Cerebral perfusion imaging in vasospasm

Diagnosis of Delayed Cerebral Ischemia in Patients with Aneurysmal Subarachnoid Hemorrhage and Triggers for Intervention

INTRODUCTION

Delayed cerebral ischemia (DCI) is a major determinant for poor neurological outcome after aneurysmal subarachnoid hemorrhage (aSAH). Detection and treatment of DCI is a key component in the neurocritical care of patients with aSAH after initial aneurysm repair.

METHODS

Narrative review of the literature.

RESULTS

Over the past 2 decades, there has been a paradigm shift away from macrovascular (angiographic) vasospasm as a main diagnostic and therapeutic target. Instead, the pathophysiology of DCI is hypothesized to derive from several proischemic pathomechanisms. Clinical examination remains the most reliable means for monitoring and treatment of DCI, but its value is limited in comatose patients. In such patients, monitoring of DCI is usually based on numerous neurophysiological and/or radiological diagnostic modalities. Catheter angiography remains the gold standard for the detection of macrovascular spasm. Computed tomography (CT) angiography is increasingly used instead of catheter angiography because it is less invasive and may be combined with CT perfusion imaging. CT perfusion permits semiquantitative cerebral blood flow measurements, including the evaluation of the microcirculation. It may be used for prediction, early detection, and diagnosis of DCI, with yet-to-prove benefit on clinical outcome when used as a screening modality. Transcranial Doppler may be considered as an additional noninvasive screening tool for flow velocities in the middle cerebral artery, with limited accuracy in other cerebral arteries. Continuous electroencephalography enables detection of early signs of ischemia at a reversible stage prior to clinical manifestation. However, its widespread use is still limited because of the required infrastructure and expertise in data interpretation. Near-infrared spectroscopy, a noninvasive and continuous modality for evaluation of cerebral blood flow dynamics, has shown conflicting results and needs further validation. Monitoring techniques beyond neurological examinations may help in the detection of DCI, especially in comatose patients. However, these techniques are limited because of their invasive nature and/or restriction of measurements to focal brain areas.

CONCLUSION

The current literature review underscores the need for incorporating existing modalities and developing new methods to evaluate brain perfusion, brain metabolism, and overall brain function more accurately and more globally.

Multimodality Monitoring for Delayed Cerebral Ischemia in Subarachnoid Hemorrhage: A Mini Review

Aneurysmal subarachnoid hemorrhage is a disease with high mortality and morbidity due in large part to delayed effects of the hemorrhage, including vasospasm, and delayed cerebral ischemia. These two are now recognized as overlapping yet distinct entities, and supportive therapies for delayed cerebral ischemia are predicated on identifying DCI as quickly as possible. The purpose of this overview is to highlight diagnostic tools that are being used in the identification of DCI in the neurocritical care settings.

Detection of Cerebral Vasospasm Following Aneurysmal Subarachnoid Hemorrhage Using Motor Evoked Potentials

BACKGROUND

Early detection of vasospasm (VS) following aneurysmal subarachnoid hemorrhage (aSAH) is vital to trigger therapy and to prevent infarction and subsequent permanent neurological deficit. Although motor evoked potentials (MEPs) are a well-established method for intraoperative detection of cerebral VS and cerebral ischemia during aneurysm surgery, there are no studies investigating the diagnostic value of MEPs for detecting delayed VS following aSAH in an intensive care unit.

OBJECTIVE

A prospective study was conceived to assess the diagnostic accuracy of MEPs in comparison with digital subtraction angiography.

METHODS

MEP threshold changes were determined in patients both with and without angiographic VS following high-grade aSAHs. Sensitivity, specificity, and the positive and negative predictive values of significant MEP threshold increases, which indicate angiographic VS, were calculated.

RESULTS

In all patients experiencing VS of the arteries supplying cerebral motor areas, a minimal MEP threshold increase of 50 mA (mean 66.25 mA) was observed, whereas a maximum MEP threshold increase of 30 mA was observed in patients without VS. Therefore, an increase from a baseline of ≥50 mA was considered significant and resulted in a sensitivity of 0.83, a specificity of 0.92, a positive predictive value of 0.83, and a negative predictive value of 0.92.

CONCLUSION

VS following aSAH can be detected accurately by using MEPs. MEPs are a feasible bedside tool for online VS detection in an intensive care unit and, therefore, may complement existing diagnostic tools.

