ability of TCD to noninvasively identify patients with right-to-left cardiac shunts with 100% sensitivity and specificity (compared with transesophageal echocardiography), makes it suitable for the selection of patients for closure procedures and also to screen patients before sitting-position craniotomy.8 TCD during sitting-position craniotomy can also be used to identify reduced cerebral perfusion and intraoperative venous air embolism. Similarly, TCD can identify cerebral hypoperfusion during shoulder surgery in the beach-chair position.8 Surgical procedures requiring steep head-down positions, and laparoscopic surgeries requiring the creation of a pneumoperitoneum, may be associated with reduced venous return and consequent intracranial hypertension.9 Intraoperative TCD can identify cerebral blood flow changes in real time in such situations. Intracranial pressure elevations during laparoscopy can also be identified by measuring the ONSD.9 TCD can also be used to assess the functioning of ventriculo-peritoneal shunts during the perioperative period, and the pulsatility index has been shown to correlate well with ventricular size in patients with hydrocephalus.10
The clinical applications of brain echography in perioperative medicine and critical care have expanded enormously over the past decade. Yet, its utilization outside the neurocritical care setting remains limited. It is important to identify the barriers to wider incorporation of brain echography in order to explore strategies to advance evidence-based clinical applications of this technique. Until then, brain echography is at risk of remaining largely confined to neurocritical care, preventing access to this technique by patients who might benefit from it.
Chiara Robba, PhD*
Aarti Sarwal, MD†
Deepak Sharma, PhD‡
*Policlinico San Martino, IRCCS For Oncology and Neuroscience, Department of Integrated Surgical and Diagnostic Science, University of Genova, Genova, Italy
†Department of Neurology, Wake Forest Baptist Medical Center, Winston Salem, NC
‡Department of Anesthesiology & Pain Medicine and Neurological Surgery, University of Washington, Seattle, WA
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