Evidence-based cerebral vasospasm surveillance

Subarachnoid hemorrhage related to aneurysmal rupture (aSAH) carries significant morbidity and mortality, and its treatment is focused on preventing secondary injury. The most common—and devastating—complication is delayed cerebral ischemia resulting from vasospasm. In this paper, the authors review the various surveillance technologies available to detect cerebral vasospasm in the days following aSAH. First, evidence related to the most common modalities, including transcranial doppler ultrasonography and computed tomography, are reviewed. Continuous electroencephalography and older instruments such as positron emission tomography, xenon-enhanced CT, and single-photon emission computed tomography are also discussed. Invasive strategies including brain tissue oxygen monitoring, microdialysis, thermal diffusion, and jugular bulb oximetry are examined. Lastly, near-infrared spectroscopy, a recent addition to the field, is briefly reviewed. Each surveillance tool carries its own set of advantages and limitations, and the concomitant use of multiple modalities serves to improve diagnostic sensitivity and specificity.

Dobutamine-induced high cardiac index did not prevent vasospasm in subarachnoid hemorrhage patients: a randomized controlled pilot study

BACKGROUND

Prevention of vasospasm is a challenging issue in subarachnoid hemorrhage (SAH) patients and the objective is to compare high dobutamine-induced cardiac index (CI) and high norepinephrine-induced hypertension for the prevention of vasospasm in SAH patients.

METHODS

Single center, single blind, controlled randomized study. Patients hospitalized in intensive care unit (ICU) for a moderate to severe SAH (WFNS grade ≥2) were randomized to receive dobutamine to reach a 25 % increase in cardiac index or norepinephrine to reach a mean arterial pressure ≥100 mmHg. Norepinephrine could be used in the dobutamine group to maintain a cerebral pressure perfusion ≥60 mmHg. Primary endpoint was the incidence of angiographic vasospasm in the first 14 days in the ICU setting, which was upheld by a cerebral arteriography. Secondary endpoints were the incidence of delayed ischemic deficits, duration of mechanical ventilation, and ICU length of stay (LOS).

RESULTS

During the 3-year study, 41 patients were randomized. Six patients were excluded--3 because of consent withdrawal and 3 who did not receive the assigned treatment. Respectively, 17 and 18 patients in the dobutamine and the norepinephrine group were kept in analysis. Five (28 %) patients in the dobutamine group and 5 (27 %) patients in the norepinephrine group developed vasospasm in ICU (p = 1). Seven (41 %) patients presented a delayed ischemic deficit in the dobutamine group and 9 (50 %) in the norepinephrine group (p = 0.24). Duration of mechanical ventilation was 8 [0.5-11.5] days in the dobutamine group and 19 [2.7-23] days in the norepinephrine group (p = 0.01). ICU LOS was 11 [6-15] days in the dobutamine group and 21 [9-28] days in the norepinephrine group (p = 0.01).

CONCLUSION

As compared to norepinephrine-induced hypertension, dobutamine-induced high CI did not reduce the rate of vasospasm in SAH patients. Dobutamine may reduce durations of mechanical ventilation and ICU LOS.

A phantom approach to interscanner comparability of computed tomographic brain perfusion parameters

OBJECTIVE

This study aimed to create a phantom, which allows reproducible computed tomography perfusion experiments, and to identify the influence of contrast bolus configuration, scan parameters, and scanner hardware on the calculation of perfusion parameters.

METHODS

A discoid perfusion phantom with centrifugally directed flow was constructed. Brain parenchyma was simulated by inert polyoxymethylene spheres. Repeated measurements were performed with variations of the above-mentioned factors, and their effects on perfusion results were analyzed.

RESULTS

Calculated flow values measured during experiments were reproducible and showed good correlation with the true flow (R = 0.995, P < 0.01). Tube voltage, injection rate of the contrast agent, the mathematical perfusion algorithm, and the hardware of the scanner hardware had a reproducible influence on calculated perfusion results.

CONCLUSIONS

In the long term, perfusion phantoms might be helpful in identifying hardware-specific and protocol-related factors of different computed tomography scanners to improve comparability of different scanners and scanning protocols.

Advanced imaging modalities in the detection of cerebral vasospasm

The pathophysiology of cerebral vasospasm following aneurysmal subarachnoid hemorrhage (SAH) is complex and is not entirely understood. Mechanistic insights have been gained through advances in the capabilities of diagnostic imaging. Core techniques have focused on the assessment of vessel caliber, tissue metabolism, and/or regional perfusion parameters. Advances in imaging have provided clinicians with a multifaceted approach to assist in the detection of cerebral vasospasm and the diagnosis of delayed ischemic neurologic deficits (DIND). However, a single test or algorithm with broad efficacy remains elusive. This paper examines both anatomical and physiological imaging modalities applicable to post-SAH vasospasm and offers a historical background. We consider cerebral blood flow velocities measured by Transcranial Doppler Ultrasonography (TCD). Structural imaging techniques, including catheter-based Digital Subtraction Angiography (DSA), CT Angiography (CTA), and MR Angiography (MRA), are reviewed. We examine physiologic assessment by PET, HMPAO SPECT, ¹³³Xe Clearance, Xenon-Enhanced CT (Xe/CT), Perfusion CT (PCT), and Diffusion-Weighted/MR Perfusion Imaging. Comparative advantages and limitations are discussed.

Intensive Care Management of Subarachnoid Haemorrhage

Aneurysmal subarachnoid haemorrhage (SAH) is a devastating disease associated with high mortality and poor outcome in many survivors. Aggressive treatment by a comprehensive multidisciplinary team is associated with improved outcome, but the intensive care management of SAH presents significant challenges. Multimodal neuromonitoring may detect secondary insults before irreversible neuronal damage has occurred, and is increasingly being used to guide treatment. This article reviews current trends in the intensive care management of SAH from aspects of initial resuscitation to recent developments in the prevention and management of complications, including delayed cerebral ischaemia. Evidence from clinical trials and recent consensus guidance is reviewed.

Intensive care management of patients with subarachnoid haemorrhage

PURPOSE OF REVIEW

The aim of this article is to summarize recent concepts regarding the intensive care management of patients with subarachnoid haemorrhage, emphasizing the detection and treatment of cerebral vasospasm and the management of systemic complications.

RECENT FINDINGS

Aneurysmal subarachnoid haemorrhage is a potentially devastating disease that requires complex treatment strategies and extended monitoring. The prognosis of subarachnoid haemorrhage depends on the severity of the initial bleed, the success of the procedure to secure the aneurysm and the occurrence and severity of sequelae, including cerebral vasospasm. Patients with subarachnoid haemorrhage benefit from multidisciplinary neurointensive care where management is targeted at securing the ruptured aneurysm, optimizing cardiovascular variables, detecting and treating cerebral vasospasm and managing systemic complications.

SUMMARY

The complex treatment strategies applied after subarachnoid haemorrhage call for interdisciplinary collaboration between neurosurgeons, neuroradiologists, neurointensivists and specialist nurses. Specialized neuromonitoring and neuroimaging techniques must also be available. The neurointensive care unit serves as the focal point for these combined efforts.

Measurement of cerebral perfusion territories using arterial spin labelling

The ability to assess the perfusion territories of major cerebral arteries can be a valuable asset to the diagnosis of a number of cerebrovascular diseases. Recently, several arterial spin labeling (ASL) techniques have been proposed for determining the cerebral perfusion territories of individual arteries by three different approaches: (1) using a dedicated labeling radio frequency (RF) coil; (2) applying selective inversion of spatially confined areas; (3) employing multidimensional RF pulses. Methods that use a separate labeling RF coil have high signal-to-noise ratio (SNR), low RF power deposition, and unrestricted three-dimensional coverage, but are mostly limited to separation of the left and right circulation, and do require extra hardware, which may limit their implementation in clinical systems. Alternatively, methods that utilize selective inversion have higher flexibility of implementation and higher arterial selectivity, but suffer from imaging artifacts resulting from interference between the labeling slab and the volume of interest. The goal of this review is to provide the reader with a critical survey of the different ASL approaches proposed to date for determining cerebral perfusion territories, by discussing the relative advantages and disadvantages of each technique, so as to serve as a guide for future refinement of this promising methodology.

Clinical review: Prevention and therapy of vasospasm in subarachnoid hemorrhage

Vasospasm is one of the leading causes of morbidity and mortality following aneurysmal subarachnoid hemorrhage (SAH). Radiographic vasospasm usually develops between 5 and 15 days after the initial hemorrhage, and is associated with clinically apparent delayed ischemic neurological deficits (DID) in one-third of patients. The pathophysiology of this reversible vasculopathy is not fully understood but appears to involve structural changes and biochemical alterations at the levels of the vascular endothelium and smooth muscle cells. Blood in the subarachnoid space is believed to trigger these changes. In addition, cerebral perfusion may be concurrently impaired by hypovolemia and impaired cerebral autoregulatory function. The combined effects of these processes can lead to reduction in cerebral blood flow so severe as to cause ischemia leading to infarction. Diagnosis is made by some combination of clinical, cerebral angiographic, and transcranial doppler ultrasonographic factors. Nimodipine, a calcium channel antagonist, is so far the only available therapy with proven benefit for reducing the impact of DID. Aggressive therapy combining hemodynamic augmentation, transluminal balloon angioplasty, and intra-arterial infusion of vasodilator drugs is, to varying degrees, usually implemented. A panoply of drugs, with different mechanisms of action, has been studied in SAH related vasospasm. Currently, the most promising are magnesium sulfate, 3-hydroxy-3-methylglutaryl-CoA reductase inhibitors, nitric oxide donors and endothelin-1 antagonists. This paper reviews established and emerging therapies for vasospasm.